From 27 July to 10 August 2017, the airborne StratoClim mission took place in Kathmandu, Nepal, where eight mission flights were conducted with the M-55 Geophysica up to altitudes of 20 km. New particle formation (NPF) was identified by the
abundant presence of nucleation-mode aerosols, with particle diameters
dp smaller than 15 nm, which were in-situ-detected by means of condensation
nuclei (CN) counter techniques. NPF fields in clear skies as well as in the
presence of cloud ice particles
(dp> 3 µm) were
encountered at upper troposphere–lowermost stratosphere (UTLS) levels and
within the Asian monsoon anticyclone (AMA). NPF-generated nucleation-mode
particles in elevated concentrations (Nnm) were frequently found
together with cloud ice (in number concentrations Nice of up to
3 cm-3) at heights between ∼ 11 and 16 km. From a
total measurement time of ∼ 22.5 h above 10 km altitude,
in-cloud NPF was in sum detected over ∼ 1.3 h
(∼ 50 % of all NPF records throughout StratoClim). Maximum
Nnm of up to ∼ 11 000 cm-3 was detected coincidently
with intermediate ice particle concentrations Nice of
0.05–0.1 cm-3 at comparatively moderate carbon monoxide (CO)
contents of ∼ 90–100 nmol mol-1. Neither under
clear-sky nor during in-cloud NPF do the highest Nnm concentrations
correlate with the highest CO mixing ratios, suggesting that an elevated
pollutant load is not a prerequisite for NPF. Under clear-air conditions,
NPF with elevated Nnm (> 8000 cm-3) occurred slightly less often than within
clouds. In the presence of cloud ice, NPF with Nnm between
1500–4000 cm-3 was observed about twice as often as under clear-air
conditions. NPF was not found when ice water contents exceeded 1000 µmol mol-1 in very cold air (< 195 K) at tropopause levels. This indicates a reduction in NPF once deep convection is prevalent together with the presence of mainly liquid-origin ice particles. Within in situ cirrus near the cold point
tropopause, recent NPF or intense events with mixing ration nnm larger than 5000 mg-1 were observed only in about 6 % of the in-cloud NPF
data. In determining whether the cloud-internal NPF is attenuated or
prevented by the microphysical properties of cloud elements, the integral
radius (IR) of the ice cloud population turned out to be indicative. Neither
the number of ice particles nor the free distance between the ice particles
is clearly related to the NPF rate detected. While the increase in ice
particles' mass per time dmdt is proportional to the IR and mainly due to the condensation of water vapour, additional condensation of NPF precursors proceeds at the expense of the NPF rate as the precursor's saturation ratio declines. Numerical simulations show the impact of the IR on the supersaturation of a condensable vapour, such as sulfuric acid, and furthermore illustrate that the IR of the cloud ice determines the effective limitation of NPF rates.
Introduction
The process of gas-to-particle conversion, also denoted as homogeneous
aerosol nucleation and most commonly known as new particle formation (NPF),
is a major source of atmospheric aerosols and cloud condensation nuclei,
which could promote the cloud formation at intermediate and upper
tropospheric altitudes (e.g. Spracklen et al., 2006;
Merikanto et al., 2009; Dunne et al., 2016; Gordon et
al., 2017). Sulfuric acid (H2SO4) and water (H2O) are
important chemical compounds involved in the NPF process, which is aided
when ions come into play at elevated altitudes and cold temperatures within
the atmosphere (Lee et al., 2003; Kazil et al., 2008; Weigel et al., 2011; Duplissy et al., 2016). It was
suggested that a ternary nucleation process involves, apart from sulfuric
acid and water, an additional constituent such as ammonia (NH3; Ball
et al., 1999; Benson et al., 2009; Höpfner et al., 2019).
Experimental studies at the CLOUD (Cosmics Leaving OUtdoor Droplets) chamber
confirmed that NPF rates are substantially elevated within this ternary
H2SO4–H2O–NH3 system (e.g. Kirkby et al., 2011;
Kürten et al., 2016; Kürten, 2019). In addition
to sulfuric acid and ammonia, organic species (e.g. Metzger et al., 2010; Kerminen et al., 2010) or amines (Kürten et al., 2018) may
also promote particle nucleation and growth. Considering the quantities of
organic aerosols (Murphy et al., 2006) and ammonia species
(Höpfner et al., 2019) that were frequently found in aerosol
particles at upper troposphere (UT)–tropical transition layer (TTL) heights in the AMA during StratoClim 2017, NPF is likely
promoted by such species in the UT and TTL region.
The results of individual CLOUD experiments (Kürten et al., 2015, 2016) under a variety of controlled
conditions and at different and elevated concentrations of the
H2SO4 solution, always at supersaturated states, show how strongly
the nucleation rates are associated with the precursor concentrations. The
time series of a nucleation event within the CLOUD chamber (supplementary
material of Kirkby et al., 2011) shows that the nucleation rate remains
elevated as long as the quantity of precursors is kept at a constant level.
Under real conditions in the atmosphere, however, the concentration of
precursor material is spatially and temporally highly variable (e.g.
Speidel et al., 2007; Ranjithkumar et
al., 2021; Höpfner et al., 2019). Besides the precursor gas
abundance, temperature determines the degree of supersaturation, which
implies that even high precursor concentrations can yield weak NPF rates.
Additionally, also temperature fluctuations at any (low) precursor
concentration can increase the local supersaturation and induce intense NPF
(see Weigel et al., 2021).
For ternary or multi-component NPF, the degree of supersaturation as a
function of temperature remains indeterminable if the respective
concentration of the different substances is unknown as so far is the case
for most atmospheric observations. The chamber experiments allow for
studying the nucleation rate as a function of the precursor concentration at
different temperatures, i.e. at varying supersaturation ratios, which are
specific but mostly unknown, with respect to the system of nucleating
substances (involving H2SO4, H2O, and NH3). The
complexity increases with sulfuric acid nucleation systems involving
besides NH3 also nitric acid (HNO3) (Wang et al., 2020) or
oxidised organic vapours (Riccobono et al., 2014), all of which are
reported as promoting NPF at supersaturations lower than required for pure
H2SO4 solutions. The role of organic substances in connection with
NPF is of particular importance in the tropical upper troposphere–lowermost stratosphere (UTLS), as has been indicated
by Schulz et al. (2018) and Andreae et al. (2018). The influence of a third substance (or even more substances) possibly involved in the NPF process is not
conclusively detectable or quantifiable in the nucleation-mode particle
population due to the current lack of instrumentation capable of directly
analysing the chemical composition of such small particles.
By means of ground-based as well as airborne in situ measurements, NPF was
frequently observed to occur at various conditions and atmospheric altitudes
(Kerminen et al., 2018). Recently, Williamson et al. (2019) compiled a comprehensive data set of in situ NPF observations at altitudes
from 180 m above sea level to up to ∼ 12 km, thereby covering
a latitude range from 80∘ N to 70∘ S alongside
the Americas and probing air over both oceans, the Pacific and the
Atlantic. In tropical regions, most of the in situ NPF observations were made below
the level of zero net radiative heating, i.e. at altitudes where subsidence
or cloud formation is still well capable of efficiently removing or scavenging aerosol particles. Investigations at high altitudes (i.e. > 12 km) concerning the occurrence of NPF within clouds or in their immediate
vicinity are sparse; most of such observations are limited to tropospheric
altitudes (e.g. Clarke and Kapustin, 2002). The region above
tropospheric clouds seems favourable for NPF to occur, and possible reasons
for this are discussed by Wehner et al. (2015). They found that
the majority of their near-cloud NPF observations correlated with increased
ultraviolet irradiance, so they concluded cloud edges to be a favourable
environment for the production of precursor gases for the formation of new
particles (Wehner et al., 2015). These authors argued that nucleation and particle growth are promoted by turbulence at the cloud edges, which also Radke and
Hobbs (1991) already observed coincidently with abundant particles at
increased relative humidity. Furthermore, NPF was found to be an important
process inside the convective outflows (e.g. Twohy et al., 2002;
Waddicor et al., 2012). From measurements in the upper troposphere it is
commonly assumed that the occurrence of NPF is directly connected to deep-convective cloud systems (e.g. de Reus et al., 2001; Clarke
and Kapustin, 2002; Weigelt et al., 2009; Andreae et al., 2018).
The relationship between NPF and ice clouds is discussed in this study,
whilst the immediate connection of NPF and deep-convective events is
addressed in Weigel et al. (2021).
During in situ measurements aboard the NASA high-altitude research aircraft WB-57,
Lee et al. (2004) observed nucleation events inside subtropical and
tropical cirrus clouds between 7 and 16 km over Florida. The same authors
summarise that they found the recent occurrence of NPF in 72 % of their
measurements within clouds. Despite the conceptual notion that the presence
of cloud elements generally inhibits the formation of new particles,
Kazil et al. (2007) demonstrated by means of model simulations
that new sulfate aerosol can form within ice clouds such as cirrus. New
particles are also produced in the anvil region and cirrus decks of
mesoscale convective systems (MCSs) over West Africa (Frey et al., 2011).
The particular role of mid-latitude MCSs as a source of freshly formed
aerosol within the upper troposphere was already suggested by Twohy et
al. (2002), based on the detection of increased concentrations of particles
with size diameter (dp) greater than 25 nm, concurrently with elevated
particle volatility. In the region of the tropical transition layer (TTL)
over South America, Australia, and West Africa, the in situ measurements by
Weigel et al. (2011) revealed nucleation-mode particles in elevated
number concentrations from recent NPF. Based on coincident detections of
abundant nucleation-mode particles together with cloud elements (i.e. ice
particles of diameters 2.7 µm <dp< 1.6 mm) at
ice number concentrations always below ∼ 2 cm-3, the
authors concluded that the occurrence of NPF is mainly limited by the number
of cloud particles. The underlying notion is that the surfaces of the
cloud elements either scavenge the NPF-produced aerosol particles or remove
the nucleating vapour molecules prior to the NPF process.
Regarding the occurrence of NPF in conjunction with the presence of upper
tropospheric ice clouds, several unspecified details remain.
What are the sets of chemical species acting as NPF precursors?
Does NPF require (or not) contributions by cosmic radiation, by ions
(Lovejoy et al., 2004; Kazil et al., 2008; Weigel et al., 2011),
or by chemical agents or catalysts (e.g. Kürten, 2019)?
Which are the advantageous thermodynamic conditions for NPF within a cloud?
What are the conditions under which NPF is suppressed by the presence of ice
particles of certain size and/or number?
What are the relative contributions from clear-air or in-cloud NPF to the
aerosol population in the UTLS?
Furthermore, it is of interest how the nucleation-mode particles from
in-cloud NPF are processed:
Are the nucleation-mode particles dispersed as a contribution to the clear-air
background aerosol as soon as the cloud elements evaporate?
Or are the nucleation-mode particles scavenged by present ice particles?
A comprehensive understanding of these relationships and their influences
under real atmospheric conditions is necessary, particularly for modelling
purposes. Such insights allow for narrowing down the cloud type and
properties as well as the location in the cloud where NPF preferentially
occurs in order to obtain estimates of the importance of NPF in the cloud.
In the context of the Asian monsoon anticyclone (AMA) it is important to
clarify the origin of observed aerosol enhancements in the embedded Asian
tropopause aerosol layer (ATAL; cf. Vernier et al., 2011 and Vernier et al., 2018). NPF is an important source of aerosol particles
which are then available for further processing to form the ATAL
(Höpfner et al., 2019; He et al., 2019; Mahnke et al., 2021). Furthermore, the
relative contribution of in-cloud versus clear-air NPF is of importance in
this context.
The Asian monsoon anticyclone (AMA) is a meteorological structure, which
determines the regional circulation in the UTLS between June and September.
The AMA is associated with extensive deep convection capable of transporting
polluted air from the regional boundary layer (BL) to high altitudes (e.g.
Randel and Park, 2006; Park et al., 2007; Vogel
et al., 2014, 2019). The vertical upward transport within
the Asian monsoon circulation is an effective pathway for young air from the
BL (Vogel et al., 2019) to UTLS altitudes, accompanied by pollutants,
further gaseous material (Pan et al., 2016), and water vapour
(Ploeger et al., 2013). The constituents of the uplifted young air from
BL altitudes also comprise precursor material from anthropogenic (Vernier
et al., 2015; Yu et al., 2015; Höpfner et al., 2019;
Mahnke et al., 2021) and other sources to develop
and maintain the observed ATAL.
This study reports on the frequent occurrence of NPF in the presence of
cloud ice in the tropopause region over the Indian subcontinent during the
Asian monsoon season of the year 2017. All measurement data shown herein
were acquired during StratoClim (in July/August 2017) based in Kathmandu,
Nepal, and conducted with the M-55 Geophysica that operates up to 20 km altitude. NPF
was observed to an almost equivalent extent in clear air as well as in the
midst of cloud ice particles. This investigation summarises the various
conditions under which NPF was observed coincidently with cloud ice
particles. The caveats limiting the magnitude of encountered NPF are
examined, as are the possibly constraining mechanisms imposed by the cloud
elements' microphysical properties. The frequency of NPF observations in
coincidence with elevated ice particle densities as well as in clear air
highlights the importance of the tropopause region within the AMA as an
effective source region of aerosols.
The StratoClim field campaign, instruments, and methods
During the Asian monsoon season, between 27 July and 10 August 2017, a total
of eight scientific flights were conducted above parts of the Indian
subcontinent, out of Kathmandu, Nepal (27∘42′3′′ N,
85∘21′42′′ E), throughout StratoClim 2017 (see Fig. 1). Some of these flights also led out of
the Nepalese airspace, to eastern India and Bangladesh, and to the northernmost part of the Bay of Bengal. The occurrence of NPF was encountered (see Fig. 1) during each flight, either in clear air
or in the presence of cloud (ice) particles.
The flight patterns of the M-55 Geophysica during StratoClim 2017 over the Indian subcontinent. New particle formation
(NPF) encountered in clear air along the flight tracks is indicated by
orange colour in the main panel (a). All NPF events coinciding with the detection of cloud (ice) particles are coloured in blue. The general
perspective (b) exhibits the patterns of the eight StratoClim flights over Nepal, north-eastern India, Bangladesh, and the Bay of Bengal. For more details, see Table 1.
Number concentration of sub-micrometre-sized particles
The four-channel continuous flow condensation particle counter COPAS
(COndensation PArticle counting System; Weigel et al., 2009) was used
for measuring aerosol particle number concentrations. Particle detection and
data storage occurred at 1 Hz frequency. The COPAS channels were set to
different 50 % detection particle diameters dp50 (i.e. 6, 10,
and 15 nm). By counting aerosols (with dp50= 10 nm) downstream of a
heated (∼ 270 ∘C) sample flow line, a fourth COPAS
channel measured particle concentrations of non-volatile (nv) or refractory
particles (e.g. soot, mineral dust, and metallic aerosol material as well as,
for example, organic material mixtures not evaporating at 270 ∘C). The measured data revealed that potential artefacts on the aerosol
measurements due to the presence of ice particles, as suggested by
Williamson et al. (2019), are largely excludable for the
StratoClim data set (see Appendix A). For further details on the operation
of COPAS during StratoClim 2017, the companion paper
(Weigel et al., 2021) provides further insights,
as does the article with the technical introduction and characterisation of
the COPAS device, the aerosol inlet system, and the particle vaporiser
(Weigel et al., 2009). COPAS is an established instrument for high-altitude application, and its data were used and discussed in various studies
(e.g. in Curtius et al., 2005; Borrmann et al., 2010; Frey et al., 2011; Weigel et al., 2011, 2014; Schumann et al., 2017; Höpfner et
al., 2019).
Terminology and notations
Measured particle number concentrations N are provided in units of particle
number per cubic centimetre of sampled air (ambient conditions). To compare
aerosol observations from different pressure altitudes and, for example, for
correlations with mixing ratios of trace gases, COPAS measurements are also
given as mixing ratio n in units of particles per milligram of air
(mg-1) as calculated based on the 1 Hz resolved data of ambient static
pressure and temperature (see Sect. 2.5). With
N6 (N15) as the number concentration of sub-micrometre-sized
particles with diameter greater than 6 nm (15 nm), the number concentration
of nucleation-mode particles (denoted as Nnm) is calculated from the difference N6-N15=N6-15. This concentration of
nucleation-mode particles indicates recent NPF if the designated NPF
criterion (Eq. 1) is met:
0.8⋅N6-1.2⋅N15>0.
This criterion was reassessed for the StratoClim 2017 data set and accounts
for the COPAS detectors' signal-to-noise ratio and the counting statistics.
Further details concerning the criterion and the corrections applied to
COPAS data are provided in Weigel et al. (2021).
If compliant with the NPF criterion, a series of data points is a designated
NPF event if measured number concentrations (or mixing ratios) of nucleation-mode
particles continuously remain greater than zero over at least 5
consecutive seconds. The term NPF event duration refers to the contiguous and uninterrupted
measurement time (the sum of consecutive measurement seconds) for which the
definition of in-cloud NPF applies. Due to the detector's
signal-to-noise ratio and counting statistics, the given quantity and
durations of events that are too short (over 1–5 s) bear uncertainties in the
resulting number concentrations of newly formed particles and the event
duration. With the mean airspeed of the M-55 Geophysica (∼ 154 ± 39 m s-1), the event definition implies that within 5 s a
horizontal distance of ∼ 770 m (in flight direction) is
covered. The total of 308 individual detections of elevated Nnm coincide
with the presence of cloud elements, 104 of which fulfilled the event
criterion. Note that the in-cloud NPF events discussed herein are partially
embedded in larger NPF fields, which are identified by successive and
uninterrupted detections of elevated Nnm. One or more in-cloud NPF
events can be subsets of widespread NPF events as those discussed by
Weigel et al. (2021), where also further details
are provided concerning the persistence of the freshly formed particles in
the nucleation mode and the presence of non-volatile particles under NPF
conditions during StratoClim 2017.
The NPF rate and, hence, the intensity of NPF vary with the degree of
supersaturation of the NPF precursor (Kirkby et al., 2011; Kürten
et al., 2016). For the StratoClim 2017 data set the strength of an NPF event
is classified as
intense NPF (often used synonymously with most recent NPF) if detected aerosol densities of
nucleation-mode particles exceed
mixing ratios of 10 000 mg-1 or
number concentrations of 5000 cm-3;
intermediate NPF when number densities of nucleation-mode particles are in the ranges of
mixing ratios of 1000 mg-1<nnm< 10 000 mg-1 or
number concentrations Nnm between 500 and 5000 cm-3; and
weak NPF when
mixing ratios nnm remain below 1000 mg-1, or
number concentrations Nnm of less than 500 cm-3 are detected.
As the persistence of the particles in the nucleation mode is short (i.e. a
few hours only due to coagulation; cf. Weigel et
al., 2021), an intense NPF event could still be in process when observed,
or it had expired recently, i.e. 1–2 h prior to the detection. For NPF
encounters with low or intermediate nnm (or Nnm), the conclusions
concerning the event's age remain ambiguous since they can result from a
proceeding event with a low NPF rate or from an event that had expired
several hours previously.
Cloud particle and water vapour detection
The NIXE-CAPS (New Ice eXpEriment: Cloud and Aerosol Particle Spectrometer,
in the following denoted as NIXE) was deployed during StratoClim for
measuring the number size distribution in the cloud particles' diameter size
range of 3–930 µm with 1 Hz resolution (Luebke
et al., 2016; Costa et al., 2017; Afchine et al., 2018). The
NIXE-CAPS consist of two detectors: the NIXE-CAS-DPOL (Cloud and Aerosol
Spectrometer with Detection of POLarization) and the NIXE-CIPg (Cloud
Imaging Probe – grayscale). The compiled measurement data of both
independent detectors delivers microphysical properties, in terms of size
and number, of particles with diameters ranging from 0.61 to
937 µm. The methods of post-flight data processing and corrections
were described by Luebke et al. (2016).
In the StratoClim 2017 data set, cloud particle detections were recognised
as such when particles of diameters > 3 µm were
encountered in numbers greater than zero. The number concentration of ice
particles is denoted as Nice (i.e. N3–937µm for the number
concentration of ice particles with diameters of 3 µm <dp< 937 µm). The data of ice water content (IWC) used
herein were ascertained by using the relationship of cloud particles' mass
(mp) to diameter (dp) (Krämer et al., 2016; Luebke et al., 2016; Afchine et al., 2018).
The closed-path Lyman-α photo-fragment fluorescence hygrometer FISH
(Fast In situ Stratospheric Hygrometer; cf. Zöger et
al., 1999; Meyer et al., 2015) allows for 1 Hz resolved
measurements of the atmosphere's gaseous- and solid-phase water, denoted as
total water or H2Otot. The FISH detection of H2Otot
covers mixing ratios of 1–1000 µmol mol-1 over atmospheric
pressures ranging from 50 to 500 hPa with an accuracy and precision of
6 %–8 % and 0.3 µmol mol-1. The IWC was
calculated by subtracting the H2OGas (measured by another
Lyman-α detector, FLASH, the FLuorescent Airborne Stratospheric
Hygrometer) from H2Otot. For further details concerning the data
processing, see Afchine et al. (2018). Dependent on ambient temperatures,
the smallest IWC detectable by the FISH instrument is approximately between
1 × 10-3 and 20 × 10-3µmol mol-1, which corresponds to approximately 1–20 × 10-4 mg m-3 (Afchine et al., 2018).
To cover the wide range of IWC observed during the StratoClim mission (from
thousandths to thousands of µmol mol-1) the complementary data
sets of NIXE-CAPS and FISH concerning IWC were merged (see Krämer et al., 2020).
Carbon monoxide
In the troposphere, carbon monoxide (CO) is a component of atmospheric
pollution (Park et al., 2009), the main sources of which are
both natural and anthropogenic (including combustion and the oxidation of
hydrocarbons). Measured CO mixing ratios are often used as a dynamic tracer
for air parcel transport. Typical CO mixing ratios range from unpolluted
50 nmol mol-1 up to mixing ratios well exceeding 700 nmol mol-1 in
close vicinity of emission sources (Clerbaux et al., 2008; Park et al., 2009). Inside the AMA and up to 15 km altitude,
CO mixing ratios remain comparatively high (100 nmol mol-1), while
between 15 and 20 km altitude the CO mixing ratios decrease monotonically
down to ∼ 40 nmol mol-1 (Park et al.,
2009).
During the StratoClim mission, the mixing ratio of CO was measured by means
of the tunable diode laser (TDL) technique implied in the revised version of
the cryogenically operated laser diode (COLD) spectrometer. Compared to the
previous instrument version (4 s temporal resolution; Viciani
et al., 2008), COLD-2 integrates improvements (Viciani et al.,
2018) regarding
a measurement resolution increased by a factor of 4,
an enhanced in-flight sensitivity of the COLD-2 spectrometer (ranking at
∼ 2 nmol mol-1 at integration times of 1 s), and
an accuracy of 3 % specified for the CO measurement with COLD-2.
In the data set of simultaneous measurements of COPAS and COLD-2, minimum
and maximum CO mixing ratios of 14 and 153 nmol mol-1
are included.
Data of ambient temperature and static pressure
The atmospheric temperature and pressure data were taken from the Unit for
Connection with the Scientific Equipment (UCSE; Sokolov and
Lepuchov, 1998), a part of the navigational system of the M-55
Geophysica. UCSE data are provided as 1 Hz resolved ambient pressure (with an
accuracy of ±1 hPa) and temperature (±2 K accuracy).
The potential temperature θ is calculated with 1 Hz resolution in
compliance with the definition by the World Meteorological Organization
(WMO, 1966). Note that for the given vertical temperature gradients
and over the θ range covered during StratoClim 2017 (i.e. up to
∼ 477 K), the WMO-recommended calculation of θ
differs only by up to ∼ 1 K from the values obtained by using
the recently reappraised θ calculation
(Baumgartner et al., 2020).
Observations and results
During StratoClim 2017, eight mission flights were conducted with a total of
36.6 flight hours, whereas over a total of 6.42 h ice clouds were
encountered at air temperatures colder than 240 K. The cirrus cloud
observations are described and discussed by Krämer et al. (2020). Most of the
in-cloud measurements during StratoClim 2017 were performed at temperatures
≲ 205 K, corresponding to potential temperatures above ∼ 355 K
and geometric altitudes higher than ∼ 12 km. The clouds
observed during the Asian monsoon season include (1) in situ cirrus, which had
formed in dynamically calm situations associated with very slow updraught, as
well as (2) liquid-origin cirrus, the formation of which is connected to deep (including
overshooting) convection with elevated uplift velocities (see Sect. 5.2),
including ice clouds (e.g. anvils) associated with convective outflow.
Number concentrations (1 Hz resolved) of aerosol particles in the nucleation-mode size range (Nnm) and of cloud (ice) particles (Nice) of the eight StratoClim flights compiled in
one time series ranging from 03:30 to 12:30 UTC. Kathmandu's (Nepal)
local noontime is indicated by the vertical orange line (corresponding to
06:15 UTC, or 22 500 s of the day; UTC). (b) Incidences of
concentrations Nnm exceeding 500, 1000, and 5000 cm-3 within 15 min time intervals. Data points of Nnm in
black whenever Nice (cyan) equals zero; otherwise Nnm is coloured in red. The dashed blue line (c) indicates the median of Nice (0.031 cm-3) over all cloud particle detections during StratoClim 2017 (Krämer et al., 2020).
At temperatures colder than 205 K, Nice and IWC often reached values above
their respective median of 0.031 cm-3 (dashed blue line in
Fig. 2c) and ∼ 0.2–2 µmol mol-1 (see Fig. 6). The highest observed IWC values at these temperatures reach up to 1000 µ mol mol-1 with a maximum Nice as high as 30 cm-3. Moreover, the ice crystal sizes
(not shown here) exceed their corresponding median; hence, comparatively
large ice crystals were found up to and around the cold point tropopause.
Such large particles were detected during flights in strong convection.
The distribution of NPF and the presence of cloud ice particles over daytime
During a total of ∼ 22.5 h of COPAS measurement time at
altitudes above ∼ 10 km (θ≳ 350 K) a total duration of 2 h and 38 min was spent under NPF conditions in the TTL region
(∼ 11–17.5 km, ∼ 355–400 K; see Weigel et al., 2021). Throughout the
StratoClim 2017 mission, elevated number densities of nucleation-mode
particles were observed coincidently with cloud particles
(Nice> 0 cm-3) over a total of about 1 h and
17 min (see Table 1). The encountered in-cloud
NPF events at altitudes between approximately 11 and 16.5 km
(∼ 355–385 K) had a mean event duration of 14.5 s (ranging from 1 s to a maximum of about 300 s; median duration: 2 s).
NPF data set of StratoClim 2017, separated by event detection under
clear-air (i.e. Nice= 0 cm-3) and in-cloud conditions (i.e. Nice> 0 cm-3). Discussed in-cloud NPF events (104 incidents that comply with introduced NPF criterion; Sect. 2.2) are
partially embedded in larger clear-air NPF fields with continuously elevated
Nnm. The total number of measurement seconds with NPF detections under either of both conditions is scaled to the total data set of the condensation nuclei (CN)
measurements and the total duration of NPF encounters. The mean horizontal
distance is calculated from the event duration based on a mean flight speed
of the M-55 Geophysica (154 ± 39 m s-1), providing an equivalent horizontal
extension of NPF. Geometric heights are interpolated values with maximum
range of scattering from UCSE data of the eight flights.
In Fig. 2, all NPF detections throughout the
StratoClim mission are compiled in a 1 d time series. The range of this
time series is limited to the schedules of the eight mission flights between
03:30 and 12:30 UTC, corresponding to local times of 09:15 to
18:15 LT. The encounter of NPF is considered to be a clear-air observation (black data points in Fig. 2) when cloud (ice) particle number concentration Nice remained at
0 cm-3. Coincident observations of NPF and cloud (ice) particles
(Nice> 0 cm-3) are highlighted by red points in
Fig. 2a and c. The number of in-cloud
NPF encounters exceeding different thresholds of measured particle number
concentration Nnm (500, 1000, and 5000 cm-3;
Fig. 2b) shows for StratoClim 2017 that
the intense events of in-cloud NPF occurred predominantly in the late
morning, well before local noon. The incidences of in-cloud NPF accumulate
in the later morning hours as well as in the local afternoon. Temporal
dependencies on daytime were not observed for the occurrence, severity, or
frequency of NPF.
Vertical distribution of nucleation-mode particles in presence or absence of cloud ice particles
Figure 3 displays the vertical distribution of
NPF-generated nucleation-mode aerosols in terms of the mixing ratio
nnm as a function of potential temperature. Figure 3 a depicts the clear-air observations of elevated nnm (black) together with those when ice particles
were coincidently detected (red). The coincident observation of ice particles and
nucleation-mode aerosols is vertically limited to a range of potential
temperatures from 355 to 385 K (see also Table 1).
Thereby, in-cloud NPF of intermediate strength was encountered together with
convective overshooting up to altitudes above the mean tropopause height
(∼ 380 K, averaged over the StratoClim 2017 period and area
of operation). Further above (above 385 K and up to ∼ 400 K) and at altitudes below 355 K, exclusively clear-air NPF was sampled.
As already indicated by Fig. 2, also the vertical
profiles in Fig. 3 suggest that the strength of
NPF was largely independent from the presence of cloud elements. The
intermediate panels (c and d) in Fig. 3 show the
StratoClim NPF data after their separation into clear-air and in-cloud
conditions. Figure 3c shows that in-cloud NPF
observations were made during each of the eight mission flights (see Fig. 1). During the first four flights (from 27 July through 2 August) no in-cloud NPF was found above 365 K since deep
convection occurred more sparsely in the first half of the StratoClim
mission period than in the second half (Bucci et
al., 2020). During the second half of the mission flights (4 to 10 August), the frequency and the spatial extent of in-cloud NPF events were
increased.
Vertical profiles of the mixing ratio (1 Hz resolved) of aerosols in the nucleation-mode size range (nnm) versus the potential temperature (θ). (a) All data separated concerning coincident detection of cloud (ice) particles (black: Nice= 0 cm-3; red: Nice> 0 cm-3) and (b) all data coloured in reference to the flight date (c) exclusively for Nice> 0 cm-3 and (d) when
Nice= 0 cm-3. In panels (e) and (f), in-cloud and clear-air measurements are distinguished as in intermediate panels (c, d) and coloured with reference to carbon monoxide (CO) mixing ratios.
The comparison of CO mixing
ratios and NPF occurrence in the tropical UTLS over West Africa (Weigel
et al., 2011) suggested a link between NPF rate and ground-level sources of
NPF precursors. These precursors (likely sulfur compounds, possibly also
organics) are thought to be efficiently lifted into the TTL region by
convection and not completely removed by scavenging. NPF should most
frequently occur in air enriched with precursor material and which
experienced vertical uplift. According to Fig. 3e and f, neither clear-air nor in-cloud NPF exhibits nnm maxima coincidently with the highest CO mixing ratios. This
is not a typical characteristic of only in-cloud NPF, as is discussed in more
detail in Weigel et al. (2021). During in-cloud
NPF, the highest densities of nucleation-mode particles were observed at
moderate CO mixing ratios of ∼ 90–100 nmol mol-1. In
air masses with the lowest CO content (∼ 40–60 nmol mol-1), NPF was observed only above the tropopause
(θ> 380 K) and in the absence of ice particles with
nnm ranging from 300 mg-1 to a maximum of 2000 mg-1.
The most intense NPF, i.e. with the highest densities of nucleation-mode aerosols,
was found below the tropopause (∼ 380 K). In the presence of
ice particles (as in clear air), intermediate nnm values were also
encountered at CO mixing ratios below ∼ 70 nmol mol-1 at
potential temperatures of 370–380 K. Under clear-air conditions, NPF
occurred even at much lower CO mixing ratios (mainly from measurements on 29
and 31 July), which is shown by the nnm vertical profile at altitudes above 385 K (Fig. 3f). According to Fig. 3, in-cloud NPF was predominantly found in an altitude band between 355 and 385 K (corresponding to ∼ 9–16.5 km) with nnm in the range of about 1000 to 50 000 mg-1
(∼ 500–11 000 cm-3). Thennm values of NPF in ice
clouds generally do not differ from those of NPF under clear-sky conditions.
Statistics of NPF events in the presence of ice particles
The frequency of NPF occurrence in coincidence with ice particles is
illustrated in Fig. 4. The upper panel (Fig. 4a) exhibits the absolute frequency of occurrence of number concentrations Nnm observed during NPF events. The
graphs compile all measurements (more than 4600 samples of 1 Hz resolved
data; see Table 1) which comply with the NPF
criterion (black) for a comparison with clear-air NPF events (green) and
those which were coincidently detected with ice particles (red). At heights
of in-cloud NPF observations (i.e. between 355 and 385 K), the number
concentrations of particles larger than the nucleation mode, i.e. N15
and N65, were in the range (by median) of ∼ 200 cm-3<N15< 1000 cm-3 (COPAS) and ∼ 60 cm-3<N65< 150 cm-3 (from measurements with the modified ultra-high-sensitivity aerosol spectrometer, UHSAS;
Mahnke et al., 2021). Two features are apparent:
Number concentrations Nnm of more than ∼ 8000 cm-3
seem to be observed more frequently (about 1.5 times more often) in
clear-air conditions. As the number of in-cloud NPF observations with
Nnm> 8000 cm-3 is comparably low (≤ 10
encounters), the statistics are likely insufficient for drawing further
conclusions from this. It is discussed in Sect. 6 whether the presence of cloud ice constrains the
chance to detect very recent NPF (resulting in high Nnm).
For NPF in the presence of cloud ice, number concentrations Nnm between 1500–4000 cm-3 were observed about twice as often as under clear-air
conditions (Fig. 4a).
The highest Nnm values are found mainly in the absence of deposition
surfaces, which ice particles would provide. It seems less understandable
why NPF should generate a particular range of Nnm more frequently in the presence of cloud ice. Further discussion on this issue is provided in
Sect. 6.
Histograms of the occurrence frequency of number concentrations Nnm of all NPF detections (1 Hz resolved) throughout StratoClim 2017. (a) All data of Nnm in general (black) and
separated concerning coincident detection of cloud (ice) particles in the
diameter size range 3 µm <dp< 937 µm
(green: Nice=N3-937μm= 0 cm-3; red:
Nice> 0 cm-3). The sum of the green and red curve
yields the black curve, the vertical bars of which represent the square root
of counts. (b) Relative occurrence frequency of Nnm for in-cloud NPF (if detected coincidently with Nice> 0 cm-3), normalised with respect to all NPF detections, i.e. the ratio of the absolute occurrence frequencies (in red and black in panel a). (c) Relative occurrence frequency of Nnm for in-cloud NPF if detected coincidently with various Nice levels, which were normalised with respect to those
NPF detections with Nice> 0 cm-3, (in red; panel b).
Until this point, the presence or absence of ice particles was distinguished
by the criteria Nice= 0 cm-3 or Nice> 0 cm-3, respectively. Figure 4b depicts the
occurrence frequency of Nnm with ice particles Nice> 0 cm-3 normalised to the occurrence frequency of Nnm of all NPF events (black curve in Fig. 4a). More than
75 % of observed NPF cases with 2000 cm-3<Nnm< 4000 cm-3 (∼ 200 samples) were
detected while ice particles were present. In Fig. 4c, the occurrence frequencies of Nnm are compiled for various levels
of number densities Nice, which were normalised to Nnm at Nice> 0 cm-3 (red curve in
Fig. 4a). Thresholds of Nice are set with
stepwise-increasing number concentrations (by 1 order of magnitude) to
investigate whether the NPF is eventually constrained or influenced by the
ice particle number density.
Although very faint, so-called sub-visible cirrus clouds were found to
comprise very small ice particle number concentrations of
10-5 cm-3 (corresponding to 0.1 L-1; cf. Kübbeler et
al., 2011; Spreitzer et al., 2017). Sub-visible cirrus with
Nice< 10-3 cm-3 are assumed to have negligible
influence on the NPF process, as is also to conclude from
Fig. 4c. Therefore, a first threshold level is
set to Nice> 10-3 cm-3 (magenta curve), followed
by the threshold level of Nice> 10-2 cm-3
(corresponding to 1–10 ice particles per litre; blue curve), which still
represents a comparatively small amount of Nice within sub-visible cirrus clouds (cf. Thomas et al., 2002;
Peter et al., 2003;
Davis et al., 2010; Frey et al., 2011). The
maximum observed Nice reached up to ∼ 3 cm-3.
Concerning the frequency of observed Nnm, the difference between
Nice> 0 cm-3 and Nice> 10-3 cm-3 appears negligibly small. This leaves us to conclude that elevated Nnm was mostly observed coincidently with ice crystal number densities greater than 10-3 cm-3. With rising Nice level
(above 10-2 cm-3), the occurrence frequency of the highest
Nnm (>∼ 5000 cm-3) decreased. When
Nice exceeds 10-1 cm-3, the occurrence of
Nnm> 4500 cm-3 is significantly reduced, and
Nnm> 8500 cm-3 was absent. At the highest observed
Nice of ∼ 3 cm-3, NPF with Nnm> 250 cm-3 was not detected anymore.
Hence, events with the highest NPF rates occurred preferentially at low ice
particle concentrations or in clear air. At a certain Nice level
(∼ 3 cm-3), the process of NPF appears to be suppressed,
in general agreement with earlier findings (Weigel et al., 2011),
indicating the limitation of NPF by 2 cm-3 of cloud ice particles with
diameter larger than 2 µm. Among other incidences, a singularly
observed event was discussed (Weigel et al., 2011), during which NPF appeared to be suppressed
by abundant cloud ice particles, while upon leaving the cloud the NPF
re-emerged at almost previously observed concentrations of nucleation-mode
particles. These findings suggest that NPF is entirely prevented in cases
when Nice exceeds 2–3 cm-3.
In-cloud NPF related to IWC and cloud particle number densitiesThe relationship between cloud ice and aerosols
Based on in situ measurements over northern Australia and over West Africa, de
Reus et al. (2009) investigated the relationship between the number density
of sub-micrometre-sized aerosol particles and the abundance of cloud particles at UTLS
levels. The authors provided aerosol and ice particle number concentrations
averaged over the duration of various cloud encounters to determine the
proportion of sub-micrometre-sized particles that potentially convert into
cloud ice. In the context of homogeneous ice nucleation, a specific
relationship between the number concentration of aerosol and of ice
particles cannot be expected (Kärcher and Lohmann, 2002), whereas
such a relationship is inherent in the ice clouds' heterogeneous freezing
process. From their analyses, de Reus et al. (2009) concluded that a
similar range of ice–aerosol ratios is observable in the convective outflow
of both ordinary tropical convection (Australia) and large mesoscale convective systems (MCSs; West Africa).
Figure 5 depicts the StratoClim 2017 data
correspondingly to the data presentation by de Reus et al. (2009) from UT
measurements in 2005 during SCOUT-O3 over Darwin, Australia. Reference
lines are included, which indicate the number of
encountered cloud particles per number of sub-micrometre-sized aerosol
particles. The two panels in Fig. 5 comprise the
identical set of data points of ice cloud encounters during StratoClim 2017,
each of which are averaged over at least 10 s and up to
∼ 23 min.
StratoClim 2017 data of the total number concentration N10 together with coincident detections of Nice (i.e.
N3–937µm) by the NIXE-CAPS. The vertical bars represent the standard deviation over the averaging periods. Data points are colour-coded in panel (a) with reference to IWC. (b) NPF encounters (orange) throughout the averaging period (otherwise grey). Shaded blue areas in both panels indicate the range of most of the data points provided by de Reus et al. (2009). Reference lines for concentration ratios of 1 : 300 and 1 : 30 000 (as in de Reus et al., 2009) and here additionally for 1 : 500 000 and 1 : 5 000 000 are
provided.
Several occasions were identified by de Reus et al. (2009) when
comparatively high ratios with up to a few hundred aerosol particles
remained non-activated per single ice particle. The cloud ice–aerosol ratios which were found in the Asian monsoon's convective outflow region agree with previous observations (de Reus et al., 2009), which were
limited to the shaded blue area in Fig. 5. Total
aerosol numbers of significantly less than a few hundred per single ice
particle were observed neither during StratoClim 2017 nor by de Reus
et al. (2009). Up to N10 of 700 cm-3 almost all StratoClim data
result from measurements at mean ambient temperatures colder than -75 ∘C (i.e. the temperatures at which the observations by de Reus et al., 2009, were made). Frequent observations were made at aerosol concentrations
below 1000 cm-3. Compared to previous findings, the StratoClim data set
comprises more observations at cloud ice–aerosol ratios between
1 : 3000 and 1 : 500 000, including frequent events of elevated N10
(> 103 cm-3). High N10 of more than
6000 cm-3 was observed at IWC values mostly below 10 µmol mol-1 (i.e. log (IWC; µmol mol-1) ≈ 1;
Fig. 5a). The majority of observations were made
at mean IWC values below ∼ 300 µmol mol-1 (i.e.
log (IWC; µmol mol-1) ≈ 2.5), which rules out that the
measured N10 was impacted by shattering artefacts from ice particles
(see Appendix A). The majority of NPF occurrences (mostly at ambient air
temperatures between -50 and -80 ∘C) coincide
with cloud ice–aerosol ratios between 1 : 3000 and 1 : 500 000 (see Fig. 5b). The data points with in-cloud NPF
concentrate between ratios of 1 : 30 000 and 1 : 500 000 because as a
consequence of NPF, the aerosol proportion in the cloud ice–aerosol ratio is strongly elevated. Concentration N10 of more than
1000 cm-3 were not detected at ratios greater than 1 : 3000. For
N10 above 500 cm-3 and for cloud ice–aerosol ratios smaller
than 1 : 30 000, i.e. where elevated total aerosol concentrations mostly
coincide with lower ice particle densities (∼ 10-3–10-1 cm-3), the observations occurred during NPF. Cloud ice–aerosol ratios greater than 1 : 3000 were reached mostly in
the absence of NPF.
As pointed out by de Reus et al. (2009), there are caveats inherent to
this kind of analysis. The strength or efficiency of the aerosol activation
is not straightforward to deduce from provided ratios of total aerosol and
cloud particle numbers. Many interdependencies exist that may impact the
illustrated relationship, such as
the altering of the aerosol particles (coagulation, condensation) or of the
cloud elements (sedimentation)
the mixing of air masses with different aerosol and/or variable vapour
saturation characteristics (entrainment).
The type of ice formation process (liquid-origin or in situ) and the convection dynamics
additionally affect the relationship of cloud elements and interstitial
aerosol. Assigning nucleation-mode particles of thousands per
cubic centimetre (or more) to result from NPF is straightforward. In
contrast, Nnm of a few tens to hundreds of particles per cubic centimetre is potentially filtered by
the NPF criterion and is probably not identified as an NPF event if detected
together with total aerosol concentrations (N10) of comparable numbers.
Apart from demonstrating the reproducibility of earlier findings (de Reus
et al., 2009), the data set was extended by new observations at different
conditions (including NPF) obtained from StratoClim measurements.
NPF in the IWC–<inline-formula><mml:math id="M443" display="inline"><mml:mi>T</mml:mi></mml:math></inline-formula> parameter space
Analyses in earlier cirrus-related studies concerning the clouds' ice water
content (IWC) as a function of ambient air temperature provide insight into the processes inherent to the cirrus formation (Krämer et al., 2016).
As introduced by Luebke et al. (2016), Krämer et
al. (2016), and Wernli et al. (2016), a distinction of cirrus
clouds regarding their formation mechanism is obtainable within the IWC–T
parameter space. The cirrus forms in situ at elevated altitudes and instantaneously
at sufficiently cold temperatures. The liquid-origin cirrus cloud forms on convective
uplift from initially liquid droplets at lower altitudes (and less cold
temperatures). More specifically, Wernli et al. (2016)
distinguish between the following.
Liquid-origin cirrus. Initially well-sized liquid cloud droplets freeze at almost
thermodynamic equilibrium in the ambient temperature range 235 K <T< 273 K under nearly saturated conditions with respect to liquid
water (relative humidity RHw of ∼ 100 %) but at high
supersaturation with respect to ice (RHi≫ 100 %), while at
freezing level, the water can coexist in each of its three phases.
In situ cirrus. Under exclusion of pre-existing large liquid cloud droplets, ice
crystals nucleate heterogeneously (due to deposition freezing) or freeze
homogeneously from tiny super-cooled aqueous solution droplets
(Koop et al., 2000), which are designated as “too small to be
considered as cloud droplets” (Wernli et al., 2016).
In Fig. 6 the IWC versus ambient air temperatures is
displayed for all cloud encounters throughout StratoClim 2017 as a function
(colour code) of
the mixing ratio of nucleation-mode particles (i.e.
n6-15=nnm; Fig. 6a),
the total mixing ratio n6 of particles with dp> 6 nm (Fig. 6b), and
the CO mixing ratio (Fig. 6c), respectively.
NPF in the IWC–T parameter space (Krämer et al., 2016):
measured ice water content (IWC) coincidently detected with COPAS data as a
function of ambient air temperature throughout StratoClim 2017 (1 Hz resolved). Data points are colour-coded referring to (a) the detected mixing ratios of nucleation-mode particles, nnm; (b) the total mixing ratio n6; and (c) the carbon monoxide (CO) mixing ratio. Note: in panel (a), the data points are grey if data of n6-15 are available, while colours are apportioned only to those nnm (i.e. n6-15) complying with the NPF
criterion. Generally, the black lines represent the median (solid) and the
upper- and lowermost bounds (dashed) of the core IWC band, respectively, as
obtained from earlier measurements at other locations (Krämer et al., 2016).
The upper panel of Fig. 6 includes two data sets:
(1) all data from StratoClim 2017 in 1 Hz resolution (grey data points)
and (2) only the resulting nnm complying with the NPF criterion (colour-coded data points). At very low ambient air temperatures (∼ 200 K and colder) and for comparatively high IWC values, the n6-15 (grey)
data were available, but many failed the NPF criterion. The absolute values
of the mixing ratio n6 of sub-micrometre-sized particles were relatively
high (Fig. 6b). The detection of likewise
excessive mixing ratios n15 (without illustration) resulted in
n6-15 that did not exceed the specified threshold of the NPF criterion
(see Sect. 1.1). Nevertheless, most of the n6-15 data points which
failed the NPF criterion (see the grey points in
Fig. 6a) coincide with the mixing ratios
n6 reaching up to several thousands of particles per milligram. It is not deducible
from COPAS measurements how the enriched particle densities (n6 and
n15) distribute over the diameter spectrum of the sub-micrometre-sized
aerosols. It therefore remains open whether the restrained n6-15 is due
to expired NPF with particles' rapid coagulation (with background aerosol
and cloud ice) out of the nucleation-mode size range
(Weigel et al., 2021) or whether the particle
enrichment (consistently in n6 and n15) is due to larger particles
that were lifted with deep convection. The main findings from these
juxtapositions can be summarised as follows:
The absence of NPF with IWC exceeding 1000 µmol mol-1 in very cold
air (Fig. 6) suggests that NPF is constrained as
soon as deep convection prevails due to the presence of predominantly
liquid-origin ice particles. IWC exceeding 1000 µmol mol-1 at air temperatures
colder than 200 K indicates that deep convection had occurred. This high
IWC originated from cloud ice that formed from liquid droplets at lower
altitudes as the amount of water vapour in the air at such cold
temperatures is not sufficient to achieve comparable IWC values. Thus, the
formation of encountered cirrus cannot be attributed to any other process than the
liquid-origin process. This feature was observed during the flights on 27 July and on 10 August 2017, respectively. Within the same temperature range (T< 200 K), only a few NPF events with intermediate nnm of more than
∼ 4000 mg-1 (log (nnm; mg-1) ≳ 3.6; yellow and
reddish colours in Fig. 6a) were encountered
offside from strong convection.
In the presence of in-situ-formed cirrus particles at cold temperatures
(185–200 K), i.e. in or around the cold point tropopause region, NPF
events with nnm> 5000 (i.e. log (nnm;
mg-1) > 3.7; orange and reddish colours in Fig. 6a) or
recent NPF bursts were rarely observed. When the cloud ice has formed
in situ (CO < 80 nmol mol-1; yellow, greenish, and blue colours in
Fig. 6c), mostly weak NPF with
nnm< 1500 mg-1 (i.e. log (nnm; mg-1) < 3.2; bluish colours of data points in Fig. 6a)
was observed. These data also indicate that NPF proceeds in air with low CO
content.
Suppression of NPF by cloud particles (due to the large total surface area from their number density and particle size) could explain why the number of nucleation-mode particles remained below the NPF criterion threshold at high IWC although total particle mixing ratios (n6 and n15) were significantly elevated. It is unlikely that the abundance of sub-micrometre-sized particles
of up to 11 000 cm-3 originates from interstitial (non-activated)
aerosols carried in the cloud without contributions from NPF. The large
particle quantities observed (103–104 mg-1) and the
moderate CO content of the air sampled (≲ 100 nmol mol-1) indicate a
source of these particles at high altitudes. About 4 h after an NPF
event has expired, the event may not be detectable anymore due to the short
persistence of the particles in the nucleation-mode size range
(Weigel et al., 2021). Hence, if the IWC values
remained high over several hours due to deep convection and if NPF had
happened more than 4 h prior to the measurements, then the nucleation-mode particles have certainly coagulated to sizes beyond 15 nm in diameter.
Air's low pollutant load is indicated by comparatively moderate or low CO
mixing ratios between 50 and about 100 nmol mol-1 at ambient air
temperatures of < 200 K (Fig. 6c). For
comparison, the NPF observed during the West African monsoon was associated with CO levels between 60 and 90 nmol mol-1 (Weigel et al., 2011). Observation of intermediate NPF (nnm< 1500 mg-1,
log (nnm; mg-1) ≲ 3.3) in the midst of in-situ-formed cloud ice in air with
low pollutant load (CO < 80 nmol mol-1) indicates that recent
convective uplift of polluted air is not a prerequisite for NPF to occur.
Advection of air from elsewhere or chemical and/or photochemical conversion
causes the accumulation of NPF precursors at UTLS levels. In air with the
highest CO content (> 100 nmol mol-1), the IWC–T values (for
T> 200 K, i.e. at lower altitudes) remain in expected ranges, and
they scatter within the limits of most frequent observations (dashed black
lines in Fig. 6) as obtained from earlier
analyses (Krämer et al., 2016). At the highest CO content
(> 100 nmol mol-1), the nnm values predominantly
remained below 5000 mg-1.
The dependency of NPF on the proximity to ice particlesNPF as a function of mean free distance between ice elements
Surfaces such as those of ice particles constitute sinks for the gaseous
precursor species such as the H2SO4–H2O system (Bogdan et
al., 2006, 2013), and the abundance of
condensation surface reduces or even prevents the NPF process.
The free distance between the ice particles is quantified based on the
measurements of Nice and of the ice particles' mean mass radius
rice‾ (consider rice‾3∼IWCNice). The
mean free volume in between the ice particles (the inter-crystal volume,
ICV, per cubic centimetre of air) is calculated with the number
Nice∗ of ice particles (instead of the particles'
number concentration) as
ICV=V-43⋅π⋅r‾ice3⋅Nice∗Nice∗,
which basically subtracts the total ice volume from the sampled air volume
(V= 1 cm3), and the division by Nice∗ yields the ICV. Consequently, the ICV represents the mean particle-free volume
assuming a homogeneous distribution of ice crystals within the air volume.
With a maximum of measured ice particles (Nice∗= 3)
together with the maximum detected ice particle radius of 100 µm, the subtraction V-43⋅π⋅r‾ice3⋅Nice∗ corresponds
by the order of magnitude to a subtraction of 10-11 cm-3 from
1 cm-3. Hence, the volume of ice is insignificant compared to the
volume of air, and the ICV may be considered to be a function of
Nice∗ only. The mean inter-crystalline distance (ICD; in centimetres) is then calculated by
ICD=ICV43⋅π3,
and the ICV is assumed as a sphere around each individual ice particle. The
radius of each sphere represents the mean ice-free distance in any direction
from the individual ice particle. Conceptually, this approach corresponds to
the definition of the cloud elements' distance provided by Baumgartner
and Spichtinger (2018).
Figure 7a depicts the number concentration of
nucleation-mode particles (Nnm) as a function of the calculated ICD. The continuous colour transition of the data points in the x direction together with
unchanged colouring in y direction demonstrates the independence of
Nnm from the ICD and rather illustrates the correlation between the number
of ice particles and their distance. The present ice particles compete for
the limited amount of available water vapour such that elevated number
concentrations of ice particles mainly result from the abundance of small
ice particles. Hence, by means of the number of ice particles only, it is
not possible to constrain the occurrence and/or strength of NPF as a wide
scattering of Nnm concentrations was observed at any ICD between about 1 and 10 cm.
Concentrations of in-cloud-detected nucleation-mode aerosols (Nnm) in 1 Hz resolution as a function of the mean inter-crystal distance, ICD, between encountered cloud (ice) particles colour-coded with reference (a) to the number concentration of cloud ice particles, (b) to the IWC, and (c) to the mean ice particles' radius.
Figure 7b shows the ice particles' mean mass radius
rice‾ as a function of the ICD and the number of nucleation-mode particles. By means of the mean mass radius rice‾, two different
cases were distinguished:
For the smallest ice particle sizes (∼ 3 µm <rice‾<∼ 20 µm,
log (rice‾; µm) ≲ 1.3), a dependency of the ICD on the particle
size was discernible. For instance, the smallest ice particles (bluish
rice‾) predominantly coincided with short ICD of about 1 cm at elevated Nice. Towards larger ICD, ice particle sizes continuously
increased up to rice‾≈ 20 µm, which reflects the
competition of the ice crystals for the available water vapour. Within the
same interval of ice particle sizes (rice‾<∼ 20 µm), the concentrations Nnm scattered over
almost 2 orders of magnitude (from ∼ 100 to
∼ 10 000 cm-3) up to ICD of ∼ 10 cm without
any obvious systematics.
In the presence of larger ice particles (rice‾>∼ 30 µm 1.3 < log (rice‾; µm) ≲ 1.4, orange and reddish colours), the ICD ranged from ∼ 1 cm
to values above ∼ 10 cm. Hence, not only did rice‾
determine the resulting ICD, but Nice increasingly contributed as well. The concentrations Nnm were not at the highest when ICD values reached their
maximum at ∼ 10 cm. For the largest particle sizes
(rice‾>∼ 30 µm), the values of
Nnm accumulate at ∼ 400–4000 cm-3 over the entire
range of ICDs.
As long as the mean ice particle radius remained below a few dozen micrometres, NPF was encountered with almost any resulting Nnm concentration. As
shown in Fig. 4 and summarised in Sect. 4.4, a wide scatter of Nnm was observed to occur
largely independent from coincidently detected number Nice of ice
particles. Hence, the in-cloud NPF observed during StratoClim 2017 was
almost unaffected by the ice particle number as long as the mean ice
particle size remained small enough (i.e. with rice‾< 20 µm).
The IWC combines both microphysical parameters of the observed ice clouds,
particle size and number concentration. If Nnm over ICD is analysed as a
function of IWC, systematics become visible (Fig. 7c).
At lower IWC (< 1 µmol mol-1, log (IWC; nmol mol-1) ≲ 0;
bluish and green colours) the ICDs were at the largest, and observed NPF was of the highest intensity (Nnm of several thousands per cubic centimetre). Between 1 and 10 µmol mol-1 (yellow colours), the maximum of Nnm throughout observed NPF events was reduced. The maximum Nnm was further reduced when IWC increased to values beyond
10 µmol mol-1. This result demonstrates that the maximum
Nnm reached throughout in-cloud NPF is determined (in addition to the
precursor gas concentration) by the combination of both the ice particles'
number concentration Nice and their mean mass radius rice‾.
NPF as a function of cloud elements' integral radius IR
The combined effect of cloud ice particles' number density and size on the
detectable Nnm during in-cloud NPF motivates the investigation of Nnm values as a function of the integral radius IR =rice‾⋅Nice of the ice particle population. The parameter IR was
described, e.g. by Manton (1979) or
Politovich and Cooper (1988), and is frequently used to
characterise clouds' microphysical properties (e.g. Korolev
and Mazin, 2003; Krämer et al., 2009). IWC and IR are
expected to be strongly related (also visible by the systematic sorting of
data in Fig. 8a) as the diffusive growth rate of
an ice particle dmdt is proportional to IR (see
e.g. Pruppacher and Klett, 2012). The IR is the direct control variable
for the mass increase per time by condensation (mainly of water vapour) on
the surface of a cloud ice particle and thus for the particle's growth rate.
At supersaturated NPF conditions, the NPF precursors condense on the cloud
ice particles, and the change in the ice particle's mass
dmdt from the condensation of a gaseous
precursor converts into a reduction in the gaseous precursor concentration.
Concentrations of nucleation-mode aerosols (Nnm) in 1 Hz resolution as a function of the cloud (ice) particles' integral radius, IR =rice‾⋅Nice colour-coded in correspondence to detected ice water content (IWC; panel a) and to measured CO mixing ratio (b); in the absence of CO values
the data points are blackened. The diagonal, grey-coloured bars indicate a
limiting range beyond which the probability of NPF observations decreases,
with two exceptional encounters of very recent or just proceeding NPF (see
text for details).
For almost all IRs below 1 µm cm-3, the
Nnm concentrations were unsystematically scattered over the entire
interval between ∼ 100 and ∼ 10 000 cm-3. Towards the highest IR (> 1 µm cm-3), the maximum of observed Nnm continuously decreased. This
reflects a limiting influence by the cloud ice on the maximum strength of
occurring NPF (indicated by the diagonal shaded grey bars in
Fig. 8). An exceptional feature is exhibited in
Fig. 8a with a high signal of Nnm
(∼ 3000–4000 cm-3) amongst elevated IR (between
∼ 4 and 10 µm cm-3). This cluster of data points
resulted from the measurements of two individual mission flights, on 27 July
(∼ 3000 cm-3<Nnm<∼ 3500 cm-3) and on 6 August (∼ 3500 cm-3<Nnm<∼ 4000 cm-3), respectively. During these measuring periods, ice particle
densities (Nice) and the mean ice particle sizes (i.e. the particles' mean mass radius rice‾) did not rise above 0.1–0.3 cm-3
and 25–50 µm. Neither Nnm nor the ice microphysics exceeded the
range of moderate values. The two independent exceptions in the
observational data indicate a local and temporal state of imbalance that could
have been caused by
intermediate NPF which was just proceeding when measured or which had been completed very recently (cf. Weigel et al., 2021; in such a case, the observed Nnm should rapidly (< 1 h)
decay to values of ∼ 1000 cm-3 due to coagulation);
ice particles which sediment from high altitudes into an area of currently
active NPF; or
cooling of air accompanied by nucleation of ice, while the cooling is due
to the air parcel's vertical displacement, which results from deep
convection or gravity wave activity (see Weigel
et al., 2021).
The limiting influence of the cloud ice on the maximum strength of NPF as
indicated by the majority of observations is explainable by the reduction in
NPF precursor material due to its condensation onto present ice particle
surfaces. The question arises whether the distance between the ice particles
allows efficient absorption and sustained reduction in NPF precursor
molecules or whether such an effect exists only in the immediate vicinity
of an ice particle. The effectiveness of such a process strongly depends on
the diffusivity of the NPF precursor molecules. If the molecules of the NPF
precursor are absorbed before the thermodynamic conditions for NPF are
reached, then these molecules are removed and missing in the formation of
molecular clusters as an initial step in the nucleation process. Numerical
analyses concerning the reduction in the saturation ratio of H2SO4
due to the presence of ice particles which are coated with H2SO4
(as is typical for cirrus particles at 10–20 km altitude; cf. Bogdan et al., 2006, 2013) are described in Appendix B (see also Fig. B1).
Although the binary H2SO4–H2O nucleation process alone is
assumed as insufficient to explain atmospheric NPF (Bianchi et al., 2016; Kirkby et al., 2011), the numerical analysis qualitatively
applies also to saturated condensable vapours containing compounds other
than H2SO4 (see Riccobono et al., 2014).
The numerical analysis yielded that the precursor's saturation ratio
decreases rapidly with increasing IR. As long as the ice particles' size
remains small (radii < 10 µm) their influence on the
saturation ratio of the NPF precursor is comparatively weak. As demonstrated
for H2SO4 (cf. Appendix B), rising IR (combining ice particle size
and number) constrains the production of high Nnm or inhibits NPF altogether. Note that only completely uncoated ice particles of pure water (which do not exist in the UTLS; cf. Bogdan et al., 2006, 2013) would be ineffective condensation surfaces
for H2SO4 vapour.
According to Fig. 8a, the Nnm range of
500–3000 cm-3 is most frequently observed over the range of detected
IR values. Regarding Fig. 4, Sect. 4.4, Fig. 8, and the simulation of Appendix B, the
following conclusions seem likely:
The maximum Nnm resulting from in-cloud NPF is determined by IR. Abundant
ice particles of sufficient size are capable of reducing the saturation
ratio of NPF precursors within timescales ranging from half an hour to a
few hours. Consequently, intermediate or weak NPF events with low Nnm
production occur most frequently in the presence of cloud ice. The
probability of instrumentally identifying weak NPF events decreases with
decreasing Nnm.
These NPF limitations by the IR result from the StratoClim 2017 data set and
may not necessarily be of general validity. Further investigations at other
locations and under variable conditions and dedicated laboratory experiments
are necessary to confirm these limitations marked as grey bars in
Fig. 8, which do not represent sharply
quantifiable relationships.
Coagulation additionally affects Nnm on timescales of a few hours (see Weigel et al., 2021).
According to the results in Fig. 8, IR values of
about 24 µm cm-3 (corresponding to Nice of about
0.7–0.8 cm-3 and rice‾ of about 32 µm) constituted the
uppermost limit for in-cloud NPF observation during StratoClim 2017. Below
the IR limits marked with grey bars, in-cloud NPF is encountered largely
unaffected by the presence of ice particles. It is emphasised that the grey
bars primarily mark a region in the IR–nnm parameter space where the duration of an exceedance of marked levels decreases with increasing IR and/or nnm. Hence, the detection of these points becomes less likely, or the probability increases to miss such events when the values cross the marked
levels.
Figure 8b depicts Nnm as a function of IR with
reference to the CO mixing ratio. Neither samples
with the highest Nnm nor samples with the highest IR were directly ascribable to
polluted air recently lifted from the surface. Intense NPF (with
Nnm> 5000 cm-3) was observed at CO mixing ratios
ranging between ∼ 90 and 100 nmol mol-1, which indicates
the air's moderate pollutant load or its moderate age. In less polluted air
(CO mixing ratios below∼ 70 nmol mol-1), the IR reaches
the highest values (up to ∼ 24 µm cm-3), which
were observed together with elevated IWC (up to ∼ 750 µmol mol-1, i.e. log (IWC; nmol mol-1) ≈ 0.88). Within
lightly polluted air, cloud ice particles mostly form in situ. It is conceivable that the in situ cloud ice formation and NPF happen simultaneously and are induced by the same process: e.g. by updraughts due to subjacent convection (pileus effect) or by (local) cooling due to gravity waves (see Weigel et al., 2021). At CO mixing ratios below
70 nmol mol-1, the observed Nnm values rank at a few hundred per cubic centimetre but systematically below 1000 cm-3.
Based on NPF encountered during StratoClim, the air masses with low
pollutant loads therefore still contain sufficient amounts of precursor
material to supply intermediate NPF
(100 cm-3<Nnm< 1000 cm-3). This differs
from earlier findings from ground-based measurements at high mountain sites
(at about 5 km altitude) in the Himalaya region by Venzac et al. (2008)
or at the Jungfraujoch station (∼ 3.5 km altitude) in the
Swiss Alps by Bianchi et al. (2016), who attributed their frequent NPF
observations to the advection of polluted air which rises up from the
valleys towards the research stations. Williamson et al. (2019) found frequent NPF during measurements over the Atlantic and the
Pacific, i.e. at a certain distance away from sources of industrial pollution.
Like for StratoClim 2017, low levels of pollution here were sufficient to
support NPF.
Summary and conclusions
Between 27 July and 10 August 2017 the airborne StratoClim mission took
place in Kathmandu, Nepal, comprising eight mission flights (∼ 22.5 h of COPAS measurement time above 10 km, θ≳ 350 K) up to
altitudes of 20 km (θ≈ 475 K) with the Russian
high-altitude research aircraft M-55 Geophysica. New particle formation in the
presence of cloud ice particles was analysed as it was encountered in the
UTLS region of the Asian monsoon anticyclone (AMA) over northern India,
Nepal, and Bangladesh. Over the StratoClim observation period, in-cloud NPF
was a frequently occurring phenomenon within the AMA associated with
predominantly large convective cloud systems over the Himalayan foothills.
Elevated concentrations of nucleation-mode particles (Nnm) generated by
NPF were observed in hitherto unreported frequency together with ice
particles (Nice> 0 cm-3) at altitudes between
∼ 11 km and 16.5 km (∼ 355–385 K) and mainly
at ambient temperatures colder than ∼ 230 K. During
StratoClim 2017, a total of 104 in-cloud NPF events were observed over
a total duration of 1 h and 17 min (∼ 5 % of the
total data set, ∼ 49 % of all observed NPF cases). Maximum
concentrations of nucleation-mode particles of up to ∼ 11 000 cm-3 (≈ 50 000 mg-1) were detected coincidently
with ice particles in concentrations Nice of 0.05–0.1 cm-3
(correspondent to 50–100 ice particles per litre) at heights of
approximately 15.5 km (∼ 370 K).
The observations indicate the Nnm range of 500–3000 cm-3 as most
frequently observed during in-cloud NPF. Weak events with low NPF rate occur
most frequently in the presence of cloud ice, whilst the probability of
instrumentally identifying such weak events decreases with Nnm. Coagulation additionally affects elevated Nnm at timescales of a few hours (see Weigel et al., 2021). Consequently, the
supposedly preferred Nnm range results from superimposed effects, and it is a matter of probability and timing (delay between NPF event and
observation) that the Nnm range of 500–3000 cm-3 is most frequently
observed in the presence of cloud ice.
Analyses of the StratoClim data set concerning the relationship between
interstitial aerosol and the abundance of cloud particles in the UTLS are
consistent with the findings from earlier measurements (de Reus et al.,
2009) and extended these by new observations under different conditions.
When ice particles are abundant (Nice> 0.5 cm-3),
total aerosol number concentrations (N10) remain generally between
∼ 200 and 700 cm-3. In agreement with earlier
findings (de Reus et al., 2009), the ratio of ice particle number and the
number of sub-micrometre-sized aerosols did not significantly rise above 300
sub-micrometre-sized aerosols per ice particle at low air temperatures
(< 200 K). Intense NPF, generating nucleation-mode particles of
several thousands per cubic centimetre, substantially decreases the
ratio of number concentrations of ice particles to aerosols. However, such
intense NPF was not observed at ratios larger than 1 : 3000, which indicates
that the presence of cloud ice imposes limitations to NPF.
In-cloud NPF appears limited in the presence of predominantly
liquid-origin ice particles with increased ice water content resulting from deep
convection up to cold point tropopause levels. This is confirmed by
coincidently measured CO content of the air sample: air's pollutant load
and/or its recent surface contact do not determine the strength of in-cloud
NPF. Otherwise, the most intensive NPF events should have been found more
frequently in air masses with the highest CO content. When the cloud ice has
formed in situ, at low CO mixing ratios, NPF was observed although with reduced
strength. However, it is not yet conclusively clarified whether the direct
convective supply of precursor material from pollution in the boundary layer
is an essential prerequisite for the occurrence of NPF in the UTLS or
whether NPF together with the ice cloud formation is initialised in
processed and diluted air masses. The observations suggest that sufficient
amounts of NPF precursor material accumulate at UTLS altitude, which is not
necessarily connected to air's recent vertical uplift. It remains
speculative, and the extent to which the vertically lifted ice particles themselves contribute as a carrier for soluble NPF precursor gases such as SO2, H2SO4,
or others, e.g. if dissolved in the cloud elements' liquid phase at lower
heights and released again at TTL altitudes after the cloud ice has
sublimated, should be the subject of suitable numerical analyses. Comparatively slow processes as air mass transport from
elsewhere or the chemical and/or photochemical conversion at elevated
altitudes may suffice to supply the reservoir of NPF precursors at UTLS
altitudes. NPF of the highest intensity, however, was observed at moderate CO
mixing ratios. Intense NPF seems suppressed in strong convective updraughts
(cf. Sect. 4.2) either because of the intense
dynamics inherent to convection or because the precursor's saturation
ratio of recently uplifted air does not suffice for NPF.
At the moment of observation, the age of the nucleation-mode aerosols (the
delay between the NPF burst and the instrumental detection) as well as the
aerosol's processing history is unknown. While the aerosol's persistence in
the nucleation mode is limited, it is conceivable that the abundance of
aerosols influences the local formation of ice particles or that ice
particles are coated by nucleation-mode aerosol material due to coagulation.
Above certain sizes, the cloud ice elements are increasingly subject to
sedimentation. Upon sedimentation to warmer ambient temperatures, the ice
particles sublimate. The remnants of sublimated cloud ice consist of
materials attributed to the initially NPF-generated nucleation-mode
aerosols. It remains speculative whether or not, in terms of
physico-chemical characteristics, the released aerosol material is
comparable with the primary NPF-generated aerosol. The sublimation of coated
ice particles and the release of aerosol material at intermediate altitudes
provide nuclei for cloud entrainment and/or for cloud formation. It remains
unquantified whether NPF near the surface (see Venzac et al., 2008;
Bianchi et al., 2016) or the NPF at UTLS altitudes contributes the
most to the availability of cloud condensation nuclei (CCN), which are
supposed to promote cloud formation (Andreae et al., 2018) at the cloud
condensation levels. The specific source contributions to the abundance of
available CCN are as variable as the chemical species that may be involved
in the NPF process.
Ice particles in sufficient number and size are well capable of reducing the
saturation ratio of an NPF precursor such as H2SO4. This implies
two conclusions: (1) in-cloud NPF is limited by abundant ice particles, and (2) not only the number of ice particles limits the NPF occurrence but also the ice particles' size. The strength of in-cloud NPF depends on the integral
radius IR
(=rice‾⋅Nice), which constitutes the control
value of the ice particle's growth dmdt. Up
to IR of ∼ 1 µm cm-3 the occurrence of NPF of any
strength (with ∼ 100 <Nnm< 10 000 cm-3) seems independent of the presence of ice particles altogether. At larger IR (> 1 µm cm-3) the presence of ice
particles limits the maximum of Nnm from NPF. This result refines
earlier conclusions (Weigel et al., 2011), according to which mainly the
number of ice particles would limit the occurrence of NPF.
Exclusion of sampling artefacts due to the presence of cloud ice
During the herein discussed NPF events, the detected total number
concentration of cloud elements never exceeded ∼ 2–3 cm-3. Thus, the number density of cloud elements was always at
least 2 orders of magnitude smaller compared to detected aerosol number
concentrations. At ambient air temperatures ranging from 187 to 235 K, the
clouds entirely consisted of ice particles. In other studies, however, the
discussions on NPF are restricted to measurements under cloud-free
(clear-air) conditions as the cloud particles are suspected to possibly
impact onto the aircraft's hull or the aerosol inlet, this way possibly
generating artefacts on the aerosol measurements
(Williamson et al., 2019, referring to
Weber et al., 1998). Regarding the in-cloud NPF
observations throughout StratoClim 2017, the following aspects are
noteworthy:
At typical flight speeds of the M-55 Geophysica (154 ± 39 m s-1),
sub-micrometre-sized particles are not subject to impaction on parts of the
aircraft structure (nose, wing's leading edge, etc.) as the particles follow
the airstream around such flow obstacles (Kulkarni et al., 2011).
Furthermore, ice particles in the diameter size range of a few micrometres
(i.e. 1 µm <dp< 10 µm) partially
sublimate in the congestion region upstream of any aircraft structure (e.g.
the wings leading edge or the aerosol inlet). The diffuser-type entry of
the aerosol inlet leads to a flow deceleration inside the probe head
accompanied by a sudden temperature increase (according to fluid dynamical
simulations by up to 13 ∘C on flow deceleration from
170 to 60 m s-1; see Weigel et al., 2009, and references
therein). Hence, if a single particle with 1 µm <dp< 10 µm randomly enters the COPAS aerosol inlet,
rapid sublimation of such an ice particle can be expected to occur inside
the aerosol inlet of COPAS. The entry of the sample air into the inlet's
second diffuser additionally reduces the sampling of ice particle fragments. Due to additional heating of the air sample and during their passage through
the aerosol line to the COPAS detector (less than about 0.5 s), the
ice particles from shattering with diameters of a few micrometres evaporate
even if they are present in large numbers.
The number concentration of ice particles with diameter
dp> 10 µm mostly remained below 0.4 cm-3 when
coincidently detected with NPF. On impact and shattering of a single ice
particle of such a size, the number of generated fragments is estimated to
range between about 10 and 100 cm-3 (Korolev et al., 2013).
Hence, to substantially affect the detected number concentration of
nucleation-mode particles (on the scale of hundreds to up to 10 000 per cubic centimetre), the number of ice particles emanating
from shattering appears too low.
The probability that ice particles hit the sharp-edged tips of the COPAS
aerosol inlet (Weigel et al., 2009) appears negligibly small. The
impaction surface provided by the COPAS aerosol inlet is mainly the inlet's
ring-shaped entry with an opening diameter of ∼ 7.3 mm and a
wall thickness of ∼ 100 µm. In the unlikely case that
a single ice particle impact occurred, all generated fragments were required
to endure the temperature rise within the inlet head (cf. first argument of
this list) and the transport through the aerosol lines towards the COPAS
detectors before they can cause any effect on the measurement.
An effect of shattered large ice particles on the detection of
nucleation-mode particles is ultimately not excludable. However, despite the
reference by Williamson et al. (2019) in this context,
ice particle fragmentation was not described by Weber et
al. (1998). The same authors discuss the influence on NPF detections due to
fragmentation of super-cooled liquid-water cloud droplets and suggest a
careful discussion in such cases. In general, such an influence due to the
fragmentation of ice particles was largely ruled out or estimated as much
lower than that of liquid droplets (Weber et al., 1998).
Concerning the analyses discussed herein, however, it seems a statistical
exception that ice particle fragments emanating from shattered ice particles
crucially affect the measurement of the numbers of nucleation-mode
particles. Moreover, if the NPF detections were systematically affected by
the presence of cloud ice, the observed quantities of nucleation-mode
particles would feature systematic and larger differences during in-cloud
measurements compared to clear-air observations. None of the described
artefacts were observable in the data from StratoClim 2017.
Impact of ice particles on NPF precursors' saturation ratio
Calculations were made regarding the timescales on which the decrease in
the supersaturation of H2SO4 vapour occurs in the presence of
coated ice particles. In the closest vicinity of an ice particle, the
condensational loss of a precursor gas like sulfuric acid (H2SO4)
predominates over the NPF process. The molecules' mobility and the
condensation efficiency of the H2SO4 molecules are mainly
determined by their diffusivity under the given atmospheric conditions. The
diffusivity of H2SO4 is about a factor of 0.2–0.5 of the
diffusivity of water vapour (Tang et al., 2014).
Simulated change in the H2SO4 vapour's saturation ratio as a function of time due to the presence of entirely H2SO4-coated ice particle surfaces of various sizes and number concentrations. (a) Particles with radii rp= 1 µm, (b)rp= 10 µm, (c)rp= 100 µm. Overall, this simulation covers a range of integral radii IR (=rice‾⋅Nice) from 0.01 to 10 µm cm-3. Note: a cloud (ice) particle is assumed as coated with H2SO4 (consistent with Bogdan et al., 2006, 2013).
Presuming that the ice particles are coated with H2SO4 (Bogdan
et al., 2006, 2013), model simulations were
performed to investigate the timescales on which the coated ice particles
reduce various H2SO4 saturation ratios. The simulation results
(shown in Fig. B1) are based on constant ambient
temperature (T≈ 200 K) and pressure (p= 110 hPa) conditions. For
the same temperature conditions, the saturation vapour pressure psat of
H2SO4 is calculated according to Vehkamäki
et al. (2002). In this way, the degree of supersaturation is deducible from
the H2SO4 molecules' concentrations reported for the CLOUD (Cosmics
Leaving OUtdoor Droplets) chamber experiments (see Kürten, 2019, and references therein). According to
this study and in agreement with other references (Hamish Gordon, School of
Earth and Environment, Leeds University, UK, personal communication, October
2019), molecule concentrations of 106–107 cm-3 are required
in the CLOUD chamber at temperatures of 208 K to induce NPF with nucleation
rates of 10-2–100 cm-3 s-1 (read-out from Fig. 4 in
Kürten, 2019, from experiments at relative humidity of
38 %). Keeping possible wall effects of the laboratory experiments in
mind, for the occurrence of NPF under real atmospheric conditions, the lower
bound of required molecule concentrations (106 cm-3) may suffice,
with an uncertainty of a factor of 5 (Hamish Gordon, School of Earth and Environment, Leeds University, UK, personal communication, October 2019). At an
ambient temperature of 208 K, the molecule concentrations of
106–107 H2SO4 cm-3 (Kürten,
2019) correspond to saturation ratios of about S≈ 10–100. The
following analysis, however, comprises a much wider range of saturation
ratios between 10 and up to 5000 to account for a higher sensitivity of the
temperature dependency of S.
Based on the expression formulated by Tsagkogeorgas et al. (2017) with the
saturation vapour pressure psat of H2SO4 (above a flat
surface) and with an accommodation coefficient of α= 0.65
(Pöschl et al., 1998), the change in the fully coated ice crystal
with mass m per time unit is calculated by
dmdt=4πDrS-1LRT-1LTDK+RTα⋅psat,
which conceptually represents the change in mass (size) of the particles onto which the H2SO4 condenses and which is also consistent with
the finding that cirrus cloud elements are coated with an
H2SO4–H2O layer (Bogdan et al., 2006, 2013). The diffusivity of H2SO4 molecules in air is denoted with D, and K refers to the thermal conductivity of air, while R and
Ra are the gas constants of H2SO4 and the air, respectively.
Since the ice particles grow predominantly by the uptake of water vapour, the effective contribution to dmdt by the
condensing H2SO4 is of minor concern. The
dmdt from the condensing H2SO4
converts instead to a reduction in the saturation ratio of gaseous
H2SO4, the change in which is
dSdt=-RRappsatNicedmdt,
with the latent heat of vaporisation assumed as constant:
L=67.59⋅103Jmol-1MH2SO4,
and Nice constitutes the number density of ice particles. Here, the
sulfuric acid's molar mass is MH2SO4= 0.098078 kg mol-1. Note that the combination of the Eqs. (B2) and (B1) implies that
dSdt∼r⋅Nice; i.e.
the temporal change in the precursor's saturation ratio is proportional to
the integral radius IR considered in Sect. 5.2.
In Fig. B1 the variability of two aspects is
considered, and in the panels (a)–(c) it is distinguished between three ice
particle radii (1, 10, and 100 µm) and two
different ice particle number concentrations (0.01 and 0.1 cm-3). The
study by Ueyama et al. (2020) revealed that ice particles
(effective radii of about 15 µm) persist over 12 to 20 h at
convective outflow levels between 365 and 370 K potential temperature in
the AMA of the 2017 season.
Based on the simulation, the largest particles (rp= 100 µm)
are capable of efficiently suppressing NPF. Particles of this size and in
the highest concentrations of 0.1 cm-3 cause the saturation ratio to abate
to saturation level (i.e. S= 1) within 20–50 min. At lower
concentrations (0.01 cm-3) of particles of 100 µm radius, the
saturation ratio is reduced by more than 70 % within 1 h. Particles of
10 µm radius and in concentrations of 0.1 cm-3 are almost
equally efficient in reducing the saturation ratio by ∼ 70 % within 1 h. Smaller number concentrations of the same particle
size range and smaller particles (rp= 1 µm) require
considerably more time than 1 h to reduce the H2SO4 saturation
ratio.
In essence, cloud ice particles can rapidly reduce the saturation ratio of
H2SO4 as well as that of other condensable gases. The ranges of
Nice (0.01–0.1 cm-3) and particle size (1 µm <rp< 100 µm) considered in the simulation correspond to
the characteristics of ice particles coincidently observed with NPF
throughout the StratoClim 2017 mission (note that away from NPF, higher
Nice and larger rice‾ were found; cf. Krämer et al., 2020). About 71 % of
all ice cloud detections in coincidence with NPF had an IR (i.e.
rice‾⋅Nice) of less than 1 µm cm-3, while only about 1.5 % of the ice particle samples reached
IR values greater than 7.5 µm cm-3; the maximum IR of 24 µm cm-3 was encountered once throughout the entire mission. In general,
the cirrus cloud particles are expected as coated with an
H2SO4–H2O layer (Bogdan et al., 2006, 2013), onto which sulfuric acid can condense. Impurities by weaker
and substitutable acids (such as organic acids or HCl or HNO3) also
allow the H2SO4 uptake on the surface, which could reduce the
gaseous H2SO4 concentration, thereby suppressing NPF. Hence, in
a certain abundance the presence of cloud ice particles restrains the NPF
process when condensation prevails over the competing gas-to-particle
conversion. The efficiency of condensation onto the ice particles' surface
depends on
the size and number concentration of cloud ice particles
on the time interval during which the conditions remain at least saturated.
For the condensation of H2SO4, a partial coating of the ice
particles' surface with sulfuric acid (or organic acids, HCl, or HNO3)
suffices to supply the gaseous H2SO4 with the required attachment
points. To simplify the numerical simulation of the saturation decay, an ice
particle is assumed as entirely coated (consistent with Bogdan et al., 2006, 2013), and the (real) ice particle's
habit (e.g. asphericity, porosity, etc.) remains unconsidered. Sporadic
updraughts due to convective lifting well below the NPF level or gravity
waves cause small-scaled expansion and cooling, which increases the
precursor's supersaturation (Weigel et al.,
2021). Certain concentrations of H2SO4 molecules exceed the
supersaturation threshold for NPF, even in the presence of abundant cloud
ice as long as the NPF process occurs faster than the reduction in S due to
the present ice.
Data availability
The data shown in this study will be available from the HALO database at https://halo-db.pa.op.dlr.de/mission/101 (last access: 6 September 2021; German Aerospace Center, 2021), or they may be provided by the respective PI upon request.
Author contributions
RW evaluated and analysed the data, created the figures, and drafted the manuscript with contributions by CM, MB, MK, HT, and PS. SB participated in the data analyses and the manuscript drafting. Numerical simulations concerning the impact of ice particles on the saturation ratio of H2SO4 were performed by MB with contributions by HT. MK, NS, AA, and CR contributed with cloud microphysical and water vapour data. SV and FD'A provided the CO data. The manuscript was critically reviewed by CM, MB, MK, PS, NS, AA, CR, SV, FD'A, HT, and SB.
Competing interests
The authors declare that they have no conflict of interest.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “StratoClim stratospheric and upper tropospheric processes for better climate predictions (ACP/AMT inter-journal SI)”. It is not associated with a conference.
Acknowledgements
The contributions from the workshops of the Max Planck Institute for
Chemistry and of the Institute for Physics of the Atmosphere (Johannes Gutenberg University Mainz) were essential for this work. In particular, we acknowledge support of Thomas Böttger, Michael Flanz,
Christian von Glahn, Harald Rott, and Wilhelm A. Schneider. Also
acknowledged are the comprehensive and helpful discussions with
Miklós Szakáll. We very much thank the crew of MDB (Myasishchev
Design Bureau) and the M-55 Geophysica pilots. The extraordinary
commitment of Fred Stroh in the realisation of the campaign
and the leadership of the entire StratoClim project by Markus Rex are
gratefully acknowledged. We explicitly thank the officials of the Nepalese government
authorities, research institutions, and Tribhuvan Airport as well as of the
German Embassy for their extraordinary support and hospitality, which
enabled our field campaign and research.
Financial support
Some of our research leading
to the presented results received funding from the European Research Council
under the European Union's Seventh Framework Programme (FP/2007-2013) ERC
grant agreement no. 321040 (EXCATRO). The StratoClim project was funded by
the EU (FP7/2007–2018 grant no. 603557) and also supported by the German
“Bundesministerium für Bildung und Forschung” (BMBF) under the joint
ROMIC project SPITFIRE (01LG1205A). Manuel Baumgartner was supported by the DFG within the Transregional Collaborative Research CentreTRR165 “Waves to Weather”, Project Z2. Peter Spichtinger was supported by the DFG within the research unit Multiscale Dynamics of Gravity Waves (MS-GWaves) through grant SP 1163/5-2. Holger Tost received funding from the Carl Zeiss Foundation.
Review statement
This paper was edited by Rob MacKenzie and reviewed by two anonymous referees.
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