Liquid clouds form by condensation of water vapour on aerosol particles in the atmosphere. Even black carbon (BC) particles, which are known to be slightly hygroscopic, have been shown to readily form cloud droplets once they have acquired water-soluble coatings by atmospheric aging processes. Accurately simulating the life cycle of BC in the atmosphere, which strongly depends on the wet removal following droplet activation, has recently been identified as a key element for accurate prediction of the climate forcing of BC.
Here, to assess BC activation in detail, we performed in situ measurements
during cloud events at the Jungfraujoch high-altitude station in Switzerland
in summer 2010 and 2016. Cloud droplet residual and interstitial
(unactivated) particles as well as the total aerosol were selectively sampled
using different inlets, followed by their physical characterization using
scanning mobility particle sizers (SMPSs), multi-angle absorption photometers
(MAAPs) and a single-particle soot photometer (SP2). By calculating cloud
droplet activated fractions with these measurements, we determined the roles
of various parameters on the droplet activation of BC. The half-rise
threshold diameter for droplet activation
(
Finally, we tested the ability of a simplified theoretical model, which
combines the
Natural and anthropogenic atmospheric aerosol particles cause a global
cooling of the Earth's surface, partially compensating for the warming caused by
greenhouse gases (Boucher et al., 2013). Black carbon (BC), formed when
fossil and biogenic fuels undergo incomplete combustion, is emitted by a
large range of anthropogenic and natural sources and has unique properties
leading to complex climate effects. BC is a strong light absorber, resulting
in a positive industrial-era forcing (warming) via aerosol–radiation
interactions (ari;
Two main mechanisms can explain the incorporation of a particle into a droplet: impaction scavenging, which involves collision and coalescence; and nucleation scavenging, i.e. droplet activation occurring when supersaturation of the air surrounding the particle exceeds its critical supersaturation. Theoretical studies (e.g., Flossmann and Wobrock, 2010) and field work (Ohata et al., 2016) have shown that the latter is predominant over the former, at least for accumulation mode particles, and this applies also for BC. The present study focuses on the parameters influencing the nucleation scavenging process for BC.
The Kelvin effect describes the influence of particle size on its critical supersaturation for activation as a water droplet: a large particle has a lower critical supersaturation than a smaller one with identical chemical composition. Henning et al. (2002) used in situ cloud measurements at the Jungfraujoch, a high-altitude site at 3580 m a.s.l. in central Switzerland, to show that the dry particle diameter is indeed the main parameter determining whether a particle activates to a droplet upon cloud formation. The threshold diameter at the Jungfraujoch is typically around 90 nm (Hoyle et al., 2016).
Raoult's law describes the influence of the chemical composition of an
aqueous solution on water activity. The Köhler theory combines the Kelvin
effect and Raoult's law, thereby relating particle dry size, particle
composition and critical supersaturation to each other. Since many cloud condensation nuclei
(CCN) closure studies confirmed the applicability of the Köhler theory to
predict CCN activation of laboratory-generated and ambient aerosols (e.g.
Snider et al., 2003; Bougiatioti et al., 2009), Hammer et al. (2014a)
proposed a method to infer the effective peak supersaturation
(SS
BC is an insoluble solid; thus no reduction of water activity through Raoult's law occurs, resulting in high critical supersaturation for CCN activation at a given dry particle size (or high critical diameter for CCN activation at a given supersaturation). However, a water-soluble coating around a BC core makes it a better CCN for the size and hygroscopicity reasons explained above. Highly aged atmospheric BC has been shown to be scavenged to the same extent as the total aerosol in clouds at the Jungfraujoch (Cozic et al., 2007). However, which factors control the fraction of BC mass that activates to cloud droplets is a question that still needs to be addressed. Recently, field studies focusing on size-resolved analyses of the droplet activation behaviour of BC have tried to quantify the influence of mixing state and chemical composition on nucleation scavenging. Schroder et al. (2015) specified the minimum coating thicknesses required for droplet activation of BC in two cloud events on the Californian coast and related it to retrieved supersaturations. However, the coating thickness calculation was heavily simplified and represented a lower limit because of technical issues, which did not allow for a comprehensive description of the conditions required for activation. Roth et al. (2016) applied single-particle mass spectrometry to interstitial and cloud droplet residual particles sampled at a mountain site in central Germany (peak Schmücke; 905 m a.s.l.) to show that internally mixed inorganic salts made BC particles act as nuclei for cloud droplet formation. Zhang et al. (2017) confirmed the ability of coated BC to activate to droplets at a mountain site in southern China (1690 m a.s.l.) and found that high fractions of sulfate in the coatings facilitated activation compared to organic coatings, which are less hygroscopic than sulfate. While these studies provide information on parameters influencing the droplet activation of BC on a qualitative level, the relative contribution of each of these parameters to droplet activation remains to be elucidated more quantitatively. Moreover, there is a need for a direct assessment of the level of complexity that is required in the description of these parameters in order to predict droplet activation of BC and realistically simulate it in climate models.
Comparing the theoretically calculated critical supersaturation of particles that do or do not form droplets at a certain supersaturation offers the opportunity to assess the predictability of the droplet activation of BC. Such an approach was conducted in a laboratory study by Dalirian et al. (2018), who coated BC particles with known amounts of identified organic species and showed that they could accurately predict the CCN activity of the mixed particles. Matsui (2016) and Matsui et al. (2013) utilized the Köhler theory considering the size and mixing state of BC-containing particles in modelling studies to show an improved simulation of BC concentrations over East Asia compared to simulations in which the mixing state was not resolved and to observations from field measurements. However, it remains to be shown that BC activation in atmospheric clouds indeed obeys such theoretical predictions.
In this study, we selectively sampled and characterized interstitial
(unactivated) particles, cloud droplet residual particles and the total
aerosol (sum of interstitial plus droplet residual particles) at the
high-alpine research station Jungfraujoch. Firstly, we used this approach to
determine the relationship between the scavenged fraction of total BC mass
and SS
A field campaign was conducted at the high-alpine research station Jungfraujoch (3580 m a.s.l. in central Switzerland) from 12 June to 6 August 2016. Additional results are included from measurements conducted during the Cloud and Aerosol Characterization Experiment 2010 (CLACE2010) campaign at the same site during the same period of the year (19 June 2010 to 17 August 2010). The exact same instruments were used during both campaigns. Over the last 20 years, the Sphinx laboratory at the Jungfraujoch has hosted numerous field experiments on aerosol-related research (Bukowiecki et al., 2016), specifically addressing aerosol–cloud interactions during CLACE campaigns (e.g. Sjogren et al., 2008; Zieger et al., 2012), new particle formation (e.g. Bianchi et al., 2016; Tröstl et al., 2016), and continuous characterization of aerosol properties and trends (Collaud Coen et al., 2013). In 1995, the aerosol monitoring became part of the Global Atmosphere Watch (GAW) programme of the World Meteorological Organization (WMO). Further environmental research comprises for example a thorough study of the aerology and air mass dynamics around this site (e.g. Poltera et al., 2017), which is important to understand aerosol transport phenomena.
The Jungfraujoch is located on a mountain pass oriented in the direction
southwest–northeast between the Jungfrau (4158 m a.s.l) and Mönch
(4107 m a.s.l.) peaks. Owing to this, two main wind directions are observed
from the southeast and the northwest. The relative proximity of the
Jungfraujoch to lower-altitude pollution sources as well as its presence
within clouds about 40 % of the time (Baltensperger et al., 1997) makes
it an appropriate site to study black-carbon–cloud interactions. According to
Herrmann et al. (2015), free tropospheric (FT) conditions prevail for
39 % of the time at the Jungfraujoch but only around 20 % in summer.
Pollution injections from the planetary boundary layer (PBL) increase the
number concentration of particles larger than 90 nm from typical levels
under FT conditions of around 40 up to 1000 cm
Aerosols were sampled through three different inlets during the whole
campaign (Fig. 1): a total inlet, an interstitial inlet and a pumped
counterflow virtual impactor (PCVI). We used stainless steel lines and short
sections of electrically conductive tubing close to the instruments. The
total inlet sampled interstitial (unactivated) particles, cloud droplets and
ice crystals when mixed-phase clouds were present. This inlet was designed
for sampling droplets with diameters up to 40
Instrumental set-up. Green rectangles indicate inlets and black rectangles indicate instruments. Acronyms: PCVI, pumped counterflow virtual impactor; CPC, condensation particle counter; MAAP, multi-angle absorption photometer; SMPS, scanning mobility particle sizer; SP2, single-particle soot photometer; CCNC, cloud condensation nuclei counter. Drying of the sample air occurs through the temperature increase from outdoor to indoor.
Measurements of the refractory BC (rBC) mass and optical sizing of BC-free
and BC-containing particles were done by a single-particle soot photometer
(SP2, upgraded to eight-channel revision C version, Droplet Measurement
Technologies, Longmont, CO, USA). The SP2 detects incandescent and scattered
light from particles passing through a high-intensity intra-cavity Nd:YAG
laser (
A second, qualitative method for BC mixing state analysis classifies the BC-containing particles into two classes: one exclusively for “thickly” coated BC and the other including all remaining degrees of coating thickness from “none” through “thin” to “moderate”. This “delay time” method, described in Schwarz et al. (2006), is based on the measurement of the time difference between the scattering signal peak and the incandescence signal peak of a particle. Delay time histograms were characterized by two distinct modes corresponding to the two above-mentioned classes. The measurements of BC core mass equivalent diameter and coating thickness are based on the assumption of a spherical core and a concentric coating surrounding the core.
The incandescence and scattering detectors of the SP2 were calibrated three times during the CLACE2016 campaign: on 3 June, 17 July and 3 August 2016. A fourth calibration of the scattering detector took place on 1 July. The BC counting efficiency of the SP2 was checked against a CPC at the beginning and the end of the campaign. On 11 July, the YAG crystal had to be changed; this caused an interruption in the SP2 operation until 17 July. After that date, the SP2 was switched on only during cloud events to preserve laser power. During the CLACE2010 campaign, the scattering detector was calibrated four times: on 16 and 27 January, 8 February and 3 March. The incandescence detector was calibrated on 27 January.
Multi-angle absorption photometers (MAAPs, model 5012, Thermo Fisher
Scientific, Waltham, MA, USA) were installed downstream of the total and
interstitial inlets (two MAAPs in total). This instrument determines the
aerosol absorption coefficient at a wavelength of 637 nm by collecting
particles on a fibre filter and measuring the transmission and back
scattering of laser light at multiple angles (Petzold and Schönlinner,
2004). The firmware output at 1 min time resolution of the equivalent black
carbon (eBC) mass concentration was used, which is calculated from the
measured absorption coefficient using a mass absorption cross-section (MAC)
value of 6.6 m
One condensation particle counter (CPC, TSI Inc., Shoreview, MN, USA) was installed downstream of each inlet in order to measure the particle number concentration. Three different CPC models were used: model 3010 for the interstitial inlet (with a 50 % detection efficiency reached at 10 nm), model 3022 for the PCVI (7 nm) and model 3025 for the total inlet (3 nm). The quality of total and interstitial CPC data was controlled by comparing them during out-of-cloud conditions.
Two custom-built SMPS systems, each consisting of a differential mobility analyser (Model 3081 Long DMA, TSI Inc., Shoreview, MN, USA) and a CPC (TSI model 3775 for the total inlet and 3022A for the interstitial inlet), measured aerosol number size distributions at a time resolution of 6 min. The measured mobility diameters ranged from 22 to 604 nm. One SMPS was placed downstream of the total inlet (it is used for the continuous GAW measurements) while the other switched every 12 min (2 scans) between the interstitial inlet and the PCVI. The sizing and counting efficiencies of both SMPS systems were checked using 150 and 269 nm polystyrene latex spheres (PSL) every 2 weeks during the campaign. Quality assurance further included an intercomparison of all five CPCs at the beginning and at the end of the campaign: their number concentration readings agreed within 10 %.
Cloud condensation nuclei number concentrations in polydisperse aerosol samples were measured at four different supersaturations (0.35 %, 0.40 %, 0.50 % and 0.70 %; total measurement cycle of 225 min) with a cloud condensation nuclei counter (DMT model CCN-100, Droplet Measurement Technologies, Longmont, CO, USA; see details in Roberts and Nenes, 2005). Calibrations of the cloud condensation nuclei counter (CCNC) took place on 10 June and 4 August 2016 and gave very similar results, with less than 5 % difference between the supersaturation calibration curves. Concerning the CLACE2010 campaign, the CCNC was calibrated on 16 June 2010.
In order to detect the presence of clouds and measure the liquid water content (LWC), a particulate volume monitor (PVM-100, Gerber Scientific Inc., Reston, VA, USA; described in Gerber, 1991) was installed on the roof of the laboratory, at the same height and around 3 m away from the inlets. The PVM detects light scattered by the cloud droplets in forward direction at multiple angles to infer the LWC. It was calibrated every week with a calibration disk provided by the manufacturer.
Measurements of air temperature 2 m above the ground, wind speed and direction are continuously conducted at the Jungfraujoch and are part of the SwissMetNet network of MeteoSwiss.
The occurrence of in-cloud conditions during the campaign was determined with
the LWC measurements of the PVM. The criterion for defining a cloud event was
a minimum LWC of 0.1 g m
As discussed below, aerosol hygroscopicity and cloud peak supersaturation often varied substantially over the full duration of a cloud event. Therefore, stable periods within a cloud event were identified as periods with limited variability in key aerosol and cloud parameters. Sometimes even two distinct stable periods were identified in a single cloud event, resulting in a total of 11 “stable cloud periods” from the CLACE2016 campaign which were chosen for further detailed analysis (see Table 1). The analyses of three stable cloud periods extracted from the CLACE2010 campaign are also shown. Combining both campaigns, these periods add up to a total duration of 14.1 h.
Parameters for all 14 stable cloud periods further analysed
in this study (3 from CLACE2010 where names are associated with an asterisk,
11 from CLACE2016).
For in-cloud conditions, we define the size-dependent activated fraction,
AF(
Alternatively, the activation spectrum is inferred from the data measured
behind the PCVI inlet and the total inlet:
Fraction of particles that activated to cloud droplets as a function of particle dry diameter as derived from the measurements behind the total and interstitial inlets for four example cloud events (averaged over the complete stable period). SMPS-derived activated fractions are shown against mobility diameter and include all particles, whereas SP2-derived data are separately shown for BC-free and BC-containing particles, both against optical diameter. BC-free particles are shown against optical diameter determined with standard optical sizing and against optical diameter determined using the LEO-fit approach in order to confirm consistency between the two. Each panel shows a different cloud event. The vertical dashed line marks the SMPS-derived half-rise threshold diameter for activation. Note that these activation spectra are averaged over a duration of 36 to 54 min, which may have resulted in a smearing of the activation transition if the cut-off diameter varied slightly in time.
Cloud droplet residual particle measurements using the PCVI inlet
for the example of the 4 August cloud event.
We use the term “activated fraction” in the context of particle number, whereas we use the term “scavenged fraction” when presenting mass fractions of particulate matter incorporated into cloud droplets relative to the total aerosol. Size-resolved activated fraction and scavenged fraction of BC are identical in the special case of choosing the BC core mass equivalent diameter for the diameter scale. However, activated fraction and scavenged fraction integrated over a certain diameter range are not identical due to the different size dependence of the weighting factor when integrating number or mass.
The equilibrium size of an aerosol particle under subsaturated relative
humidity (RH) conditions and its activation threshold to a cloud droplet at
supersaturated RH conditions depend primarily on the particle dry diameter
(
The combination of total CCN number concentration at a defined
supersaturation, measured by the CCNC in a polydisperse set-up, with total
particle number size distribution, measured by the SMPS, makes it possible to
infer the critical dry diameter of the ambient aerosol for the
supersaturation set in the CCNC (Kammermann et al., 2010b). This approach was
applied for the first time at the Jungfraujoch by Jurányi et al. (2011)
under the assumption that the aerosol is internally mixed. Specifically, the
particle number size distribution was integrated from the maximum diameter to
the diameter at which the integrated particle number concentration is equal
to the simultaneously measured CCN number concentration. The lower limit of
integration matching this condition corresponds to the critical dry diameter
for CCN activation,
Hygroscopicity parameter as a function of the critical diameter
during the
The activation of aerosol particles in an ambient cloud depends on the peak
supersaturation reached during cloud formation. We applied the method
introduced by Hammer et al. (2014a) to retrieve the effective peak
supersaturation (SS
The 31 July–1 August full cloud event under northwestern wind
conditions.
Hygroscopicity parameter
The apparently circular calculation in the above approach, i.e. using the
The critical supersaturation for droplet activation is calculated for
individual BC-containing particles detected by the SP2 behind the different
inlets, following the approach described in Motos et al. (2019). Briefly, the
approach entails combining
The PCVI, described in Sect. 2.2.1, was not operational during all but two
cloud events due to technical issues linked to flow rate adjustments and
icing of the inlet (Table S1 in the Supplement). During the cloud events on
22 July and 4 August, when the PCVI functioned, the input and output flow
rates were set to 11.8 and 1.5 L min
In the present work, the uncertainties associated with MAAP- and SMPS-derived
scavenged fractions are based on propagating differences between measurements
conducted behind the interstitial and total inlets during out-of-cloud
conditions, immediately before and after each cloud event. The same approach
applies for SMPS-derived activated fractions of the total aerosol (and the
corresponding activation diameter
The uncertainties in the retrieval of
The aerosol properties observed during this study will not be discussed in detail as several comprehensive data sets of the Jungfraujoch aerosol observations have already been published (e.g. Bukowiecki et al., 2016, and references therein). By contrast, only limited data on BC properties have been published at the Jungfraujoch so far (e.g. Liu et al., 2010; Kupiszewski et al., 2016), but a more comprehensive manuscript on this topic is currently in preparation (Motos et al., 2019). Here, we focus only on the aerosol properties that are directly relevant for determining the activation behaviour of BC in clouds.
The hygroscopicity parameter
The statistics of aerosol hygroscopicity over the whole CLACE2016 campaign
are shown in Fig. 6. Mean values of
The size distribution measurements behind the total and interstitial inlets
were used to determine the size-resolved fraction of particles that activated
to cloud droplets during cloud events (Eq. 1; Sect. 3.2). The half-rise
threshold diameter (
The
Table S1 shows that the variations in SS
Fraction of particles activated to cloud droplets for each stable
cloud period of the CLACE2016 campaign as derived from particle number size
distributions measured by the SMPS behind the total and interstitial inlets.
The lines are coloured by SS
The size-resolved activated fractions averaged separately for all stable
cloud periods of the CLACE2016 campaign are plotted in Fig. 7. The mean peak
supersaturation, indicated for each period by the line colour, decreases
monotonically with increasing activation threshold diameter. This is not
surprising as the SS
The scavenging of BC, i.e. the mass fraction of BC incorporated into cloud
droplets, has previously been investigated at the Jungfraujoch. Cozic et
al. (2007) applied the same combination of interstitial and total inlets to
determine the scavenged fraction of BC (based on eBC mass measured by two
MAAPs), as well as the scavenged fraction of the total aerosol (derived from
SMPS measurements). They found close agreement between the scavenged
fractions of BC and that of the total aerosol for warm clouds with
temperature at Jungfraujoch (
Going a step further, we examined the dependence of the scavenged fractions
on SS
The SS dependence of the scavenged fraction of a hypothetical, internally mixed aerosol with log-normal size distribution and size-independent hygroscopicity (composition) will follow a Hill curve such as the dashed lines in Fig. 8b. Variations in modal size would shift the position of the Hill curve, whereas deviations from the log-normal size distribution shape would distort the shape of the curve. Similarly, variations and size dependence of aerosol hygroscopicity would also modulate the scavenging curve but are probably too small to cause modifications to the same extent. Such variations are the reasons for the substantial scatter around the Hill curves in Fig. 8b. The reverse conclusion is that size distribution and mean hygroscopicity must be known to accurately describe the supersaturation dependence of the scavenged fraction.
eBC mass scavenged fractions derived from the two MAAPs and aerosol
volume scavenged fractions derived from the two SMPSs during all liquid
clouds.
The scavenged fraction of BC mass is only expected to be equal to the total
aerosol volume scavenged fraction for all peak supersaturations if BC
contributes an equal fraction to the aerosol volume at any particle size and
if the critical activation diameters of the BC-containing particles and total
aerosol are equal. While the latter condition is closely fulfilled if BC is
internally mixed with substantial coatings, size-independent BC volume
fractions are a priori not expected. Nevertheless, the scavenged fractions
of total aerosol volume and BC mass are essentially equal on average.
However, deviations of several data points in Fig. 8a from the
The scavenged fractions of BC and the aerosol volume observed in fog events
in Zurich by Motos et al. (2019) are also included in Fig. 8b. The peak
supersaturation in fog is much lower than that of typical clouds at the
Jungfraujoch. Nevertheless, one cloud during the CLACE2016 campaign had a
peak supersaturation as low as the fog data (
Matsui (2016) used a mixing-state-resolved 3-D model to simulate the mixing state of BC-containing particles over East Asia and to estimate the critical supersaturation required for CCN activation of these particles. He concluded that almost all BC-containing particles activate to form droplets at 1.0 % supersaturation while 20 % to 50 % by number stay in the interstitial phase at 0.1 % supersaturation. He applied a theoretical approach equivalent to the one verified in the present study (see Sect. 4.4). These model results are in qualitative agreement with our observations. However, a direct, quantitative comparison of number-based activated fractions of BC over East Asia with mass-based scavenged fractions of BC at the Jungfraujoch is not justified because mass- and number-based activated fractions can differ for the same aerosol population and because the sources and levels of air pollution are different in central Europe and East Asia.
The SMPS measurements behind the interstitial and total inlets and Eq. (1) were used to infer the size-dependent activation of aerosol particles as discussed above in Sect. 4.1 and shown in Fig. 2. The SP2 measurements behind these inlets were used in an equivalent manner to specifically investigate the activation of BC-containing particles to cloud droplets. Figure 2 shows, on the basis of four example cloud events, that the SP2-based results for BC-free particles (blue and green lines) agree with the SMPS-based results for all particles (pink lines). This comparison is appropriate as the BC-free particles represent around 70 %–95 % of all particles by number (see Sect. 2.2.2 for our operational definition of “BC-free” particle). These independent measurements of activated fractions agree well because the optical diameters provided by the SP2 for BC-free particles are equal to the respective mobility diameters measured by the SMPS, which was tested by comparing corresponding size distributions from these two instruments (not shown). Such consistency in sizing is expected for spherical particles if appropriate calibration and data processing procedures are applied to the SP2 light-scattering signals. The optical sizing of BC-containing particles by the SP2 requires the more sophisticated LEO-fit technique (see Sect. 2.2.2), which was limited to optical diameters greater than 180 nm. The SP2-LEO-fit-derived size-dependent activated fractions of BC-free and BC-containing particles shown in Fig. 2 as green and black lines, respectively, are in agreement within experimental uncertainty. Such agreement indicates that the majority of the BC-containing particles with a diameter greater than 180 nm consist of small BC cores with substantial coating acquired through various processes during atmospheric transport to the remote Jungfraujoch site (through for example condensation of oxidized organic compounds, coagulation with particles or in-cloud processes). Such small insoluble cores hardly alter the hygroscopicity of the entire particle compared to a BC-free particle. Using single-particle mass spectrometry, Zhang et al. (2017) performed an equivalent comparison in southern China and also found that the activated fraction of BC-containing particles was similar or slightly lower compared to that of the total aerosol in the vacuum aerodynamic diameter range from about 200 to 1300 nm.
Activation of BC to cloud droplets.
The scavenged fraction of BC mass can be more directly understood by
analysing activated fractions as a function of BC core size rather than total
particle diameter. The finding that the BC scavenged fraction is primarily
controlled by cloud peak supersaturation, as shown in Fig. 8 and discussed in
Sect. 4.2, is also clearly shown in Fig. 9a, which shows the BC activated
fraction as a function of the rBC mass equivalent diameter for all stable
liquid cloud periods: the activated fraction increases with increasing
SS
Mixed-phase or even completely glaciated clouds may occur at lower
temperatures. Mixed-phase clouds may result in the conversion of particles
from droplets (activated particles) to interstitial aerosol through the
Wegener–Bergeron–Findeisen process (e.g. Cozic et al., 2007), thereby
potentially obscuring the causal relationship between SS
According to the Köhler theory (Sect. 3.3), the BC core diameter of an
internally mixed BC-containing particle is not the decisive parameter for its
critical supersaturation (even for a hypothetical spherical core–shell
morphology). Instead, in the absence of surfactants, the overall particle
diameter and the mean hygroscopicity are important: the acquisition of
water-soluble coatings on BC cores is expected to decrease the critical
supersaturation. In addition to the LEO-fit technique, we also applied the
delay time method, described in Sect. 2.2.2, to investigate the influence of
BC mixing state using SP2 data. This method makes it possible to split
BC-containing particles with a certain core size into two distinct classes,
one containing exclusively thickly coated BC particles and the other one
containing BC particles with thin to moderate coatings, with a
classification threshold at a BC volume fraction of
The size-segregated activation of BC cores observed in a previous study
during a fog event at an urban site in Zurich, Switzerland, is also shown in
Fig. 9 (Motos et al., 2019). The peak supersaturations in this fog event were
in the range 0.040 %–0.051 %, which is typical for mid-latitude fog
(Hammer et al., 2014b) and almost an order of magnitude lower than the
supersaturations in most of the clouds at the Jungfraujoch site. Accordingly,
the activation onset diameter above which BC cores are activated to fog
droplets is much greater, i.e. as large as
Ching et al. (2018) used the particle-resolved aerosol model PartMC-MOSAIC to
simulate the aging of BC-containing particles in urban plumes. They modelled
two-dimensional BC core size and mixing state distributions, and they then inferred
size-segregated activation curves and integrated scavenged fractions for BC
using the
The delay time method, which was applied to the SP2 data for the analyses
presented above, only provides a binary mixing state classification.
Quantitative mixing state information can be retrieved from the SP2 data
using the LEO-fit approach described in Sect. 2.2.2, though at the expense of
limiting the accessible size range. Coating thickness distributions for BC
cores with mass equivalent diameters in the range 170 nm
The results presented in the previous section demonstrate qualitatively that the acquisition of coatings by BC particles increases their ability to form cloud droplets. Here we go a step further by comparing calculated and observed droplet activation thresholds on a single BC-containing particle level. Combining the SP2 data with the CCN measurements makes it possible to predict the critical supersaturation of individual BC-containing particles as described in Sect. 3.6. This prediction can then be compared with the actual activation to cloud droplets, which is inferred from the SP2 measurements behind the total and interstitial inlets. Motos et al. (2019) performed such a closure study for the activation of BC in fog. Here we investigate activation of BC in clouds at the Jungfraujoch, which typically have much higher peak supersaturations than fog.
Activation of BC-containing and BC-free particles during the 25 June
Results for two example cloud periods are shown in Fig. 11. The properties of
individual BC-containing particles are shown in Fig. 11a1 and b1 for the
samples taken behind the total inlet (grey data points) and the interstitial
inlet (coloured by coating thickness). The cloud peak supersaturations are
shown as light blue horizontal lines. In an ideal case, i.e. perfectly well defined
and homogeneous peak supersaturation and aerosol composition, and
negligible measurement noise and bias, one would expect that no interstitial
particles show up below the light blue lines (i.e. activated
fraction
An alternative but equivalent method of performing this closure exercise is
to compare the observed activation spectrum of the BC-containing particles
(black lines in Fig. 11a2 and b2) with that of the total aerosol based on
SMPS measurements (green lines; inferred from the size-segregated activation
spectra shown in Fig. 7). The activated fraction of the total aerosol reaches
The two stable cloud periods shown in Fig. 11 have an SS
Activated fractions as a function of coating thickness for BC-containing particles with an optical diameter of 200 nm during four stable cloud periods of the CLACE2016 campaign and three periods of the CLACE2010 campaign. Triangles accompanied by horizontal dashed lines correspond to the activated fraction of 200 nm BC-free particles, derived from Fig. 2 (four example figures shown). The triangles are plotted at 100 nm coating thickness because this corresponds to 200 nm optical diameter if no BC core is present (BC-free particle). Dashed lines attached to activated fraction lines indicate the difference between experimentally observed (open circles) and theoretically predicted (crosses) coating thicknesses required for 200 nm BC-containing particles to reach activation up to half of the activation plateau.
It has to be noted that, in the present study, we have tested the validity of
the
Two field experiments with in situ cloud measurements were performed at the
high-altitude research station Jungfraujoch, central Switzerland, in summer
2010 and 2016. We selectively sampled the interstitial aerosol (unactivated
particles), cloud droplet residual particles, and the total aerosol (sum of
interstitial and droplet residual particles) using three different inlets
with the aim of investigating the influence of size and mixing state on the
activation of BC-containing particles to droplets in ambient clouds. We
showed that the cloud peak supersaturation is the main parameter controlling
the BC mass scavenged fraction. Variations in BC core size distribution and
BC mixing state also have a minor influence on the scavenged fraction,
particularly at higher supersaturations. It was qualitatively shown that, as
expected, acquisition of coating increases the ability of BC cores of a
certain size to activate to cloud droplets. Furthermore, quantitative closure
between predicted and observed threshold coating thicknesses was achieved.
Successful closure for the activation of BC was also achieved in a previous
study in fog with lower peak supersaturations (Motos et al., 2019). These
findings validate the approach of combining the
Data used in this article are available in the Supplement. Reconstructed overall particle diameter and coating thickness data on a single-particle level (using the LEO-fit analysis) are available upon request to the corresponding author.
The supplement related to this article is available online at:
MG and UB acquired the funding. MG conceptualized the study and the experiment was designed with JS and GM. MG supervised the study together with JS and UB. GM, JS and NK performed the field campaign and JCC contributed to instrument preparation and maintenance. GM analysed and validated the experimental data with support from RM, JCC, JS, MG and NK. GM prepared the manuscript and all co-authors contributed to the interpretation of the results as well as manuscript review and editing.
The authors declare that they have no conflict of interest.
This article is part of the special issue “BACCHUS – Impact of Biogenic versus Anthropogenic emissions on Clouds and Climate: towards a Holistic UnderStanding (ACP/AMT/GMD inter-journal SI)”. It is not associated with a conference.
We thank Nicolas Bukowiecki and Erik Herrmann for their help during the CLACE2016 campaign, as well as Ernest Weingartner, Zsofia Jurányi and Emanuel Hammer for their contributions to the CLACE2010 campaign. We also thank the International Foundation High Altitude Research Station Jungfraujoch and Gornergrat (HSFJG) for giving us the opportunity to perform an intensive campaign in addition to the continuous measurements in the Sphynx laboratory of the Jungfraujoch. This work was supported by the ERC under grant ERC-CoG-615922-BLACARAT and the EU FP7 project BACCHUS (grant no. 603445). Part of the observations included in this work originate from the continuous aerosol measurements at the Jungfraujoch site, which are supported by MeteoSwiss in the framework of the Swiss contributions (GAW-CH and GAW-CH-Plus) to the Global Atmosphere Watch programme of the World Meteorological Organization (WMO) and are also supported by the ACTRIS2 project (funded by the EU H2020-INFRAIA-2014-2015 grant agreement no. 654109 and by the Swiss State Secretariat for Education, Research and Innovation, SERI, under contract number 15.0159-1; the opinions expressed and arguments employed herein do not necessarily reflect the official views of the Swiss Government). Meteorological measurements from the SwissMetNet network were obtained through MeteoSwiss.
This paper was edited by Allan Bertram and reviewed by two anonymous referees.