Sea spray aerosol particles are a recognised type of
ice-nucleating particles under mixed-phase cloud conditions. Entities that
are responsible for the heterogeneous ice nucleation ability include intact
or fragmented cells of marine microorganisms as well as organic matter
released by cell exudation. Only a small fraction of sea spray aerosol is
transported to the upper troposphere, but there are indications from
mass-spectrometric analyses of the residuals of sublimated cirrus particles
that sea salt could also contribute to heterogeneous ice nucleation under
cirrus conditions. Experimental studies on the heterogeneous ice nucleation
ability of sea spray aerosol particles and their proxies at temperatures
below 235
A wealth of recent studies has substantiated early findings from the 1970s
that sea spray aerosol (SSA) particles are able to act as ice-nucleating
particles (INPs) in the immersion freezing mode for clouds in the
mixed-phase temperature regime between 273 and 235
Alphabetical list of previous ice nucleation measurements with SSA particles and their proxies at cirrus conditions, specifying the investigated substances and the employed ice nucleation measurement techniques.
Previous ice nucleation measurements with SSA particles and their proxies
featured a broad variety of approaches. They included stationary field
studies at coastal sites (e.g. Mason et al., 2015; Ladino et al., 2019;
Wex et al., 2019), ship- and aircraft-based measurements (e.g. McCluskey
et al., 2018a; Hartmann et al., 2020; Welti et al., 2020), as well as
complex laboratory experiments where phytoplankton blooms were simulated in
large seawater tanks and wave channels, generating SSA particles by plunging
sheets of water or breaking waves (e.g. DeMott et al., 2016; McCluskey et
al., 2017, 2018b). Some experiments have specifically
targeted phytoplankton organisms and their exudates (e.g. Alpert et al.,
2011b; Knopf et al., 2011; Ladino et al., 2016; Tesson and Šantl-Temkiv,
2018; Wolf et al., 2019; Ickes et al., 2020), asking whether the ice
nucleation behaviour of these species is representative of that observed for
ambient sea surface microlayer samples. The two ice nucleation measurement
techniques most frequently employed in the literature are droplet freezing
assays and continuous flow diffusion chambers (CFDCs), and the freezing data
are usually reported as the temperature-dependent number of INPs per either
droplet volume or volume of collected air. For ice nucleation measurements
under cirrus conditions (see Sect. 1.2), INP concentrations are reported as
a function of temperature and relative humidity. To quantitatively compare
the ice nucleation ability of the SSA particles with that of particles from
terrestrial sources like mineral dust, some measurements have also been
analysed within the concept of the ice nucleation active surface site
density,
The atmospheric concentration of sea salt exhibits a strong decrease with
altitude, from typically 0.3–3
Cirrus clouds can either be formed in situ or originate from other cloud
systems like deep convective clouds, where the ice phase is mostly formed
through the supercooled liquid phase
(Krämer et al., 2016). We do not
consider the latter here but focus on in situ cirrus, where ice crystals are directly formed at
In order to evaluate the available data, a distinction has to be made
between the experimental approach and the initial phase state of the
investigated particles. Three techniques have previously been adopted in
studies that specifically investigated the heterogeneous ice nucleation
behaviour of SSA particles and their proxies at cirrus conditions. The first
type of studies involved deposition nucleation experiments with cells or
cell fragments of the diatom
In the third type of experiments, the SSA particles' ice nucleation ability was probed with CFDCs (Wilson et al., 2015; Ladino et al., 2016; Kong et al., 2018; Wolf et al., 2019, 2020). In all of these studies,
particles generated from either sea surface microlayer samples or cultures
of phytoplankton and marine bacterial species in seawater were first dried
to RH
Due to the intrinsic heterogeneous ice nucleation ability of purely
inorganic, partly deliquesced sea salt particles below 220
Two further CFDC ice nucleation studies have investigated field-collected
sea surface microlayer samples. Wilson et al. (2015) probed a variety of
microlayer samples from the North Pacific and the British Columbia
coastline. The samples were aerosolised with an atomiser, and the ice
nucleation behaviour of dried, 200
The available data allow the following preliminary conclusions regarding the
ice nucleation behaviour of SSA particles and their proxies at cirrus
temperatures. The heterogeneous ice nucleation experiments with the
phytoplankton species mostly show a moderate reduction of the homogeneous
freezing level to
Our article is organised as follows. Section 2 describes the collection and
preparation of the seawater samples and the diatom culture (Sect. 2.1) as
well as the aerosol particle generation (Sect. 2.2) and the technical
details of the ice nucleation measurements (Sect. 2.3). As the central part
of our article, Sect. 3 summarises the results of the ice nucleation
experiments. Before addressing in detail the experiments under cirrus
conditions, we present a short summary of the freezing behaviour of the bulk
solutions in the mixed-phase cloud temperature region as measured with a
cold-stage instrument (Sect. 3.1). In doing so, we can assess whether the
range of freezing temperatures of our seawater samples is representative of
that from previous studies in the mixed-phase cloud region. After
aerosolisation of the bulk samples, we tested the particles' ice nucleation
behaviour at two different cirrus temperatures in the AIDA chamber, namely
at 229 and 217
Overview of the investigated samples, specifying the sampling
location, sampling time, and coordinates. The last column denotes the median
equal-volume sphere diameter,
We have probed seawater samples from three Arctic locations. The sea surface microlayer samples from the eastern Canadian Arctic and the Greenland Sea were collected with the glass plate technique during NETCARE (Irish et al., 2019) and from a hydrophilic Teflon film on a rotating drum during ACCACIA (Wilson et al., 2015) field expeditions, respectively, and some of them were already used in previous AIDA ice nucleation measurements that focussed on the mixed-phase cloud temperature region (Ickes et al., 2020). The seawater samples from Kongsfjorden were collected during rough sea conditions with a Niskin sampler placed horizontally on the water surface. Therefore, we use the term “surface seawater samples”. These samples likely contained neuston and non-living material present in the surface microlayer, but this material will have been heavily diluted with subsurface water both due to the sampling technique and the weather conditions. A closer description of the Kongsfjorden sampling site, the meteorological conditions during sampling, and the analysis of the aquatic chemistry and bacterial abundance of these samples is presented in Appendix A. For pertinent details regarding the NETCARE and ACCACIA samples, we refer to the cited literature. All investigated samples are listed in Table 2. To ensure their unique identification, we labelled the NETCARE and ACCACIA samples with the abbreviations STN and SML, respectively, as used in the original publications. The acronym KFJ was used for the Kongsfjorden samples. The measurements presented in this article were conducted during September and October 2017.
In addition to the microlayer samples, we used a laboratory-grown culture of
Number size distributions of aerosol particles generated by
nebulising the STN1, SML13, KFJ2, and SM100 bulk solutions. The size spectra
were measured at 298
For the ice nucleation measurements in the AIDA chamber, the seawater
samples and the SM100 cultures were thawed, homogenised by shaking, and
aerosolised with an ultrasonic nebuliser (GA2400, SinapTec). After passing
through a pair of silica gel diffusion dryers that reduced the ambient RH to
less than 3
In order to confirm that the sea surface microlayer and surface seawater
samples as well as the SM100 culture contained ice-active entities for
inducing heterogeneous freezing in the mixed-phase cloud temperature regime,
we investigated the freezing behaviour of 50
For a subset of samples, we diluted the suspensions by factors of 10 and 100 with ultrapure water to extend the measured
The ice nucleation experiments at cirrus conditions were conducted in the
aerosol and cloud chamber AIDA of the Karlsruhe Institute of Technology. The
operation of the AIDA chamber has been described in detail in a large number
of publications, but we want to specifically refer to its description in our
study on the ice nucleation behaviour of purely inorganic sea salt particles
at cirrus temperatures (Wagner et al., 2018), because the
modus operandi in that work was essentially the same as in our current
study. Briefly, the AIDA chamber is an 84
For the injection of the aerosol particles generated from the seawater
samples and the SM100 culture, the AIDA chamber was conditioned to almost
ice-saturated conditions (
Apart from the basic instrumentation described above, we used infrared
extinction as well as light scattering and depolarisation measurements to
probe the phase state of the added aerosol particles and to detect possible
phase changes during expansion cooling. Infrared extinction spectra of the
aerosol particles were recorded in situ from 6000 to 800
A summary of the freezing experiments with the cold-stage instrument INSEKT
is shown in Fig. 2. Panel (a) shows the temperature-dependent FF curves for
all investigated samples, corrected for the freezing point depression by the
salts (see Sect. 2.3.1). The data are colour-coded with respect to the
sampling location (magenta: STN samples from the Canadian Arctic, red: SML
samples from the Greenland Sea, blue: KFJ samples from Kongsfjorden, green:
SM100 culture, and grey: background measurement with ultrapure water). The
median freezing temperatures,
In Fig. 2b, we present the cumulative INP concentrations as a function of
temperature for our samples (
Time series of the AIDA records from the expansion cooling
experiments started at 229
Series of infrared extinction spectra that were recorded in the
first 80
Quantitative analysis of the AIDA expansion cooling runs started at
229
We first focus on the expansion cooling experiments started at about 229
Figure 3a shows the previously measured ice nucleation behaviour of
particles generated from a commercially available bulk Atlantic water sample
(data from Fig. 6, upper left panel, of Wagner et al., 2018). We consider
this measurement as the blank experiment for the intrinsic ice nucleation
behaviour of purely inorganic SSA particles when probed in an expansion
cooling run started at
How did the ice nucleation behaviour change when the sea salt aerosol
particles contained additional organic components? In Fig. 3b, we show the
AIDA records from the expansion run with the particles generated from the
undiluted SM100 culture with a concentration of
The AIDA records of three exemplary expansion cooling runs with particles
generated from the microlayer and surface seawater samples STN2, SML13, and
KFJ4 support the finding from the experiment with SM100 that there are two
distinct regimes where a heterogeneous ice nucleation activity becomes
apparent (Fig. 3d, e, and f). The particles generated from the STN2 sample
showed ice formation in both regimes, i.e. the early, weak immersion freezing mode starting at
An intriguing question is whether one can relate the heterogeneous ice
nucleation ability of the particles at cirrus conditions to the freezing
behaviour of the bulk solutions at mixed-phase cloud temperatures. To
facilitate such comparison, we have included in the last column of Table 3
the
Time series of the AIDA records from the expansion cooling
experiments started at 217
Illustration of the ice nucleation behaviour observed
during the AIDA expansion runs with the particles generated from the bulk
Atlantic water sample (blank
Quantitative analysis of the AIDA expansion cooling runs started at
217
Figure 5a shows the AIDA records of an experiment where the ice nucleation
ability of the particles generated from the bulk Atlantic water sample was
probed at a lower starting temperature of 217
As a summary of our observations, we show in Fig. 6 two diagrams in the
Homogenous and heterogeneous ice nucleation onsets from the AIDA
expansion run with the particles generated from the SM100 culture shown in
Fig. 3b (light green and blue diamonds) in comparison with data from
previous experiments with phytoplankton and marine bacterial species (see
Sect. 1.2). The onsets from the immersion freezing measurements with the
technique by Alpert et al. (2011a) were evaluated at median freezing
temperatures for homogeneous and heterogeneous ice nucleation. The
heterogeneous ice nucleation onsets from the AIDA and the various CFDC
experiments correspond to different FF values as indicated in the legend and
discussed in Sect. 4.1. The
Our new AIDA results on the ice nucleation behaviour of particles generated
from microlayer and surface seawater suspensions and diatom cultures at
cirrus temperatures show both similarities to and discrepancies with the data from the previous studies that we have summarised in Sect. 1.2. In Fig. 7,
we present a compilation of the freezing data for the experiments with
phytoplankton and marine bacterial cells as well as their exudates. The grey
line shows the trajectory of our AIDA expansion experiment with the
particles generated from the SM100 culture in the
Ladino et al. (2016) detected in their CFDC measurements a very small early
ice nucleation mode at
Whereas our ice nucleation experiments with SM100 fit well into previous
data, the same is not true for the ice nucleation experiments with our
microlayer and surface seawater samples. Both the particles from the samples
collected by Wilson et al. (2015) and by Wolf et al. (2020) showed much
higher ice-active fractions at temperatures above 220
Due to the limited number of measurements, it is premature to ascribe this
difference solely to the geographical sampling region, i.e. the Arctic region in our study versus locations in temperate and subtropical zones in
the studies by Wilson et al. (2015) and Wolf et al. (2020). A factor that
could contribute to a regional variation in the INP concentrations is the
biogeographic pattern of the phytoplankton species. As summarised in Sect.
1.2, there are notable variations in the heterogeneous ice nucleation
ability of various phytoplankton species under cirrus conditions, with e.g.
When comparing different measurements, it is important to take into account
that there can also be a strong seasonal variation in the INP concentrations
for the same sampling location and that the measured ice nucleation ability
might also depend on the thickness of the sampled microlayer, i.e. how much ice-active organic material was sampled in relation to inorganic solutes
(Irish et al., 2017, 2019), and therefore on the technique
used to acquire the sample. Nonetheless, it is remarkable that the
heterogeneous ice nucleation activity of all our samples is consistently
very low at cirrus conditions, provided that the inorganic salts are not yet
ice-active. The bulk freezing measurements with INSEKT clearly indicated that our samples contained representative amounts of ice-nucleating entities
at mixed-phase cloud conditions. A quantitative comparison between ice
nucleation under mixed-phase cloud and cirrus conditions is challenging
because different nucleation modes might be involved. As noted in the
introduction, the
A key factor in all laboratory experiments with microlayer samples and
phytoplankton species is the aerosolisation method. In Wilson et al. (2015), Wolf et al. (2020), as well as our study, the particles were
produced from well-mixed microlayer and surface seawater samples with
standard aerosol generators, so that we do not have any reason to assume
that the observed differences in the ice nucleation activity are related to
the aerosol generation method. However, all of these measurements only
represent some kind of averaged ice nucleation activity, meaning that the
ice-nucleating entities are equally distributed amongst all particles and
that there is presumably no significant variability in the particle
composition. As already mentioned in Sect. 2.2, these techniques do not
mimic the natural process of sea spray aerosol production, where the
bursting of bubble cap films can lead to the formation of highly organically
enriched particles (O'Dowd et al., 2004; Ault et al., 2013; Prather et
al., 2013). For ice nucleation experiments under cirrus conditions, the
particle composition is particularly important because it can influence the
underlying ice nucleation mode. For particles predominantly or even
exclusively composed of organics, the ice nucleation mode might change from
immersion freezing, as observed in the AIDA experiments, to deposition
nucleation, where ice formation initiates by the deposition of water vapour
on crystalline or glassy surfaces (Murray et al., 2010; Wilson et al.,
2012). For this reason, Wolf et al. (2019) have used pure organic compounds
as a proxy to represent the ice nucleation ability of the particles from the
lysed
In our previous ice nucleation experiments at mixed-phase cloud
temperatures, we have attempted a more representative way of SSA production
and have added 80 to 900
The second one of the two most important factors in future studies would be
the intercomparison of different ice nucleation measurement techniques. The
most recent laboratory workshop on the intercomparison of ice nucleation
measurements focussed on immersion freezing experiments under mixed-phase cloud conditions (DeMott et al., 2018). We consider it
equally important to perform such a study under cirrus conditions where, due
to their complex hygroscopic behaviour, SSA particles would be an
interesting and experimentally challenging INP type to be investigated. Let
us imagine an aerosol particle that was produced from a microlayer sample,
dried to a low relative humidity, and is now in a supersaturated environment
with
Organic-rich particles might prevail in a highly viscous or glassy state at
low temperature, with the result that there is a competition between water
uptake and deposition ice nucleation on the glassy, solidified organic
surface (Reid et al., 2018). The effect of kinetic
limitations of water diffusion and its impact on equilibration timescales
and modes of ice nucleation have already been investigated in various
computational studies with model organic substances (e.g. Berkemeier et al.,
2014; Lienhard et al., 2015; Price et al., 2015; Fowler et al., 2020). For
example, Price et al. (2015) modelled equilibration times for
Even if we have found that the dried particles from our microlayer samples showed no notable change in their hygroscopic behaviour compared to inorganic sea salt, we are still far from formulating this as a general statement. It was recently shown that wintertime Arctic SSA particles originating from open leads featured particularly high volume fractions of organic material that was produced as a cryoprotectant (Kirpes et al., 2019). The organic components formed a thick coating layer around the sea salt core. From a mechanistic point of view, it would be highly interesting to investigate the water uptake and ice nucleation behaviour of such internally mixed SSA particles. Apart from experiments with cloud chambers and CFDCs, the direct observation of ice nucleation on individual particles with an environmental scanning electron microscope would be another promising approach (Zimmermann et al., 2007; Wang et al., 2016).
If the SSA particles temporarily encounter a relative humidity below about
40
The influence of the salt constituents is another example that there are
manifold factors which control the ice nucleation activity of SSA particles
at cirrus conditions. The activity is not only related to the amount and
identity of the organic material that is transferred to the particle phase
by the bubble bursting process, but also to the potential crystallisation of
NaCl or
The high-latitude glacial fjord Kongsfjorden (79
The seawater samples for this study were collected on 5 July 2017 in the
transitional zone of Kongsfjorden, east of the settlement of Ny-Ålesund
in possible influence of the outflow from the Midtre Lovénbreen glacier.
The sampling took place on a small inflatable boat (Zodiac) using a Niskin
bottle sampler placed horizontally onto the water surface. The pre-cleaned
Niskin sampler was triple-rinsed with sample water prior to sample
collection. The surface seawater was filled from the Niskin sampler outlet
directly into sterile sampling bags (Whirl-Pak
Aquatic chemistry and bacterial abundance,
Table A1 summarises the measured aquatic chemistry and bacterial abundance
of the samples. For the measurement of the aquatic chemistry, a portion of
each sample was filtered through a precombusted glass fibre filter (MN GF-5,
Macherey-Nagel, 25
Here,
The ice nucleation data sets derived in this work can be downloaded from the
KITopen repository, the central publication platform for KIT (Karlsruhe
Institute of Technology) scientists, at
LI, MES, and RW designed the work. The experimental work was carried out by RW, NE, NSU, and OM. AKB, NE, EG, BJM, and MES provided the resources. RW and BJM visualized the data. RW and NE wrote the original draft of the manuscript. All the authors contributed to the review and editing.
The authors declare that they have no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
We are grateful for the continuous support by all members of the Engineering and Infrastructure group of IMK-AAF, in particular by Olga Dombrowski, Rainer Buschbacher, Tomasz Chudy, Steffen Vogt, and Georg Scheurig. We would like to thank Luis Antonio Ladino and one anonymous referee for their valuable feedback.
This work has been funded by the Helmholtz-Gemeinschaft Deutscher Forschungszentren as part of the programme “Atmosphere and Climate”. Luisa Ickes was supported by the Swiss National Science Foundation (Early Postdoc.Mobility). Benjamin J. Murray was supported by the European Research Council (MarineIce; grant no. 648661). Nsikanabasi Silas Umo was supported by the Alexander von Humboldt Foundation, Germany (grant no. 1188375). Matthew E. Salter was supported by the Swedish Research Council (grant no. 2016-05100). The article processing charges for this open-access publication were covered by the Karlsruhe Institute of Technology (KIT).
This paper was edited by Hinrich Grothe and reviewed by Luis Antonio Ladino and one anonymous referee.