Figure 4. The ice-active
site density (ns) for aerosol sampled during the cruise (see Figure 2
for key to colours) compared to n sparameterisations for desert dust [Ullrich et al. , 2017],
Icelandic dust [Sanchez-Marroquin et al. , 2020] and for
aerosol from the North Atlantic [McCluskey et al. , 2018b]
that is also consistent with values for aerosol over the Southern Ocean
[McCluskey et al. , 2018a].
3.2 Ice-nucleating particle concentrations above the surface mixed layer
Eight flights with a balloon-borne size-resolved aerosol sampler were
conducted while the Oden was at the ice station (see SI for
flight details). This sampler, known as the selective height aerosol
research kit (SHARK) was lofted to a defined height using a tethered
balloon system operated from the sea ice and aerosol samples were
collected into multiple size bins from below 0.25 µm to 10 µm with an
additional stage for particles larger than 10 µm (with a poorly defined
upper limit) [Porter et al. , 2020]. During these flights, we
used a live link to the on-board temperature and humidity measurements
to ensure that we sampled above the surface mixed layer and thus in air
decoupled from the surface, but within the boundary layer (i.e. in the
cloud mixed layer). Hence, the flights occurred at 390 m to 600 m
altitude while the ship was on the ice-floe station and we sampled for 3
to 6 hours. In addition, we paused sampling when the RH was more than 80
% to avoid sampling biases associated with hygroscopically swollen
aerosol and we also avoided sampling while the SHARK was enveloped in
cloud.
Given the surface mixed layer is often decoupled from the rest of the
boundary layer, these measurements in principle allow us to compare INP
concentrations within and above the surface mixed layer. The values ofT [INP]=0.1 are shown in Figure 2b, whereas
the INP spectra are shown in Fig SI2 for the INP summed across all the
particle size bins. It should be borne in mind that, for practical
reasons, the sampling durations on the ship and on the SHARK were not
the same, however it is still possible to draw conclusions from this
comparison. There is evidence that there are substantial differences
between the INP concentrations in the surface mixed layer compared to
above it. For example, on the 5th and
8th September theT [INP]=0.1 (summed across all sizes) was
around −18 to −19 °C above the surface mixed layer, whereas it was below
–26 °C within it. However, on the 20th August,T [INP]=0.1 was around –23 °C above the
surface mixed layer, but –14 °C within it. On all three days,
radiometer and radiosonde temperature profiles confirmed that the
surface mixed layer was decoupled from the rest of the boundary layer
(Table S2). In contrast, on 13th September, the
surface mixed-layer was mainly coupled and the INP concentrations within
and above the surface mixed layer were similar. However, on two other
days (23rd August and 10th September) the activity in
the surface mixed layer and above were similar even though it was
decoupled. Overall, out of the eight SHARK samples collected above the
surface mixed layer, there was one SHARK sample that had much lower
ice-nucleating activity than that in the surface mixed layer, three
samples with higher activity, three with similar activity and one that
was ambiguous (due to both samples being close to the baseline of
detection). This is consistent with the air at the surface sometimes
being coupled to the cloud mixed layer, allowing transport of aerosol
throughout the boundary layer, but at other times the measurements at
the surface are not representative of those above the surface mixed
layer.
The size-resolved INP activity (T [INP]=0.1)
is also shown in Figure 3b. In many locations around the world,
supermicron aerosol dominate the INP population [Porter et al. ,
2020]. However, contrary to what might be expected, the smallest size
ranges of < 0.25 μm contributed the most INPs on five out of
the eight flights, with the 2.5 to 10 μm and 0.5 to 1 μm bins both
contributing the most on one flight each. Inspection of the
corresponding INP spectra associated with each bin (Fig SI2) revealeds
that the particles < 0.25 μm made a pronounced contribution to
the INP population on the 23rd August and the
8th and 9th September. The other
flights produced data mainly in the baseline for all sizes.
The coarse mode (>2.5 µm diameter) has a relatively short
lifetime in the Arctic boundary layer, being removed effectively by wet
scavenging processes [Leck and Svensson, 2015]. Hence, it is perhaps
not so surprising that the fine mode aerosol (< 0.25 μm)
appears to be so important in this region for the INP population. While
INPs are typically thought of as being the larger particles in a size
distribution [Mason et al. , 2016; Porter et al. ,
2020], there are INPs that fall into the < 0.25 μm size
range that are also very active. For example, film droplet aerosol
resulting from wave breaking are produced in a range of sizes centred
around 100-200 nm and are often rich in organic material [O’Dowdet al. , 2004] that is known to include small ice-nucleating
entities [Schnell and Vali, 1975; Wilson et al. , 2015].
Alternatively, ice-nucleating macromolecules from terrestrial biological
sources internally mixed with other aerosol particles might fall into
this size range [O’Sullivan et al. , 2016; O’Sullivan et
al. , 2015; Pummer et al. , 2015] and it has been proposed that
fungal material, some of which is known to act as an INP [O’Sullivanet al. , 2015], can fragment to form nanoparticles [Lawleret al. , 2020].
3.3 Correlation between
INP concentrations and dimethyl sulfide, equivalent black carbon and
aerosol surface area
To investigate possible sources of the INPs we detected over the central
Arctic Ocean, we have correlated the ice-nucleating activity of the
aerosol with: i) dimethyl sulfide (DMS), a product of marine biological
activity, particularly in the marginal ice zone (MIZ, the transitional
zone between open sea and dense ice) [Leck and Persson, 1996]; ii)
equivalent black carbon (eBC), based on aerosol absorption at 637 nm;
and iii) aerosol surface area, derived from size distribution
measurements. We present the time series for aerosol particle surface
area, DMS and eBC concentrations, as well as the Pearson’s rcoefficient between ice-nucleating activity and each quantity in Figure
3a.
DMS is found in the marine atmosphere, originating from the metabolites
of some marine algae [Leck and Persson, 1996; Lohmann and Leck,
2005]. Hence, the presence of DMS indicates that an air mass has
origins in a location rich in biological activity, which may also be
expected to correlate with marine biological INP sources. DMS is thought
to be relatively short-lived in the atmosphere, with a lifetime on the
order of 1-3 days [Kerminen and Leck, 2001; Khan et al. ,
2016]. Therefore, it is a useful indicator for the interaction of air
masses with the MIZ at the outer edge of the pack ice region, and
possibly the open leads or melt ponds within the pack ice if they were
producing DMS at that time.
The concentration of DMS during the cruise was highest in the outbound
24-hour MIZ station (2nd-3rdAugust), where the ship was close to open water, but was variable whilst
in the pack ice (Figure 3), and remained relatively low in the inbound
MIZ station (19th September). The data in Fig. 2
clearly shows that there is no obvious correlation between DMS and INP
activity (r = 0.15) suggesting that MIZ marine biogenic sources
exerted little influence on the measured INP concentrations.
Equivalent BC (eBC) is a quantity derived from aerosol absorption and is
the equivalent black carbon mass concentration needed to produce the
observed absorption. Other aerosol types such as dust, brown carbon or
other organic aerosol might also produce absorption, thus potentially
contaminating the small signal we observed. However, absorption by BC is
much stronger at 637 nm than other materials, hence the signal is most
likely dominated by BC. BC is produced through a range of combustion
processes, including biomass burning, wildfires and fossil fuel
combustion, which are all remote from the central Arctic. Other
potential contributors to the absorption signal, such as dust or brown
carbon are also remote from the central Arctic. Rigorous procedures were
in place to ensure that BC (and other aerosol) from the ship stack did
not affect measurements (see methods for details). Therefore, eBC is
used here as an indicator of long-range transport. The literature
indicates that BC is a relatively ineffective ice nucleator under
mixed-phase cloud conditions [Adams et al. , 2020; Chen et
al. , 2018; Schill et al. , 2020; Vergara-Temprado et al. ,
2018a], hence we would not necessarily interpret a positive
correlation as an indication of ice nucleation by BC. However,
combustion processes are thought to be a source of ice-nucleating
aerosol, even if BC itself is not an effective INP [Barry et
al. , 2021; Jahn et al. , 2020; Umo et al. , 2015]. Thus,
a correlation between BC and INP concentrations would indicate that
aerosol particles transported along with BC from outside the central
Arctic Ocean nucleate ice. Wildfires around the Arctic are a potential
source of BC, and we note that during the cruise there were persistent
Siberian wildfires, as can be seen using the NASA Worldview satellite
imagery tool (e.g. clear skies on the 14th August
reveal widespread fires; https://worldview.earthdata.nasa.gov/). There
is also industry, shipping and mining along the Arctic coast of Russia,
but the role of gas flaring is thought to be a particularly important
source of BC [Stohl et al. , 2013].
The overall correlation between eBC and INP concentration (r =
0.65) is much stronger than for DMS. In fact, the eBC concentration
appears to track the INP concentration in Figure 3 until the
27th August, after which the INP concentrations stay
relatively low whilst the eBC remains highly variable. The decoupling of
eBC and INPs later in the campaign indicates that distant sources of BC
are not always related to distant sources of highly active INPs.
The surface area concentration of the bulk aerosol follows a similar
trend to the eBC concentrations, but with a slightly weaker correlation
with the INP activity (r = 0.52). This indicates that the
variability in INP concentrations at the North Pole is not simply driven
by aerosol surface area, rather that some specific component(s) of the
aerosol population are ice-active and these particles are most likely
associated with specific sources at latitudes further south than the
MIZ.
3.4 Trajectory analysis of aerosol collected in the central Arctic
Backward trajectories from the sampling location near the surface are
presented in Figure 5a. We only show points that are in in the boundary
layer and for a maximum of 7 days. The figure shows a clear relationship
between the origin of the aerosol – considered here as the boundary
layer points along the trajectories - and the measured INP
concentrations. The origin of the air with the most active INPs is
around the Russian Arctic coast including the Barents, Kara and Laptev
Seas. Out of the 30 filter runs, those filters with the highest INP
activity (the top 20 % of filters; −9 °C ≥T [INP]=0.1 > −13.5 °C) sampled
air masses originating over the Barents and Kara Seas. The next seven
highest (23 %; −13.5 °C ≥ T [INP]=0.1> −22 °C) filters, in terms of INP activity, sampled air
originating from over the Laptev and East Siberian Seas. The next six
filters with lower INP activity (20 % of filters; −22 °C ≥T [INP]=0.1 > −25 °C) sampled air
that originated off the eastern coast of Greenland from over both the
pack ice and open ocean. The 11 filters with the lowest INP activity
(bottom 37 % of filters; −25 °C ≥ T [INP]=0.1≥ −30 °C) all sampled air which mostly originated from the pack ice
adjacent to North America (also see Figure SI3).