Figure 7. Time series
showing [INP]ambient for both the measurements taken at ship height
(within the surface mixed layer) and using the SHARK (within the cloud
mixed layer). The temperature of the mixed layers is shown alongside
these measurements. When an INP spectrum did not extend to the
atmospheric temperature (i.e. where the highest temperature at which an
INP concentration was reported was below the atmospheric temperature),
the [INP] value associated with the highest temperature is provided
as an upper limit to [INP]ambient (open symbols).
Generally, [INP]ambient was typically below about
0.1 INP L-1 at the top of both the surface mixed layer
and the top of the cloud mixed layer. The periods of high INP
concentration before the 23rd August coincide with
periods of higher ambient inversion temperatures, whereas later in the
campaign the opposite is the case, which results in a relatively
invariant [INP]ambient. Whether this is a
coincidence, or if INP concentrations are correlated with ambient
temperature, is unclear from this limited dataset. However, there may be
a physical mechanism behind this apparent correlation. Transport of air
to the North Pole from sources further afield will likely result in
multiple cycles of cloud formation and dissipation in any one air mass,
hence INP active at above the ambient temperature of those clouds will
likely activate and be removed via precipitation (bearing in mind that
the majority of INPs relevant for mixed-phase clouds only activate to
ice in the presence of water droplets [Murray et al. , 2012]).
While transport through the boundary layer likely removes INP active
above the lowest temperatures experienced by an air parcel, further
cooling subsequent to the measurement time will lead to primary ice
production. Hence, the air masses sampled before the
23rd August with high INP concentrations have a great
potential for primary ice production if or when these air masses become
colder downwind of the sampling location. In the absence of local
sources, the INP spectrum over the central Arctic Ocean must therefore
be determined by a combination of the characteristics of the upwind
sources and the cloud temperature that these air parcels experience on
transport.
The relatively low [INP]ambient values suggest that
clouds not influenced by seeding from above should be mixed-phase, i.e.
contain a substantial proportion of liquid water with some ice crystals.
Observations of the phase of clouds during this campaign are discussed
in [Vüllers et al. , 2021]. Overall, the fraction of single
layer clouds (where seeding from above is unlikely) throughout the
troposphere that were mixed-phase was relatively constant throughout the
campaign: mixed phase frequency ~20-30 %, ice cloud
frequency ~10-30 % and liquid clouds were generally
infrequent; see Figure 15 in [Vüllers et al. , 2021]), despite
the strong decrease in temperature during the campaign. Similarly, in
the bottom few kilometres the frequency of occurrence of mixed-phase
clouds were often between 25% and 50%, with ice clouds about half as
frequent and liquid only clouds much less frequent throughout the
campaign. Overall, the observations of cloud phase for single-layer
clouds reported by [Vüllers et al. , 2021] are qualitatively
consistent with our relatively invariant
[INP]ambient measurements.
Clouds were multi-layered around 50 % of the time, with many situations
identified where seeding of ice from higher, colder clouds into lower
clouds might occur. Clouds were regularly observed up to around 8 or 9
km, where temperatures [Vüllers et al. , 2021] were low enough
for homogeneous freezing [Herbert et al. , 2015], and
frequently occurred in the mid-troposphere where heterogeneous
nucleation on INPs was most likely important. These higher clouds were
often in the free troposphere where our boundary layer INP measurements
are not necessarily relevant. Indeed, aerosol and INPs in the free
troposphere may have different sources to those in the boundary layer.
In order to obtain a more complete picture of primary ice production in
clouds in the central Arctic, INP measurements in the free troposphere
in this region would be needed and should be a target of future
campaigns.
4 Summary and conclusions
Arctic mixed-phase and supercooled clouds play a crucial role in Arctic
climate, but the processes that dictate their characteristics are poorly
understood. Here, we show that INP concentrations at 88 - 90°N are
extremely variable, and throughout the MOCCHA campaign between the
1st of August 2018 and the 18th of
September 2018 the temperature at which 0.1 INP L-1was reached varied between −9 °C and −30 °C. The highest 20% of
observed INP activity is related to air masses originating in the
ice-free ocean environment off the Russian coast, while the lowest 37 %
of observations related to air masses which originated and circled over
the pack ice north of Canada for most of the 7-day back trajectory.
Trajectories of air with intermediate INP activity also originated over
the ice-free ocean. These results indicate a strong dependence of the
measured INP concentration on the origin of the air with pack ice, open
leads, and the MIZ apparently being weak sources of INP, whereas
ice-free oceans, especially those near the Russian coast when wind
speeds were high, were a significant source.
The heat sensitivity of the most active INPs indicates the INP to be
proteinaceous, biogenic origin. This, together with the trajectory
analysis, indicates that there are strong biogenic sources of INP in the
shallow seas over the Russian continental shelf. The ice-nucleating
activity of the aerosol at the North Pole derived from off the coast of
Russia is much greater than that for sea spray aerosol in remote oceans
(such as the Southern Ocean [McCluskey et al. , 2018a] or the
North Atlantic [McCluskey et al. , 2018b]). This may indicate
the marine waters off Russia are very rich in ice-nucleating material,
perhaps related to the substantial riverine input, or alternatively the
islands in this region may be sources of biogenic INPs. More work is
needed to define what the key sources are along the Russian coast and to
see if similar sources exist elsewhere around the Arctic and Antarctic.
By making measurements of INP spectra both above and within the surface
mixed layer of decoupled boundary layers, we found that surface
measurements were often not representative of the INPs in the cloud
mixed layer. Hence, measurements at altitude, within the cloud mixed
layer, are necessary in order to define primary ice production in Arctic
mixed-phase clouds. In addition, our measurements allowed us to estimate
the INP concentration active at the temperature of the top of the
surface mixed layer and also at the top of the boundary layer. This
revealed that, despite massive variability in INP spectra, the INP
concentration at ambient temperature was typically less than 0.1
L-1, which is consistent with remote sensing
observations that indicate the persistence of mixed-phase clouds (in the
absence of seeding of ice from above). We also recommend future studies
focus on INP measurements throughout the free troposphere where primary
ice production may lead to seeding of ice in lower level clouds.
Overall, it is striking that INP concentrations at the summertime North
Pole vary from some of the lowest measured anywhere in the world, to as
high as the highest INP concentrations in terrestrial locations rich in
biological INPs such as in the UK [O’Sullivan et al. , 2018].
Since these INPs are transported from the seas off the Russian coast,
they may be sensitive to changes in climate. In particular, reduced sea,
land ice and permafrost may open up more sources for more of the year
around the Arctic, which may increase the future strength (and may
already have done so) of the sources of INPs that are important for
mixed-phase clouds in the central Arctic. More work needs to be
undertaken to understand how climate change may affect INP sources
around the periphery of the Arctic and how this may influence Arctic
clouds and feedback on Arctic climate.
Acknowledgments
This research was part of the Arctic Ocean (AO) 2018 expedition. The
Swedish Polar Research Secretariat (SPRS) provided access to the
icebreaker (I/B) Oden and logistical support in collaboration with the
U.S. National Science Foundation. We are grateful to the Chief Scientist
Patricia Matrai for planning and coordination of AO2018 (along with
coauthor Leck) as well as to the SPRS logistical staff and to I/B Oden’s
Captain Mattias Peterson and his crew for expert field support. We are
grateful for funding from the European Research Council 648661 MarineIce
(BJM), Natural Environment Research Council NE/R009686/1 (IMB and BJM),
NE/T00648X/1 (BJM), Swiss National Science Foundation grant no.
200021_169090 (JS), Swiss Polar Institute (JS),
Knut-and-Alice-Wallenberg Foundation within the ACAS project (Arctic
Climate Across Scales) project no. 2016.0024 (PZ), Bolin Centre for
Climate Research, RA2 (PZ, MES), Swedish Research Council project nos.
2018-05045 (PZ) and 2016-05100 (MES) and The Ingvar Kamprad Chair,
sponsored by Ferring Pharmaceuticals (JS).
Competing interests
All other authors declare they have no competing interests.
Data Availability Statement
All data will be made available via the Research Data Leeds Repository
and the Bolin Centre Database.
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