Figure
5. Backward trajectories over 7 days, starting at the ship location,
for the INP samples taken throughout the campaign. Trajectories were
launched every hour during the sampling period, and each point
represents an hour in time along the back trajectory. The starting
height for the trajectories was 32 m above mean sea level. Any points
along the trajectories which were above the model boundary layer were
removed, and any points along the trajectory that preceded precipitation
events (>0.1 mm h-1) were removed. Hence, any potential
sources of INP in the boundary layer are neglected if they occurred
prior to a precipitation event (we assume precipitation removes INPs) a)
The colour of the trajectories represents the temperature at which 0.1
INP L-1 was measured for that sampling period. b) The colour of the
trajectories represents the wind speed for each point along the
trajectory. The sea ice extent is from the NASA National Snow and Ice
Data Center [Maslanik and Stroeve, 1999].
It is striking that the trajectories with the lowest INP concentrations
spent most of the preceding seven days over the pack ice and to some
extent over the MIZ. These results indicate that, during this campaign,
open leads, sea ice and the MIZ were weak sources of INPs, in conflict
with previous suggestions [Bigg and Leck, 2001; Hartmann et
al. , 2020a].
The highest ice-nucleating activities from sampled aerosol originating
along the Russian coast were also correlated with high wind speeds along
the trajectories, i.e., in the region of the aerosol origin (Figure 5b).
This, together with the heat tests and INP size information, point to a
wind-driven marine biological source of INPs associated with
organic-rich film droplet sea spray aerosol. There were trajectories
with high wind speeds over the North American continent, the pack ice
and the coast of Greenland, but the ice-nucleating activity for these
was not greatly enhanced. Hence, our results are consistent with a
strong source of highly active INPs in the coastal marine waters of
northern Russia which were aerosolised during windy conditions. Marine
waters elsewhere in the world are thought to produce aerosol with
relatively low ice nucleating activities [Vergara-Temprado et
al. , 2017] (see Figure 4); however, our results suggest that the
shallow seas off the Russian coast might be relatively strong sources of
highly active INPs. Composition analysis of the aerosol during the peak
ice nucleation activity on the 11th August and the
19th August is consistent with a marine source for
many of these aerosol particles (samples were rich in Na, Cl and
sulfate; see Table S3). Hence, the question is why the aerosol from near
the coast of Russia is so much more active than aerosol derived from
other marine locations such as the North Atlantic and the Southern Ocean
[McCluskey et al. , 2018a; McCluskey et al. , 2018b].
A major difference to the North Atlantic and Southern Ocean is that the
shallow seas off the coast of northern Russia are strongly influenced by
riverine input from Russia that is rich in organic material, silt and
nutrients [Ahmed et al. , 2020; Juhls et al. , 2019]. In
fact, much of the dissolved organic matter in the Arctic Ocean is
derived from river input [Juhls et al. , 2019] and the
discharge of these rivers (and the amount of dissolved organic carbon
flowing in the sea via rivers) is increasing [Ahmed et al. ,
2020; Juhls et al. , 2020]. It has been shown that melting
permafrost, which is known to enter river water [Juhls et al. ,
2020], harbors copious quantities of warm temperature INPs [Creameanet al. , 2020]. Hence, it is possible that the highest INP
concentrations we detected at the North Pole were derived from marine
waters rich in terrestrially derived ancient biological INPs.
Alternatively, ocean biology fertilised by the nutrient rich waters on
the continental shelf may produce more INPs than are present in remote
marine locations. A measurement campaign to quantify the INP content of
the waters off the coast of Russia is clearly required.
Some of the back trajectories that had the highest INP concentrations
passed over islands in the Barents and Kara Seas, including Svalbard,
Franz Josef Land, Novaya Zemlya and Severnaya Zemlya. Many of these
locations have been identified as poorly defined dust sources [Bullardet al. , 2016] and dust from Svalbard has been shown to contain
biological ice-nucleating materials [Tobo et al. , 2019].
However, in a further analysis of the back trajectory data (Figure 6),
we find that there was little to no correlation with time spent over
land, whereas the ice-nucleating activity increased with the time the
air parcels spent over open ocean. This implies that the sources of INP
were associated with the marine environment. Having said this, we cannot
rule out relatively small island point sources being important sources
of INPs.