Figure
6. The time that air masses spent over water, ice and land derived
using back trajectory analysis. The colour represents the temperature at
which an INP concentration of 0.1 INP L-1 was reached. The black dots
correspond to individual trajectories. The same filters as in Figure 5
were applied.
Overall, the evidence indicates that there is a strong source of
biogenic INPs in the Barents, Kara and Laptev Seas off the Russian coast
that can be sporadically transported to the central Arctic Ocean. There
was high wind along the trajectories off the Russian coast which would
be consistent with both the production of INPs in sea spray from these
organic-rich seas or dust combined with terrestrial biogenic material
from the various islands in this region. We also note that a recent
study found that very active INP were produced in the Chukchi sea (near
Alaska) under high wave conditions [Inoue et al. , 2021].
Furthermore, we note that these potential sources are sensitive to a
changing climate, with river discharge, permafrost melt and organic
matter input into the ocean from the major Russian rivers increasing in
a warmer world [Jahn et al. , 2020]. In addition, the removal
of ice from the Arctic could also expose marine and terrestrial sources
of ice-nucleating aerosol around the Arctic Ocean, where they can then
be aerosolised by the action of wind [Schmale et al. , 2021].
3.5 Implications for ice production in boundary layer central Arctic
mixed-phase clouds
In this section, we assess whether the measured INP concentrations are
high enough to initiate a transition from liquid-dominated clouds to
ice-dominated clouds. Model simulations indicate that on the order of
one ice crystal per litre of air is required to remove the bulk of
liquid water from an Arctic cloud, whereas lower concentrations still
reduce the liquid water path [Stevens et al. , 2018;
Vergara-Temprado et al. , 2018b]. Hence, we use our INP
measurements, both at ship level in the surface mixed layer and those
from the SHARK in the cloud mixed layer, to estimate the concentration
of INPs that become active at the ambient temperature of the atmosphere
([INP]ambient).
The quantity [INP]ambient combines the atmospheric
temperature profiles from radiosonde measurements with the corresponding
INP spectra as a rough indicator of primary ice crystal production. This
is a crude analysis and a full cloud model would be required to
represent ice crystal formation and sedimentation as well as INP
recycling and latent heat release, to predict ice crystal concentration
given an INP spectrum, but it does give an indication of what the
measured INP spectra might mean for primary ice production in clouds.
The highest [INP]ambient will be at the cold points,
i.e. the top of the surface mixed layer and the top of the boundary
layer (top of the cloud mixed layer), hence this is where we focus this
analysis. We assume that ship level measurements are representative of
the [INP]ambient at the top of the surface mixed
layer and those from SHARK are representative of the top of the boundary
layer, the assumption being that these respective layers are
individually well-mixed.
The temperature minima within the main boundary layer were determined
from radiosonde profiles, which were made every 6 hours throughout the
entire cruise [VĂ¼llers et al. , 2021]. The calculated
[INP]ambient for the duration of the cruise are
shown in Figure 7, where the minimum temperature at the top of the cloud
mixed layer (main boundary layer) and the top of the surface mixed layer
is indicated for each filter period. In many cases, the temperature of
the atmosphere was higher than the highest INP measurement, hence we are
only able to report upper limits to [INP]ambient in
these cases.