Figure
2. Surface mixed layer INP concentrations throughout the campaign. a)
The number of INPs per litre of air sampled was calculated using data
from offline droplet freezing experiments, conducted within hours of the
samples being taken. The spectra shown in blues represent samples that
were heated to close to 100 °C for 30 min. Background values were
subtracted from the data. Sampling times varied from 6 h to 3 days and
were taken using a heated whole air inlet on the 4th deck (25 m above
mean sea level) of the Oden Icebreaker. Temperature uncertainties (not
shown) for the droplet freezing experiments were estimated to be ±0.4
°C. The format of the key is YYMMDD_hhmm; these correspond to the start
time of the filter sample in the 24-hour time format; the start and end
times are in Table S1. b) The data from this study are presented
alongside literature data for ground, ship and aircraft-based campaigns
around the Arctic [Bigg, 1996; Bigg and Leck, 2001; Borys, 1989; Conenet al. , 2016; Creamean et al. , 2019; Creamean et
al. , 2018; DeMott et al. , 2016; Flyger and Heidam, 1978;
Hartmann et al. , 2020b; Hartmann et al. , 2021; Irishet al. , 2019; Mason et al. , 2016; Porter et al. ,
2020; Prenni et al. , 2007; Rogers et al. , 2001;
Sanchez-Marroquin et al. , 2020; Si et al. , 2019; Wexet al. , 2019] and a compilation derived from precipitation
samples [Petters and Wright, 2015].
It is is remarkable to note that the highest concentrations measured
here at the North Pole are as high as the highest INP concentration
reported in INP rich environments such as the mid-latitude terrestrial
environment [O’Sullivan et al. , 2018; Petters and Wright,
2015], despite having much lower aerosol concentrations. Overall, our
INP measurements indicate that the INP concentration spectra within the
high Arctic surface mixed boundary layer can be extremely variable,
perhaps far more variable than anywhere else on Earth.
In order to test for the presence of proteinaceous biological
ice-nucleating material, we heated the most active sample suspensions to
close to 100 °C [Daily et al. , 2021]. The activity of these
samples was always reduced, with all of the activity above −20 °C being
removed (Figure 2a and Figure S1). Hence, it appears that the most
active INPs sampled close to the North Pole were most likely of
biological origin. Atmospheric INP at lower latitudes were also found to
be heat sensitive [Hartmann et al. , 2021]. Together, this
indicates that proteinaceous biological INP are important in the Artic.
The time series in Figure 3 shows the temperature at which a
concentration of 0.1 INP L-1 was measured
(T [INP]=0.1), and highlights the variability
of INP concentrations at the North Pole throughout August and September
of 2018. The first peak in ice-nucleating activity was observed during a
period in which the ship was breaking ice prior to being moored to an
ice-floe (i.e. prior to 16th August). It is reasonable
to question whether the very high INP concentrations observed during the
ice-breaking period resulted from the ice-breaking itself. Ice-breaking
involved frequent backward and forward motions, hence there is the
potential for sampling ship emissions (such as ship stack emissions,
detailed in the methodology) and aerosol resulting from ice-breaking and
the disruption of the sea surface. We manually stopped sampling if there
were activities planned that would affect sampling (such as helicopter
flights) in addition to using a pollution control system that stopped
the flow through the filters when aerosol concentrations increased
rapidly in a manner associated with sampling a ship plume, and also when
the wind was not from the correct direction (from forward of the ship’s
superstructure). Despite the precautions taken to eliminate these
sources of contamination, we cannot completely exclude the possibility
of contamination. However, there was a pause in ice-breaking when a
clean air station was established (10th August, we
sampled for 6 hours) that coincided with high INP concentrations. At
this clean air station the ship was moored facing into the wind, with
ship-based aerosol sources aft of the aerosol inlets. Hence, we were
confident that sampling of ship pollution and ice-breaking aerosol were
eliminated, increasing confidence that these high values in this period
were indeed representative of the central Arctic Ocean. In addition,
there was also a period of very high ice-nucleating activity a few days
after the ice-floe station had been established and the ship was
pointing into the wind, demonstrating that there were very active INPs
that were not related to ice-breaking or ship emissions. The time series
also highlights two distinct periods. The period up to the
23rd August was characterised by variable but often
very high INP concentrations, whereas the period after this was
characterised by much lower INP concentrations. We examine the back
trajectories associated with these different periods later in the paper.