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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