Sea Salt Aerosol above Arctic Sea Ice - sources, processes and climate impacts (SSAASI-CLIM)

Sea salt aerosol (SSA) may influence regional climate directly through scattering of radiation or indirectly via its role as cloud-forming particles. While it is well known that SSA can be cloud condensation nuclei (CCN) forming cloud droplets, it has been shown only recently that SSA can also be a source of ice nucleating particles (INP) forming ice crystals, depending on its chemical composition and surface shape. Arctic clouds are poorly represented in climate models, which is partly due to a lack of understanding of source and nucleating capability of natural aerosol in the high Arctic. Aerosol models for example do currently not capture aerosol maxima in the Arctic winter/spring observed at high latitudes. Recent field campaigns provide first evidence of a hypothesized source of SSA from salty blowing snow (BSn) above sea ice. During storms salty snow gets lofted into the air and undergoes sublimation to generate SSA. Additional but minor SSA sea ice sources are frost flowers and open leads. The impact on radiation and clouds of SSA from this new source of SSA above sea ice is not known. However, a quantitative understanding of natural aerosol processes and climate interactions is needed to provide a baseline against which to assess anthropogenic pollution reaching the Arctic and evaluate the success of mitigation measures.

We therefore propose to determine the SSA source, fate and potential impact on Arctic climate associated with blowing snow above sea ice and other sea ice sources. To do this we seek funding to participate in the year-long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) to observe aerosol processes in the central Arctic ocean throughout all seasons.

Proposed measurements on the sea ice and on-board "FS Polarstern" include particle size and concentration (sub-micron to snow particle size), INP concentrations, and a range of chemical properties using aerosol filters. Sampling of snow on sea ice, brine, frost flowers will constrain the local source of SSA. Tethered balloon launches will yield information on the fate of particles formed near the sea ice surface as they get lofted to heights where clouds may form.

The main concrete results of this project are (1) the first year-round quantitative estimates of the SSA sea ice source in the Arctic and its contribution to CCN and INP, and (2) determination of the impact of SSA from sea ice on cloud fraction and therefore climate. Thus, the main immediate beneficiaries of our research are academic ones. Our data will benefit process understanding and quantitative estimates in several fields. We have identified in particular: cryosphere scientists studying Arctic sea ice processes, atmospheric scientists studying aerosol surface sources and aerosol-cloud interactions, climate scientists modeling future climate change in the Arctic, atmospheric chemists studying tropospheric ozone and halogen chemistry, paleo-climate scientists developing proxies in ice cores and other archives for past sea ice conditions.

Our route to these people is the normal academic one of scientific papers and conference presentations. We are reaching out to the communities of aerosol scientists and cloud micro-physicists, to whom our findings will be very relevant, by involving them as Co-Is or project partners. Dissemination of results to atmosphere and cryosphere scientists studying air-snow-ice interactions will be achieved by organizing topical sessions at major international conferences. PI Frey is Co-chair of CATCH, a global activity facilitating interdisciplinary research in atmospheric chemistry and the cryosphere e.g. through special conference sessions or workshops.

Further beneficiaries include stakeholders in policy. This is because quantifying variability and sources of natural background aerosol in the Arctic is critical in order to reduce model uncertainties in climate prediction and provide a baseline against which to compare the influence of anthropogenic aerosol from the mid-latitudes reaching the Arctic, as well as to evaluate the effectiveness of climate and air quality protection policy options. Narrowing these uncertainties will help us to reduce the uncertainty range in projections of Arctic sea ice and climate. Such findings are of great interest to policymakers (e.g. DECC, DEFRA), and will emerge as the sum of inputs from many proposals (including this one) that will be aggregated within MOSAiC and beyond, and that are presented at that higher level.

Another aspect of impact is that we as scientists play a role in making science accessible and exciting to the wider public and to students. Regarding this project, BAS has a strong track record for public outreach explaining the science and societal relevance of polar climate change. BAS is part of the network of Ambassadors for Science with particular emphasis on outreach to schools. For such school talks we will gather highly visual footage while working in the central Arctic ocean, in particular of the sea ice work (snow sample collection, tethered balloon soundings).

The PI maintains a personal web site explaining his research and posts widely read field blogs. He is an active STEM ambassador and has a strong record of outreach activities related to scientific research in the polar regions. The PI will follow at least two different routes to disseminate the science emerging from this proposal and achieve impact with a broader audience. One will be through school talks as well seminars given to school teachers; e.g. since 2016 PI Frey has been engaging with the annual London Youth Scientific Forum (LYSF) by giving invited seminars / study days to approx. 500 young students from more than 30 countries. A second route also will be through live Skype lessons, which can reach hundreds of school children of various age groups and will very effectively contribute to motivate and educate the next generation of scientists.