Controls on surface aerosol number concentrations and aerosol-limited cloud regimes over the central Greenland Ice Sheet

Abstract. This study presents the first full annual cycle (2019–2020) of ambient surface aerosol number concentration (condensation nuclei > 20 nm, N20) measurements collected at Summit Station, in the centre of the Greenland Ice Sheet (72.58° N, −38.45° E, 3250 m asl). The mean surface N20 concentration in 2019 was 129 cm−3, with the 6 h mean ranging between 1 cm−3 and 1441 cm−3. The highest monthly mean concentrations occurred during the late spring and summer, with the 5 minimum concentrations occurring in February (mean: 18 cm−3), demonstrating an opposite seasonal cycle in aerosol concentrations compared to low altitude Arctic stations (with latitudes > 66° N). High aerosol concentration events are linked to anomalous anticyclonic circulation over Greenland and the descent of free tropospheric aerosol down to the surface, whereas low aerosol concentration events are linked to cyclonic circulation over south-east Greenland that drives upslope flow and enhances precipitation en-route to Summit. Fog strongly effects N20 concentrations, on average reducing N20 by 20 % during the first three hours of fog formation. Extremely low N20 concentrations (