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

Abstract

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 (< 10 cm−3) occur in all seasons, and we suggest that fog, and potentially cloud formation, can be limited by low aerosol concentrations over central Greenland.

Observations of Fog‐Aerosol Interactions Over Central Greenland

“…To explore individual fog cases in more depth we examine data from the ceilometer (CT25K, Münkel, 2006), sodar (Neff et al., 2008), total sky imager, and near‐surface temperature profiles and sensible heat flux estimates from tower‐mounted in‐situ sensors (Guy et al., 2020). Data from the millimeter cloud radar and precipitation occurrence sensor system were used to help identify fog cases during the summer of 2019, and radiosonde data were used to help constrain retrievals of continuous temperature and water vapor profiles from the AERI; both steps are described in detail in Guy et al.…”

“…The FM100 data from Cox et al (2019) are archived at https://doi.org/10.18739/A28K74W5W . Aerosol particle number concentration measurements, near surface temperature and wind profiles from the 15 m tower, and sensible heat flux measurements are available at the CEDA data archive (Guy et al, 2020). ICECAPS ceilometer data (https://doi.org/10.18739/A27659G3R) and sodar data (https:// doi.org/10.18739/A2HM52K68) are archived at the Arctic Data Center (M. Shupe, 2020aShupe, , 2020b.…”