This study presents the first full annual cycle (2019-2020) of ambient surface aerosol particle number concentration measurements (condensation nuclei 20 nm, N.sub.20) collected at Summit Station (Summit), in the centre of the Greenland Ice Sheet (72.58.sup." N, -38.45.sup." E; 3250 ma.s.l.). The mean surface concentration in 2019 was 129 cm.sup.-3, with the 6 h mean ranging between 1 and 1441 cm.sup.-3 . The highest monthly mean concentrations occurred during the late spring and summer, with the minimum concentrations occurring in February (mean: 18 cm.sup.-3). High-N.sub.20 events are linked to anomalous anticyclonic circulation over Greenland and the descent of free-tropospheric aerosol down to the surface, whereas low-N.sub.20 events are linked to anomalous cyclonic circulation over south-east Greenland that drives upslope flow and enhances precipitation en route to Summit. Fog strongly affects particle number concentrations, on average reducing N.sub.20 by 20 % during the first 3 h of fog formation. Extremely-low-N.sub.20 events ( 10 cm.sup.-3) occur in all seasons, and we suggest that fog, and potentially cloud formation, can be limited by low aerosol particle concentrations over central Greenland.