Is the summer aerosol over the Arctic controlled by regional atmospheric circulation or ice conditions? Trends and Future Implications

Abstract. Based on aerosol particle number size distributions measured ≥85º N on I/B Oden covering the summers of 1991, 1996, 2001, 2008 regional atmospheric circulation regimes (nodes) delineated with self-organizing maps (SOMs) were investigated as potential controllers of Arctic aerosol sources. The three most prominent nodes were not connected to regional source-related differences and did not vary systematically throughout the study period. Instead, the seasonal course of sea ice melt and freeze-up appeared to affect the shape of the aerosol size distributions significantly. High sub-Aitken concentrations occurred during the "freeze-up", most commonly associated with the low wind, restricted sea ice movement, and effective radiative cooling conditions. The high concentrations of newly formed particles measured during "freeze-up" were interpreted as deriving from frost flower formation. With the data on ice and atmospheric conditions and their seasonal course the study was extended to cover all years from 1991 to 2023 to enable speculations about changing aerosol source conditions in the warming Arctic climate. Over the 33 years of the study, the significant increases in sea and air temperatures nearly doubled the favorable ice conditions for new particle formation ≥85º N, lengthening both "melt" and "freeze-up" parts of the illuminated Arctic by more than a week. Whereas the sum effect of counteracting processes during the ice melt season on the airborne biogenic Arctic aerosol in a warming climate is unclear, the net effect of the changing the freeze-up of sea ice is expected to enhance the airborne biogenic Arctic aerosol in late summer/autumn.