Multiphase Chemistry Processes within Arctic Fog Droplets Can Enable Rapid Growth of Aitken Mode Particles to CCN Sizes

The Arctic marine boundary layer (AMBL) is characterized by low aerosol particle concentrations. Thus, new particle formation (NPF) and even the smallest amount of Aitken mode particle growth is capable to significantly increase the cloud condensation nuclei (CCNs) budget in the AMBL, which can significantly influence the albedo of low-level clouds. During the Arctic ship cruise campaign PASCAL in summer 2017, measurements of aerosol particles were performed and multiple regional NPF events were observed. Interestingly, a rapid growth of the further formed Aitken mode particles with > 20 nm h^-1 was observed right after a long (> 4 h) fog episode. However, the growth could not be explained by the measured values alone.

To uncover the responsible processes, detailed multiphase chemistry box model simulations with MCM/CAPRAM were performed. Together with the field analyses, the model simulations suggest a novel mechanism that explains the observed rapid Aitken mode particle growth right after the fog event. Briefly, during the fog event multiphase chemistry processes produce semi-volatile compounds. When the fog evaporates, partitioning processes occur that explain the rapid growth. For example, a redistribution of semi-volatile acidic (e.g., methanesulfonic acid) and basic (e.g., ammonia) compounds between the activated and non-activated particles (modeled to be < 102 nm in diameter) occurs. This process enables the non-activated Aitken mode particles to grow towards CCN size. Overall, the simulations indicate that, chemistry within Arctic fog and subsequent post-fog repartitioning are important processes that (i) contribute to the increase in the number of CCNs and cloud droplets, and (ii) lead to an increased albedo of Arctic clouds. Rough calculations with radiative transfer model reveal that the postulated increased CCN number affects the radiative forcing of Arctic low-level mixed phase clouds by up to 31 W m^-2.