Impact of warm air mass intrusions on atmospheric chemistry and microphysics: Observations during MOSAiC

Arctic winter is dominated by anthropogenic haze, while pollution transport from mid-latitudes decreases in summertime. As a result, aerosol concentrations reach a maximum in winter and drop to minimum in summer. What happens during the season transition, i.e., during spring, is less studied owing to the difficulty of performing measurements in the high Arctic at this time of year. Warm air mass intrusions are characteristic for springtime and they are harbingers of change, from a dry stable atmosphere to a more dynamic one with precipitation. Such air mass intrusions have been studied from a meteorological and thermodynamic perspective but not yet in full detail from a chemical one. Here, we present first results on microphysical and chemical properties of the Arctic air from observations of warm air mass intrusions during the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in April 2020. A full suite of state-of-the-art instrumentation (trace gases concentration, aerosol number size distribution, aerosol mass composition, cloud condensation nuclei) was deployed on the bow of the research vessel Polarstern as part of the MOSAiC expedition for a comprehensive characterization of the chemical and microphysical state of the Arctic atmosphere. In this study, we aim to disentangle the effects of warm air mass intrusions on trace gases concentration as well as on aerosol number, mass and composition. During the first major intrusion episode (April 15 to 16, 2020), ambient temperature increased from -30 oC to roughly 0 oC within 48 hours. The relatively ‘cold’ period in early April was characterized by stagnant northerly winds, hence aged and dry Arctic air masses, where a very stable accumulation mode composed of sulfate and organics with traces of halogens was measured. With the arrival of southerly air masses, the particle number size distribution started featuring several modes and increased concentrations as well as particle growth. Moreover, the trace gas and particle chemical composition significantly changed, featuring methane sulfonic acid in the gas phase and ammonium in the particle phase. Warm air intrusions were also systematically associated with regular ozone levels (non-depleted) which is in line with the absence of halogen signature in particles.