Aerosol microphysical and optical closure study in the Baltic Sea coastal zone

This study presents results of a field experiment in the Baltic Sea coastal area focused on optical and microphysical aerosol properties and particle water uptake. This experiment took place in Sopot (Poland) in March 2023. Hygroscopic aerosol properties were measured with two Aurora 4000 nephelometers controlled by the Aerosol Condition System (ACS1000), absorption properties with an aethalometer AE-31, and aerosol size distribution with SMPS/OPS devices. Due to the variability of weather conditions, different aerosol properties were observed, including relatively clean marine air mass with sea spray as well as local and long-range transported pollution. Hygroscopic properties defined by the scattering enhancement factor $\mathbb{}f$(RH) at 85 % of relative humidity (RH) and kappa parameters show elevated values not only during marine air masses but also during the transport of continental air masses from the south-western direction. Statistically significant Pearson correlation coefficients were found for relations between sulphate aerosol proportion in aerosol mass (from MERRA-2 reanalysis) and both hygroscopicity parameters, $\mathbb{}f$(RH = 85 %) and kappa (0.47 and 0.54, respectively).
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Experiment closure shows that the aerosol scattering coefficient obtained from numerical simulation based on aerosol size distribution from observations and refractive index estimated from MERRA-2 reanalysis agree with Aurora 4000 measurements. In the case of the aerosol absorption coefficient, the comparison with aethalometer observation is not so consistent due to the high sensitivity of this parameter to the assumed value of the imaginary part of the refractive index.