Multi-instrumental investigation of aerosol microphysical and optical evolution during a winter smog episode in Wrocław, Poland

Winter smog episodes in Central European cities are associated with a significant increase in aerosol mass and optical effects. In this study, we have analyzed winter smog episode that occurred in January in Wrocław, using an integrated set of in situ measurements taken at an urban background location. Particle size distributions in the diameter range 10 - 800 nm were measured using a scanning mobility particle sizer (SMPS, TSI model 3938) and combined with total particle number concentrations obtained from a condensation particle counter (CPC, TSI model 3750). Aerosol optical properties were characterized using light scattering and backscattering coefficients measured at three wavelengths - 450, 525, and 635 nm with an Aurora 4000 nephelometer. Absorbing aerosol was quantified as equivalent black carbon (eBC) using a filter-based aethalometer (Magee AE43). The dataset was analyzed with different time resolutions to characterize changes before, during, and after the smog episode. During the episode, total particle concentrations and scattering coefficients increased significantly, while the relative contribution of ultrafine particles (<100 nm) decreased, indicating a shift towards larger particle sizes. This was accompanied by an increase in Ångström scattering exponents and an increase in the backscattering fraction, consistent with an increase in aerosol due to condensation and coagulation processes.