Impact of atmospheric condition on aerosol optical properties in urban and rural environment

This study investigates how aerosol optical properties derived from in situ and remote sensing observations differ between an urban site (Warsaw) and a background site (Strzyżów), under varying meteorological conditions. The analysis focuses on the role of air mass advection and vertical mixing, aiming to distinguish between air mass transformation effects and urban pollution impact. For this purpose, data from nephelometers, aethalometers, radiometers, and sun photometers supported by information on air mass trajectories are used. In spite of the fact that mean values of aerosol scattering coefficient (ASC) and equivalent black carbon (eBC) concentration are significantly different in urban and background sites, during northern flow but also during daytime convective conditions, the difference is relatively small. The highest difference has been found during southern flow or during nighttime conditions. Based on differences in aerosol optical properties during air mass transport between both sites, the air mass transformation effect and the urban effect were estimated. During transport of air mass between both sites the ASC, aerosol optical depth, and eBC increase by 39%, 18%, and 32% in relative to the background site. Enhancement of these parameters due to urban effect was estimated as 32%, 15%, and 43%. Comparison of the aerosol properties at both sites during the warm season shows negligible differences in ASC during clear-sky daytime and relatively small differences in eBC, as a result of intensive vertical mixing. ASC and eBC show significant variability with planetary boundary layer height (PBLH). The ASC can be reduced even up to 5.5 times when PBLH increases from 50 to 650 m at 12 UTC. During the warm season sensitivity of ASC and eBC to PBLH variability is significantly smaller than in the cold season. • Daytime convection during the warm season improves urban air quality to level of background conditions • Both surface and columnar aerosol optical properties at urban and background site are statistically correlated • Air quality in Warsaw is more likely to be controlled by weather conditions than by temporal emission variability