City-produced and transported black carbon: Synergy of in-situ optical measurements and modeling

The implementation of air pollution mitigation strategies requires not only high-quality continuous measurements of pollutants but also proper definitions of ways to differentiate between transported and locally produced contributions, as only the latter can be effectively reduced by authorities. To address this issue, we propose a new approach for partitioning monitored black carbon (BC) concentrations into city-produced (urban) and transported fractions using a combination of measured and modeled data. Two simultaneous measurement campaigns (warm season 2022 and cold season 2022/23) were conducted in two urban environments: Vilnius (Lithuania) and Warsaw (Poland). In the cold season in Warsaw, BC mass concentration was 90% higher than in the warm season, while in Vilnius, an increase of 44% was observed, as compared to the warm season. Aerosol optical properties showed more complex aerosol mixtures of dust, BC and brown carbon (BrC) during the cold season, forming larger particles. Single scattering albedo (SSA) anti-correlated with BC FF , proving that fossil fuel (FF) combustion contributes to the warming effect in both cities. A positive correlation between the population density of the emission areas of transported BC and the BC mass concentrations in Vilnius and Warsaw was found. The impact of transported BC on the local BC levels in the cities was of 53 ± 8 % and 36 ± 28 % in the cold season and of 23 ± 7 % and 13 ± 21 % in the warm season for Warsaw and Vilnius, respectively. Thus, the approach of BC partitioning showed that in the cold season, the two cities suffered from worse air quality, in part due to more transported BC. • Massive increase in black carbon in the cold season in Warsaw. • Highly similar pattern of black carbon diurnal cycles in Vilnius and Warsaw. • Warsaw cold season air dominated by mixtures of dust, black and brown carbon. • Anti-correlation of fossil fuel combustion black carbon and single scattering albedo. • New approach to partition black carbon into city-produced and transported fractions.