Wildfires significantly enhance light absorption of black carbon and brown carbon aerosols in Moscow urban atmosphere

• Wildfire plumes significantly enhanced light absorption by BC, primary BrC and secondary BrC in Moscow. • During wildfire plumes, biomass burning contributed over 83 % of primary BrC, while secondary BrC was driven by aqueous-phase processes. • The radiative forcing of both primary BrC and secondary BrC relative to BC increased in Moscow during wildfire plumes. Large-scale wildfires are essential sources of black carbon (BC) and brown carbon (BrC), affecting aerosol-induced radiative forcing. This study investigated the impact of two wildfire plumes (Plume 1 and 2) transported to Moscow on the optical properties of BC and BrC during August 2022. During the wildfires, the total light absorption at 370 nm (b abs_370nm ) increased 2.3–3.4 times relative to background (17.30 ± 13.98 Mm − 1 ), and the BrC contribution to total absorption increased from 14 % to 42–48 %. BrC was further partitioned into primary (BrC Pri ) and secondary (BrC Sec ) components. Biomass burning accounted for ∼83–90 % of BrC Pri during the wildfires. The b abs_370nm of BrC Pri increased 5.6 times in Plume 1 and 11.5 times in Plume 2, due to the higher prevalence of peat combustion in Plume 2. b abs_370nm of BrC Sec increased 8.3–9.6 times, driven by aqueous-phase processing, as evidenced by strong correlations between aerosol liquid water content and b abs_370nm of BrC Sec . Daytime b abs_370nm of BrC Sec increased 7.6 times in Plume 1 but only 3.6 times in Plume 2, due to more extensive photobleaching, as indicated by negative correlations with oxidant concentrations and longer transport times. The radiative forcing of BrC Pri relative to BC increased 1.8 times in Plume 1 and Plume 2. In contrast, this increase for BrC Sec was 3.4 times in Plume 1 but only 2.3 times in Plume 2, due to differences in chemical processes, which may result in higher uncertainty in its radiative forcing. Future work should prioritize elucidating both the emissions and atmospheric processes to better quantify wildfire-derived BrC and its radiative forcing.