Divergent Responses to Wet Scavenging: Fossil Fuel Black Carbon Exhibits Longer Lifetime and Stronger Light Absorption than Biomass Burning BC

Wet scavenging is the major sink for black carbon (BC), while the responses of BC emitted from different sources remain poorly quantified. In this study, the below-cloud scavenging of BC from biomass burning (BCbb) and fossil fuel combustion (BCff) was investigated, using year-round in situ measurements in Nanjing, China. A statistical approach was developed to quantify the source-specific mass absorption efficiency (MAE). Results showed that BCff was less susceptible to scavenging than BCbb, with wet scavenging ratios of 0.13 and 0.45, respectively. Additionally, BCff exhibited a stronger light-absorbing ability than BCbb, with MAE values of 8.0–12.0 and 3.4–8.0 m2 g–1 (25–75th percentiles), respectively. Consequently, the MAE of BC increased significantly with precipitation amount, driven by a combination of a rising BCff proportion, a higher intrinsic MAE of BCff, and enhancement of the MAE for both BC types due to high humidity. Neglecting the source-specific MAE could lead to substantial biases in the estimated direct radiative forcing, which can exceed 20% in regions with high BCbb contributions. This study strongly suggests that BCff and BCbb should be treated separately in models to improve the simulation of atmospheric loadings and climate impacts.