Comment on acp-2022-26

Abstract

Black carbon (BC) has a strong light absorption ability and is known as the second strongest light-absorbing substance in the atmosphere after CO2. Atmospheric motion plays an important role in the ambient mass concentrations of pollutants. The relationship between atmospheric motion and BC aerosols is complex, and detailed investigation of the impact of different scales of motion on BC is still insufficient. Thus, an intensive observation was launched in a typical river-valley city. Equivalent BC (eBC) source apportionment was conducted using the aethalometer model with the site-dependent absorption Ångström exponents (AAEs) and the mass absorption cross-sections (MACs) which were retrieved using a positive matrix factorization (PMF) model based on observed chemical data and optical data. The derived AAEs were 1.07 for diesel vehicular emissions, 2.13 for biomass burning, 1.74 for coal combustion, and 1.78 for mineral dust. The mean values of eBCfossil and eBCbiomass were 2.46 μg m-3 and 1.17 μg m-3 respectively. A self-organizing map showed that four featured atmospheric motions categories were identified at the sampling site. A further analysis of eBC under the four motion categories showed that the BC pollution was more likely to happen when the influence of local-scale motion outweighed that of regional-scale motion. The trajectory clusters indicate that air mass direction could post divergent impacts under different scales of atmospheric motion. The direct radiative effects (DRE) of source-specific eBCs were lower when the influence of regional-scale motion outweighed that of the local one. However, the DRE efficiencies under the dominance of regional scale motion were ~1.5 times higher than those under the dominance of local scale motion. This study revealed the disproportional change between BC mass concentration and its DRE. The DRE efficiency of BC was enhanced during the regional transport which could lead to greater consequences in receptor regions. It highlights the regionally transported BC and its potentially enhanced climatic effect are worthy of attention.