Characteristics of urban black carbon aerosols in the Yangtze River Delta of China based on long-term observations

Black carbon (BC) aerosols are important absorbing components that can impact the regional climate and environment. To better understand the effects of BC, long-term variations in BC concentrations and the relationships between BC and other air pollutants were investigated in urban Nanjing in the Yangtze River Delta (YRD) based on near-nine-year observations using a seven-channel aethalometer (AE-31). The BC concentrations in YRD city had substantial seasonal and diurnal variations, which are higher in winter (rush hours) and lower in summer (at noon). They had a significant declining trend since 2013 when the Action Plan for Air Pollution Prevention and Control was carried out, which was reduced by at least 45% from 2013 to 2019, and the same applies to heavy pollution episodes. The aerosol absorption coefficient (AAC) had temporal variations similar to those of the BC concentration because BC was the dominant component of absorbing aerosols (>80% at 550 nm). Investigations also indicated that the average contributions of BC from biomass burning (BCbb) and fossil fuel (BCff) to the total BCs concentration could be of equal importance. However, the ratios of BCbb to the total BCs in colder seasons were much higher than those in summer. Nevertheless, both BCbb and BCff might significantly contribute to the high BC loadings during particulate pollution episodes. Further comparisons showed that BCs had substantial positive correlations with CO and PM2.5. The relationships between BC and CO imply that the sources of fossil fuel BC aerosols in Nanjing might mainly came from the combustion of industrial coal and gasoline vehicles. An extremely high BC loading or higher BC/PM2.5 ratio always corresponded to a lower O3 concentration, implying that BC might have a substantial influence on O3 formation.