Assessment of carbonaceous aerosols in suburban Nanjing under air pollution control measures: Insights from long-term measurements

Abstract

The concentrations of organic carbon (OC) and elemental carbon (EC) in fine particulate matter (PM2.5) were analyzed using a semicontinuous carbon analyzer to characterize their carbonaceous components at the Nanjing University site from August 2013 to December 2018. OC was divided by the minimum R squared (MRS) method into primary organic carbon (POC) and secondary organic carbon (SOC). The results showed that annual mean POC and EC concentrations declined from 10.00 to 3.62 μg m−3 and from 6.73 to 3.40 μg m−3, respectively, during 2013–2018. The apparent reduction in POC and EC concentrations indicated that the implementation of air pollution control measures helped reduce carbonaceous aerosol pollution. Higher concentrations of POC and EC were recorded during the cold season and lower in the warm season. The annual mean SOC concentrations varied between 4.35 and 3.18 μg m−3 from 2013 to 2018. Elevated SOC was observed during the warm season, most likely attributable to the enhanced photochemical activity at high temperatures. Regarding the diurnal variation, the high concentrations of POC and EC were observed at night and in the morning due to stronger primary emissions and accumulations of pollutants with low boundary-layer heights, while the peak of SOC was observed at approximately noon due to the increases in photochemical activity. Nonparametric wind regression analysis showed the higher concentrations of POC, SOC and EC in the northwesterly, southwesterly to southeasterly, and southwesterly winds with high speeds. Concentration-weighted trajectory (CWT) analysis suggests that the areas with potentially high contributions to POC and EC changed from the north to the western areas of China, and that northern China played an increasingly important role in the SOC concentration of Nanjing. These results demonstrate that controlling emissions from the western and the northern areas in China may further alleviate carbonaceous aerosol pollution in Nanjing.