Characteristics and source origins of carbonaceous aerosol in fine particulate matter in a megacity, Sichuan Basin, southwestern China

This paper reports the temporal variations, sources and transport characteristics of carbonaceous aerosol in Chongqing, a megacity in the Sichuan Basin. Hourly organic carbon (OC) and elemental carbon (EC) mass concentrations in PM2.5 were measured at an urban supersite from November 2019 to October 2020. The annual mean PM2.5, OC and EC concentrations (+/- 1SD) were 38.50 +/- 25.79 mu g/m(3), 9.03 +/- 5.73 and 2.45 +/- 1.47 mu gC/m(3), respectively. An intensive influence of biomass combustion was found during the observation period. Strong seasonality of carbonaceous aerosol with highest concentrations in winter and lowest concentrations in summer was observed. Meanwhile, two diverse pathways for secondary formation dominated in different seasons. One was highly related to gas-phase photochemical oxidation under high temperature and radiation, and another was highly related to heterogeneous reactions, while the latter was more significant, especially in winter. Diurnal and quarterly variation patterns for carbonaceous aerosol showed that the development of planetary boundary layer strongly influenced carbonaceous aerosol concentrations. Moreover, the regional sources from the northeast within the basin were identified as major contributors of the primary carbonaceous aerosol to Chongqing, while secondary components were more from local sources than regional transport. This study highlights the importance of prioritising the abatement of gaseous precursors for carbonaceous aerosol and an urgent need for the inter-regional prevention and control measures of the cities located in the Sichuan basin, especially the cities in the northeast of urban Chongqing.