A two-year study of carbonaceous aerosols in ambient PM2.5 at a regional background site for western Yangtze River Delta, China

• Hourly OC and EC were continuously measured for two years in western YRD.
• Relatively low concentrations and ratios of OC to EC were observed for the region.
• Ambient OC and EC were reduced from improved emission controls during YOG.
• The ambient EC and OC are largely influenced by transportation and biomass burning.

To analyze the characteristics of regional background carbonaceous aerosols in western Yangtze River Delta (YRD), hourly organic carbon (OC) and elemental carbon (EC) in fine particular matter (PM2.5) were measured with a semi-continuous carbon analyzer at a suburban site in upwind Nanjing from June 2013 to May 2015. Relatively low OC, EC and OC/EC were observed compared to other studies conducted in Nanjing. The reasons include the limited primary emissions around the observation site, the improved emission controls in recent years, and the use of denuder to reduce positive artifact in OC measurement. Resulting from the stable atmosphere conditions and emission variations, the highest concentrations of carbonaceous aerosols were found in both winters, with average OC and EC observed at 11.8 ± 10.0 and 5.9 ± 3.4 μg/m3 for the first one, and 8.1 ± 5 and 4.5 ± 2.4 μg/m3 for the second one, respectively. Compared to 2013, reduced OC and EC were found in summer and autumn 2014, demonstrating the benefits of emission control polices implemented for the Nanjing Youth Olympic, while elevated OC observed in spring 2015 was attributed probably to the increased biomass burning. For the hazy event in winter 2013, the back trajectories of air masses suggested that heavy pollution were from eastern Jiangsu, northern Anhui and Jiangsu, downtown Nanjing, and Shanghai. Secondary aerosol formation played an important role indicated by the larger mass fraction of OC and increased OC/EC in PM2.5 during the heavy pollution period. In the harvest season, biomass burning was estimated to contribute 51% and 16% of OC and EC concentrations, respectively.