[Changes in Carbonaceous Aerosol in the Northern Suburbs of Nanjing from 2015 to 2019]

Abstract

Carbonaceous aerosol is an important component of atmospheric fine particles that has an important impact on air quality, human health, and climate change. In order to explore the long-term changes in carbonaceous aerosol under the background of emission reduction, this study measured the mass concentrations of organic carbon (OC) and elemental carbon (EC) of PM_2.5, which collected in the northern suburbs of Nanjing for five years (December 17, 2014 to January 5, 2020). The results showed that the five-year average ρ(OC) and ρ(EC) were (10.2±5.3) μg·m^-3 and (1.6±1.1) μg·m^-3, accounting for 31.1% and 5.2% of PM_2.5, respectively. OC and EC concentrations were both high in winter and low in summer. According to the nonparametric Mann-Kendall test and Sen's slope, the mass concentrations of OC and PM_2.5 decreased significantly[OC:P<0.0001, -0.79 μg·(m³·a)^-1, -0.29%·a^-1; PM_2.5:P<0.0001, -4.59 μg·(m³·a)^-1, -1.58%·a^-1]. Although EC had an upward trend, the significance and range of change were not obvious[P=0.02, 0.05 μg·(m³·a)^-1, 0.02%·a^-1]. OC and EC decreased significantly during winter from 2014 to 2019[OC:P<0.0001, -2.05 μg·(m³·a)^-1, -0.74%·a^-1; EC:P=0.001, -0.15 μg·(m³·a)^-1, -0.05%·a^-1], and the decline was more obvious than the whole. The correlation between OC and EC showed that the sources in winter and summer were more complex than those in spring and autumn. According to the characteristic ratio of OC and EC, the contribution of coal combustion and biomass burning decreased from 2015 to 2019, whereas the impact of industrial sources and vehicle emissions became more significant. Corresponding to this was the obvious decline in OC and the slight recovery of EC. The OC/EC ratio was over 2.0, indicating that there was secondary pollution in the study area. Further calculation revealed that the variation in SOC was consistent with that in OC, showing a significant decrease[P<0.0001, -0.47 μg·(m³·a)^-1, -0.17%·a^-1]. The average mass concentration of SOC was (5.0±3.5) μg·m^-3, accounting for 49.2% of OC. These changes indicate clear effects of the prevention and control of air pollution in Nanjing in recent years. Furthermore, future control can focus on the emissions of VOCs to reduce secondary pollution.

Keywords: elemental carbon(EC); long-term changes; northern suburb of Nanjing; organic carbon(OC); secondary organic carbon(SOC).