Impact of air pollution control measures and regional transport on carbonaceous aerosols in fine particulate matter in urban Beijing, China: Insights gained from long-term measurement
Authors: Ji Dongsheng , Gao Wenkang , Maenhaut Willy , He Jun , Wang Zhe , Li Jiwei , Du Wupeng , Wang Lili , Sun Yang , Xin Jinyuan , Hu Bo , Wang Yuesi 
Published: February 2019

Abstract. As major chemical components of airborne fine particulate matter (PM2.5), organic carbon (OC) and elemental carbon (EC) have vital impacts on air quality, climate change, and human health. Because OC and EC are closely associated with fuel combustion, it is helpful for the scientific community and policymakers assessing the efficacy of air pollution control measures to study on the impact of the control measures and regional transport on the OC and EC levels. In this study, hourly mass concentrations of OC and EC associated with PM2.5 were semi-continuously measured from March 2013 to February 2018. The results showed that annual mean OC and EC concentrations declined from 14.0 to 7.7 μg/m3 and from 4.0 to 2.6 μg/m3, respectively, from March 2013 to February 2018. In combination with the data of OC and EC in previous studies, an obvious decreasing trend in OC and EC concentrations was found, which was caused by clean energy policies and effective air pollution control measures. However, no obvious change in the ratios of OC and EC to the PM2.5 mass (on average, 0.164 and 0.049, respectively) was recorded, suggesting that inorganic ions still contributed a lot to PM2.5. Based on the seasonal variations of OC and EC, it appeared that higher OC and EC concentrations were still observed in the winter months, with the exception of winter of 2017–2018. Traffic policies executed in Beijing resulted in nighttime peaks of OC and EC, caused by heavy-duty vehicles and heavy-duty diesel vehicles being permitted to operate from 0:00 to 6:00. In addition, the fact that there was no traffic restriction in weekends led to higher concentrations in weekends compared to weekdays. Significant correlations between OC and EC were observed throughout the study period, suggesting that OC and EC originated from common emission sources, such as exhaust of vehicles and fuel combustion. OC and EC levels increased with enhanced SO2, CO and NOx concentrations while the O3 and OC levels enhanced simultaneously when O3 concentrations were higher than 50 μg/m3. Nonparametric wind regression analysis was performed to examine the sources of OC and EC in the Beijing area. It was found that there were distinct hot spots in the northeast wind sector at wind speeds of approximately 5 km/h, as well as diffuse signals in the southwestern wind sectors, highlighting probable trans-boundary transport from highly industrialized regions upwind of the Hebei province, such as Baoding, Shijiazhuang and Handan, which were the most polluted cities in China. This was consistent with their higher potential as source areas, as determined by the potential source contribution function (PSCF) analysis. A high-potential source area was precisely pinpointed, which was located in the northwestern and southern areas of Beijing in 2017 instead of solely in the southern areas of Beijing in 2013. This work shows that improvement of the air quality in Beijing benefits from strict control measures; however, joint prevention and control of regional air pollution in the regions is needed for further improving the air quality. The results provide a reference for controlling air pollution caused by rapid economic development in developing countries.