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: July 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 the impact of control measures
and regional transport on 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 m−3 and from 4.0 to 2.6 µg m−3, 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 in 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 00:00
to 06:00 (China standard time, UTC+8, for all times throughout the paper). In addition, the fact that there was no traffic restriction in
weekends led to higher concentrations on 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 were enhanced simultaneously when O3 concentrations
were higher than 50 µg m−3. Non-parametric 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 0–6 km h−1, as well as diffuse signals in the
southwestern wind sectors. Source areas further away from Beijing were
assessed by 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.