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. As major chemical components of airborne fine particulatematter (PM 2.5 ), organic carbon (OC) and elemental carbon (EC) havevital impacts on air quality, climate change, and human health. Because OCand EC are closely associated with fuel combustion, it is helpful for thescientific community and policymakers assessing the efficacy of airpollution control measures to study the impact of control measuresand regional transport on OC and EC levels. In this study, hourly massconcentrations of OC and EC associated with PM 2.5 weresemi-continuously measured from March 2013 to February 2018. The resultsshowed 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, fromMarch 2013 to February 2018. In combination with the data of OC and EC inprevious studies, an obvious decreasing trend in OC and EC concentrationswas found, which was caused by clean energy policies and effective airpollution control measures. However, no obvious change in the ratios of OCand EC to the PM 2.5 mass (on average, 0.164 and 0.049, respectively)was recorded, suggesting that inorganic ions still contributed a lotto PM 2.5 . Based on the seasonal variations in OC and EC, it appearedthat higher OC and EC concentrations were still observed in the wintermonths, with the exception of winter of 2017–2018. Traffic policies executedin Beijing resulted in nighttime peaks of OC and EC, caused by heavy-dutyvehicles and heavy-duty diesel vehicles being permitted to operate from 00:00to 06:00 (China standard time, UTC + 8, for all times throughout the paper). In addition, the fact that there was no traffic restriction inweekends led to higher concentrations on weekends compared to weekdays.Significant correlations between OC and EC were observed throughout thestudy period, suggesting that OC and EC originated from common emissionsources, such as exhaust of vehicles and fuel combustion. OC and EC levelsincreased with enhanced SO2 , CO, and NOx concentrations while the O3 and OC levels were enhanced simultaneously when O3 concentrationswere higher than 50  µg m−3 . Non-parametric wind regression analysiswas performed to examine the sources of OC and EC in the Beijing area. Itwas found that there were distinct hot spots in the northeast wind sector atwind speeds of approximately 0–6 km h −1 , as well as diffuse signals in thesouthwestern wind sectors. Source areas further away from Beijing wereassessed by potential source contribution function (PSCF) analysis. Ahigh-potential source area was precisely pinpointed, which was located inthe northwestern and southern areas of Beijing in 2017 instead of solely inthe southern areas of Beijing in 2013. This work shows that improvement ofthe air quality in Beijing benefits from strict control measures; however,joint prevention and control of regional air pollution in the regions isneeded for further improving the air quality. The results provide areference for controlling air pollution caused by rapid economic developmentin developing countries.

atmospheric chemistry and physics

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