Estimated contribution of vehicular emissions to carbonaceous aerosols in urban Beijing, China

Worldwide attention has been focused on air pollution associated with vehicle emissions. With the highest number of vehicles, Beijing has experienced air pollution related to vehicle emissions. To better recognize the contributions of vehicle emissions to organic carbon (OC) and elemental carbon (EC), hourly measurements of OC and EC in PM2.5 were conducted in urban Beijing from June 1, 2016 to May 31, 2017 in addition to auxiliary measurements of water-soluble ions and elements. The average concentrations of OC and EC were 11.9 ± 11.1 and 3.7 ± 3.6 μg/m3 during the study period, respectively, while those of OC and EC during the nighttime (0:00–6:00), morning rush hour (7:00–9:00) and evening rush hour (17:00–19:00) were 12.9 ± 12.9 μg/m3 and 4.4 ± 4.3 μg/m3, 10.4 ± 9.8 μg/m3 and 3.7 ± 3.1 μg/m3, and 12.3 ± 8.7 μg/m3 and 3.5 ± 2.4 μg/m3, respectively. The diurnal variations in OC and EC concentrations showed higher values during the nighttime, morning rush hour and evening rush hour. Strong correlations were found between OC and CO (r2 = 0.76–0.79) and between OC and NOx (r2 = 0.63–0.68); strong correlations were also found between EC and CO (r2 = 0.76–0.81) and between EC and NOx (r2 = 0.63–0.68) during the nighttime, morning rush hour and evening rush hour. The results of source apportionment based on the hourly PM2.5-associated chemical composition by a positive matrix factorization model showed that vehicle emissions accounted for 23.0% of PM2.5. From the profiles of the source apportionment of PM2.5, it could be found that vehicle emissions contributed 51.2%, 51.3% and 25.2% of the concentrations of OC, EC and PM2.5 on average during the nighttime, respectively. The contributions of vehicle emissions to OC, EC and PM2.5 during the nighttime were higher than or close to those during the evening rush hour and the morning rush hour. This result indicated that vehicular emissions play an important role in carbonaceous aerosols and PM2.5 during the nighttime. The above results provide insight into the quantitative evaluation of the contribution of vehicular emissions to OC, EC and PM2.5 and thus could provide references for pollution control in areas with high vehicular emissions.