Source Apportionment of Fine Organic Carbon

Organic aerosol is known to be a major component of fine aerosols in China. This study was designed to investigate the seasonal characteristics and source apportionment of organic aerosols during both clean days and haze events. Time-resolved concentrations of carbonaceous aerosols and organic molecular tracers were measured at Pinggu, a rural site of Beijing during the winter of 2016 and summer 2017 within an intensive sampling campaign. The Chemical Mass Balance (CMB) receptor model was applied to estimate the average source contributions to organic carbon (OC) at Pinggu based on molecular markers for primary and secondary organic aerosols. The concentration of OC in winter is comparable with previous studies, but decreased significantly during summer. The CMB model apportioned to seven separate primary sources, which explained on average 73.8% on haze days and 81.2% on clean days of the organic carbon in winter, including vegetative detritus, biomass burning, gasoline vehicles, diesel vehicles, industrial coal combustion, residential coal combustion and cooking. A slightly lower percentage of OC was apportioned in the summer campaign with 64.5% and 78.7% accounted for on moderate and good days. The other unapportioned OC is considered to consist of secondary organic aerosol (SOA) formed through chemical reactions in the atmosphere. During haze episodes in winter, coal combustion and SOA were the dominant sources of organic carbon with 23.3% and 26.2% of OC, respectively, followed by biomass burning emissions (20%), indicating that combustion activities and secondary formation play significant roles in particulate matter accumulation, whereas in summer, industrial coal combustion and SOC are important sources. Diurnal contribution cycles for coal combustion and biomass burning showed a peak at 6-9 pm, suggesting cooking and heating were predominantly responsible for organic aerosols in this rural area. The backward trajectory analysis showed that high mass concentrations of OC were measured when the air mass was from the south, suggesting that the organic aerosols in Pinggu were mainly affected by both local emissions and regional transport from central Beijing and Hebei province during haze episodes. The source apportionment by CMB is compared with the results of a PMF analysis of ACSM data for non-refractory PM1, showing generally good agreement.