Abstract. To improve air quality, the Beijing government has employed several air pollution control measures since the 2008 Olympics. In order to investigate organic aerosol sources after the implementation of these measures, ambient fine particulate matter was collected at a regional site in Changping (CP) and an urban site at the Peking University Atmosphere Environment Monitoring Station (PKUERS) during the “Photochemical Smog in China” field campaign in summer 2016. Chemical mass balance (CMB) modeling and the tracer yield method were used to apportion primary and secondary organic sources. Our results showed that the particle concentration decreased significantly during the last few years. The apportioned primary and secondary sources explained 62.8 ± 18.3 and 80.9 ± 27.2 % of the measured OC at CP and PKUERS, respectively. Vehicular emissions served as the dominant source. Except for gasoline engine emissions, the contributions of all the other primary sources decreased. In addition, the anthropogenic SOC, i.e., toluene SOC, also decreased, implying that deducting primary emissions can reduce anthropogenic SOA. In contrast to the SOA from other regions in the world where biogenic SOA was dominant, anthropogenic SOA was the major contributor to SOA, implying that deducting anthropogenic VOC emissions is an efficient way to reduce SOA in Beijing. Back-trajectory cluster analysis results showed that high mass concentrations of OC were observed when the air mass was from the south. However, the contributions of different primary organic sources were similar, suggesting regional particle pollution. The ozone concentration and temperature correlated well with the SOA concentration. Different correlations between day and night samples suggested different SOA formation pathways. Significant enhancement of SOA with increasing particle water content and acidity was observed in our study, suggesting that aqueous-phase acid-catalyzed reactions may be the important SOA formation mechanism in summer in Beijing.

1To improve the air quality, Beijing government has employed several air pollution 2 control measures since 2008 Olympics. In order to investigate the organic aerosol 3 sources after the implementation of these measures, ambient fine particulate matters 4 were collected at a regional site Changping (CP) and an urban site Peking University 5Atmosphere Environment MonitoRing Station (PKUERS) during the "Photochemical 6Smog in China" field Campaign in summer of 2016. A chemical mass balance (CMB) 7 modeling and the tracer yield method were used to apportion the primary and 8 secondary organic sources. Our results showed that the particle concentration 9 decreased significantly during the last a few years. The apportioned primary and 10 secondary sources explained 62.8  18.3% and 80.9  27.2% of the measured OC at 11 CP and PKUERS, respectively. Vehicular emissions served as the dominant sources. 12Except gasoline engine emission, the contributions of all the other primary sources 13 decreased. Besides, the anthropogenic SOC, i.e. toluene SOC, also decreased, 14 implying that deducting primary emission can reduce anthropogenic SOA. Different 15 from the SOA from other regions in the world, where biogenic SOA was dominant, 16 anthropogenic SOA was the major contributor to SOA, implying that deducting 17 anthropogenic VOCs emissions is an efficient way to reduce SOA in Beijing. Back 18 trajectory cluster analysis results showed that high mass concentrations of OC were 19 observed when the air mass was from south. However, the contributions of different 20 primary organic sources were similar, suggesting the regional particle pollution. The 21 ozone concentration and temperature correlated well with the SOA concentration. 22Different correlations between day and night samples suggested the different SOA 23 formation pathways. Significant enhancement of SOA with increasing particle water 24 content and acidity were observed in our study, suggesting the aqueous phase 25 acid-catalyzed reactions may be the important SOA formation mechanism in summer 26 of Beijing. 27Atmos. Chem. Phys. Discuss., https://doi