Contribution and Formation of Biogenic and Anthropogenic Secondary Organic Aerosols in Beijing

Ambient aerosol samples were collected at an urban site and an upwind rural site of Beijing during the CAREBEIJING-2008 (Campaigns of Air Quality REsearch in BEIJING and surrounding region) summer field campaign. Contributions of primary particles and secondary organic aerosols (SOA) were estimated by chemical mass balance (CMB) modeling and tracer-yield method. The apportioned primary and secondary sources explain 73.8%±9.7% and 79.6%±10.1% of the measured OC at the urban and rural sites, respectively. Secondary organic carbon (SOC) contributes to 32.5±15.9% of the organic carbon (OC) at the urban site, with 17.4±7.6% from toluene, 9.7±5.4% from isoprene, 5.1±2.0% from α-pinene, and 2.3±1.7% from β-caryophyllene. At the rural site, the secondary sources are responsible for 38.4±14.4% of the OC, with the contributions of 17.3±6.9%, 13.9±9.1%, 5.6±1.9%, and 1.7±1.0% from toluene, isoprene, α-pinene, and β-caryophyllene, respectively. The aerosol gaseous precursor concentrations and temperature correlate well with SOA, both affecting SOA formation. The significant SOA enhancement with increasing water uptake and acidification indicates that the aqueous-phase reactions are largely responsible for SOA formation in Beijing. Source control policy during Olympic Games remarkably reduced the contributions of coal burning, gasoline cars and diesel cars by 60%, 38.5% and 12.5%. However, these strategies seemed have no significant effect on SOA contributions. A variety of other source apportionment techniques were also used to estimate secondary organic aerosols, including EC tracer method, water soluble organic carbon content, chemical mass balance model, and AMS-PMF method. Possible reasons for the differences between these methods were discussed.