Carbonaceous species in PM2.5 at a pair of rural-urban sites in Beijing, 2005–2008
1College of Earth Science, Graduate University of Chinese Academy of Sciences, Beijing 100049, China
2Atmospheric Science and Technology Directorate/Science and Technology Branch, Environment Canada, Toronto, ON M3H 5T4, Canada
3School of Environment, Tsinghua University, Beijing 100084, China
Abstract. One-week integrated PM2.5 samples were collected at a pair of rural (MY)-urban (TH) sites in Beijing over four years between 2005 and 2008. Weekly OC and EC in PM2.5 were compared to investigate their respective levels and temporal patterns at the two sites, and differences in the factors contributing to them were discussed. A systematic decrease of annual mean concentration of OC and an opposite trend for EC at both sites, and the significantly lower fractions of TCM (total carbonaceous mass) in PM2.5 mass than those measured at TH in 1999, indicate that the relative importance of carbonaceous species in PM2.5 was probably weakened whereas that of EC in TC was steadily enhanced. Clear seasonal variations were found for both OC and EC concentrations (varying seasonally by factors of 1.35~3.0) at TH with higher weekly concentrations and fluctuations in winter and much lower values in summer and spring. The minimum seasonal urban excesses of OC (3.0 μg m−3) and EC (1.2 μg m−3), which were only one-ninth to one-eighth of their corresponding maxima, both occurred in 2008 summer. The noticeably more positive stable carbon isotope values (δ13C) of EC at TH in that summer relative to the preceding summers puts in new evidence that the contribution to carbonaceous particles from mobile sources was substantially reduced due to a concerted effort to reduce emissions from new and existing vehicles before, during and after the Summer Olympics. No consistent seasonal patterns of OC and EC concentrations without strong correlations and their high ratios (OC/EC) at the MY site reflect their complex and variable major sources and formation/production in the rural area compared to the urban area in Beijing, such as biomass burning during the harvest seasons, widely used high-polluting family stoves and small boiler for cooking and heating, and high potential formation of SOA.