Seasonal and diurnal variations of organic carbon in PM2.5 in Beijing and the estimation of secondary organic carbon

Organic carbon (OC) and elemental carbon (EC) in PM2.5 were measured hourly with a semicontinuous thermal-optical analyzer in Beijing, China, during four seasons from November 2005 to October 2006. The average OC concentrations measured at Peking University (PKU) site in urban Beijing were 20 +/- 19, 12 +/- 7, 10 +/- 5, and 18 +/- 11 mu gC/m(3) in winter, spring, summer, and autumn, respectively. Distinct seasonal and diurnal patterns of OC were observed, and their concentrations were significantly influenced by the seasonal and diurnal changes in sources, emission rates, and mesoscale meteorology. The impacts of meteorological parameters such as wind and precipitation on OC and EC concentrations are also discussed. OC and EC concentrations were found to be well correlated in winter, spring, and autumn, indicating their common origins, while their correlation was poorer in summer, especially in the afternoon, indicating that the influence of secondary organic carbon (SOC) was significant for this period. SOC concentrations were estimated by using the EC-tracer method as about 2.6 +/- 3.1, 2.8 +/- 2.4, 4.5 +/- 2.9, and 4.1 +/- 4.0 mu gC/m(3), accounting for 19%, 27%, 45%, and 23% of total organic carbon in winter, spring, summer, and autumn, respectively. This result showed that the relative contribution of SOC to OC in PM2.5 at this urban location was only comparable to that of primary organic carbon (POC) to OC in summer, while in the other three seasons, OC was dominated by POC.