Seasonal and diurnal variations of organic carbon in PM_(2.5)inBeijing and the estimation of secondary organic carbon

摘要

Organic carbon (OC) and elemental carbon (EC) in PM_(2.5)were measured hourly witha semicontinuous thermal-optical analyzer in Beijing, China, during four seasons fromNovember 2005 to October 2006. The average OC concentrations measured at PekingUniversity (PKU) site in urban Beijing were 20 ± 19, 12 ± 7, 10 ± 5, and 18 ± 11 ,ugC/m~3in winter, spring, summer, and autumn, respectively. Distinct seasonal and diurnal patternsof OC were observed, and their concentrations were significantly influenced by theseasonal and diurnal changes in sources, emission rates, and mesoscale meteorology. Theimpacts of meteorological parameters such as wind and precipitation on OC and ECconcentrations are also discussed. OC and EC concentrations were found to be wellcorrelated in winter, spring, and autumn, indicating their common origins, while theircorrelation was poorer in summer, especially in the afternoon, indicating that the influenceof secondary organic carbon (SOC) was significant for this period. SOC concentrationswere 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 μgC/m~3, accounting for 19%, 27%, 45%, and 23% of total organic carbon inwinter, spring, summer, and autumn, respectively. This result showed that the relativecontribution of SOC to OC in PM_(2.5)at this urban location was only comparable to that ofprimary organic carbon (POC) to OC in summer, while in the other three seasons, OC wasdominated by POC.