Chemical composition, sources, and aging process of submicron aerosols in Beijing: Contrast between summer and winter

Abstract

To investigate the seasonal characteristics of submicron aerosol (PM 1) in Beijing urban areas, a high‐resolution time‐of‐flight aerosol‐mass‐spectrometer (HR‐ToF‐AMS) was utilized at an urban site in summer (August to September 2011) and winter (November to December 2010), coupled with multiple state of the art online instruments. The average mass concentrations of PM 1 (60–84 µg m−3) and its chemical compositions in different campaigns of Beijing were relatively consistent in recent years. In summer, the daily variations of PM 1 mass concentrations were stable and repeatable. Eighty‐two percent of the PM 1 mass concentration on average was composed of secondary species, where 62% is secondary inorganic aerosol and 20% secondary organic aerosol (SOA). In winter, PM 1 mass concentrations changed dramatically because of the different meteorological conditions. The high average fraction (58%) of primary species in PM 1 including primary organic aerosol (POA), black carbon, and chloride indicates primary emissions usually played a more important role in the winter. However, aqueous chemistry resulting in efficient secondary formation during occasional periods with high relative humidity may also contribute substantially to haze in winter. Results of past OA source apportionment studies in Beijing show 45–67% of OA in summer and 22–50% of OA in winter can be composed of SOA. Based on the source apportionment results, we found 45% POA in winter and 61% POA in summer are from nonfossil sources, contributed by cooking OA in both seasons and biomass burning OA (BBOA) in winter. Cooking OA, accounting for 13–24% of OA, is an important nonfossil carbon source in all years of Beijing and should not be neglected. The fossil sources of POA include hydrocarbon‐like OA from vehicle emissions in both seasons and coal combustion OA (CCOA) in winter. The CCOA and BBOA were the two main contributors (57% of OA) for the highest OA concentrations (>100 µg m−3) in winter. The POA/ΔCO ratios in winter and summer are 11 and 16 µg m−3 ppm−1, respectively, similar to ratios from western cities. Higher OOA/Ox (= NO 2 + O 3) ratio (0.49 µg m−3 ppb−1) in winter study than these ratios from western cities (0.03–0.16 µg m−3 ppb−1) was observed, which may be due to the aqueous reaction or extra SOA formation contributed by semivolatile organic compounds from various primary sources (e.g., BBOA or CCOA) in Beijing. The evolution of oxygen to carbon ratio (O/C) with photochemical age allows to estimate an equivalent rate constant for chemical aging of OA in summer as kOH ~ 4.1 × 10−12 cm 3 molecule−1 s−1, which is of the same order as obtained in other anthropogenic influenced areas and may be useful for OA modeling.