Diurnal variations of total carbon, dicarboxylic acids, ketoacids and α-dicarbonyls in aerosols in the northern vicinity of Beijing

Abstract. Aerosol samples (TSP, n=58) were collected on day- and night-time basis at Mangshan in the north of Beijing, China in autumn 2007 to better understand the status of air quality and the influence of urban pollutants in the northern vicinity of Beijing. The samples were analyzed for aerosol mass, total carbon (TC), low molecular weight α, ω-dicarboxylic acids (C₂-C₁₂), ketoacids (ωC₂-ωC₉, pyruvic acid), α-dicarbonyls (glyoxal and methylglyoxal), as well as aromatic (phthalic, iso- and tere-phthalic) diacids. Aerosol mass and TC concentrations are higher in daytime than in nighttime. TC/aerosol mass ratios in this study are lower than those reported in megacities in East Asia, but higher than those reported in marine aerosols. Molecular distributions of diacids demonstrated that oxalic (C₂) acid was the most abundant species, comprising 38–77% of total diacids, followed by succinic (C₄) and malonic (C₃) acids. For most compounds, the concentrations were higher in daytime than nighttime, indicating that diacids are produced in daytime by photochemical oxidation of organic precursors emitted from anthropogenic sources in Beijing during the transport to Mangshan area by the northward wind. However, we found that C₂ concentrations are higher in nighttime than in daytime. A positive correlation of C₂ to glyoxylic acid (ωC₂) was obtained at night when relative humidity increased up to 100%, suggesting that aqueous phase production of C₂ occurs in nighttime via the oxidation of ωC₂. Depletion of C₂ by photolysis of Fe-oxalato complexes might be another reason for the lower concentrations of C₂ in daytime samples. High phthalic acid/C₄ ratios in the aerosol samples suggest that automobile combustion and coal burning products are important sources, which are subjected to photochemical oxidation during the atmospheric transport of urban aerosols from Beijing. In contrast, higher concentrations of methylglyoxal in nighttime than daytime may suggest that isoprene emitted from the northern forest area is oxidized in daytime and then transported to the sampling area at night by northerly winds. This study demonstrates that secondary organic aerosols are significantly produced and aged in the vicinity of Beijing during atmospheric transport.