Vertical distribution of particle-phase dicarboxylic acids, oxoacids and α-dicarbonyls in the urban boundary layer based on the 325m tower in Beijing

Vertical distributions of dicarboxylic acids, oxoacids, alpha-dicarbonyls and other organic tracer compounds in fine aerosols (PM2.5) were investigated at three heights (8, 120 and 260 m) based on a 325m meteorological tower in urban Beijing in the summer of 2015. Results showed that the concentrations of oxalic acid (C-2), the predominant diacid, were more abundant at 120m (210 +/- 154 ng m(-3)) and 260m (220 +/- 140 ng m(-3)) than those at the ground surface (160 +/- 90 ng m(-3)). Concentrations of phthalic acid (Ph) decreased with the increase in height, indicating that local vehicular exhausts were the main contributor. Positive correlations were noteworthy for C-2 / total diacids with mass ratios of C-2 to main oxoacids (Pyr and omega C-2) and alpha-dicarbonyls (Gly and MeGly) in polluted days (0.42 <= r(2) <= 0.65), especially at the ground level. In clean days, the ratios of carbon content in oxalic acid to water-soluble organic carbon (C-2-C / WSOC) showed larger values at 120 and 260m than those at the ground surface. However, in polluted days, the C-2-C / WSOC ratio mainly reached its maximum at ground level. These phenomena may indicate the enhanced contribution of aqueous-phase oxidation to oxalic acid in polluted days. Combined with the influence of wind field, total diacids, oxoacids and alpha-dicarbonyls decreased by 22%-58% under the control on anthropogenic activities during the 2015 Victory Parade period. Furthermore, the positive matrix factorisation (PMF) results showed that the secondary formation routes (secondary sulfate formation and secondary nitrate formation) were the dominant contributors (37 %-44 %) to organic acids, followed by biomass burning (25 %-30 %) and motor vehicles (18 %-24 %). In this study, the organic acids at ground level were largely associated with local traffic emissions, while the long-range atmospheric transport followed by photochemical ageing contributed more to diacids and related compounds in the urban boundary layer than the ground surface in Beijing.