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Abstract. Vertical distributions of dicarboxylic acids, oxoacids, α-dicarbonyls and other organic tracer compounds in fine aerosols(PM2.5) were investigated at three heights (8, 120 and260 m) based on a 325 m meteorological tower in urbanBeijing in the summer of 2015. Results showed that the concentrationsof oxalic acid (C2), the predominant diacid, were moreabundant at 120 m (210±154 ng m−3) and260 m (220±140 ng m−3) than those at theground surface (160±90 ng m−3). Concentrations ofphthalic acid (Ph) decreased with the increase in height, indicatingthat local vehicular exhausts were the main contributor. Positivecorrelations were noteworthy for C2 ∕ total diacids with massratios of C2 to main oxoacids (Pyr and ωC2) and α-dicarbonyls (Gly and MeGly) in polluteddays (0.42≤r2≤0.65), especially at the groundlevel. In clean days, the ratios of carbon content in oxalic acid towater-soluble organic carbon (C2−C ∕ WSOC) showed largervalues at 120 and 260 m than those at the groundsurface. However, in polluted days, the C2−C ∕ WSOC ratiomainly reached its maximum at ground level. These phenomena mayindicate the enhanced contribution of aqueous-phase oxidation tooxalic acid in polluted days. Combined with the influence of windfield, total diacids, oxoacids and α-dicarbonyls decreased by22 %–58 % under the control on anthropogenic activities duringthe 2015 Victory Parade period. Furthermore, the positive matrixfactorisation (PMF) results showedthat the secondary formation routes (secondary sulfate formation andsecondary nitrate formation) were the dominant contributors(37 %–44 %) to organic acids, followed by biomass burning(25 %–30 %) and motor vehicles (18 %–24 %). In thisstudy, the organic acids at ground level were largely associatedwith local traffic emissions, while the long-range atmospherictransport followed by photochemical ageing contributed more to diacidsand related compounds in the urban boundary layer than the groundsurface in Beijing.

atmospheric chemistry and physics

Vertical distribution of particle-phase dicarboxylic acids, oxoacids and &amp;lt;i&amp;gt;α&amp;lt;/i&amp;gt;-dicarbonyls in the urban boundary layer based on the 325 m tower in Beijing