Field Observation of Heterogeneous Formation of Dicarboxylic acids, Keto-carboxylic acids, α-Dicarbonyls and Nitrate in Xi’an, China during Asian dust storm periods

Abstract:

To understand the formation mechanism of secondary organic aerosols (SOA) on dust surfaces, this study investigated the concentrations and compositions of dicarboxylic acids (C₂-C₁₁), keto-carboxylic acids (C₃-C₇), α-dicarbonyls and inorganic ions in size-segregated aerosols (9-stages) collected in Xi’an, China during the nondust storm and dust storm periods of 2009 and 2011. During the events the ambient particulate dicarboxylic acids were 932−2240 ng m^-3, which are comparable and even higher than those in nondust periods. Molecular compositions of the above SOA are similar to those in nondust periods with oxalic acid being the leading species. In the presence of the dust storms, all the above mentioned SOA species in Xi’an were predominantly enriched on the coarse particles (>2.1μm), and oxalic acid well correlated with NO₃^- (R²=0.72, p<0.001) rather than SO₄^2-.This phenomenon differs greatly from the SOA in any other nondust period that is characterized by an enrichment of oxalic acid in fine particles and a strong correlation of oxalic acid with SO₄^2-. Our results further demonstrate that NO₃^- in the dust periods in Xi’an was mostly derived from secondary oxidation, whereas SO₄^2- during the events was largely derived from surface soil of Gobi deserts. We propose a formation pathway to explain these observations, in which nitric acid and/or nitrogen oxides react with dust to produce Ca(NO₃)₂ and form a liquid phase on the surface of dust aerosols via water vapor-absorption of Ca(NO₃)₂, followed by a partitioning of the gas-phase water-soluble organic precursors (e.g.,glyoxal and methylglyoxal) into the aqueous-phase and a subsequent oxidation into oxalic acid. To the best of our knowledge, we found for the first time the enrichment of glyoxal and methylglyoxal on dust surface. Our data suggest an important role of nitrate in the heterogeneous formation process of SOA on the surface of Asian dust.