Chemical characterization of atmospheric aerosols in spring Shanghai: Formation mechanism of secondary organic aerosols on Asian dust surface

Abstract

The chemical evolution and secondary reaction of inorganic aerosol during East Asian dust long-range transport have been extensively investigated. However, knowledge about the impact of dust on secondary organic aerosols (SOA) formation remains unclear. By combining online chemical observation of submicron aerosols and offline measurements of size-segregated aerosols in Shanghai, this study revealed that SOA dominated organic aerosols (OA) during the campaign and comprised biogenic SOA (BSOA), oxygenated OA (OOA) and aqueous-phase SOA (aqSOA). During the non-dust storm (NDS) periods, aqSOA prevailed, followed by OOA and BSOA, while in the dust storm (DS) periods OOA was dominant, followed by BSOA with a minor aqSOA. BSOA presented a similar size distribution pattern in both NDS and DS periods, with a predominant peak in the fine mode (<2.1 μm) and a minor peak in the coarse mode (>2.1 μm). However, OOA displayed a distinct size distribution pattern with a large peak in the fine mode and a small peak in the coarse mode during the NDS periods and only a large coarse-mode peak during the DS periods, respectively. HR-AMS analysis results further suggested that during the campaign OOA are composed of carboxylic acid-like compounds, which were produced largely by photochemical oxidation, BSOA was formed by photochemical oxidation of isoprene, while aqSOA was produced mainly through aqueous-phase reactions of carbonyls. Compared to those in the NDS periods, during the DS periods OOA formation was enhanced through reactive uptake by alkaline dust particles, particularly in the coarse mode. In contrast, aqSOA and BSOA formation was suppressed mainly due to dry conditions and reduced aerosol acidity in the DS periods.