Measurement report: Simultaneous measurement on gas- and particle-phase water-soluble organics in Shanghai: Enhanced light absorption of transported Asian dust

Abstract

To better understand the physicochemical evolution of Asian dust particles during long-range transport, water-soluble organic compounds (WSOCs) in gas- (WSOCg) and particle-phase (WSOCp) in the spring atmosphere of Shanghai during the 2023 dust storm period (DS) and haze events (HE) were simultaneously measured with a 3 h time resolution, and characterized for their optical properties and size distribution. Our results showed that gas-to-particle-phase partitioning coefficients (Fp) of WSOCs in DS (0.30 ± 0.06) was comparable to that in HE (0.32 ± 0.06), although both temperature and relative humidity in DS were not favorable for the partitioning, indicating a promoting role of dust particles in the transformation process of WSOCg from gas to particle phase. Fp variation was largely driven by aerosol liquid water content in HE but by aerosol acidity in DS. WSOCp and its light absorption at λ365nm dominated at the fine mode (< 2.1 μm) in non-DS period and the coarse mode (> 2.1 μm) in DS, respectively. Mass absorption coefficient (MAC) of the coarse mode of WSOCp at λ365nm in DS was 0.8 m2 g-1, which is four times that (0.20 ± 0.09 m2 g-1) in the source region of Tengger Desert, suggesting a remarkably increase in light absorbing ability of Asian dust during long-range transport. Sharp co-increases of nitroaromatics, imidazoles, and water-soluble organic nitrogen at the coarse mode in the DS period further revealed that such an increasing MAC is mainly caused by adsorption and heterogeneous formation of light absorbing nitrogen-containing organics on the dust surface during long-range transport.