Measurement report: Simultaneous measurement on atmospheric gas- and aerosol-phase water-soluble organics in Shanghai: Remarkable increase in light absorbing of Asian dust aerosols during long-range transport

Abstract. To better understand physicochemical evolution of Asian dust particles during long-range transport, water-soluble organic compounds (WSOCs) in gas- (WSOC_g) and particle-phase (WSOC_p) were simultaneously measured with a 1-hr time resolution in Shanghai during the 2023 dust storm (DS) and haze event (HE), and characterized for their optical properties and size distribution. Our results showed that gas-to-particle-phase partitioning coefficients (F_p) of WSOCs in DS (0.3 ± 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 WSOC_g from the gas to the particle phase. F_p variation was largely driven by aerosol liquid water content in HE but by aerosol acidity in DS. WSOC_p and its light absorption at l_365nm dominated at the fine mode (< 2.1 µm) in HE and the coarse mode (> 2.1 µm) in DS, respectively. Mass absorption coefficient (MAC) of the coarse mode of WSOC_p at l_365nm 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.