Fine particle chemistry under a special dust transport event: impacts from unusually enhanced ozone and air mass backflows over the ocean

Abstract. A five-days long-lasting dust event was observed with a synergy of field measurements techniques in Shanghai in the autumn of 2019. Different from most dust events, this dust was an unusual one characterized of low wind speed, high relative humidity, high concentrations of gaseous precursors, and contrasting wind vectors between low and high altitudes. Three dust stages were identified and the first stage was a normal dust invasion with high particulate concentrations and short duration. In contrast, unusual enhancement of ozone was observed in the second stage, due to compound causes of weak synoptic system, transport from the ocean, and subsidence of high-altitude O₃ down drafted by dust. As a result, sulfate and nitrate moderately correlated with O₃ while had almost no correlation with aerosol liquid water content, indicating the dominant role of gas phase oxidations. During the third stage of dust, a special phenomenon of dust backflow was observed that the dust plume drifted from the Shandong Peninsula and travelled slowly over the Yellow Sea and the East China Sea, finally returning to Shanghai. The dust backflow was evidenced by the enrichment of marine vessel emissions (V and Ni) and increased solubility of calcium. Under the humid oceanic breezes, the formation of nitrate was dominated by aqueous processing, while the strong correlation between SO₄^2- and Na⁺ suggested that a considerable part of sulfate was aged and directly transported. Based on the thermodynamic modeling, sea salts probably involved more in the secondary aerosol formation than the dust heterogeneous reactions. By developing an upstream-receptor relationship method, the amounts of transported and secondarily formed aerosol species were separated. This study highlights that the transport pathway of dust and environmental conditions could significantly modify the aerosol properties, especially at the complex land-sea interface.