Chemical evaluation of aerosol particles in an intense Asian dust storm in a coastal city: Direct vs. reverse transport stages.

An intense Asian dust storm invaded Qinhuangdao, a coastal city of North China on April 11-14, 2023. We analyzed the bulk water-soluble inorganic ions (WSII) of PM10 and PM2.5 using an Ion Chromatograph and investigated individual particle morphology and elemental composition using a scanning electron microscope coupled with energy dispersive X-ray. The results showed when the dust was directly transported to the sampling site from northwestern China (direct transport stages), the PM10 mass concentrations were high (675.0 µg/m3) but the relative humidity (RH) was low. Total WSII were 19.54 µg/m3 and 11.17 µg/m3, accounting for 2.90 % and 7.25 % of the PM10 and PM2.5, respectively. In addition, the proportion of S-containing individual mineral dust was low (28.7 %-35.3 %). The results suggested there were limited secondary inorganic aerosol formation. However, when the dust was transported back to the sampling site across the Bohai Sea in the following days (reverse transport stages), both the PM10 mass concentrations (462.3 µg/m3) and RH were high. Total WSII were 43.75 µg/m3 and 18.71 µg/m3, accounting for 9.62 % and 14.45 % of the total PM10 and PM2.5, respectively. The proportion of S-containing particles was 51.5 %-72.5 %. In addition, the concentration of NO3- was much higher than that of SO42-. The results suggested both the sulfate and nitrate formation occurred on coarse mineral dust but the nitrate formation exceeded sulfate formation. Interestingly, most mineral dust, including Si-rich particles contained Cl when they passed through the marine atmosphere. The results highlighted the heterogeneous reactions of dust varied in different transport stages.