# Ammonium nitrate promotes sulfate formation through uptake kinetic regime

Although the anthropogenic emissions of SOinline-formula2 have decreased significantly in China, the decrease in SOinline-formula $M2inlinescrollmathml{}_{normal 4}^{normal 2-}$ 13pt17ptsvg-formulamathimg63f11f9b60a09208ebeae789905b238b acp-21-13269-2021-ie00001.svg13pt17ptacp-21-13269-2021-ie00001.png in PMinline-formula2.5 is much smaller than that of SOinline-formula2. This implies an enhanced formation rate of SOinline-formula $M5inlinescrollmathml{}_{normal 4}^{normal 2-}$ 13pt17ptsvg-formulamathimg277c1427ed297c2e5c45f7f988764cfe acp-21-13269-2021-ie00002.svg13pt17ptacp-21-13269-2021-ie00002.png in the ambient air, and the mechanism is still under debate. This work investigated the formation mechanism of particulate sulfate based on statistical analysis of long-term observations in Shijiazhuang and Beijing supported with flow tube experiments. Our main finding was that the sulfur oxidation ratio (SOR) was exponentially correlated with ambient RH in Shijiazhuang (SOR inline-formula=inline-formula $M7inlinescrollmathmlnormal 0.15+normal 0.0032×\mathrm{exp}\left(\mathrm{normal RH}/normal 16.2\right)$ 147pt14ptsvg-formulamathimg4a6136bb53ae3e48c5694250a1cbee2f acp-21-13269-2021-ie00003.svg147pt14ptacp-21-13269-2021-ie00003.png ) and Beijing (SOR inline-formula=inline-formula $M9inlinescrollmathml-normal 0.045+normal 0.12×\mathrm{exp}\left(\mathrm{normal RH}/normal 37.8\right)$ 149pt14ptsvg-formulamathimg327e454893203ce23db8fcb0b3dcbeec acp-21-13269-2021-ie00004.svg149pt14ptacp-21-13269-2021-ie00004.png ). In Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water content (AWC) in PMinline-formula2.5 (SOR inline-formula=inline-formula $M12inlinescrollmathmlnormal 0.15+normal 0.40×\mathrm{normal AWC}/chem{\mathrm{normal PM}}_{normal 2.5}$ 126pt14ptsvg-formulamathimga515fb43e81956eaba2748e5738edfee acp-21-13269-2021-ie00005.svg126pt14ptacp-21-13269-2021-ie00005.png ). Our results suggest that uptake of SOinline-formula2 instead of oxidation of S(IV) in the particle phase is the rate-determining step for sulfate formation. NHinline-formula4NOinline-formula3 plays an important role in the AWC and the change of particle state, which is a crucial factor determining the uptake kinetics of SOinline-formula2 and the enhanced SOR during haze days. Our results show that NHinline-formula3 significantly promoted the uptake of SOinline-formula2 and subsequently the SOR, while NOinline-formula2 had little influence on SOinline-formula2 uptake and SOR in the presence of NHinline-formula3.

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Liu, Yongchun / Feng, Zemin / Zheng, Feixue / et al: Ammonium nitrate promotes sulfate formation through uptake kinetic regime. 2021. Copernicus Publications.

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