Ammonium nitrate promotes sulfate formation through uptake kinetic regime,Atmospheric Chemistry and Physics

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Ammonium nitrate promotes sulfate formation through uptake kinetic regime
Atmospheric Chemistry and Physics ( IF 5.2 ) Pub Date : 2021-09-07 , DOI: 10.5194/acp-21-13269-2021
Yongchun Liu , Zemin Feng , Feixue Zheng , Xiaolei Bao , Pengfei Liu , Yanli Ge , Yan Zhao , Tao Jiang , Yunwen Liao , Yusheng Zhang , Xiaolong Fan , Chao Yan , Biwu Chu , Yonghong Wang , Wei Du , Jing Cai , Federico Bianchi , Tuukka Petäjä , Yujing Mu , Hong He , Markku Kulmala

Although the anthropogenic emissions of SO₂ have decreased significantly in China, the decrease in SO

_{4}^{2 -}

in PM_2.5 is much smaller than that of SO₂. This implies an enhanced formation rate of SO

_{4}^{2 -}

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 =

0.15 + 0.0032 \times \exp (RH / 16.2)

) and Beijing (SOR =

- 0.045 + 0.12 \times \exp (RH / 37.8)

). In Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water content (AWC) in PM_2.5 (SOR =

0.15 + 0.40 \times AWC / {PM}_{2.5}

). Our results suggest that uptake of SO₂ instead of oxidation of S(IV) in the particle phase is the rate-determining step for sulfate formation. NH₄NO₃ plays an important role in the AWC and the change of particle state, which is a crucial factor determining the uptake kinetics of SO₂ and the enhanced SOR during haze days. Our results show that NH₃ significantly promoted the uptake of SO₂ and subsequently the SOR, while NO₂ had little influence on SO₂ uptake and SOR in the presence of NH₃.

更新日期：2021-09-07

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