Ammonium nitrate promotes sulfate formation through uptake kinetic regime

Yongchun Liu,Yan Zhao,Yunwen Liao,Xiaolong Fan,Pengfei Liu,Federico Bianchi,Hong He,Yusheng Zhang,Yanli Ge,Chao Yan,Zemin Feng,Feixue Zheng,Yonghong Wang,Biwu Chu,Xiaolei Bao,Yujing Mu,Markku Kulmala,Jing Cai,Wei Du,Jianhua Tao ,Tuukka Petäj̈ä

doi:10.5194/acp-21-13269-2021

Abstract

Abstract. Although the anthropogenic emissions of SO2 have decreased significantly in China, the decrease in SO42- in PM2.5 is much smaller than that of SO2. This implies an enhanced formation rate of SO42- 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×exp⁡(RH/16.2)) and Beijing (SOR = -0.045+0.12×exp⁡(RH/37.8)). In Shijiazhuang, the SOR is linearly correlated with the ratio of aerosol water content (AWC) in PM2.5 (SOR = 0.15+0.40×AWC/PM2.5). 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.

Highlights

Atmospheric particulate matter (PM) is a worldwide concern due to its adverse effect on human health, such as its association with respiratory and cardiovascular diseases, lung cancer, and premature death (WHO, 2013; Lelieveld et al, 2015)
The composition of the model particles is much simpler than that of ambient particles, it is still meaningful, because we mainly focused on the influence of relative humidity (RH) or aerosol water content (AWC) on uptake kinetics of SO2
Based on 1 year of observations, we confirmed that high PM2.5 mass concentration in pollution events usually coincided with the high sulfate concentration, the fraction of sulfate, and the sulfur oxidation ratio (SOR) in both Beijing and Shijiazhuang

Summary

Introduction

Atmospheric particulate matter (PM) is a worldwide concern due to its adverse effect on human health, such as its association with respiratory and cardiovascular diseases, lung cancer, and premature death (WHO, 2013; Lelieveld et al, 2015). The annual PM2.5 concentration in Beijing decreased from 89.5 μg m−3 in 2013 to 58 μg m−3 in 2017 due to the stringent reduction of local and regional emissions (Cheng et al, 2019; Ji et al, 2019). The PM2.5 concentrations in most regions of China Y. Liu et al.: Ammonium nitrate promotes sulfate formation the World Health Organization (WHO) (WHO, 2006). Haze events occur with high frequency, especially, in autumn and winter

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