Effects of relative humidity on heterogeneous reaction of SO2 with CaCO3 particles and formation of CaSO4·2H2O crystal as secondary aerosol

Yang Yue,Jingru Cheng,Kang Soo Lee,Roman Stocker,Xu He,Maosheng Yao,Jing Wang

doi:10.1016/j.atmosenv.2021.118776

Abstract

Haze related air pollution has become one of major environmental concerns in some Asian countries. Sulfate is a main component of ambient particulate matter (PM) in the urban environment during haze episodes. Among the pollutants, sulfur dioxide (SO2) is an important precursor of sulfate and new atmospheric particle formation. However, under different atmospheric conditions its underlying formation mechanisms are not clearly elucidated. In the current work, we collected ambient PM in two cities: Zurich (Switzerland) and Beijing (China). The PM morphology and the composition were investigated by scanning electron microscopy, energy dispersive X-Ray spectroscopy and Raman micro-spectroscopy. In addition, a series of lab controlled experiments were also performed to study the SO2 reaction with CaCO3 aerosol. We found different types of CaSO4·2H2O crystals in the Chinese urban samples, with clearly different compositions than those from Zurich. The experimental data showed that the relative humidity played a significant role on the new CaSO4·2H2O crystal formation including its size, morphology and composition. Relative humidity (RH) above 80% can significantly promote SO2 oxidation on the CaCO3 particles and form the CaSO4·2H2O crystals. In contrast, at relative humidity below 40%, only few CaCO3 particles can be converted to CaSO4 particles. The results of this study facilitate the understanding of secondary inorganic aerosol formation by the reaction of CaCO3 particles with SO2 with different RHs in different city environments, and provide useful information for air pollution control.

Highlights

In many Asian countries, including cities: Zurich (Switzerland) and Beijing (China) and India, the main air pollution is the haze induced by ambient particulate matter (PM) (Boys et al, 2014; Huang et al, 2014; Jayarathne et al, 2016; Li et al, 2017, 2019)
At relative humidity below 40%, only few CaCO3 particles can be converted to CaSO4 particles
Zurich PM samples tended to be more homogeneous in morphology and were mainly composed of carbon, oxygen, nitrogen, silicon, iron, etc

Summary

Introduction

In many Asian countries, including China and India, the main air pollution is the haze induced by ambient particulate matter (PM) (Boys et al, 2014; Huang et al, 2014; Jayarathne et al, 2016; Li et al, 2017, 2019). The haze events occurred frequently and influenced large areas in China, India and other counties. These haze events led to millions of premature deaths and inestimable economic losses (Balakrishnan et al, 2019; Liu et al, 2019a). G., SO2, NOx, NH3, dust and smoke), local stagnant meteorological conditions and fast formation of secondary aerosol, especially sulfate (SO42− ) and nitrate (NO3− ) (Cheng et al, 2016; Guo et al, 2014; Huang et al, 2014). Heterogeneous oxidation of SO2 on solid or aqueous particles is thought to be a potentially important source of sulfate PM in the atmosphere (Takahashi et al, 2008; Yu et al, 2018; Zhao et al, 2018)

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Conclusion