Measurement report: Vertical distribution of atmospheric particulate matter within the urban boundary layer in southern China – size-segregated chemical composition and secondary formation through cloud processing and heterogeneous reactions

Shengzhen Zhou,Shiguo Jia,Xuemei Wang,Jun Tao,Yanning Chen,Pingqing Fu,Zuzhao Huang,Weihua Chen,Luolin Wu,Jinpu Zhang,Junchen Guo,Yafang Cheng,Yele Sun,Jun Zhao,Junxia Kuang

doi:10.5194/acp-20-6435-2020

Abstract

Abstract. Many studies have recently been done on understanding the sources and formation mechanisms of atmospheric aerosols at ground level. However, vertical profiles and sources of size-resolved particulate matter within the urban boundary layer are still lacking. In this study, vertical distribution characteristics of size-segregated particles were investigated at three observation platforms (ground level, 118 m, and 488 m) on the 610 m high Canton Tower in Guangzhou, China. Size-segregated aerosol samples were simultaneously collected at the three levels in autumn and winter. Major aerosol components, including water-soluble ions, organic carbon, and elemental carbon, were measured. The results showed that daily average fine-particle concentrations generally decreased with height. Concentrations of sulfate and ammonium in fine particles displayed shallow vertical gradients, and nitrate concentrations increased with height in autumn, while the chemical components showed greater variations in winter than in autumn. The size distributions of sulfate and ammonium in both seasons were characterized by a dominant unimodal mode with peaks in the size range of 0.44–1.0 µm. In autumn, the nitrate size distribution was bimodal, peaking at 0.44–1.0 and 2.5–10 µm, while in winter it was unimodal, implying that the formation mechanisms for nitrate particles were different in the two seasons. Our results suggest that the majority of the sulfate and nitrate is formed from aqueous-phase reactions, and we attribute coarse-mode nitrate formation at the measurement site to the heterogeneous reactions of gaseous nitric acid on existing sea-derived coarse particles in autumn. Case studies further showed that atmospheric aqueous-phase and heterogeneous reactions could be important mechanisms for sulfate and nitrate formation, which, in combination with adverse weather conditions such as temperature inversion and calm wind, led to haze formation during autumn and winter in the Pearl River Delta (PRD) region.

Highlights

IntroductionAir pollution is of serious environmental concern in China and is often characterized by high concentrations of many pollutants, among which fine particulate matter (particles with an aerodynamic diameter of 2.5 μm and smaller or PM2.5) is currently the primary pollutant in most cities
Air pollution is of serious environmental concern in China and is often characterized by high concentrations of many pollutants, among which fine particulate matter is currently the primary pollutant in most cities
The objectives of this study are to (1) analyze the vertical mass size distribution of the particulate matter (PM) chemical components and the factors that affect their vertical variations; (2) investigate the roles of in-cloud processes and multiphase reactions in secondary aerosol formation and the implications for haze pollution in subtropical urban areas; and (3) evaluate the simulation performance of the Weather Research and Forecasting Model coupled with online chemistry (WRFChem) in the vertical based on the measurement data

Summary

Introduction

Air pollution is of serious environmental concern in China and is often characterized by high concentrations of many pollutants, among which fine particulate matter (particles with an aerodynamic diameter of 2.5 μm and smaller or PM2.5) is currently the primary pollutant in most cities. The condensation sub-mode originates from primary emissions and the growth of smaller particles by coagulation and condensation, while the droplet sub-mode mainly results from cloud/fog processing or the coagulation of smaller particles (Seinfeld and Pandis, 2006). Numerous studies have shown that in-cloud processes or multiphase reactions are plausible mechanisms for the formation of droplet-mode particles (Meng and Seinfeld, 1994; Zhuang et al, 1999a; Yao et al, 2003; Guo et al, 2010; Tian et al, 2016). Coarse-mode particles are primarily produced by mechanical processes like sea spay, mineral particles, and plant debris; coarse-mode secondary sulfates and nitrates have been observed, and their formation has been attributed to heterogeneous and multiphase reactions (Pakkanen, 1996; Liu et al, 2008)

Objectives

Methods

Results

Conclusion