Chemical characteristics of cloud water and the impacts on aerosol properties at a subtropical mountain site in Hong Kong SAR

Tao Li,Jia Guo,Hartmut Herrmann,Chen Wu,Weihao Wang,Tao Wang,Men Xia,Yiheng Liang,Hui Yun,Yan Wang,Chuan Yu,Yaru Wang,Zhe Wang

doi:10.5194/acp-20-391-2020

Abstract

Abstract. To investigate the cloud water chemistry and the effects of cloud processing on aerosol properties, comprehensive field observations of cloud water, aerosols, and gas-phase species were conducted at a mountaintop site in Hong Kong SAR in October and November 2016. The chemical composition of cloud water including water-soluble ions, dissolved organic matter (DOM), carbonyl compounds (refer to aldehydes and acetone), carboxylic acids, and trace metals was quantified. The measured cloud water was very acidic with a mean pH of 3.63, as the ammonium (174 µeq L−1) was insufficient for neutralizing the dominant sulfate (231 µeq L−1) and nitrate (160 µeq L−1). Substantial DOM (9.3 mgC L−1) was found in cloud water, with carbonyl compounds and carboxylic acids accounting for 18 % and 6 % in carbon molar concentrations, respectively. Different from previous observations, concentrations of methylglyoxal (19.1 µM; µM is equal to µmol L−1) and glyoxal (6.72 µM) were higher than that of formaldehyde (1.59 µM). The partitioning of carbonyls between cloud water and the gas phase was also investigated. The measured aqueous fractions of dicarbonyls were comparable to the theoretical estimations, while significant aqueous-phase supersaturation was found for less soluble monocarbonyls. Both organics and sulfate were significantly produced in cloud water, and the aqueous formation of organics was more enhanced by photochemistry and under less acidic conditions. Moreover, elevated sulfate and organics were measured in the cloud-processed aerosols, and they were expected to contribute largely to the increase in droplet-mode aerosol mass fraction. This study demonstrates the significant role of clouds in altering the chemical compositions and physical properties of aerosols via scavenging and aqueous chemical processing, providing valuable information about gas–cloud–aerosol interactions in subtropical and coastal regions.

Highlights

Clouds in the troposphere play a key role in atmospheric aqueous-phase chemistry by acting as efficient media for the in-cloud formation of sulfate and secondary organic aerosol (SOA) (Harris et al, 2013; Ervens, 2015)
A total of 32 cloud water samples in six cloud events were collected at Tai Mo Shan (TMS) in Hong Kong during the campaign (Fig. S1 in the Supplement)
The abundant Cl− in cloud water can be ascribed to potential anthropogenic chloride sources in the Pearl River Delta (PRD) region (Wang et al, 2016)

Summary

Introduction

Clouds in the troposphere play a key role in atmospheric aqueous-phase chemistry by acting as efficient media for the in-cloud formation of sulfate and secondary organic aerosol (SOA) (Harris et al, 2013; Ervens, 2015). Numerous studies on cloud and fog chemistry have been conducted in Europe and North America since the 1990s (Collett et al, 2002; Ervens, 2015; van Pinxteren et al, 2016). Studies of the compositions of cloud and fog water, cloud scavenging, and aqueous-phase reactions have been carried out in Asia, in China and Japan (Aikawa et al, 2007; Guo et al, 2012). Li et al.: Chemical characteristics of cloud water et al, 2008; Herckes et al, 2013), because of the significant contribution of chemical cloud processing to the formation of aqueous SOA (aqSOA) in high-humidity environments (Ervens et al, 2011; Huang et al, 2011; Tomaz et al, 2018)

Methods

Results

Conclusion