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

Abstract

<p><strong>Abstract.</strong> To investigate the cloud water chemistry and the effects of cloud processes on aerosol properties, comprehensive field observations of cloud water, aerosols, and gas-phase species were conducted at a mountaintop site in Hong Kong in October and November 2016. The chemical composition of cloud water including water-soluble ions, dissolved organic matter (DOM), carbonyl compounds, 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<sup>−1</sup>) was insufficient for neutralizing the dominant sulfate (230 μeq L<sup>−1</sup>) and nitrate (160 μeq L<sup>−1</sup>). Substantial DOM was found in cloud water, with carbonyl compounds and carboxylic acids accounting for 18.2 % and 5.6 %, respectively. Different from previous observations, concentrations of methylglyoxal (19.1 μM) 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, suggesting complicated effects of both physical and chemical processes. In-cloud oxidation played an important role in increasing DOM and sulfate in the cloud water. Abundant glyoxal is suggested to be the most likely precursor of cloud water organics. The aqueous formation of organics was enhanced by photochemistry and under less-acidic conditions. Moreover, as a result of the cloud processes, DOM mass fractions were found to be significantly elevated in in-cloud aerosols, which was likely to contribute to the increase in droplet-mode mass fraction of cloud processed aerosols. This study demonstrates the significant role of clouds in altering the chemical composition and physical properties of aerosols via scavenging and aqueous processes, and provides valuable information about aerosol–cloud interactions in subtropical and coastal regions.</p>

Different Formation Pathways of Nitrogen-containing Organic Compounds in Aerosols and Fog Water in Northern China

“…Although it is virtually impossible to identify the functional groups in the formula list obtained by FT-ICR MS, carbonyls have been widely detected in fog and cloud water (Ervens et al, 2013;Van Pinxteren et al, 2005). More importantly, a high LWC in cloud water may promote the dissolution of some carbonyl compounds (e.g., methylglyoxal) from gas phase and increase their aqueous concentrations (Li et al, 2020b). The enhanced formation of imidazole, a typical product of carbonyls and ammonium/amine, was also observed in cloud droplets in southern China (Lian et al, 2021).…”

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