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

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.

Formation and sink of glyoxal and methylglyoxal in a polluted subtropical environment: observation-based photochemical analysis and impact evaluation

“…NO and NO 2 were detected using a chemiluminescence instrument (Thermo Scientific 42i) with a photolytic converter (Xu et al, 2013). The method detection limits for non-methane hydrocarbons (NMHCs), carbonyls, CO, SO 2 , O 3 , NO and NO 2 were 20-300, 20-450, 4000, 100, 500, 60 and 300 pptv, respectively (Yun et al, 2018;Chang et al, 2019;Li et al, 2020). The measurement method uncertainty in the retrieved Gly and Mgly mixing ratios was estimated around 15 % (Chang et al, 2019;Li et al, 2020).…”

“…The method detection limits for non-methane hydrocarbons (NMHCs), carbonyls, CO, SO 2 , O 3 , NO and NO 2 were 20-300, 20-450, 4000, 100, 500, 60 and 300 pptv, respectively (Yun et al, 2018;Chang et al, 2019;Li et al, 2020). The measurement method uncertainty in the retrieved Gly and Mgly mixing ratios was estimated around 15 % (Chang et al, 2019;Li et al, 2020). Furthermore, hourly meteorological parameters including temperature, wind speed, wind direction, pressure and relative humidity were recorded using a pyranometer (CMP22, Kipp & Zonen B.V., Holland) and a portable weather station (model WXT520, Vaisala, Finland).…”

Partitioning of hydrogen peroxide in gas-liquid and gas-aerosol phases

“…However, it is noteworthy that the predicted liquid-phase concentration of H 2 O 2 in rainwater using Henry's law was not enough to account for the measured level, and a large amount of liquid-phase H 2 O 2 was produced from other reactions (Liang et al, 2013). Chung et al (2005) demonstrated that the salting-in effect could double the solubility of H 2 O 2 in salt solutions with concentrations up to 10 M, but the ionic strength in rainwater was too low to impose the salting-in effect (Li et al, 2020).…”

Insights into tropical cloud chemistry at Reunion Island (Indian Ocean): results from the BIO-MAÏDO campaign

“…Since the late 1990s, substantial efforts have been made to further understand the chemical composition and reactivity of dissolved matter in cloud droplets through in situ measurements and laboratory investigations (Aleksic et al, 2009;Bianco et al, 2017;Brege 70 et al, 2018;Brüggemann et al, 2005;Cini et al, 2002;Deguillaume et al, 2014;Fomba et al, 2015;Gilardoni et al, 2014;Gioda et al, 2013;Herckes et al, 2013Herckes et al, , 2002Lee et al, 2012;Li et al, 2020;van Pinxteren et al, , 2020Renard et al, 2020;Vaïtilingom et al, 2013;Viana et al, 2014). Some of these studies were focused on the role of clouds in the lifecycle of inorganic compounds (i.e., ions, metals, and oxidants).…”

“…Given the complexity and the evolving nature of clouds, their measurement and chemical characterization represent a challenge and have resulted in significant uncertainties in our understanding of formation/evolution processes, chemical composition, fate, as well as potential impacts. Some well-established measurement observatories located at high altitudes have been carried out in numerous studies about cloud chemical composition, such as puy de Dôme (PUY) observatory in France (Bianco et al, 2017;Deguillaume et 100 al., 2014;Renard et al, 2020;Vaïtilingom et al, 2013;, Schmücke Mountain in Germany (Brüggemann et al, 2005;Roth et al, 2016;Whalley et al, 2015), East Peak in Puerto Rico (Gioda et al, 2013) , Mount Tai in China (Li et al, 2017;Liu et al, 2012a;Shen et al, 2012), Whiteface Mountain New York (Aleksic et al, 2009;Dukett et al, 2011;Lance et al, 2020), Po Valley (Brege et al, 2018;Gilardoni et al, 2014) and Tai Mo Shan in Hong Kong SAR (Li et al, 2020). Nevertheless, most of the aforementioned observatories are located in the mid-northern latitudes 105 with significant influence of continental emissions from natural and anthropogenic origins.…”