Aqueous Secondary Organic Aerosol Formation in Ambient Cloud Water Photo-Oxidations

Abstract

The current understanding of aqueous secondary organic aerosol (aqSOA) formation is based largely on laboratory investigations of very simple surrogate cloud water solutions that aid mechanistic understanding of aqueous oxidation but may not accurately reflect the influence of the complex ambient matrix present in authentic cloud waters on organic chemistry. In this study, unaltered ambient cloud water and 'biogenically influenced' ambient cloud water (with added pinonic acid) were photo-oxidized, atomized, and dried to simulate the formation of aqSOA in clouds, then analyzed using an Aerodyne Aerosol Mass Spectrometer. Two major chemical regimes were identified: in the first, particle organic mass is gained, then lost; sustained increases in highly oxidized fragments indicate overall organic acid formation, while increases in nominally volatile fragments suggest that evaporation may contribute to the observed mass decrease. In the second regime, the oxidation level of cloud water organic matter decreases as mass decreases, suggesting that oxidized functional groups are fragmented and lost to evaporation. Overall, the rate of aqSOA production in unaltered cloud water decreases as oxygenation increases, until organic mass loss beginning at consistent values of f44 > 0.23 ± 0.05 and O:C > 0.61 ± 0.05. We hypothesize that there may be a parameterizable 'maximum oxidation level' for cloud water above which functional group fragmentation is dominant. These experiments are among the first to quantify organic mass production in ambient cloud water and employ the most atmospherically relevant oxidant concentrations to date.

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

“…Although the DOM also showed higher concentration in lower pH conditions, the DOM / SO 2− 4 ratios clearly decreased at a lower pH range. It is well known that in-cloud oxidation of S(IV) by H 2 O 2 is the predominant pathway for sulfate formation at pH < 5, within which the oxidation rate is independent of pH (Seinfeld and Pandis, 2006;Shen et al, 2012). The reduced DOM / SO 2− 4 ratios with pH suggest that DOM production was reduced compared to the sulfate in the more acidic condition.…”

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

“…The DOM/SO 4 2ratios reduced under more acidic conditions (lower pH values due to increased sulfate). As the predominant pathway of in-cloud sulfate oxidation by H 2 O 2 in the pH range of 3-6 is independent of pH (Seinfeld and Pandis, 2006;Shen et al, 2012), the reduced DOM/SO 4 2ratios with lowered pH suggest that DOM production slowed, probably due https://doi.org/10.5194/acp-2019-481 Preprint. Discussion started: 3 June 2019 c Author(s) 2019.…”

“…It has been suggested that highly oxidized cloud water organics readily remain in evaporating cloud droplets and contribute to aqSOA mass production, whereas semi-volatile products are prone to escape into the gas phase (Schurman et al, 2018).…”

Isotopic constraints on the atmospheric sources and formation of nitrogenous species in clouds influenced by biomass burning

“…Globally, over 50 % of the NO x emissions derive from combustion of fossil fuels (∼ 25 Tg N yr −1 ; Jaegle et Richter et al, 2005;Duncan et al, 2016) with the remainder being primarily soil-related emissions (∼ 9 Tg N yr −1 ; Lamsal et al, 2011;Price et al, 1997;Yienger and Levy, 1995;Miyazaki et al, 2017) or deriving from biomass burning (∼ 6 Tg N yr −1 ) and lightning (2-6 Tg N yr −1 ) (Anenberg et al, 2017;Levy et al, 1996). The atmospheric sinks of NO x include the production of HNO 3(g) and the formation of aerosol NO − 3 (Seinfeld and Pandis, 2012), the partitioning of which can vary with time (Morino et al, 2006). As for NH 3 , over 90 % of the NH 3 emissions in terrestrial ecosystems originate from agricultural production, such as livestock breeding and NH 3 -based fertilizer application (Paulot and Jacobs, 2014; Kang et al, 2016;Reis et al, 2009;Bouwman et al, 1997;Heald et al, 2012;Balasubramanian et al, 2015;Huang et al, 2011).…”