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Chemical Composition of Isoprene SOA Under Acidic and Non-Acidic Conditions: Effect of Relative Humidity
Citation:
Nestorowicz, K., M. Jaoui, K. Rudzinski, M. Lewandowski, Tad Kleindienst, G. Spolnik, W. Danikiewicz, AND R. Szmigielski. Chemical Composition of Isoprene SOA Under Acidic and Non-Acidic Conditions: Effect of Relative Humidity. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 18(4):18101-18121, (2018). https://doi.org/10.5194/acp-18-18101-2018
Impact/Purpose:
Secondary organic aerosol (SOA), which is formed through complex physico-chemical reactions of volatile organic compounds (VOCs) emitted into the atmosphere from biogenic and anthropogenic sources, constitutes a great part of the continental aerosol mass (Guenther et al., 1995; Goldstein and Galbally, 2007). Isoprene (ISO) is one of the most studied atmospheric hydrocarbons in ambient SOA formation because it has been found to be the most abundant non-methane hydrocarbon emitted to the atmosphere and the most effective contributor to ambient SOA among the biogenic hydrocarbons (Guenther et al., 1995; Henze and Seinfeld, 2006; Fu et al., 2008; Carlton et al., 2009; Hallquist et al., 2009). The primary removal of ISO in the atmosphere is through the gas-phase reactions with hydroxyl radicals (OH), nitrate radicals (NO3) and ozone (O3) which result in the formation of numerous oxidized SOA components, including 2-methyltetrols, 2-methylglyceric acid, C5-alkene triols and C4/C5 organosulfates (OSs). These compounds were identified in ambient PM2.5 (particulate matter with diameter < 2.5 μm) in several places around the world while SOA generated from isoprene was reported to account up to 20 – 50% of the overall SOA budget (Clayes et al., 2004a; Wang et al., 2005; Henze and Seinfeld, 2006; Kroll et al., 2006; Surratt et al., 2006; Hoyle et al., 2007).
Description:
The effect of acidity and relative humidity on bulk isoprene aerosol parameters has been investigated in several studies; however, few measurements have been conducted on individual aerosol compounds. The focus of this study has been the examination of the effect of acidity and relative humidity on secondary organic aerosol (SOA) chemical composition from isoprene photooxidation in the presence of nitrogen oxide (NOx). A detailed characterization of SOA at the molecular level was also investigated. Experiments were conducted in a 14.5 m3 smog chamber operated in flow mode. Based on a detailed analysis of mass spectra obtained from gas chromatography–mass spectrometry of silylated derivatives in electron impact and chemical ionization modes, ultra-high performance liquid chromatography/electrospray ionization/time-of-flight high-resolution mass spectrometry, and collision-induced dissociation in the negative ionization modes, we characterized not only typical isoprene products but also new oxygenated compounds. A series of nitroxy-organosulfates (NOSs) were tentatively identified on the basis of high-resolution mass spectra. Under acidic conditions, the major identified compounds include 2-methyltetrols (2MT), 2-methylglyceric acid (2mGA), and 2MT-OS. Other products identified include epoxydiols, mono- and dicarboxylic acids, other organic sulfates, and nitroxy- and nitrosoxy-OS. The contribution of SOA products from isoprene oxidation to PM2.5 was investigated by analyzing ambient aerosol collected at rural sites in Poland. Methyltetrols, 2mGA, and several organosulfates and nitroxy-OS were detected in both the field and laboratory samples. The influence of relative humidity on SOA formation was modest in non-acidic-seed experiments and stronger under acidic seed aerosol. Total secondary organic carbon decreased with increasing relative humidity under both acidic and non-acidic conditions. While the yields of some of the specific organic compounds decreased with increasing relative humidity, others varied in an indeterminate manner from changes in the relative humidity.
URLs/Downloads:
DOI: Chemical Composition of Isoprene SOA Under Acidic and Non-Acidic Conditions: Effect of Relative Humidity