Chemical composition of isoprene SOA under acidic and non-acidic conditions: effect of relative humidity

Abstract

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 (NO x ). A detailed characterization of SOA at the molecular level was also investigated. Experiments were conducted in a 14.5 m 3 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 highresolution 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 PM 2.5 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.

Strong anthropogenic control of secondary organic aerosol formation from isoprene in Beijing

“…The identity of the OS at m/z 182 (C5H10SO5) is currently unknown and the product ion MS 475 provides little additional information other than sulfate-related fragment ions at m/z 97 and m/z 80. The OS at m/z 198 (C5H10SO6) was identified as an IEPOX-related OS in chamber experiments by Nestorowicz et al (2018), but at relatively low concentrations compared to the 2-MT-OS (1-4 %). This is very different to the observed ratio is Beijing, where the C5H10SO6 average concentration was more than double that of 2-MT-OS, as shown in Figure 4c.…”

“…A strong linear relationship was observed between the 2-MG-OS monomer and dimer concentrations (R 2 =0.83) with a dimer:monomer ratio of 0.02. Formation of oligomers from reactions of 2-MG and HMML has been shown to be reduced in chamber experiments under humid conditions (Schwantes et al, 2019;Nestorowicz et al, 2018). The average RH during the afternoon of the campaign was ~40 %, which may 460 account for the relatively low formation of the dimer OS compared to the monomer (see SI Figure 9).…”

Organosulfates in Atlanta, Georgia: anthropogenic influences on biogenic secondary organic aerosol formation

“…A. P. S. Hettiyadura et al: Organosulfates in Atlanta, Georgia fate radicals (Nozière et al, 2010;Schindelka et al, 2013) and nucleophilic substitution of nitrate groups by sulfate (Darer et al, 2011;Hu et al, 2011). Biogenic VOC precursors of organosulfates include isoprene, monoterpenes, sesquiterpenes, 2-methyl-3-butene-2-ol (MBO), and green leaf volatiles (Zhang et al, 2012;Surratt et al, 2008;Chan et al, 2011;Iinuma et al, 2009;Shalamzari et al, 2014).…”

A biogenic secondary organic aerosol source of cirrus ice nucleating particles

“…The fraction of ice-active 2-MTs, 2-MT OSs, and IEPOX particles at −46°C scales with increasing ice supersaturation (Fig. 4) 29,30,48 . Fractional activation within the deposition freezing regime falls within the range found in other studies on SOA and organic analogs 19,21,49,50 .…”

“…The average aerosol mass loadings during these sampling measurements were quantified for eight SOA species. Components of isoprene-derived SOA [28][29][30][31] included first-generation multiphase chemical products of isoprene epoxydiols (IEPOX), specifically 2methyltetrols (2-MTs, C 5 H 12 O 4 ), 2-methyltetrol organosulfates (2-MT OSs, C 5 H 12 O 7 S) 32 , as well as heterogeneous hydroxyl (OH) radical oxidation products of particulate 2-MT OSs 33 Table 3; Supplementary Fig. 4).…”