Laboratory studies of the chemical composition and cloud condensation nuclei (CCN) activity of secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA)

Abstract

Secondary organic aerosol (SOA) and oxidized primary organic aerosol (OPOA) were produced in laboratory experiments from the oxidation of fourteen precursors representing atmospherically relevant biogenic and anthropogenic sources. The SOA and OPOA particles were generated via controlled exposure of precursors to OH radicals and/or O<sub>3</sub> in a Potential Aerosol Mass (PAM) flow reactor over timescales equivalent to 1–20 days of atmospheric aging. Aerosol mass spectra of SOA and OPOA were measured with an Aerodyne aerosol mass spectrometer (AMS). The fraction of AMS signal at <i>m/z</i> = 43 and <i>m/z</i> = 44 (<i>f</i><sub>43</sub>, <i>f</i><sub>44</sub>), the hydrogen-to-carbon (H/C) ratio, and the oxygen-to-carbon (O/C) ratio of the SOA and OPOA were obtained, which are commonly used to characterize the level of oxidation of oxygenated organic aerosol (OOA). The results show that PAM-generated SOA and OPOA can reproduce and extend the observed <i>f</i><sub>44</sub>–<i>f</i><sub>43</sub> composition beyond that of ambient OOA as measured by an AMS. Van Krevelen diagrams showing H/C ratio as a function of O/C ratio suggest an oxidation mechanism involving formation of carboxylic acids concurrent with fragmentation of carbon-carbon bonds. Cloud condensation nuclei (CCN) activity of PAM-generated SOA and OPOA was measured as a function of OH exposure and characterized as a function of O/C ratio. CCN activity of the SOA and OPOA, which was characterized in the form of the hygroscopicity parameter κ<sub>org</sub>, ranged from 0.003 to 0.28 over measured O/C ratios ranging from 0.05 to 1.42. This range of κ<sub>org</sub> and O/C ratio is significantly wider that has been previously obtained. To first order, the κ<sub>org</sub>-to-O/C relationship is well represented by a linear function of the form κ<sub>org</sub> = (0.17 ± 0.04) × O/C + 0.04, suggesting that a simple, semi-empirical parameterization of OOA hygroscopicity and oxidation level can be defined for use in chemistry and climate models

Chemical aging of single and multicomponent biomass burning aerosol surrogate particles by OH: implications for cloud condensation nucleus activity

“…Further exposure (≥ 4 × 10 11 molecules cm −3 s −1 ) does not significantly enhance MNC κ, which suggests that MNC or the particle surface is fully oxidized (Slade and Knopf, 2014) and that the reaction products reach a maximum in κ. Similar enhancements in κ and subsequent constant κ values with increasing OH exposure have been observed for organic aerosol with initially low hygroscopicity Lambe et al, 2011). For example, George et al (2009) observed that κ of BES increased from ∼ 0.008 to ∼ 0.08 for an OH exposure of ∼ 1.5 × 10 12 molecule cm −3 s, and κ of stearic acid increased from ∼ 0.004 to ∼ 0.04 due to an OH exposure of ∼ 7.5 × 10 11 molecule cm −3 s. The enhancement in MNC κ following OH exposure may be linked to the formation of more hydrophilic chemical functionalities.…”

“…For example, George et al (2009) observed that κ of BES increased from ∼ 0.008 to ∼ 0.08 for an OH exposure of ∼ 1.5 × 10 12 molecule cm −3 s, and κ of stearic acid increased from ∼ 0.004 to ∼ 0.04 due to an OH exposure of ∼ 7.5 × 10 11 molecule cm −3 s. The enhancement in MNC κ following OH exposure may be linked to the formation of more hydrophilic chemical functionalities. Strongly linked to enhancements in OA hygroscopicity are larger O : C ratios (Massoli et al, 2010;Lambe et al, 2011;Mei et al, 2013a, b;Suda et al, 2014). Neglecting the oxygen atoms in the -nitro functionality of MNC (Suda et al, 2014), the O : C ratio of pure MNC is ∼ 0.29, close to the lower end in O : C where transitions from low κ to high κ typically occur (Suda et al, 2014).…”

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

“…For T1 and T3, reductions in Δ -2 , Δ 0 , and Δ 6 , which may be partially attributed to alcohols (Table S3), suggests at least that alcohol addition does not exceed consumption of unsaturated hydrocarbons and carbonyls (at Δ 0 , Δ -2 ), and implies that carboxylic addition with C-C fragmentation is a more important reaction pathway for this dataset; this fragmentation may also contribute to the organic mass loss observed if the non-functionalized fragments are volatile. These slopes are similar to gas-phase chamber oxidations of both generated and ambient OA Lambe et al, 2011;Ng et al, 2011).…”

Prediction of cloud condensation nuclei activity for organic compounds using functional group contribution methods

“…Laboratory (George and Abbatt, 2010;Poulain et al, 2010;Cappa et al, 2011;Massoli et al, 2010;Lambe et al, 2011;Duplissy et al, 2011;Kuwata et al, 2013;Rickards et al, 2013;Suda et al, 2014) and field studies (Jimenez et al, 2009;Chang et al, 2010;Mei et al, 2013) have demonstrated a robust link between the aerosol oxidation state and the ability of the organic fraction to promote hygroscopic water uptake and CCN activity. Proxies from mass spectrometry such as the fragmentation peak f 44 or the atomic oxygento-carbon ratio are often used to model the increase in hygroscopicity.…”