Investigation of Secondary Organic Aerosol Formation from Different Sources and Ambient Environments

Ortega, Amber Marie

Graduate Thesis Or Dissertation

Investigation of Secondary Organic Aerosol Formation from Different Sources and Ambient Environments Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/jq085k01r

Abstract

To investigate atmospheric processing of direct urban and biomass burning emissions, I modified and deployed an oxidation flow reactor with submicron aerosol size and chemical composition measurements during FLAME-3, a biomass-burning study at USDA Fire Sciences Laboratory in Missoula, MT, and CalNex, a field study investigating the nexus of air quality and climate change at a receptor site in the LA-Basin at Pasadena, CA. The reactor produces OH concentrations up to 4 orders of magnitude higher than in ambient air, achieving aging by OH reactions in less than 5 minutes that is equivalent to ~2 weeks in the atmosphere. The OH exposure (OH_exp) was stepped every 20 min in both field studies. Results show the value of this approach as a tool for in-situ evaluation of changes in OA concentration and composition due to photochemical processing.

In FLAME-3, the average OA enhancement factor was 1.42 ± 0.36 of the initial POA. Reactive VOCs, such as toluene, monoterpenes, and acetaldehyde, decreased with increased OH_exp; however, formic acid, acetone, and some unidentified OVOCs increased after significant exposure. Net SOA formation in the photochemical reactor increased with OHₑₓp, typically peaking around 3 days of equivalent atmospheric photochemical age (OH_exp ~3.9×10¹¹ molecules cm^-3 s), then leveling off at higher exposures. Unlike other studies, no decrease in OA is observed at high exposure, likely due to lower max OH_exp in this study due to very high OH reactivity. The amount of additional OA mass added from aging is positively correlated with initial POA concentration, but not with the total VOC concentration or the concentration of known SOA precursors. The mass of SOA formed often exceeded the mass of the known VOC precursors, indicating the likely importance of primary semivolatile/intermediate volatility species, and possibly of unidentified VOCs as SOA precursors in biomass burning smoke.

Results from CalNex show enhancement of OA and inorganic aerosol from gas-phase precursors. The OA mass enhancement from aging was highest at night and correlated with trimethylbenzene concentrations, indicating the dominance of highly reactive VOC emissions as SOA precursors in the LA Basin. Aging in the reactor mimics atmospheric processing as the elemental composition of ambient and reactor measurements, when plotted in a Van Krevelen diagram, follow similar slopes; additionally, reactor measurements extend over a larger range of oxygen-to-carbon ratios (O/C) compared to that observed in the LA-Basin. While reactor aging always increases O/C, often beyond maximum ambient levels, we observe a transition from functionalization to fragmentation at intermediate OH_exp, with fragmentation dominating at very high OH_exp. Maximum net SOA production is observed between 0.8–4 days of aging and decreases at higher exposures. A traditional SOA model with mostly aromatic precursors underpredicts the amount of SOA formed by an order-of-magnitude, which is consistent with model evaluations for ambient air at many polluted locations.

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