Constraining Predicted Secondary Organic Aerosol Formation and Processing Using Real-Time Observations of Aging Urban Emissions in an Oxidation Flow Reactor

Tuesday, 16 December 2014: 5:45 PM
Amber M Ortega1,2, Brett B Palm1,2, Patrick L Hayes1,2, Douglas A Day2,3, Michael Cubison2,4, William H Brune5, Weiwei Hu1,2, Martin Graus6,7, Carsten Warneke2,7, Jessica Gilman2,7, Joost A De Gouw2,7 and Jose L Jimenez1,2, (1)University of Colorado at Boulder, Boulder, CO, United States, (2)Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States, (3)University of Colorado at Boulder, Dept. of Chemistry and Biochemistry, Boulder, CO, United States, (4)Tofwerk AG, Thun, Switzerland, (5)Pennsylvania State University Main Campus, University Park, PA, United States, (6)Universität Innsbruck, Institut für Meteorologie und Geophysik, Innsbruck, Austria, (7)NOAA Earth System Research Lab, Boulder, CO, United States
To investigate atmospheric processing of urban emissions, we deployed an oxidation flow reactor with measurements of size-resolved chemical composition of submicron aerosol during CalNex-LA, a field study investigating air quality and climate change at a receptor site in the Los Angeles Basin. The reactor produces OH concentrations up to 4 orders of magnitude higher than in ambient air, achieving equivalent atmospheric aging of hours to ~2 weeks in 5 minutes of processing. The OH exposure (OHexp) was stepped every 20 min to survey the effects of a range of oxidation exposures on gases and aerosols. This approach is a valuable tool for in-situ evaluation of changes in organic aerosol (OA) concentration and composition due to photochemical processing over a range of ambient atmospheric conditions and composition. Combined with collocated gas-phase measurements of volatile organic compounds, this novel approach enables the comparison of measured SOA to predicted SOA formation from a prescribed set of precursors.

Results from CalNex-LA show enhancements of OA and inorganic aerosol from gas-phase precursors. The OA mass enhancement from aging was highest at night and correlated with trimethylbenzene, indicating the importance of relatively short-lived VOC (OH lifetime of ~12 hrs or less) as SOA precursors in the LA Basin. Maximum net SOA production is observed between 3-6 days of aging and decreases at higher exposures. Aging in the reactor shows similar behavior to atmospheric processing; the elemental composition of ambient and reactor measurements follow similar slopes when plotted in a Van Krevelen diagram. Additionally, for air processed in the reactor, oxygen-to-carbon ratios (O/C) of aerosol extended over a larger range compared to ambient aerosol observed in the LA Basin. While reactor aging always increases O/C, often beyond maximum observed ambient levels, a transition from net OA production to destruction occurs at intermediate OHexp, suggesting a transition from functionalization/condensation at low-to-moderate OHexp to fragmentation/evaporation dominating at very high OHexp. A traditional SOA model with mostly aromatic precursors underpredicts the amount of SOA formed in the reactor by an order-of-magnitude, which is consistent with model evaluations for ambient air at many polluted locations.