Journal cover Journal topic
Atmospheric Chemistry and Physics An interactive open-access journal of the European Geosciences Union
Atmos. Chem. Phys., 16, 7411-7433, 2016
https://doi.org/10.5194/acp-16-7411-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
15 Jun 2016
Real-time measurements of secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor in the Los Angeles area
Amber M. Ortega1,2, Patrick L. Hayes3, Zhe Peng1,4, Brett B. Palm1,4, Weiwei Hu1,4, Douglas A. Day1,4, Rui Li1,2,5,a, Michael J. Cubison1,4,b, William H. Brune6, Martin Graus1,5,c, Carsten Warneke1,5, Jessica B. Gilman1,5, William C. Kuster1,5,*, Joost de Gouw1,5, Cándido Gutiérrez-Montes7, and Jose L. Jimenez1,4 1Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA
2Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA
3Department of Chemistry, Université de Montréal, Montréal, Québec, Canada
4Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA
5Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA
6Department of Meteorology, Pennsylvania State University, University Park, PA, USA
7Departamento de Ingeniería, Mecánica y Minera, Universidad de Jaen, Jaen, Spain
anow at Markes International Inc., Cincinnati, OH 45242, USA
bnow at: Tofwerk AG, Thun, Switzerland
cnow at: Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria
*retired
Abstract. Field studies in polluted areas over the last decade have observed large formation of secondary organic aerosol (SOA) that is often poorly captured by models. The study of SOA formation using ambient data is often confounded by the effects of advection, vertical mixing, emissions, and variable degrees of photochemical aging. An oxidation flow reactor (OFR) was deployed to study SOA formation in real-time during the California Research at the Nexus of Air Quality and Climate Change (CalNex) campaign in Pasadena, CA, in 2010. A high-resolution aerosol mass spectrometer (AMS) and a scanning mobility particle sizer (SMPS) alternated sampling ambient and reactor-aged air. The reactor produced OH concentrations up to 4 orders of magnitude higher than in ambient air. OH radical concentration was continuously stepped, achieving equivalent atmospheric aging of 0.8 days–6.4 weeks in 3 min of processing every 2 h. Enhancement of organic aerosol (OA) from aging showed a maximum net SOA production between 0.8–6 days of aging with net OA mass loss beyond 2 weeks. Reactor SOA mass peaked at night, in the absence of ambient photochemistry and correlated with trimethylbenzene concentrations. Reactor SOA formation was inversely correlated with ambient SOA and Ox, which along with the short-lived volatile organic compound correlation, indicates the importance of very reactive (τOH  ∼  0.3 day) SOA precursors (most likely semivolatile and intermediate volatility species, S/IVOCs) in the Greater Los Angeles Area. Evolution of the elemental composition in the reactor was similar to trends observed in the atmosphere (O : C vs. H : C slope  ∼  −0.65). Oxidation state of carbon (OSc) in reactor SOA increased steeply with age and remained elevated (OSC  ∼  2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (ΔCO, ambient CO above regional background) vs. photochemical age is similar to previous studies at low to moderate ages and also extends to higher ages where OA loss dominates. The mass added at low-to-intermediate ages is due primarily to condensation of oxidized species, not heterogeneous oxidation. The OA decrease at high photochemical ages is dominated by heterogeneous oxidation followed by fragmentation/evaporation. A comparison of urban SOA formation in this study with a similar study of vehicle SOA in a tunnel suggests the importance of vehicle emissions for urban SOA. Pre-2007 SOA models underpredict SOA formation by an order of magnitude, while a more recent model performs better but overpredicts at higher ages. These results demonstrate the value of the reactor as a tool for in situ evaluation of the SOA formation potential and OA evolution from ambient air.

Citation: Ortega, A. M., Hayes, P. L., Peng, Z., Palm, B. B., Hu, W., Day, D. A., Li, R., Cubison, M. J., Brune, W. H., Graus, M., Warneke, C., Gilman, J. B., Kuster, W. C., de Gouw, J., Gutiérrez-Montes, C., and Jimenez, J. L.: Real-time measurements of secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor in the Los Angeles area, Atmos. Chem. Phys., 16, 7411-7433, https://doi.org/10.5194/acp-16-7411-2016, 2016.
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Short summary
An oxidation flow reactor (OFR) was deployed to study secondary organic aerosol (SOA) formation and aging of urban emissions at a wide range of OH exposures during the CalNex campaign in Pasadena, CA, in 2010. Results include linking SOA formation to short-lived reactive compounds, similar elemental composition of reactor-aged emissions to atmospheric aging, changes in OA mass due to condensation of oxidized gas-phase species and heterogeneous oxidation of particle-phase species.
An oxidation flow reactor (OFR) was deployed to study secondary organic aerosol (SOA) formation...
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