Document Type

Article

Publication Date

2016

Publication Title

Atmospheric Chemistry and Physics

ISSN

1680-7324

Volume

16

Issue

11

DOI

http://dx.doi.org/10.5194/acp-16-7411-2016

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 3aEuro-min of processing every 2aEuro-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 2aEuro-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 O-x, which along with the short-lived volatile organic compound correlation, indicates the importance of very reactive (tau(OH)aEuro- aEuro-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 (OaEuro-:aEuro-C vs. HaEuro-:aEuro-C slope aEuro--0.65). Oxidation state of carbon (OSc) in reactor SOA increased steeply with age and remained elevated (OS(C)aEuro- aEuro-2) at the highest photochemical ages probed. The ratio of OA in the reactor output to excess CO (Delta CO, 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.

Comments

Authors

A. M. Ortega (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
A. M. Ortega (Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA)
P. L. Hayes (Department of Chemistry, Université de Montréal, Montréal, Québec, Canada)
Z. Peng (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
Z. Peng (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)
B. B. Palm (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
B. B. Palm (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)
W. Hu (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
W. Hu (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)
D. A. Day (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
D. A. Day (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)
R. Li (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
R. Li (Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA)
R. Li (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
R. Li (now at Markes International Inc., Cincinnati, OH 45242, USA)
M. J. Cubison (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
M. J. Cubison (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)
M. J. Cubison (now at: Tofwerk AG, Thun, Switzerland)
W. H. Brune (Department of Meteorology, Pennsylvania State University, University Park, PA, USA)
M. Graus (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
M. Graus (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
M. Graus (now at: Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria)
C. Warneke (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
C. Warneke (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
J. B. Gilman (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
J. B. Gilman (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
W. C. Kuster (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
W. C. Kuster (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
W. C. Kuster (retired)
J. de Gouw (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
J. de Gouw (Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA)
C. Gutiérrez-Montes (Departamento de Ingeniería, Mecánica y Minera, Universidad de Jaen, Jaen, Spain)
J. L. Jimenez (Cooperative Institute for Research in the Environmental Sciences, University of Colorado, Boulder, CO, USA)
J. L. Jimenez (Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO USA)

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