real-time measurements of secondary organic aerosol formation and aging from ambient air in an oxidation flow reactor in the los angeles area
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ID: 205448
2016
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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.
| Reference Key |
ortega2016atmosphericreal-time
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| Authors | ;A. M. Ortega;A. M. Ortega;P. L. Hayes;Z. Peng;Z. Peng;B. B. Palm;B. B. Palm;W. Hu;W. Hu;D. A. Day;D. A. Day;R. Li;R. Li;R. Li;R. Li;M. J. Cubison;M. J. Cubison;M. J. Cubison;W. H. Brune;M. Graus;M. Graus;M. Graus;C. Warneke;C. Warneke;J. B. Gilman;J. B. Gilman;W. C. Kuster;W. C. Kuster;W. C. Kuster;J. de Gouw;J. de Gouw;C. Gutiérrez-Montes;J. L. Jimenez;J. L. Jimenez |
| Journal | Journal of agricultural and food chemistry |
| Year | 2016 |
| DOI |
10.5194/acp-16-7411-2016
|
| URL | |
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