Journal article

Characterization of submicron particles influenced by mixed biogenic and anthropogenic emissions using high-resolution aerosol mass spectrometry: Results from CARES

Setyan A, Zhang Q, Merkel M, Knighton W, Sun Y, Song C, Shilling J, Onasch T, Herndon S, Worsnop D, Fast J, Zaveri R, Berg L, Wiedensohler A, Flowers B, Dubey M, Subramanian R ...see all

Atmospheric Chemistry and Physics, vol. 12, issue 17 (2012) pp. 8131-8156

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An Aerodyne high resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative
Effects Study (CARES) that took place in northern California in June
2010. We present results obtained at Cool (denoted as the T1 site of the
project) in the foothills of the Sierra Nevada Mountains, where intense
biogenic emissions are periodically mixed with urban outflow transported
by daytime southwesterly winds from the Sacramento metropolitan area.
During this study, the average mass loading of submicrometer particles
(PM1) was 3.0 mu g m(-3), dominated by organics (80 %) and sulfate (9.9
%). The organic aerosol (OA) had a nominal formula of
C1H1.38N0.004O0.44, thus an average organic mass-to-carbon (OM/OC) ratio
of 1.70. Two distinct oxygenated OA factors were identified via Positive
matrix factorization (PMF) of the high-resolution mass spectra of
organics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a
surrogate for secondary OA (SOA) influenced by biogenic emissions
whereas the less oxidized LO-OOA (O/C=0.42) was found to represent SOA
formed in photochemically processed urban emissions. LO-OOA correlated
strongly with ozone and MO-OOA correlated well with two 1st generation
isoprene oxidation products (methacrolein and methyl vinyl ketone),
indicating that both SOAs were relatively fresh. A hydrocarbon like OA
(HOA) factor was also identified, representing primary emissions mainly
due to local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for
91% of the total OA mass and 72% of the PM1 mass observed at Cool.
Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool)
were identified using the Weather Research and Forecasting model coupled
with Chemistry (WRF-Chem). The average PM1 mass loading was considerably
higher in urban plumes than in air masses dominated by biogenic SOA. The
change in OA mass relative to CO (Delta OA/Delta CO) varied in the range
of 5-196 mu g m(-3) ppm(-1), reflecting large variability in SOA
production. The highest Delta OA/Delta CO was reached when air masses
were dominated by anthropogenic emissions in the presence of a high
concentration of biogenic volatile organic compounds (BVOCs). This
ratio, which was 97 mu g m(-3) ppm(-1) on average, was much higher than
when urban plumes arrived in a low BVOC environment (similar to 36 mu g
m(-3) ppm(-1)) or during other periods dominated by biogenic SOA (35 mu
g m(-3) ppm(-1)). These results demonstrate that SOA formation is
enhanced when anthropogenic emissions interact with biogenic precursors.

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  • Qi ZhangUniversity of California - Davis

  • A. Setyan

  • Q. Zhang

  • M. Merkel

  • W. B. Knighton

  • C. Song

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