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| Titel |
The Pasadena Aerosol Characterization Observatory (PACO): chemical and physical analysis of the Western Los Angeles basin aerosol |
| VerfasserIn |
S. P. Hersey, J. S. Craven, K. A. Schilling, A. R. Metcalf, A. Sorooshian, M. N. Chan, R. C. Flagan, J. H. Seinfeld |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 11, no. 15 ; Nr. 11, no. 15 (2011-08-01), S.7417-7443 |
| Datensatznummer |
250009954
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| Publikation (Nr.) |
 copernicus.org/acp-11-7417-2011.pdf |
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| Zusammenfassung |
| The Pasadena Aerosol Characterization Observatory (PACO) represents
the first major aerosol characterization experiment centered in the
Western/Central Los Angeles Basin. The sampling site, located on the campus of
the California Institute of Technology in Pasadena, was positioned to
sample a continuous afternoon influx of transported urban aerosol with
a photochemical age of 1–2 h and generally free from major local
contributions. Sampling spanned 5 months during the summer of 2009,
which were broken into 3 regimes on the basis of distinct
meteorological conditions. Regime I was characterized by a series of
low pressure systems, resulting in high humidity and rainy periods
with clean conditions. Regime II typified early summer meteorology,
with significant morning marine layers and warm, sunny
afternoons. Regime III was characterized by hot, dry conditions with
little marine layer influence. Regardless of regime, organic aerosol (OA) is the most significant constituent of nonrefractory submicron Los
Angeles aerosol (42, 43, and 55 % of total submicron mass in regimes
I, II, and III, respectively). The overall oxidation state
remains relatively constant on timescales of days to weeks
(O:C = 0.44 ± 0.08, 0.55 ± 0.05, and 0.48 ± 0.08
during regimes I, II, and III, respectively), with no difference in
O:C between morning and afternoon periods. Periods characterized
by significant morning marine layer influence followed by
photochemically favorable afternoons displayed significantly higher
aerosol mass and O:C ratio, suggesting that aqueous processes may
be important in the generation of secondary aerosol and oxidized
organic aerosol (OOA) in Los Angeles. Online analysis of water soluble organic carbon (WSOC) indicates that water soluble organic mass (WSOM) reaches maxima near
14:00–15:00 local time (LT), but the percentage of AMS organic mass
contributed by WSOM remains relatively constant throughout the
day. Sulfate and nitrate reside predominantly in accumulation mode
aerosol, while afternoon SOA production coincides with the appearance
of a distinct fine mode dominated by organics. Particulate
NH4NO3 and (NH4)2SO4 appear to be
NH3-limited in regimes I and II, but a significant excess of
particulate NH4+ in the hot, dry regime III suggests
less SO42− and the presence of either organic
amines or NH4+-associated organic acids. C-ToF-AMS data were analyzed by Positive Matrix Factorization (PMF), which
resolved three factors, corresponding to a hydrocarbon-like OA (HOA),
semivolatile OOA (SV-OOA), and low-volatility OOA (LV-OOA). HOA
appears to be a periodic plume source, while SV-OOA exhibits a strong
diurnal pattern correlating with ozone. Peaks in SV-OOA concentration
correspond to peaks in DMA number concentration and the appearance of
a fine organic mode. LV-OOA appears to be an aged accumulation mode
constituent that may be associated with aqueous-phase processing,
correlating strongly with sulfate and representing the dominant
background organic component. Periods characterized by high SV-OOA and LV-OOA were analyzed by filter analysis, revealing a complex mixture of species during periods dominated by SV-OOA and LV-OOA, with LV-OOA periods characterized by
shorter-chain dicarboxylic acids (higher O:C ratio), as well as
appreciable amounts of nitrate- and sulfate-substituted
organics. Phthalic acid was ubiquitous in filter samples, suggesting
that PAH photochemistry may be an important SOA pathway in Los
Angeles. Aerosol composition was related to water uptake characteristics, and it is concluded that hygroscopicity is largely
controlled by organic mass fraction (OMF). The hygroscopicity parameter
κ averaged 0.31 ± 0.08, approaching 0.5 at low OMF and 0.1 at high OMF,
with increasing OMF suppressing hygroscopic growth and increasing critical dry
diameter for CCN activation (Dd). An experiment-averaged κorg of
0.14 was calculated, indicating that the highly-oxidized organic fraction of
aerosol in Los Angeles is appreciably more hygroscopic than previously reported
in urban areas. Finally, PACO will provide context for results forthcoming from
the CalNex field campaign, which involved ground sampling in Pasadena during
the spring and summer of 2010. |
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