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| Titel |
Aerosol characterization over the southeastern United States using high-resolution aerosol mass spectrometry: spatial and seasonal variation of aerosol composition and sources with a focus on organic nitrates |
| VerfasserIn |
L. Xu, S. Suresh, H. Guo, R. J. Weber, N. L. Ng |
| 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 ; 15, no. 13 ; Nr. 15, no. 13 (2015-07-08), S.7307-7336 |
| Datensatznummer |
250119873
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| Publikation (Nr.) |
 copernicus.org/acp-15-7307-2015.pdf |
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| Zusammenfassung |
We deployed a High-Resolution Time-of-Flight Aerosol Mass Spectrometer
(HR-ToF-AMS) and an Aerosol Chemical Speciation Monitor (ACSM) to
characterize the chemical composition of submicron non-refractory particulate matter
(NR-PM$_{1}$) in the southeastern USA. Measurements were performed in both
rural and urban sites in the greater Atlanta area, Georgia (GA), and
Centreville, Alabama (AL), for approximately 1 year as part of
Southeastern Center for Air Pollution and Epidemiology study (SCAPE) and
Southern Oxidant and Aerosol Study (SOAS). Organic aerosol (OA) accounts for
more than half of NR-PM1 mass concentration regardless of sampling
sites and seasons. Positive matrix factorization (PMF) analysis of
HR-ToF-AMS measurements identified various OA sources, depending on location
and season. Hydrocarbon-like OA (HOA) and cooking OA (COA) have important,
but not dominant, contributions to total OA in urban sites (i.e., 21–38 %
of total OA depending on site and season). Biomass burning OA (BBOA)
concentration shows a distinct seasonal variation with a larger enhancement
in winter than summer. We find a good correlation between BBOA and brown
carbon, indicating biomass burning is an important source for brown carbon,
although an additional, unidentified brown carbon source is likely present
at the rural Yorkville site. Isoprene-derived OA factor (isoprene-OA) is only
deconvolved in warmer months and contributes 18–36 % of total OA. The
presence of isoprene-OA factor in urban sites is more likely from local
production in the presence of NOx than transport from rural sites.
More-oxidized and less-oxidized oxygenated organic aerosol (MO-OOA and
LO-OOA, respectively) are dominant fractions (47–79 %) of OA in all sites.
MO-OOA correlates well with ozone in summer but not in winter, indicating
MO-OOA sources may vary with seasons. LO-OOA, which reaches a daily maximum
at night, correlates better with estimated nitrate functionality from
organic nitrates than total nitrates.
Based on the HR-ToF-AMS measurements, we estimate that the nitrate
functionality from organic nitrates contributes 63–100 % to the total
measured nitrates in summer. Furthermore, the contribution of organic nitrates
to total OA is estimated to be 5–12 % in summer, suggesting that organic
nitrates are important components in the ambient aerosol in the southeastern
USA. The spatial distribution of OA is investigated by comparing simultaneous
HR-ToF-AMS measurements with ACSM measurements at two different sampling
sites. OA is found to be spatially homogeneous in summer due possibly to
stagnant air mass and a dominant amount of regional secondary organic
aerosol (SOA) in the southeastern USA. The homogeneity is less in winter,
which is likely due to spatial variation of primary emissions.
We observe that the seasonality of OA concentration shows a clear
urban/rural contrast. While OA exhibits weak seasonal variation in the urban
sites, its concentration is higher in summer than winter for rural sites.
This observation from our year-long measurements is consistent with 14 years
of organic carbon (OC) data from the SouthEastern Aerosol Research and
Characterization (SEARCH) network. The comparison between short-term
measurements with advanced instruments and long-term measurements of basic
air quality indicators not only tests the robustness of the short-term
measurements but also provides insights in interpreting long-term
measurements. We find that OA factors resolved from PMF analysis on
HR-ToF-AMS measurements have distinctly different diurnal variations. The
compensation of OA factors with different diurnal trends is one possible
reason for the repeatedly observed, relatively flat OA diurnal profile in
the southeastern USA. In addition, analysis of long-term measurements shows
that the correlation between OC and sulfate is substantially stronger in
summer than winter. This seasonality could be partly due to the effects of
sulfate on isoprene SOA formation as revealed by the short-term intensive
measurements. |
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