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| Titel |
Strong wintertime ozone events in the Upper Green River basin, Wyoming |
| VerfasserIn |
B. Rappenglück, L. Ackermann, S. Alvarez, J. Golovko, M. Buhr, R. A. Field, J. Soltis, D. C. Montague, B. Hauze, S. Adamson, D. Risch, G. Wilkerson, D. Bush, T. Stoeckenius, C. Keslar |
| 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 ; 14, no. 10 ; Nr. 14, no. 10 (2014-05-20), S.4909-4934 |
| Datensatznummer |
250118724
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| Publikation (Nr.) |
 copernicus.org/acp-14-4909-2014.pdf |
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| Zusammenfassung |
| During recent years, elevated ozone (O3) values have been observed
repeatedly in the Upper Green River basin (UGRB), Wyoming, during wintertime.
This paper presents an analysis of high ozone days in late winter 2011
(1 h average up to 166 ppbv – parts per billion by volume). Intensive operational periods (IOPs) of
ambient monitoring were performed, which included comprehensive surface and
boundary layer measurements. On IOP days, maximum O3 values are
restricted to a very shallow surface layer. Low wind speeds in combination
with low mixing layer heights (~ 50 m above ground level
around noontime) are essential for accumulation of pollutants within the
UGRB. Air masses contain substantial amounts of reactive nitrogen (NOx)
and non-methane hydrocarbons (NMHC) emitted from fossil fuel exploration
activities in the Pinedale Anticline. On IOP days particularly in the morning hours, reactive nitrogen (up to 69%), aromatics and alkanes
(~ 10–15%; mostly ethane and propane) are major
contributors to the hydroxyl (OH) reactivity. Measurements at the Boulder
monitoring site during these time periods under SW wind flow conditions show
the lowest NMHC / NOx ratios (~ 50), reflecting a
relatively low reactive NMHC mixture, and a change from a NOx-limited
regime towards a NMHC-limited regime as indicated by photochemical
indicators, e.g., O3 /NOy, O3 /NOz, and O3 / HNO3
and the EOR (extent of reaction). OH production on IOP days is mainly due to
nitrous acid (HONO). On a 24 h basis and as determined for a measurement
height of 1.80 m above the surface HONO photolysis on IOP days can
contribute ~ 83% to OH production on average, followed by
alkene ozonolysis (~ 9%). Photolysis by ozone and HCHO
photolysis contribute about 4% each to hydroxyl formation. High HONO
levels (maximum hourly median on IOP days: 1096 pptv – parts per trillion by volume) are favored by a
combination of shallow boundary layer conditions and enhanced photolysis
rates due to the high albedo of the snow surface. HONO is most likely formed
through (i) abundant nitric acid (HNO3) produced in atmospheric
oxidation of NOx, deposited onto the snow surface and undergoing
photo-enhanced heterogeneous conversion to HONO (estimated HONO production:
10.2 ± 40% ppbv h−1) and (ii) combustion-related emission of HONO
(estimated HONO production: ~ 0.1 ± 30% ppbv h−1). HONO
production is confined to the lowermost 10 m of the boundary layer. HONO,
serves as the most important precursor for OH, strongly enhanced due to the
high albedo of the snow cover (HONO photolysis rate 10.7 ± 30%
ppbv h−1). OH radicals will oxidize NMHCs, mostly aromatics (toluene,
xylenes) and alkanes (ethane, propane), eventually leading to an increase in
ozone. |
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