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| Titel |
Vertical profiles of nitrous acid in the nocturnal urban atmosphere of Houston, TX |
| VerfasserIn |
K. W. Wong, H.-J. Oh, B. L. Lefer, B. Rappenglück, J. Stutz |
| 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. 8 ; Nr. 11, no. 8 (2011-04-18), S.3595-3609 |
| Datensatznummer |
250009637
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| Publikation (Nr.) |
 copernicus.org/acp-11-3595-2011.pdf |
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| Zusammenfassung |
Nitrous acid (HONO) often plays an important role in tropospheric
photochemistry as a major precursor of the hydroxyl radical (OH) in early
morning hours and potentially during the day. However, the processes leading
to formation of HONO and its vertical distribution at night, which can have
a considerable impact on daytime ozone formation, are currently poorly
characterized by observations and models. Long-path differential optical
absorption spectroscopy (LP-DOAS) measurements of HONO during the 2006
TexAQS II Radical and Aerosol Measurement Project (TRAMP), near downtown
Houston, TX, show nocturnal vertical profiles of HONO, with mixing ratios of
up to 2.2 ppb near the surface and below 100 ppt aloft. Three nighttime
periods of HONO, NO2 and O3 observations during TRAMP were used to
perform model simulations of vertical mixing ratio profiles. By adjusting
vertical mixing and NOx emissions the modeled NO2 and O3
mixing ratios showed very good agreement with the observations.
Using a simple conversion of NO2 to HONO on the ground, direct HONO
emissions, as well as HONO loss at the ground and on aerosol, the observed
HONO profiles were reproduced by the model for 1–2 and 7–8 September
in the nocturnal boundary layer (NBL). The unobserved increase of
HONO to NO2 ratio (HONO/NO2) with altitude that was
simulated by the initial model runs was found to be due to HONO uptake being
too small on aerosol and too large on the ground. Refined model runs, with
adjusted HONO uptake coefficients, showed much better agreement of HONO and
HONO/NO2 for two typical nights, except during morning rush hour, when
other HONO formation pathways are most likely active. One of the nights
analyzed showed an increase of HONO mixing ratios together with decreasing
NO2 mixing ratios that the model was unable to reproduce, most likely
due to the impact of weak precipitation during this night.
HONO formation and removal rates averaged over the lowest 300 m of the
atmosphere showed that NO2 to HONO conversion on the ground was the
dominant source of HONO, followed by traffic emission. Aerosol did not play
an important role in HONO formation. Although ground deposition was also a
major removal pathway of HONO, net HONO production at the ground was the
main source of HONO in our model studies. Sensitivity studies showed that in
the stable NBL, net HONO production at the ground tends to increase with
faster vertical mixing and stronger NOx emission. Vertical transport was found
to be the dominant source of HONO aloft. |
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