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| Titel |
Daytime HONO vertical gradients during SHARP 2009 in Houston, TX |
| VerfasserIn |
K. W. Wong, C. Tsai, B. Lefer, C. Haman, N. Grossberg, W. H. Brune, X. Ren, W. Luke, 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 ; 12, no. 2 ; Nr. 12, no. 2 (2012-01-16), S.635-652 |
| Datensatznummer |
250010520
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| Publikation (Nr.) |
 copernicus.org/acp-12-635-2012.pdf |
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| Zusammenfassung |
Nitrous Acid (HONO) plays an important role in tropospheric chemistry as a
precursor of the hydroxyl radical (OH), the most important oxidizing agent
in the atmosphere. Nevertheless, the formation mechanisms of HONO are still
not completely understood. Recent field observations found unexpectedly high
daytime HONO concentrations in both urban and rural areas, which point to
unrecognized, most likely photolytically enhanced HONO sources. Several
gas-phase, aerosol, and ground surface chemistry mechanisms have been
proposed to explain elevated daytime HONO, but atmospheric evidence to favor
one over the others is still weak. New information on whether HONO
formation occurs in the gas-phase, on aerosol, or at the ground may be
derived from observations of the vertical distribution of HONO and its
precursor nitrogen dioxide, NO2, as well as from its dependence on solar
irradiance or actinic flux.
Here we present field observations of HONO, NO2 and other trace gases
in three altitude intervals (30–70 m, 70–130 m and 130–300 m) using UCLA's
long path DOAS instrument, as well as in situ measurements of OH, NO,
photolysis frequencies and solar irradiance, made in Houston, TX, during the
Study of Houston Atmospheric Radical Precursor (SHARP) experiment from 20
April to 30 May 2009. The observed HONO mixing ratios were often ten times
larger than the expected photostationary state with OH and NO. Larger HONO
mixing ratios observed near the ground than aloft imply, but do not clearly
prove, that the daytime source of HONO was located at or near the ground.
Using a pseudo steady-state (PSS) approach, we calculated the missing
daytime HONO formation rates, Punknown, on four sunny days. The
NO2-normalized Punknown, Pnorm, showed a clear symmetrical
diurnal variation with a maximum around noontime, which was well correlated
with actinic flux (NO2 photolysis frequency) and solar irradiance. This behavior,
which was found on all clear days in Houston, is a strong indication of a
photolytic HONO source. [HONO]/[NO2] ratios also showed a clear diurnal
profile, with maxima of 2–3% around noon. PSS calculations show that this
behavior cannot be explained by the proposed gas-phase reaction of photoexcited NO2 (NO2*)
or any other gas-phase or aerosol photolytic process occurring
at similar or longer wavelengths than that of HONO photolysis. HONO
formation by aerosol nitrate photolysis in the UV also seems to be unlikely.
Pnorm correlated better with solar irradiance (average
R2 = 0.85/0.87 for visible/UV) than with actinic flux (R2 = 0.76) on
the four sunny days, clearly pointing to HONO being formed at the ground
rather than on the aerosol or in the gas-phase. In addition, the observed
[HONO]/[NO2] diurnal variation can be explained if the formation of
HONO depends on solar irradiance, but not if it depends on the actinic flux.
The vertical mixing ratio profiles, together with the stronger correlation with
solar irradiance, support the idea that photolytically
enhanced NO2 to HONO conversion on the ground was the dominant source
of HONO in Houston. |
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