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| Titel |
CARIBIC DOAS observations of nitrous acid and formaldehyde in a large convective cloud |
| VerfasserIn |
K.-P. Heue, H. Riede, D. Walter, C. A. M. Brenninkmeijer, T. Wagner, U. Frieß, U. Platt, A. Zahn, G. Stratmann, H. Ziereis |
| 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. 13 ; Nr. 14, no. 13 (2014-07-01), S.6621-6642 |
| Datensatznummer |
250118853
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| Publikation (Nr.) |
 copernicus.org/acp-14-6621-2014.pdf |
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| Zusammenfassung |
The chemistry in large thunderstorm clouds is influenced by local
lightning-NOx production and uplift of boundary layer air. Under
these circumstances trace gases like nitrous acid (HONO) or formaldehyde
(HCHO) are expected to be formed or to reach the tropopause region.
However, up to now only few observations of HONO at this altitude have
been reported.
Here we report on a case study where enhancements in HONO, HCHO
and nitrogen oxides (NOx) were observed by the CARIBIC flying
laboratory (Civil Aircraft for the Regular Investigation of the atmosphere
Based on an Instrument Container). The event took place in a convective
system over the Caribbean Sea in August 2011. Inside the cloud the light path
reaches up to 100 km. Therefore the DOAS instrument on CARIBIC was very
sensitive to the tracers inside the cloud. Based on the enhanced slant column
densities of HONO, HCHO and NO2, average mixing
ratios of 37, 468 and 210 ppt, respectively, were calculated. These data
represent averages for constant mixing ratios inside the cloud. However, a
large dependency on the assumed profile is found; for HONO a mixing
ratio of 160 ppt is retrieved if the total amount is assumed to be situated
in the uppermost 2 km of the cloud.
The NO in situ instrument measured peaks up to 5 ppb NO inside
the cloud; the background in the cloud was about 1.3 ppb, and hence clearly
above the average outside the cloud (≈ 150 ppt). The
high variability and the fact that the enhancements were observed over a
pristine marine area led to the conclusion that, in all likelihood, the high
NO concentrations were caused by lighting. This assumption is
supported by the number of flashes that the World Wide Lightning Location
Network (WWLLN) counted in this area before and during the overpass.
The chemical box model CAABA is used to estimate the NO and
HCHO source strengths which are necessary to explain our
measurements. For NO a source strength of
10 × 109 molec cm−2 s−1 km−1 is found, which
corresponds to the lightning activity as observed by the World Wide Lightning
Location network, and lightning emissions of 5 × 1025 NO molec flash−1 (2.3–6.4 × 1025).
The uncertainties are determined by a change of the input
parameters in the box model, the cloud top height and the flash density. The
emission rate per flash is scaled up to a global scale and 1.9 (1.4–2.5) tg N a−1
is estimated. The HCHO updraught is of the order of
120 × 109 molec cm−2 s−1 km−1. Also isoprene and
CH3OOH as possible HCHO sources are discussed. |
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