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| Titel |
The impact of parameterising light penetration into snow on the photochemical production of NOx and OH radicals in snow |
| VerfasserIn |
H. G. Chan, M. D. King, M. M. Frey |
| 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. 14 ; Nr. 15, no. 14 (2015-07-17), S.7913-7927 |
| Datensatznummer |
250119909
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| Publikation (Nr.) |
 copernicus.org/acp-15-7913-2015.pdf |
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| Zusammenfassung |
Snow photochemical processes drive production of chemical trace gases in snowpacks,
including nitrogen oxides (NOx = NO + NO2)
and hydrogen oxide radical (HOx = OH + HO2), which are then released to the lower
atmosphere. Coupled atmosphere–snow modelling
of theses processes on global scales
requires simple parameterisations of actinic flux in snow to reduce
computational cost. The disagreement between a physical radiative-transfer
(RT) method and a parameterisation based upon the e-folding depth of
actinic flux in snow is evaluated. In particular, the photolysis of the
nitrate anion (NO3-), the nitrite anion (NO2-) and
hydrogen peroxide (H2O2) in snow and nitrogen dioxide (NO2)
in the snowpack
interstitial air are considered.
The emission flux from the snowpack is estimated as the product of the
depth-integrated photolysis rate coefficient, v,
and the concentration of photolysis precursors in the snow.
The depth-integrated photolysis rate coefficient is calculated (a) explicitly with
an RT model (TUV), vTUV, and
(b) with a simple parameterisation based on e-folding depth,
vze. The metric for the evaluation is based upon the deviation of the
ratio of the depth-integrated photolysis rate coefficient determined by the two
methods, vTUV/vze, from unity.
The ratio depends primarily on the position of the peak in the
photolysis action spectrum of chemical species, solar zenith angle and physical
properties of the snowpack, i.e. strong dependence on
the light-scattering cross section and
the mass ratio of light-absorbing impurity (i.e. black carbon and HULIS)
with a weak dependence on density.
For the photolysis of NO2, the NO2- anion, the
NO3- anion and H2O2 the ratio
vTUV/vze varies within the range of
0.82–1.35, 0.88–1.28, 0.93–1.27 and 0.91–1.28 respectively. The e-folding
depth parameterisation underestimates for small solar zenith angles
and overestimates at solar zenith angles around 60° compared to the RT method.
A simple
algorithm has been developed to improve the parameterisation which
reduces the ratio vTUV/vze to
0.97–1.02, 0.99–1.02, 0.99–1.03 and 0.98–1.06 for photolysis of NO2,
the NO2- anion, the NO3- anion and H2O2
respectively. The e-folding depth parameterisation may give
acceptable results for the photolysis of the NO3- anion and
H2O2 in cold polar snow with large solar zenith angles, but it can be
improved by a correction based on solar zenith angle and for cloudy skies. |
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