Atmos. Chem. Physics, vol. 8, issue 14 (2008) pp. 4061-4068
We have studied the impact of the recently observed reaction NO+ HO2
-> HNO3 on atmospheric chemistry. A pressure and temperature-dependent
parameterisation of this minor channel of the NO+ HO2 -> NO2+ OH
reaction has been included in both a 2-D stratospheretroposphere
model and a 3-D tropospheric chemical transport model (CTM). Significant
effects on the nitrogen species and hydroxyl radical concentrations
are found throughout the troposphere, with the largest percentage
changes occurring in the tropical upper troposphere ( UT). Including
the reaction leads to a reduction in NOx everywhere in the troposphere,
with the largest decrease of 25% in the tropical and Southern Hemisphere
UT. The tropical UT also has a corresponding large increase in HNO3
of 25%. OH decreases throughout the troposphere with the largest
reduction of over 20% in the tropical UT. The mean global decrease
in OH is around 13%, which is very large compared to the impact
that typical photochemical revisions have on this modelled quantity.
This OH decrease leads to an increase in CH4 lifetime of 5%. Due
to the impact of decreased NOx on the OH: HO2 partitioning, modelled
HO2 actually increases in the tropical UT on including the new reaction.
The impact on tropospheric ozone is a decrease in the range 5 to
12%, with the largest impact in the tropics and Southern Hemisphere.
Comparison with observations shows that in the region of largest
changes, i.e. the tropical UT, the inclusion of the new reaction
tends to degrade the model agreement. Elsewhere the model comparisons
are not able to critically assess the impact of including this reaction.
Only small changes are calculated in the minor species distributions
in the stratosphere.
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