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Impact of the new {HNO}_3-forming channel of the {HO}_2+{NO} reaction on tropospheric {HNO}_3, {NOx}, {HOx} and ozone

by D Cariolle, M J Evans, M P Chipperfield, N Butkovskaya, A Kukui, G Le Bras
Atmos. Chem. Physics ()
  • ISSN: 1680-7316


We have studied the impact of the recently observed reaction NO+ HO2\n-> HNO3 on atmospheric chemistry. A pressure and temperature-dependent\nparameterisation of this minor channel of the NO+ HO2 -> NO2+ OH\nreaction has been included in both a 2-D stratospheretroposphere\nmodel and a 3-D tropospheric chemical transport model (CTM). Significant\neffects on the nitrogen species and hydroxyl radical concentrations\nare found throughout the troposphere, with the largest percentage\nchanges occurring in the tropical upper troposphere ( UT). Including\nthe reaction leads to a reduction in NOx everywhere in the troposphere,\nwith the largest decrease of 25% in the tropical and Southern Hemisphere\nUT. The tropical UT also has a corresponding large increase in HNO3\nof 25%. OH decreases throughout the troposphere with the largest\nreduction of over 20% in the tropical UT. The mean global decrease\nin OH is around 13%, which is very large compared to the impact\nthat typical photochemical revisions have on this modelled quantity.\nThis OH decrease leads to an increase in CH4 lifetime of 5%. Due\nto the impact of decreased NOx on the OH: HO2 partitioning, modelled\nHO2 actually increases in the tropical UT on including the new reaction.\nThe impact on tropospheric ozone is a decrease in the range 5 to\n12%, with the largest impact in the tropics and Southern Hemisphere.\nComparison with observations shows that in the region of largest\nchanges, i.e. the tropical UT, the inclusion of the new reaction\ntends to degrade the model agreement. Elsewhere the model comparisons\nare not able to critically assess the impact of including this reaction.\nOnly small changes are calculated in the minor species distributions\nin the stratosphere.

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