Abstract
Abstract. 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 stratosphere-troposphere 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.
Highlights
It is well established that the reaction of HO2 with NO plays a central role in atmospheric chemistry: NO + HO2 → OH + NO2 (R1)In the stratosphere, this reaction moderates the effectiveness of the cycle involving HOx (OH, HO2) radicals that is an important removal mechanism of ozone
Another potential significant chain termination reaction is the minor HNO3-forming channel of the reaction of HO2 with NO that has been observed in laboratory experiments by Butkovskaya et al (2005): enger of OH produced in Reaction (R1), preventing formation of HNO3 in the secondary reaction of OH with NO2 (Reaction R4)
The HNO3 formation in the OH + NO2 reaction contributed up to 10% of the yield from HO2 + NO reaction at typical concentrations of CO, O2 and NO used in these experiments, as calculated by numerical simu
Summary
It is well established that the reaction of HO2 with NO plays a central role in atmospheric chemistry: NO + HO2 → OH + NO2. P =7t6y0picaanl dβ vPal=ue5s0asTaorfru,ncrteiosnpeocftaivlteitluyd.e fTrohme thaerrEoawrthc’sorsurerfsapcoe nds typicatol tβhevtarolpuoepsaaussearefguionnc.tion of altitude from the Earth’s surface itloar to that occurring in ptoeroxy radicals in the chemical amplifiers used to measure atmosphere Another potential significant chain termination reaction is the minor HNO3-forming channel of the reaction of HO2 with NO that has been observed in laboratory experiments by Butkovskaya et al (2005): enger of OH produced in Reaction (R1), preventing formation of HNO3 in the secondary reaction of OH with NO2 (Reaction R4). The branching ratio, or rate constant ratio, β=kR1b/kR1, for the new Reaction (R1b) was found to range from ∼0.2 to 0.8% from 300 K to 200 K, at a pressure of 200 Torr These first data led to the suggestion that Reaction (R1b) could be a significant loss process of HOx radicals in the upper troposphere. This model has a more detailed treatment of tropospheric chemical processes and allows an assessment of the impact of Reaction (R1b) under a wider range of conditions and a more critical comparison against tropospheric observations
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