Large scale atmospheric models which include chemical processes require the use of detailed reaction kinetics and mechanisms which are typically measured under controlled laboratory conditions. The large number of possible interactions between reactive chemical species means that such laboratory measurements have been ongoing for the past 50 years. Advances in chemical instrumentation and techniques often lead to new insights into important reactions which are thought to be relatively well understood. One such reaction is the rapid conversion of NO to NO2 via the reduction of the HO2 radical, which a dominant process under atmospheric conditions.

Recent studies using Chemical Ionization Mass Spectroscopy (CIMS) have shown that HNO3 maybe directly formed as a minor product via the reaction of NO with HO2. This temperature and pressure dependant reaction channel has the potential to perturb in-situ O3 formation in the upper troposphere/lower stratosphere (UTLS). Recently, as part of integrated EU project SCOUT-O3, Jason Williams at KNMI has tested whether any significant effect is introduced as a result of including this reaction channel into the global CTM TM4. Figure 1 shows that there is a reduction in the total tropospheric ozone column as a direct result. In order to evaluate whether this improves the performance of the model, comparisons have been made versus co-located ozonesondes, which are part of the SHADOZ network. Figure 2 shows all year comparisons for the stations Nairobi (1.3°S, 36.8°E), Irene (25.9°S, 28.2°E), Paramaribo (5.8°N, 55.2°W) and Fiji (18.1°S, 178.4°E), where the blue profile indicates the default simulation and the red profile the simulation with the additional HNO3 formation channel. In general it can be seen that the for the upper troposphere, where the reaction is most dominant, the agreement gets worse as a result of the reduction in [NO2], whose photo-dissociation is an efficient means of forming O3. These results should be viewed as preliminary as there are indications that the efficiency of this reaction channel is affected by [H2O], possibly via the formation of the intermediate HO2-H2O. This would enhance the effect in the lower troposphere. These results were recently presented at the fourth annual SCOUT-O3 research meeting in Potsdam, Germany.

See poster. (pdf)