Effect of sulfate aerosol on tropospheric NOx and ozone budgets: Model simulations and TOPSE evidence

The distributions of NO x and O 3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N 2 O 5 on sulfate aerosols has on tropospheric NO x and O 3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NO x concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NO x concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N 2 O 5 on sulfate aerosol plays an important role in controlling the tropospheric NO x and O 3 budgets. The calculated reduction of NO x attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NO x , O 3 concentrations are also decreased. The maximum O 3 reduction occurs in spring although the maximum NO x reduction occurs in winter when photochemical O 3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NO x due to these uncertainties are much smaller than the impact of hydrolysis of N 2 O 5 on sulfate aerosol. The effect that hydrolysis of N 2 O 5 on global NO x and O 3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NO x budget decreases 50% due to this reaction in winter and 5% in summer. The average O 3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NO x and O 3 budgets of the Southern Hemisphere.