Past and future simulations of NO2 from a coupled chemistry-climate model in comparison with observations

Trends in NO2 derived from a 45 year integration of a chemistry-climate model (CCM) run have been compared with ground-based NO2 measurements at Lauder (45degrees S) and Arrival Heights (78degrees S). Observed trends in NO2 at both sites exceed the modelled trends in N2O, the primary source gas for stratospheric NO2. This suggests that the processes driving the NO2 trend are not solely dictated by changes in N2O but are coupled to global atmospheric change, either chemically or dynamically or both. If CCMs are to accurately estimate future changes in ozone, it is important that they comprehensively include all processes affecting NOx (NO+NO2) because NOx concentrations are an important factor affecting ozone concentrations. Comparison of measured and modelled NO2 trends is a sensitive test of the degree to which these processes are incorporated in the CCM used here. At Lauder the 1980-2000 CCM NO2 trends (4.2% per decade at sunrise, 3.8% per decade at sunset) are lower than the observed trends (6.5% per decade at sunrise, 6.0% per decade at sunset) but not significantly different at the 2sigma level. Large variability in both the model and measurement data from Arrival Heights makes trend analysis of the data difficult. CCM predictions (2001-2019) of NO2 at Lauder and Arrival Heights show significant reductions in the rate of increase of NO2 compared with the previous 20 years (1980-2000). The model results indicate that the partitioning of oxides of nitrogen changes with time and is influenced by both chemical forcing and circulation changes.