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

Trends in NO2 derived from a 45 year integrationof a chemistry-climate model (CCM) run have beencompared with ground-based NO2 measurements at Lauder(45 S) and Arrival Heights (78 S). Observed trends in NO2at both sites exceed the modelled trends in N2O, the primarysource gas for stratospheric NO2. This suggests thatthe processes driving the NO2 trend are not solely dictatedby changes in N2O but are coupled to global atmosphericchange, either chemically or dynamically or both. If CCMsare to accurately estimate future changes in ozone, it is importantthat they comprehensively include all processes affectingNOx (NO+NO2) because NOx concentrations are animportant factor affecting ozone concentrations. Comparisonof measured and modelled NO2 trends is a sensitive test ofthe degree to which these processes are incorporated in theCCM used here. At Lauder the 19802000 CCM NO2 trends(4.2% per decade at sunrise, 3.8% per decade at sunset) arelower than the observed trends (6.5% per decade at sunrise,6.0% per decade at sunset) but not significantly different atthe 2 level. Large variability in both the model and measurementdata from Arrival Heights makes trend analysis ofthe data difficult. CCM predictions (20012019) of NO2 atLauder and Arrival Heights show significant reductions in therate of increase of NO2 compared with the previous 20 years(19802000). The model results indicate that the partitioningof oxides of nitrogen changes with time and is influenced byboth chemical forcing and circulation changes.