Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 4 4 2004 10.5194/acpd-4-4545-2004 http://www.atmos-chem-phys-discuss.net/4/4545/2004/ http://www.atmos-chem-phys-discuss.net/4/4545/2004/acpd-4-4545-2004.html http://www.atmos-chem-phys-discuss.net/4/4545/2004/acpd-4-4545-2004.pdf 4545 4579 2004-08-20 Past and future simulations of NO₂ from a coupled chemistry-climate model in comparison with observations H. Struthers h.struthers@niwa.co.nz K. Kreher J. Austin R. Schofield G. Bodeker P. Johnston H. Shiona A. Thomas National Institute of Water and Atmospheric Research, Private Bag 50061, Omakau, New Zealand Geophysical Fluid Dynamics Laboratory, Princeton Forrestal Campus Rte.1, 201 Forrestal Rd., Princeton, NJ 08542-0308, USA NOAA Aeronomy Laboratory, 325 Broadway, R/AL8, Boulder CO 80305, USA 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 (45° S) and Arrival Heights (78° S). Observed trends in NO₂ at both sites exceed the trends in N₂O, the primary source gas for stratospheric NO₂, suggesting that processes driving the NO₂ trend are more complex than direct conversion of N₂O to NO₂. If CCMs are to accurately estimate future changes in ozone, it is important that they comprehensively include these N₂O→NO₂ processes since NO_x (NO+NO₂) 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, 3.9% 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 2σ 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.