ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-4-2227-2004 Past and future simulations of NO<sub>2</sub> from a coupled chemistry-climate model in comparison with observations Struthers H. 1 Kreher K. 1 Austin J. 2 Schofield R. 3 Bodeker G. 1 Johnston P. 1 Shiona H. 1 Thomas A. 1 National Institute of Water and Atmospheric Research, Private Bag 50061, Omakau, New Zealand Geophysical Fluid Dynamics Lab., Princeton Forrestal Campus Rte.1, 201 Forrestal Rd., Princeton, NJ 08542-0308, USA NOAA Aeronomy Laboratory, 325 Broadway, R/AL8, Boulder CO 80305, USA 22 11 2004 4 8 2227 2239 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/4/2227/2004/acp-4-2227-2004.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/4/2227/2004/acp-4-2227-2004.pdf

Trends in derived from a 45 year integration of a chemistry-climate model (CCM) run have been compared with ground-based measurements at Lauder (45&deg; S) and Arrival Heights (78&deg; S). Observed trends in at both sites exceed the modelled trends in N<sub>2</sub>O, the primary source gas for stratospheric NO<sub>2</sub>. This suggests that the processes driving the trend are not solely dictated by changes in 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 NO<sub>x</sub> (NO+NO<sub>2</sub>) because NO<sub>x</sub> concentrations are an important factor affecting ozone concentrations. Comparison of measured and modelled NO<sub>2</sub> 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 NO<sub>2</sub> 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 2&sigma; 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 NO<sub>2</sub> at Lauder and Arrival Heights show significant reductions in the rate of increase of NO<sub>2</sub> 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.