CO has a strong thermodynamic potential for reducing HNO3 to HONO. If the reaction of HNO3 with CO does proceed via heterogeneous catalysis on sulfuric acid aerosols in our atmosphere, then this data assimilation study shows that the model is better able to reproduce the observed NOx/HNO3 ratio even with a &ggr; value as low as 1¿10-4. This is particularly true in the upper troposphere and lower stratosphere. We would like to highlight the possibility that elements such as iron deposited in the lower stratosphere by meteorites may be catalyzing this and other reactions within sulfate aerosols. ¿ 2000 American Geophysical Union