Modeling surface-mediated renoxification of the atmosphere via reaction of gaseous nitric oxide with deposited nitric acid

Air quality models consider the formation and deposition of nitric acid (HNO3) on surfaces to be an irreversible sink of atmospheric nitrogen oxides (NOx) and therefore an effective termination step in the ozone formation cycle. However, experimental evidence suggests that the reaction of gaseous nitric oxide with nitric acid on surfaces may convert HNO3 to photochemically active NOx. A first-order simulation of this surface-mediated renoxification process is performed using an air quality model of the South Coast Air Basin of California. Peak ozone concentrations are predicted closer to observed values in regions regularly underpredicted by base case models. In certain regions, ozone predictions are enhanced by as much as 30 ppb or 20% compared to the baseline simulation. These results suggest that renoxification processes may be a key to resolving long-standing shortcomings of air quality models, in addition to reconciling [HNO3]/[NOx] ratios in remote regions. This study also illustrates that the surface terrain may play a more active chemical role than hitherto considered in air quality models.