A model study of ATMOS observations and the heterogeneous loss of N2O5 by the sulphate aerosol layer

The heterogeneous removal of N₂O₅ by sulphuric acid aerosols as been invoked to explain the decline of mid-latitude ozone in the last decade. We have used a photochemical model to study measurements of odd-nitrogen made by Spacelab 3. The gas-phase photochemical model overestimates the amount of N₂O₅ present. The loss of N₂O₅ by aerosols does reduce N₂O₅, but is likely to be slower than assumed in WMO (1992). The sunset measurements at 25.5 km cannot be explained by heterogeneous loss of N₂O₅ and is more likely to be due to a faster photolysis than assumed. New absorption cross-sections of HNO₃ reduce the photolysis of HNO₃ so that the model with gas-phase chemistry only gives better agreement at 19 km, than a model including heterogeneous chemistry.