Global simulation of tropospheric O3‐NOx‐hydrocarbon chemistry: 2. Model evaluation and global ozone budget

Results from a global three‐dimensional model for tropospheric O 3 ‐NO x ‐hydrocarbon chemistry are presented and evaluated with surface, ozonesonde, and aircraft measurements. Seasonal variations and regional distributions of ozone, NO, peroxyacetylnitrate (PAN), CO, ethane, acetone, and H 2 O 2 are examined. The model reproduces observed NO and PAN concentrations to within a factor of 2 for a wide range of tropospheric regions including the upper troposphere but tends to overestimate HNO 3 concentrations in the remote troposphere (sometimes several fold). This discrepancy implies a missing sink for HNO 3 that does not lead to rapid recycling of NO x ; only in the upper troposphere over the tropical South Atlantic would a fast conversion of HNO 3 to NO x improve the model simulation for NO x . Observed concentrations of acetone are reproduced in the model by including a large biogenic source (15 Tg C yr −1 ), which accounts for 40% of the estimated global source of acetone (37 Tg C yr −1 ). Concentrations of H 2 O 2 in various regions of the troposphere are simulated usually to within a factor of 2, providing a test for HO x chemistry in the model. The model reproduces well the observed concentrations and seasonal variations of ozone in the troposphere, with some exceptions including an underestimate of the vertical gradient across the tropical trade wind inversion. A global budget analysis in the model indicates that the supply and loss of tropospheric ozone are dominated by photochemistry within the troposphere and that NO x . emitted in the southern hemisphere is twice as efficient at producing ozone as NO x emitted in the northern hemisphere.