Annual average sensitivity of total tropospheric O<sub>3</sub> to cruise NO<sub><em>x</em></sub> emissions (9–12 km) in kg(O<sub>3</sub>) / kg(NO<sub><em>x</em></sub>)

<p><strong>Figure 2.</strong> Annual average sensitivity of total tropospheric O<sub>3</sub> to cruise NO<sub><em>x</em></sub> emissions (9–12 km) in kg(O<sub>3</sub>) / kg(NO<sub><em>x</em></sub>).</p> <p><strong>Abstract</strong></p> <p>Aviation NO<sub><em>x</em></sub> emissions promote tropospheric ozone formation, which is linked to climate warming and adverse health effects. Modeling studies have quantified the relative impact of aviation NO<sub><em>x</em></sub> on O<sub>3</sub> in large geographic regions. As these studies have applied forward modeling techniques, it has not been possible to attribute O<sub>3</sub> formation to individual flights. Here we apply the adjoint of the global chemistry–transport model GEOS-Chem to assess the temporal and spatial variability in O<sub>3</sub> production due to aviation NO<sub><em>x</em></sub> emissions, which is the first application of an adjoint to this problem. We find that total aviation NO<sub><em>x</em></sub> emitted in October causes 40% more O<sub>3</sub> than in April and that Pacific aviation emissions could cause 4–5 times more tropospheric O<sub>3</sub> per unit NO<sub><em>x</em></sub> than European or North American emissions. Using this sensitivity approach, the O<sub>3</sub> burden attributable to 83 000 unique scheduled civil flights is computed individually. We find that the ten highest total O<sub>3</sub>-producing flights have origins or destinations in New Zealand or Australia. The top ranked O<sub>3</sub>-producing flights normalized by fuel burn cause 157 times more normalized O<sub>3</sub> formation than the bottom ranked ones. These results show significant spatial and temporal heterogeneity in environmental impacts of aviation NO<sub><em>x</em></sub> emissions.</p>