Variation of radiative forcings and global warming potentials from regional aviation NOx emissions

The response to hemispherical and regional aircraft NOx emissions is explored by using two climate metrics: radiative forcing (RF) and Global Warming Potential (GWP). The global chemistry transport model, MOZART-3 CTM, is applied in this study for a series of incremental aircraft NOx emission integrations to different regions. It was found that the sensitivity of chemical responses per unit emission rate from regional aircraft NOx emissions varies with size of aircraft NOx emission rate and that climate metric values decrease with increasing aircraft NOx emission rates, except for Southeast Asia. Previous work has recognized that aircraft NOx GWPs may vary regionally. However, the way in which these regional GWPs are calculated are critical. Previous studies have added a fixed amount of NOx to different regions. This approach can heavily bias the results of a regional GWP because of the well-established sensitivity of O3 production to background NOx whereby the Ozone Production Efficiency (OPE) is greater at small background NOx. Thus, even a small addition of NOx in a clean-air area can produce a large O3 response. Using this ‘fixed addition’ method of 0.035 Tg(N) yr−1, results in the greatest effect observed for North Atlantic and Brazil, ∼10.0 mW m−2/Tg(N) yr−1. An alternative ‘proportional approach’ is also taken that preserves the subtle balance of local NOx–O3–CH4 systems with the existing emission patterns of aircraft and background NOx, whereby a proportional amount of aircraft NOx, 5% (N) yr−1, is added to each region in order to determine the response. This results in the greatest effect observed for North Pacific that with its net NOx RF of 23.7 mW m−2/Tg(N) yr−1 is in contrast with the ‘fixed addition’ method. For determining regional NOx GWPs, it is argued that the ‘proportional’ approach gives more representative results. However, a constraint of both approaches is that the regional GWP determined is dependent on the relative global emission pattern, so if that changes in the future, the regional NOx GWP will change