Trading off Aircraft Fuel Burn and NO Emissions for Optimal Climate Policy.

Aviation emits pollutants that affect the climate, including CO and NO , NO indirectly so, through the formation of tropospheric ozone and reduction of ambient methane. To improve the fuel performance of engines, combustor temperatures and pressures often increase, increasing NO emissions. Conversely, combustor modifications to reduce NO may increase CO. Hence, a technology trade-off exists, which also translates to a trade-off between short-lived climate forcers and a long-lived greenhouse gas, CO. Moreover, the NO -O-CH system responds in a nonlinear manner, according to both aviation emissions and background NO . A simple climate model was modified to incorporate nonlinearities parametrized from a complex chemistry model. Case studies showed that for a scenario of a 20% reduction in NO emissions the consequential CO penalty of 2% actually increased the total radiative forcing (RF). For a 2% fuel penalty, NO emissions needed to be reduced by >43% to realize an overall benefit. Conversely, to ensure that the fuel penalty for a 20% NO emission reduction did not increase overall forcing, a 0.5% increase in CO was found to be the "break even" point. The time scales of the climate effects of NO and CO are quite different, necessitating careful analysis of proposed emissions trade-offs.