Evaluation of aviation NOx-induced radiative forcings for 2005 and 2050

Aviation NOx emissions lead to the production of ozone (O3) and an OH increase in the troposphere and lower stratosphere. This OH increase results in a reduction of methane (CH4) which causes a longer-term reduction in O3 concentrations and lower stratospheric water vapor (SWV). Here we report new evaluations of a more comprehensive suite of aviation NOx-induced steady-state radiative forcing (RF) for 2006 aviation NOx emissions and for two different projections of 2050 NOx emissions and compare them with previously published analyses of aviation NOx-induced steady-state RFs. To do so, we evaluate the aviation NOx-induced steady-state RFs using the three-dimensional (3-D) global climate-chemistry Community Atmosphere Model (CAM), CAM4Chem. Moreover, we determine time dependent (transient) RFs for 2005 and the two 2050 emission scenarios by utilizing a parameterization based on specific RFs (steady-state RFs per unit of NOx emission) calculated in the 3-D model simulations, time-dependent NOx emissions data, and considering the lifetime of the perturbed species. For aviation NOx-induced effects we find that the net transient RFs differ from the linearly scaled steady-state RFs (specific steady-state RFs multiplied by NOx emissions) by +10%–36% for the different emission scenarios used in this study. These results indicate the importance of not using the transient and steady-state RF interchangeably, especially when comparing the aviation contribution in RF changes to contributions from other transportation sectors. Also, these results indicate that careful consideration is necessary when choosing whether to use the steady-state or transient RF when evaluating aviation NOx-induced RFs.