Aviation NOx-induced CH4 effect: Fixed mixing ratio boundary conditions versus flux boundary conditions.

Atmospheric chemistry-climate models are often used to calculate the effect of aviation NOx emissions on atmospheric ozone (O 3 ) and methane (CH 4 ). Due to the long (∼10 yr) atmospheric lifetime of methane, model simulations must be run for long time periods, typically for more than 40 simulation years, to reach steady-state if using CH 4 emission fluxes. Because of the computational expense of such long runs, studies have traditionally used specified CH 4 mixing ratio lower boundary conditions (BCs) and then applied a simple parameterization based on the change in CH 4 lifetime between the control and NOx-perturbed simulations to estimate the change in CH 4 concentration induced by NOx emissions. In this parameterization a feedback factor (typically a value of 1.4) is used to account for the feedback of CH 4 concentrations on its lifetime. Modeling studies comparing simulations using CH 4 surface fluxes and fixed mixing ratio BCs are used to examine the validity of this parameterization. The latest version of the Community Earth System Model (CESM), with the CAM5 atmospheric model, was used for this study. Aviation NOx emissions for 2006 were obtained from the AEDT (Aviation Environmental Design Tool) global commercial aircraft emissions. Results show a 31.4 ppb change in CH 4 concentration when estimated using the parameterization and a 1.4 feedback factor, and a 28.9 ppb change when the concentration was directly calculated in the CH 4 flux simulations. The model calculated value for CH 4 feedback on its own lifetime agrees well with the 1.4 feedback factor. Systematic comparisons between the separate runs indicated that the parameterization technique overestimates the CH 4 concentration by 8.6%. Therefore, it is concluded that the estimation technique is good to within ∼10% and decreases the computational requirements in our simulations by nearly a factor of 8. [ABSTRACT FROM AUTHOR]