Radiative forcing from aircraft NO x emissions: Mechanisms and seasonal dependence

A chemistry‐climate model has been applied to study the radiative forcings generated by aircraft NO x emissions through changes in ozone and methane. Four numerical experiments, where an extra pulse of aircraft NO x was emitted into the model atmosphere for a single month (January, April, July, or October), were compared to a control experiment, allowing the aircraft impact to be isolated. The extra NO x produces a short‐lived (few months) pulse of ozone that generates a positive radiative forcing. However, the NO x and O 3 both generate OH, which leads to a reduction in CH 4 . A detailed analysis of the OH budget reveals the spatial structure and chemical reactions responsible for the generation of the OH perturbation. Methane's long lifetime means that the CH 4 anomaly decays slowly (perturbation lifetime of 11.1 years). The negative CH 4 anomaly also has an associated negative O 3 anomaly, and both of these introduce a negative radiative forcing. There are important seasonal differences in the response of O 3 and CH 4 to aircraft NO x , related to the annual cycle in photochemistry; the O 3 radiative forcing calculations also have a seasonal dependence. The long‐term globally integrated annual mean net forcing calculated here is approximately zero, although earlier work suggests a small net positive forcing. The model design (e.g., upper tropospheric chemistry, convection parameterization) and experimental setup (pulse magnitude and duration) may somewhat influence the results: further work with a range of models is required to confirm these results quantitatively.