Global sensitivity of aviation NO_x effects to the HNO₃-forming channel of the HO₂ + NO reaction

Abstract. The impact of a recently proposed HNO₃-forming channel of the HO₂ + NO reaction on atmospheric ozone, methane and their precursors is assessed with the aim to investigate its effects on aviation NO_x induced radiative forcing.

The first part of the study addresses the differences in stratospheric and tropospheric HO_x-NO_x chemistry in general, by comparing a global climate simulation without the above reaction to two simulations with different rate coefficient parameterizations for HO₂ + NO → HNO₃. A possible enhancement of the reaction by humidity, as found by a laboratory study, particularly reduces the oxidation capacity of the atmosphere, increasing methane lifetime significantly. Since methane lifetime is an important parameter for determining global methane budgets, this might affect estimates of the anthropogenic greenhouse effect.

In the second part aviation NO_x effects are isolated independently for each of the three above simulations. Warming and cooling effects of aircraft NO_x emissions are both enhanced when considering the HNO₃-forming channel, but the sum is shifted towards negative radiative forcing. Uncertainties associated with the inclusion of the HO₂ + NO → HNO₃ reaction and with its corresponding rate coefficient propagate a considerable additional uncertainty on estimates of the climate impact of aviation and on NO_x-related mitigation strategies.