Global sensitivity of aviation NOx effects to the HO2 + NO → HNO3 reaction

Abstract

The impact of a recently proposed HNO3-forming channel of the HO2 + NO reaction (Butkovskaya et al., 2005, 2007) on atmospheric mixing ratios of ozone, methane and their precursors is assessed with a global stratosphere-troposphere chemistry-climate model. Previous modelling studies applied a rate coefficient that depends only on pressure and temperature. We additionally considered a possible enhancement of the reaction by humidity, as found by a laboratory study (Butkovskaya et al., 2009). This particularly reduces the oxidation capacity of the atmosphere, increasing methane lifetime significantly. The effects of aircraft NOx emissions on atmospheric chemistry are altered when considering the above reaction, resulting in a negative net radiative forcing relative to an atmosphere without aviation NOx. 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 NOx-related mitigation strategies.