Abstract
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 NOx induced radiative forcing. The first part of the study addresses the differences in stratospheric and tropospheric HOx-NOx 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 NOx effects are isolated independently for each of the three above simulations. Warming and cooling effects of aircraft NOx 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 NOx-related mitigation strategies.
Highlights
N(UOT2L)Sp)e, aakndinththeeruespupletirntgroNpGoOsexpohinescrcereiaaensned tilimofiwpcearctsstroantotshpeherarestratospheric and tropospheric HOx-NOx 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
5.3 Net radiative forcing from aviation NOx effects
Net radiative forcing (RF) from aviation NOx induced perturbations is the sum from large positive and large negative terms, giving a small net forcing with relatively big uncertainty ranges
Summary
This study is based on the global ECHAM/MESSy Atmospheric Chemistry (EMAC) model. EMAC is a numerical. Chemistry and climate simulation system that includes submodels describing tropospheric and middle atmosphere processes and their interaction with oceans, land and human influences (Jockel et al, 2006). The applied chemical mechanism included full stratospheric complexity, but neglected the sulphur and halogen families in the troposphere It has been evaluated by Jockel et al (2006), showing that it is a reasonably realistic representation of atmospheric chemistry. Equation (4) is based on an empirical fit to laboratory data and is valid for dry conditions, in the pressure range 93–800 hPa and the temperature range 223–298 K. Both reaction rates depend on temperature and pressure in this case.
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