Estimations of global NO(x) emissions and their uncertainties

Abstract

The AERONOX programme investigated the impact of NO(x) emissions from aircraft on the atmosphere and included an extensive modelling programme. In the model comparisons undertaken within the AERONOX programme, a 'standard' set of emissions of NO(x) from both aviation and non-aviation sources was required so that differences between the models could be examined. This paper describes the data sets used in the study. These were: fossil fuel combustion from stationary and mobile sources at Earth's surface (22 Tg N yr-1), tropical biomass burning (5 Tg N yr-1), soil microbial production of NO (4 Tg N yr-1), lightning (5 Tg N yr-1) and the stratospheric decomposition of nitrous oxide (0.6 Tg N yr-1). However, global emission inventories of trace gases are developing rapidly: this paper also presents some emission estimates updated since the AERONOX study and also attempts to quantify uncertainties. The lightning source was constructed using convective cloud-top height from a GCM and differential rates of NO production calculated for cloud-to-cloud, and cloud-to-ground strikes. A revised biomass inventory including deforestation, savanna burning, agricultural waste burning and biofuel combustion results in approximately 8 Tg N yr-1. This estimate includes sources beyond the tropics. Both extrapolation of measurements of soil NO fluxes by biome type, and a further refinement of the AERONOX soils emission model resulted in an emission of approximately 7 Tg N yr-1. Ammonia oxidation as a source of NO, is calculated to be 0.9 N Tg yr-1 with a range of 0-1.6 Tg N yr-1, which shows that this is a relatively unimportant source of NO(x) in the troposphere. Uncertainty estimates for all sources have been given and discussed. The global source term for NO(x) for all sources (including the revisions) is estimated to be 44 Tg N yr-1 with an uncertainty range of 23-81 Tg N yr-1. A future scenario for fossil fuel combustion is given for 2025 resulting in an emission term of 46.5 Tg N for this source, showing a pronounced shift in distribution to Asia and the Far-East.