Quantifying the impacts of shipping NOₓ emissions on tropospheric O₃, CH₄ and their radiative forcing

Hilaire, Jerome D. J.-M. (2012) Quantifying the impacts of shipping NOₓ emissions on tropospheric O₃, CH₄ and their radiative forcing. Doctoral thesis (PhD), Manchester Metropolitan University.

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Abstract

International shipping is a cornerstone of the global economy since it carries about 80% of the world trade by volume. It is also a growing sector that releases significant amounts of pollutants into the atmosphere. Shipping NOx emissions could represent about 15% of the total anthropogenic NOx emitted in 2000 and are particularly important since they perturb the concentrations of O3 and CH4, two greenhouse gases present in the atmosphere. Studies focusing on the impacts of shipping NOx emissions on tropospheric chemistry and radiative forcing remain however quite recent and still bear significant uncertainties. The aim of this study is to provide an extended view of the problem by making use of a state-of-art global chemistry-transport model (MOZART-4) in conjunction with a radiative transfer model (Edwards-Slingo). Since MOZART-4 has never been used before to analyse shipping NOx impacts, a thorough comparison with the material reported in previous studies was carried out. The global model was further tested by performing a sensitivity analysis of shipping perturbations against various uncertainties in atmospheric processes and model changes. In addition, impacts from the implementation of a ship plume parameterisation based on the concept of effective reaction rates were assessed. The O3 and CH4 radiative forcings resulting from shipping NOx emissions were then estimated with the help of Edwards-Slingo in 2000 and in the future by using the RCP scenarios. This last step laid the ground for a linearization of the global shipping NOx-O3-CH4 system. The assessment of MOZART-4 revealed that the model is suitable for the analysis of shipping NOx impacts. Although it simulates larger O3 perturbations in the free troposphere as well as a larger CH4 lifetime perturbation, estimates remain within 1-σ standard deviation of the results reported in a previous multimodel study. Current uncertainties in shipping emissions and particularly their magnitude remain the largest contributor to the uncertainty in shipping O3 and CH4 perturbations ([-39%, 67%] and [-37%, 59%], respectively). The shipping emission parameterisation developed by Dr. Huszar was found to be unsuitable for usage in MOZART-4, partly because of the absence of NOx sink processes. This was corrected by implementing the approach designed by Dr. Moldanova which lead to NOx and O3 reductions larger than 20% at the surface of the oceans. A net radiative forcing from shipping NOx emissions of -60 mW/m2 was computed with Edwards-Slingo in 2000. Under the RCP scenarios, the net radiative forcing range between -96 mW/m2 and -15 mW/m2. A linear relationship was finally derived between shipping NOx emissions and the radiative forcing of shipping O3 and CH4. Estimates calculated with the linear model remain within 15% of the values computed with the global models. Further work should focus on trying to reduce the uncertainties linked to the magnitude and spatial distribution of shipping emissions. The shipping emission parameterisation based on the concept of effective reaction rates still bear problems which should be addressed in the future. Finally, it would be interesting to estimate the future radiative forcings from shipping NOx emissions under the SRES scenarios since these forecast values are larger shipping emissions. This would potentially increase the robustness of the linear model.