Investigating Key Uncertainties in Models of Tropospheric Photochemistry

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Our understanding of the impacts of atmospheric composition on climate and air quality relies on simulations from numerical models of atmospheric chemistry and transport. The photochemical components of such models are uncertain, due to the reductions of detailed chemical mechanisms introduced to meet computing demands, and also due to uncertainties in empirically-derived kinetic parameters used in their representations of photochemistry. Validation of the ozone photochemistry component of models by comparison with observations is difficult, since the long lifetime of ozone in the free troposphere (~20 days), means that variability in its concentration at a given point in the atmosphere is generally dominated by transport rather than photochemistry. However, validation of our understanding of ozone photochemistry is critical to our predictions of future climate and air quality, particularly under future changes to ozone precursor sources, and any changes to other parts of the tropospheric ozone budget, such as stratospheric input and surface deposition. This proposal exploits a unique set of observations made during the ITCT Lagrangian-2K4 (Intercontinental Transport and Chemical Transformation) experiment. These provide measurements of a suite of trace gases and aerosol in single air masses at multiple stages during their advection across the North Atlantic from North America to Europe. These linked observations allow ozone photochemistry to be examined in a 'flow-relative', pseudo-Lagrangian frame of reference, removing the influence of advection on the observed ozone change. Simulations of ozone change within the observed air masses using a Lagrangian chemical transport model (CTM) will for the first time allow uncertainties in model photochemistry to be assessed over a timescale of several days. Comparison of Monte-Carlo type model simulations, perturbed by model uncertainties, with observed composition changes within air masses, will test the consistency of model photochemistry with observations. Information will be gained on which uncertainties have the largest impacts on ozone photochemistry. Recommendations will then be made of where effort should be focused in reducing uncertainties in laboratory or atmospheric measurement. The proposal envisages a short 1-year project, using existing tools to investigate these issues, which are of central importance to our confidence in models of air quality and climate. The model framework has already been developed to run locally in Leeds, and the full set of observations is available for use immediately. The project will benefit from expertise from one of the leading atmospheric modelling groups in the UK, in addition to local expertise in kinetic uncertainties and parameterisations from close links with groups in the School of Chemistry.