How is surface ozone in Europe linked to Asian and North American NOx emissions?

Source–receptor relationships linking North American and Asia NOx emissions with surface ozone in Europe have been explored using a global Lagrangian chemistry-transport model. We used the model to perform a set of simulations, each 21 months in duration, which were comprised of a base case and a series of emission perturbation experiments. In each of the perturbation runs, an additional month-long NOx emission pulse from a particular 10° × 10° region was included. Overall, results from 42 different emission regions in both boreal summer and winter were analysed. Shortly after the start of each NOx emission pulse, increased ozone mixing ratios were detected at surface sites across Europe. The extra ozone peaked and then decayed away to leave a small but persistent ozone deficit that persisted throughout the year-long model experiments. The ozone responses varied spatially by over three orders of magnitude, depending on the location of the NOx emission pulses in each continent and on the receptor location within Europe. Source–receptor relationships also varied markedly with season. NOx emissions from lower latitudes, especially in the boreal summer, were found to decrease European ozone, when both the short- and long-term responses were considered. In this study, it has been possible to begin the process of examining the likely influences on ozone levels across Europe resulting from precursor emission controls in North America and Asia and, in turn, their possible impacts on meeting ozone air quality targets over Europe.