Intercontinental Source−Receptor Relationships for Ozone Calculated with a Tagging Approach

HTAP Phase 2 is a set of cooperative multi-model experiments organized by the Hemispheric Transport of Air Pollution (HTAP) Task Force, and is intended to inform the Convention on Long-range Transboundary Air Pollution (LRTAP) and other multi-lateral cooperative efforts, as well as national governments on actions to decrease air pollution and its impacts. The standard HTAP2 experiment protocol specifies perturbations to emissions of ozone precursors in a number of continental source regions in order to determine the multi-model response of ozone in a number of sub-continental receptor regions around the Northern Hemisphere. Here we present a set of complementary simulations performed with an alternative approach. Using the Community Atmosphere Model with Chemistry (CAM-Chem), we implement a tagged chemical mechanism which allows the labelling of anthropogenic ozone precursor emissions (here primarily NOx) from HTAP source regions, and the attribution of subsequently produced ozone to these emissions. By simultaneously labelling all anthropogenic and natural precursor emissions, we construct a complete source-receptor matrix based on the contribution of labeled ozone to the total ozone mixing ratio in each receptor region. In most HTAP receptor regions, locally-emitted anthropogenic NOx is primarily responsible for the modelled summertime maximum in surface ozone mixing ratio. Biogenic NOx from soils can also contribute up to 10 ppb to mean summertime surface ozone in many receptor regions. During springtime, surface ozone is dominated by long range transport. A springtime maximum is seen for ozone of stratospheric origin, contributing up to 5 ppb to surface ozone mixing ratios in most HTAP receptor regions. Ozone produced from remotely emitted anthropogenic NOx also shows a springtime maximum in most receptor regions. This springtime maximum in long range transported ozone contributes to a broadening of the peak in the annual cycle of surface ozone. In some receptor regions, especially at high northern latitudes where local photochemistry is slow, the annual maximum in surface ozone occurs during spring and is due in large part to remotely emitted anthropogenic precursors. International shipping is also found to contribute significantly to surface ozone (up to 10 ppb), especially in receptor regions on the western sides of continents. Our tagging approach produces results which are broadly consistent with more common perturbation methods, while also providing a fully closed diagnostic of the modelled tropospheric ozone budget with respect to precursor emissions. If this method were to be implemented in other global models, this would provide a powerful tool for model intercomparison.