Source Attribution and Radiative Impacts of the Mediterranean Summertime Ozone Maximum: a Satellite and Model Perspective

Abstract. The Mediterranean troposphere exhibits a marked and localised summertime ozone maximum, which has the potential to strongly impact regional air quality and radiative forcing. The Mediterranean region can be perturbed by long-range pollution import from Northern Europe, North America and Asia, in addition to local emissions, which may all contribute to regional ozone enhancements. We exploit ozone profile observations from the Tropospheric Emission Spectrometer (TES) and the Global Ozone Monitoring Experiment-2 (GOME-2) satellite instruments, and an offline 3-D global chemical transport model (TOMCAT) to investigate the geographical and vertical structure of the summertime tropospheric ozone maximum over the Mediterranean region. We show that both TES and GOME-2 are able to detect enhanced levels of ozone in the lower troposphere over the region during the summer. These observations, together with surface measurements, are used to evaluate the TOMCAT model's ability to capture the observed ozone enhancement. The model is used to quantify contributions to the ozone maximum from anthropogenic and natural volatile organic compound (VOC) emissions, anthropogenic NOx emissions, wildfire emissions and long-range import of ozone and precursors. Our results show a dominance of natural VOC emissions on ozone in the Mediterranean Basin over anthropogenic VOC emissions. However, local anthropogenic NOx emissions are the overall dominant contribution to near-surface ozone. We also show that in the lower troposphere, global VOC emissions account for 40% of the VOC contribution to ozone in the region, whereas, for NOx the global contribution is only 10% at these altitudes. However, in the mid and upper troposphere almost all of the ozone comes from long-range transport for all emission sources. In terms of radiative effects on regional climate, ozone contributions from non-local sources are more important with Asian monsoon outflow having the greatest impact. Our results allow improved understanding of the large-scale processes controlling air quality and climate in the region of the Mediterranean Basin.