Ozone source apportionment during peak summer events over
southwestern Europe
Authors: Pay Maria Teresa , Gangoiti Gotzon , Guevara Marc , Napelenok Sergey , Querol Xavier , Jorba Oriol , Pérez García-Pando Carlos 
Published: September 2018

Abstract. It is well established that in Europe, high O3 concentrations are most pronounced in southern/Mediterranean countries due to the more favorable climatological conditions for its formation. However, the contribution of the different sources of precursors to O3 formation within each country relative to the imported (regional and hemispheric) O3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O3 Target Value set by the European Air Quality Directive. O3 source attribution is a challenge because the concentration at each location and time results not only from local biogenic and anthropogenic precursors, but also from the transport of O3 and precursors from neighbouring regions, O3 regional and hemispheric transport and stratospheric O3 injections. Our study applies and thoroughly evaluates a countrywide O3 source apportionment method implemented in a Chemical Transport Model (CTM) at high resolution (4 × 4 km) over the Iberian Peninsula (IP) to understand and quantify the origin of peak O3 events over a 10-day period covering the most frequent synoptic summer conditions in the IP. The method tags both O3 and its gas precursor emissions from source sectors within one simulation and each tagged species is subject to the typical physical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NOx local sources to high O3 concentrations compared to the contribution of imported O3. We show for the first time that imported O3 is the largest input to the ground-level O3 concentration in the IP, accounting for 46 % to 68 % of the daily mean O3 concentration during exceedances of the European Target Value. The hourly imported O3 increases during typical northwestern advections (70–90 %, 60–80 µg/m3), and decreases during typical stagnant conditions (30–40 %, 30–60 µg/m3) due to the local NO titration effect. During stagnant conditions, the anthropogenic precursors control the O3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O3 concentrations are strongly affected by vertical mixing of O3-rich layers present in the free troposphere, which result from local/regional layering and accumulation, and continental/hemispheric transport. Indeed, vertical mixing largely explains the presence of imported O3 at ground level in the Iberian Peninsula. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O3 concentrations for local O3 management, being the O3 source apportionment an essential analysis prior to the design of O3 mitigation plans in any non-attainment area. To achieve the European O3 objectives in southern Europe, ad hoc local actions should be complemented by decided national and European-wide strategies.