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It is well established that in Europe, high O-3 concentrations are most pronounced in southern/Mediterranean countries due to the more favourable climatological conditions for its formation. However, the contribution of the different sources of precursors to O-3 formation within each country relative to the imported (regional and hemispheric) O-3 is poorly quantified. This lack of quantitative knowledge prevents local authorities from effectively designing plans that reduce the exceedances of the O-3 target value set by the European air quality directive.O-3 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 O-3 and precursors from neighbouring regions, O-3 regional and hemispheric transport and stratospheric O-3 injections. The main goal of this study is to provide a first quantitative estimation of the contribution of the main anthropogenic activity sectors to peak O-3 events in Spain relative to the contribution of imported (regional and hemispheric) O-3. We also assess the potential of our source apportionment method to improve O-3 modelling. Our study applies and thoroughly evaluates a countrywide O-3 source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 x 4 km(2)) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (IP). The method tags both O-3 and its gas precursor emissions from source sectors within one simulation, and each tagged species is subject to the typical physico-chemical 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 O-3 concentrations compared with the contribution of imported O-3. We show, for the first time, that imported O-3 is the largest input to the ground-level O-3 concentration in the IP, accounting for 46 %-68 % of the daily mean O-3 concentration during exceedances of the European target value. The hourly imported O-3 increases during typical northwestern advections (70 %-90 %, 60-80 mu g m(-3)), and decreases during typical stagnant conditions (30 %-40 %, 30-60 mu g m(-3)) due to the local NO titration. During stagnant conditions, the local anthropogenic precursors control the O-3 peaks in areas downwind of the main urban and industrial regions (up to 40 % in hourly peaks). We also show that ground-level O-3 concentrations are strongly affected by vertical mixing of O-3-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 O-3 at ground level in the IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O-3 concentrations for local O-3 management, and show O-3 source apportionment to be an essential analysis prior to the design of O-3 mitigation plans in any non-attainment area.

Achieving the European O-3 objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies.