In summer, the complex layout of the coasts and mountains surrounding the western Mediterranean basin favors the development of mesoscale atmospheric recirculations and the formation of ozone reservoir layers above the coastal areas and the sea. Tropospheric ozone cycles vary here according to location and exposure of the monitoring station in relation to the flows and reservoir layers, and large differences can be encountered within tens of kilometers. The basic premise for this work is that the representativeness of any station is determined by the (fore)knowledge of the processes affecting the site, at the proper timescales and space scales within its region. Thus, available data have been combined with mesoscale analysis and modeling to interpret the observed summer ozone cycles for the monitoring network at Castellón, on the Spanish east coast. The area is approximately 120 km by 120 km, is backed by coastal mountains, and includes the following: a conurbation, industries, and a densely trav... Abstract In summer, the complex layout of the coasts and mountains surrounding the western Mediterranean basin favors the development of mesoscale atmospheric recirculations and the formation of ozone reservoir layers above the coastal areas and the sea. Tropospheric ozone cycles vary here according to location and exposure of the monitoring station in relation to the flows and reservoir layers, and large differences can be encountered within tens of kilometers. The basic premise for this work is that the representativeness of any station is determined by the (fore)knowledge of the processes affecting the site, at the proper timescales and space scales within its region. Thus, available data have been combined with mesoscale analysis and modeling to interpret the observed summer ozone cycles for the monitoring network at Castellón, on the Spanish east coast. The area is approximately 120 km by 120 km, is backed by coastal mountains, and includes the following: a conurbation, industries, and a densely trav...

Mesometeorological information obtained in several research projects in southern Europe has been used to analyze perceived changes in the western Mediterranean summer storm regime. A procedure was developed to disaggregate daily precipitation data into three main components: frontal precipitation, summer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processes involved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive to variations in surface airmass temperature and moisture. Land-use perturbations that accumulated over historical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed vertical recirculations where feedback mechanisms favor the loss of storms over the coastal mountains and additional heating of the sea surface temperature during summer. This, in turn, favors Mediterranean cyclogenesis and torrential rains in autumn–winter. Because these intense rains and floods can occur anywhere in the basin, perturbations to the hydrological cycle in any part of the basin can propagate to the whole basin and adjacent regions. Furthermore, present levels of air pollutants can produce greenhouse heating, amplifying the perturbations and pushing the system over critical threshold levels. The questions raised are relevant for the new European Union (EU) water policies in southern Europe and for other regions dominated by monsoon-type weather systems.

It is well established that in Europe, high O₃ 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 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. The main goal of this study is to provide a first quantitative estimation of the contribution of the main anthropogenic activity sectors to peak O₃ events in Spain relative to the contribution of imported (regional and hemispheric) O₃. We also assess the potential of our source apportionment method to improve O₃ modelling. Our study applies and thoroughly evaluates a countrywide O₃ source apportionment method implemented in the CALIOPE air quality forecast system for Spain at high resolution (4 × 4 km²) over a 10-day period characterized by typical summer conditions in the Iberian Peninsula (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 physico-chemical processes (advection, vertical mixing, deposition, emission and chemistry) as the actual conditions remain unperturbed. We quantify the individual contributions of the largest NO_x local sources to high O₃ concentrations compared with 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 %–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 m⁻³), and decreases during typical stagnant conditions (30 %–40 %, 30–60 µg m⁻³) due to the local NO titration. During stagnant conditions, the local 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 IP. Our results demonstrate the need for detailed quantification of the local and remote contributions to high O₃ concentrations for local O₃ management, and show O₃ source apportionment to be an essential analysis prior to the design of O₃ mitigation plans in any non-attainment area. Achieving the European O₃ objectives in southern Europe requires not only ad hoc local actions but also decided national and European-wide strategies.