Meteorology and photochemical air pollution in Southern Europe: Experimental results from EC research projects

The MECAPIP project of EC has served to document the summer atmospheric circulations and the related air pollution dynamics over Spain and the Western Mediterranean. The first includes surface wind convergence over the Iberian peninsula, large-scale compensatory subsidence over the surrounding coastal areas, and the formation of re-circulatory cells as a result of the sea breezes combining with up-slope winds and their return and compensatory flows. With respect to the second, experimental evidence indicates that, (a) during the day the sea breezes can transport ozone 60 to 100, or more, km inland, (b) during the afternoon, photo-oxidants are injected 3 to 5 km into the mid-troposphere over the Spanish Central plateau, and (c) stratified reservoir layers, stacked up to approximate to 2-3 km high, build up along the Mediterranean coast. These retain ozone from one day to the next and following days. The lowermost layers are drawn inland by the sea breeze during the day, and the aged pollutants can react with new coastal emissions. All of these processes have a marked diurnal cycle and can recur for many consecutive days or weeks. Tracer experiments show that the average return times for pollutants can be of the order of 2-3 d. This includes first emission on the coast in the morning, travel inland with sea breeze, convective and/or orographic injection in return flows followed by layering and subsidence over the sea, and their final re-entry with the sea breeze a few days later. All together, those mechanisms can be considered to form a large ''photochemical reactor'' which operates almost every day from spring to fall. It can generate ozone levels 2 to 3 times higher than EC directives for damages to vegetation 65 mu g m(-3) (approximate to 32 ppbv), 24 h avg. This work presents a summary of results illustrating some of the above-mentioned processes. Similar situations are also expected to occur in other Mediterranean regions. Copyright (C) 1996 Elsevier Science Ltd