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The southern Mediterranean region frequently experiences critical levels ofphotochemical pollutants during summertime. In order to account for thecontribution of different atmospheric processes during this type ofepisodes, the WRF-ARW/HERMES/CMAQ modelling system was applied with highresolution (1 km2, 33 sigma vertical layers, 1 h) to assess thedifferent dynamics in a coastal environment and an inland-continental zone:the North-Eastern and Central Iberian Peninsula (NEIP and CIP,respectively). The former is characterized by a very complex terrain, whilethe latter behaves as a flat area, which clearly affects the pattern oflocal flows. A representative type of photochemical pollution episode(occurring over 78% of summer days) which occurred during 17–18 June,2004 is selected as the study period. The CMAQ Integrated Process Rateprovides the hourly contributions of atmospheric processes to net O3,NOx and NMVOCs concentrations. The O3 photochemical formationoccurs mainly in downwind areas from the main NOx emission sourcesduring midday. At surface level it accounts for 50 to 75 μg m−3 h−1.The urban areas and main roads, as main sources of NOxemissions, act as O3 sinks, quenching up to −200 μg m−3 perhour during the traffic circulation peaks. The O3 concentrationgradient generated, larger during daytime, increases the contribution ofdiffusion processes to ground-level O3 (up to 200 μg m−3 h−1fluxes, mainly from upper vertical layers). The maximum positivecontributions of gas-phase chemistry to O3 occur in the coastal domainat high levels (around 500 to 1500 m a.g.l.), while in the continental domainthey take place in the whole atmospheric column under the PBL. The transportof ozone precursors by advective flows determines the location of themaximum O3 surface concentrations. The O3 chemical formationinvolves the oxidation of less NMVOCs in the NEIP than in the CIP domains,due to differences in chemical sensitivity between these areas. The drydeposition is an important sink in the lowest layer of the model, togetherwith vertical diffusion flows. Finally, the contributions from cloudprocesses, wet deposition and heterogeneous chemistry are negligible duringthe whole episode, characterized by a high solar radiation and neitherprecipitation nor cloudiness. This process analysis provides newquantitative information about the origin of the peaks of O3 and itsprecursors, aiding the design of abatement strategies in South-WesternEurope

atmospheric chemistry and physics

Contribution of atmospheric processes affecting the dynamics of air pollution in South-Western Europe during a typical summertime photochemical episode