Durante el verano, la región sur de la cuenca mediterránea experimenta con frecuencia niveles altos de contaminantes químicos en el aire. "Los peores niveles de calidad del aire se observan en zonas a sotavento de Barcelona y Madrid, debido a la contaminación urbana".
En ambas ciudades los episodios de contaminación acústica son frecuentes, y proceden de las emisiones urbanas como el tráfico, las industrias y la actividad portuaria. En Barcelona, los contaminantes se dispersan gracias a las brisas; en Madrid, lo hacen por el ascenso y descenso del aire que genera el calentamiento superficial.
En las observaciones realizadas, se registraron mayores concentraciones de ozono sobre Madrid y niveles mayores de dióxido de nitrógeno y de partículas de polvo sobre Barcelona. El trabajo, dirigido por el investigador del BSC José Maria Baldasano, se ha publicado en ‘Atmospheric Chemistry and Physics’.
Los investigadores seleccionaron uno de los episodios de contaminación fotoquímica más agudos en estas áreas, el 17 y 18 de junio de 2004, un fenómeno meteorológico "que sucede en un 78% de las situaciones de verano", subraya la investigadora.
Los principales focos de contaminación atmosférica provienen de las emisiones derivadas de las actividades urbanas, sobre todo el tráfico, "aunque no se pueden descartar los puntos de emisión industrial ni las actividades desarrolladas en el puerto, especialmente en Barcelona", recalca Gonçalves.
Los óxidos de nitrógeno emitidos en las áreas urbanas y en la red viaria "son transportados a sotavento donde se consumen por reacción química o se depositan a nivel superficial", manifiesta Gonçalves.
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Atmos. Chem. Phys., 9, 849-864, 2009
www.atmos-chem-phys.net/9/849/2009/
© Author(s) 2009. This work is distributed
under the Creative Commons Attribution 3.0 License.
Contribution of atmospheric processes affecting the dynamics of air pollution in South-Western Europe during a typical summertime photochemical episode
M. Gonçalves1, P. Jiménez-Guerrero2, and J. M. Baldasano1,2
1Projects Engineering Department, Technical University of Catalonia, Barcelona, Spain
2Barcelona Supercomputing Center-Centro Nacional de Supercomputación, Barcelona, Spain
Abstract. The southern Mediterranean region frequently experiences critical levels of photochemical pollutants during summertime. In order to account for the contribution of different atmospheric processes during this type of episodes, the WRF-ARW/HERMES/CMAQ modelling system was applied with high resolution (1 km2, 33 sigma vertical layers, 1 h) to assess the different 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, while the latter behaves as a flat area, which clearly affects the pattern of local 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 Rate provides the hourly contributions of atmospheric processes to net O3, NOx and NMVOCs concentrations.
The O3 photochemical formation occurs mainly in downwind areas from the main NOx emission sources during 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 NOx emissions, act as O3 sinks, quenching up to −200 μg m−3 per hour during the traffic circulation peaks. The O3 concentration gradient generated, larger during daytime, increases the contribution of diffusion processes to ground-level O3 (up to 200 μg m−3 h−1 fluxes, mainly from upper vertical layers).
The maximum positive contributions of gas-phase chemistry to O3 occur in the coastal domain at high levels (around 500 to 1500 m a.g.l.), while in the continental domain they take place in the whole atmospheric column under the PBL. The transport of ozone precursors by advective flows determines the location of the maximum O3 surface concentrations. The O3 chemical formation involves the oxidation of less NMVOCs in the NEIP than in the CIP domains, due to differences in chemical sensitivity between these areas.
The dry deposition is an important sink in the lowest layer of the model, together with vertical diffusion flows. Finally, the contributions from cloud processes, wet deposition and heterogeneous chemistry are negligible during the whole episode, characterized by a high solar radiation and neither precipitation nor cloudiness. This process analysis provides new quantitative information about the origin of the peaks of O3 and its precursors, aiding the design of abatement strategies in South-Western Europe.
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