Analysis Of Ozone Measurements In TheMadrid Airshed

Abstract

This paper focuses on the results of an experimental study about the formation and transport of ozone in the Madrid airshed. Data from a year survey on three rural stations show a transport of ozone, with concentrations often exceeding hourly values of 90 ppb, generated from daily photochemical reactions of precursors emitted in the metropolitan area, mainly during late spring and summer months. The mean ozone production within the urban plume is calculated on the basis of maximum downwind and upwind values on polluted days. The knowledge of prevailing surface wind flows during episodic summer conditions leads to a preliminary characterization of the dynamics of the polluted airmass. Finally, the vertical meteorological and ozone structure in episodic summer conditions is presented from soundings performed in July 1993. MEASURING SITES AND INSTRUMENTATION The Madrid Greater Area is located in the center of the Iberian Peninsula, within the airshed defined by a high mountain range of a maximum height of 2400 m. (Sierra de Guadarrama), 40 km away from the city and with a NW-SE orientation. The urban area is settled over a non homogeneous basin approximately 700 meters high. The metropolitan area has a population of 5 million inhabitants. Traffic, heating and small industries are the main pollutant sources. In the past, the city of Madrid has undergone serious pollution problems derived from coal consumption in domestic heating, and recently some studies have addressed the increasing photochemical pollution in the area (1). From November 1991 to December 1992, meteorological towers were deployed on five rural sites to document local flows as well as the Transactions on Ecology and the Environment vol 4, © 1994 WIT Press, www.witpress.com, ISSN 1743-3541 340 Pollution Control and Monitoring interaction between wind field and topography in the basin. Simultaneously, continuous ozone and complementary NOx measurements were performed by means of automatic analyzers on three of these sites, approx. 30 km away from the urban center, Villanueva (620 m. ASL, West), Hoyo (1050 m. ASL, NW) and San Martin (515 ASL, SE). Additional available ozone data of three urban stations provided by the Town Council have also been analyzed: Casa de Campo (West), Moratalaz (E) and Avda. Aragon (NE). Figure 1 is a bidimensional view showing the topographical features of the Madrid airshed, the metropolitan area and the location of the three rural measuring sites and the three urban monitoring stations. During a week in July 1993, meteorological and ozone soundings up to a maximum height of 2 km. were carried out in the Villanueva site. Ozone measurements were performed using a Mast-Brewer electrochemical sonde. This sonde was previously calibrated in the laboratory in order to obtain the response factor of the sonde measuring known ozone concentrations in mixtures with dry and clean air. Laboratory and field tests of the sonde showed an accuracy of ±20%. RESULTS AND DISCUSSION Surface measurements The series of hourly averaged NOx and 0% concentrations during 1992 on the rural and urban monitoring sites are summarized on table 1. The difference between rural and urban monitoring sites is clearly appreciable on the levels of NO and O% concentrations. Moratalaz and Avda. Aragon sites are located near busy roads and experience high levels of NO emitted by traffic. The urban station of Casa de Campo, located on a great park area, experiences the highest NOg values, and Villanueva presents the Og and NO% maximum values of the three rural sites. These two sites are located in the west of the metropolitan area. TABLE 1. Hourly averaged values during 1992 (ppb)