Environ Monit Assess (2008) 136:3–11 DOI 10.1007/s10661-007-9723-1 Analysis of tropospheric ozone concentration on a Western Mediterranean site: Castellon (Spain) Nuria Castell · Enrique Mantilla · Millan M. Millan Received: 5 February 2006 / Accepted: 26 December 2006 / Published online: 16 May 2007 © Springer Science + Business Media B.V. 2007 Abstract Ozone dynamics in our study area (Castellon, Spain) is both strongly bound to the mesoscale circulations that develop under the effect of high insolation (especially in summer) and conditioned by the morphological character- istics of the Western Mediterranean Basin. In this work we present a preliminary analysis of ozone time series on five locations in Castellon for the period 1997–2003. We study their temporal and spatial variations at different scales: daily, weekly, seasonally and interannually. Because both the O 3 concentration and its temporal variation depend on the topographic location of the observing sta- tion, they can show large differences within tens of kilometer. We also contrast the variation in the ozone concentration with the variations found for meteorological variables such as radiation, tem- perature, relative humidity and recirculation of the air mass. The link between elevated ozone concentrations and high values of the recircula- tion factor (r = 0.7 − 0.9) shown the importance N. Castell · E. Mantilla · M. M. Millan Centro de Estudios Ambientales del Mediterraneo, Paterna, Valencia, Spain N. Castell (B ) Fundacion CEAM, Parque Tecnologico Paterna, C. Charles R. Darwin, 14, E-46980, Paterna, Valencia, Spain e-mail: nuria@ceam.es of recirculating flows on the local air pollution episodes. Keywords Ozone time series · Mesoscale circulations · Meteorological variables · Recirculation factor Introduction Pollution by ozone is the result of a complex process that involves chemical reactions between nitrogen oxides, volatile organic compounds and oxygen in the presence of solar light. The highest surface ozone concentrations occur mainly in the seasons of the year with greatest insolation and are usually associated with poor-ventilation situ- ations. The meteorological conditions favouring these situations reduce the dispersion of the pol- luting agents and thus increase the probability that they will react. Pollution on a local or regional scale is determined by a great number of factors that can be combined into three large groups: orographic, meteorological and precursor sources. The Mediterranean area is located in subtropical lati- tudes. In the case of Castellon, a strong altitudinal gradient is registered, from 0 to 1,500 m in 100 km. Also characteristic of the zone is the complexity of its topography, abounding in river channels that provide penetration paths for the coastal