Surface ozone measurements in the southwest of the Iberian Peninsula (Huelva, Spain)

Introduction Photochemical ozone pollution of the lower troposphere (LT) is a very complex process involving meteorological, topographic emissions and chemical parameters. Ozone is considered the most important air pollutant in rural, suburban and industrial areas of many sites in the world since it strongly affects human health, vegetation and forest ecosystems, and its increase during the last decades has been significant. In addition, ozone is a greenhouse gas that contributes to climate change. For these reasons, it is necessary to carry out investigations that determine the behaviour of ozone at different locations. The aim of this work is to understand the levels and temporal variations of surface ozone in an industrial-urban region of the Southwest Iberian Peninsula. Materials and methods The study is based on ozone hourly data recorded during a 6-year period, 2000 to 2005 at four stations and meteorological data from a coastal station. The stations used were El Arenosillo and Cartaya—both coastal stations, Huelva—an urban site and Valverde—an inland station 50 km away from the coastline. The general characteristics of the ozone series, seasonal and daily ozone cycles as well as number of exceedances of the threshold established in the European Ozone Directive have been calculated and analysed. Results Analysis of the meteorological data shows that winter-autumn seasons are governed by the movement of synoptic weather systems; however, in the spring-summer seasons, both synoptic and mesoescale conditions exist. Average hourly ozone concentrations range from 78.5 ± 0.1 μg m⁻³ at Valverde to 57.8 ± 0.2 μg m⁻³ at Huelva. Ozone concentrations present a seasonal variability with higher values in summer months, while in wintertime, lower values are recorded. A seasonal daily evolution has also been found with minimum levels around 08:00 UTC, which occurs approximately 1-1.5 h after sunrise, whereas the maximum is reached at about 16:00 UTC. Furthermore, during summer, the maximum value at El Arenosillo and Valverde stations remains very uniformed until 20:00 UTC. These levels could be due to the photochemical production in situ and also to the horizontal and vertical ozone transport at El Arenosillo from the reservoir layers in the sea and in the case of Valverde, the horizontal transport, thanks to the marine breeze. Finally, the data have been evaluated relative to the thresholds defined in the European Ozone Directive. The threshold to protect human health has been exceeded during the spring and summer months mainly at El Arenosillo and Valverde. The vegetation threshold has also been frequently exceeded, ranging from 131 days at Cartaya up to 266 days at Valverde. Discussion The results in the seasonal and daily variations demonstrate that El Arenosillo and Valverde stations show higher ozone concentrations than Cartaya and Huelva during the spring and summer months. Under meteorological conditions characterized by land-sea breeze circulation, the daytime sea breeze transports the emissions from urban and industrial sources in the SW further inland. Under this condition, the area located downwind to the NE is affected very easily by high ozone concentrations, which is the case for the Valverde station. Nevertheless, according to this circulation model, the El Arenosillo station located at the coast SE from these sources is not directly affected by their emissions. The ozone concentrations observed at El Arenosillo can be explained by the ozone residual layer over the sea, similar to other coastal sites in the Mediterranean basin. Conclusions The temporal variations of the ozone concentrations have been studied at four measurement sites in the southwest of the Iberian Peninsula. The results obtained point out that industrial and urban emissions combined with specific meteorological conditions in spring and summer cause high ozone levels which exceed the recommended threshold limits and could affect the vegetation and human health in this area. Recommendations and perspectives This work is the first investigation related to surface ozone in this region; therefore, the results obtained may be a useful tool to air quality managers and policy-makers to apply possible air control strategies towards a reduction of ozone exceedances and the impact on human health and vegetation. Due to the levels, variability and underlying boundary layer dynamics, it is necessary to extend this research in this geographical area with the purpose of improving the understanding of photochemical air pollution in the Western Mediterranean Basin and in the south of the Iberian Peninsula.