Ozone levels in the Spanish Sierra de Guadarrama mountain range are above the thresholds for plant protection: analysis at 2262, 1850, and 995 m a.s.l.

The Sierra de Guadarrama mountain range, located at 60 km from Madrid City (Spain), includes high valuable ecosystems following an altitude gradient, some of them protected under the Sierra de Guadarrama National Park. The characteristic Mediterranean climatic conditions and the precursors emitted from Madrid favor a high photochemical production of ozone (O) in the region. However, very little information is available about the patterns and levels of O and other air pollutants in the high elevation areas and their potential effects on vegetation. Ozone levels were monitored at three altitudes (2262, 1850, and 995 m a.s.l.) for at least 3 years within the 2005-2011 period. NO and SO were also recorded at the highest and lowest altitude sites. Despite the inter-annual and seasonal variations detected in the O concentrations, the study revealed that SG is exposed to a chronic O pollution. The two high elevation sites showed high O levels even in winter and at nighttime, having low correlation with local meteorological variables. At the lower elevation site, O levels were more related with local meteorological and pollution conditions. Ozone concentrations at the three sites exceeded the thresholds for the protection of human health and vegetation according to the European Air Quality Directive (EU/50/2008) and the thresholds for vegetation protection of the CLRTAP. Ozone should be considered as a stress factor for the health of the Sierra de Guadarrama mountain ecosystems. Furthermore, since O levels at foothills differ from concentration in high elevation, monitoring stations in mountain ranges should be incorporated in regional air quality monitoring networks.