Spatiotemporal variations of ozone exposure and its risks to vegetation and human health in Cyprus: an analysis across a gradient of altitudes

Ground-level ozone (O-3) affects vegetation and threatens environmental health when levels exceed critical values, above which adverse effects are expected. Cyprus is expected to be a hotspot for O-3 concentrations due to its unique position in the eastern Mediterranean, receiving air masses from Europe, African, and Asian continents, and experiencing a warm Mediterranean climate. In Cyprus, the spatiotemporal features of O-3 are poorly understood and the potential risks for forest health have not been explored. We evaluated O-3 and nitrogen oxides (NO and NO2) at four regional background stations at different altitudes over 2014-2016. O-3 risks to vegetation and human health were estimated by calculating accumulated O-3 exposure over a threshold of 40 nmol mol(-1) (AOT40) and cumulative exposure to mixing ratios above 35 nmol mol(-1) (SOMO35) indices. The data reveal that mean O-3 concentrations follow a seasonal pattern, with higher levels in spring (51.8 nmol mol(-1)) and summer (53.2 nmol mol(-1)) and lower levels in autumn (46.9 nmol mol(-1)) and winter (43.3 nmol mol(-1)). The highest mean O-3 exposure (59.5 nmol mol(-1)) in summer occurred at the high elevation station Mt. Troodos (1819 m a.s.l.). Increasing (decreasing) altitudinal gradients were found for O-3 (NOx), driven by summer-winter differences. The diurnal patterns of O-3 showed little variation. Only at the lowest altitude O-3 displayed a typical O-3 diurnal pattern, with hourly differences smaller than 15 nmol mol(-1). Accumulated O-3 exposures at all stations and in all years exceeded the European Union's limits for the protection of vegetation, with average values of 3-month (limit: 3000 nmol mol(-1) h) and 6-month (limit: 5000 nmol mol(-1) h) AOT40 for crops and forests of 16,564 and 31,836 nmol mol(-1) h, respectively. O-3 exposures were considerably high for human health, with an average SOMO35 value of 7270 nmol mol(-1) days across stations and years. The results indicate that O-3 is a major environmental and public health issue in Cyprus, and policies must be adopted to mitigate O-3 precursor emissions at local and regional scales.