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 (O3) 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 O3 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 O3 are poorly understood and the potential risks for forest health have not been explored. We evaluated O3 and nitrogen oxides (NO and NO2) at four regional background stations at different altitudes over 2014-2016. O3 risks to vegetation and human health were estimated by calculating accumulated O3 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 O3 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 O3 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 O3 (NOx), driven by summer-winter differences. The diurnal patterns of O3 showed little variation. Only at the lowest altitude O3 displayed a typical O3 diurnal pattern, with hourly differences smaller than 15 nmol mol-1. Accumulated O3 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. O3 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 O3 is a major environmental and public health issue in Cyprus, and policies must be adopted to mitigate O3 precursor emissions at local and regional scales.