Spatiotemporal trends in ground-level ozone concentrations and metrics in France over the time period 1999-2012

The hourly ozone (O-3) data from 332 background monitoring stations, spread in France, were analyzed over the period 1999-2012 and short-term trends were calculated. In the current climate change context, the calculation of human health- and vegetation-relevant metrics, and of associated trends, provides a consistent method to establish proper and effective policies to reduce the adverse O-3 effects. The generation of optimal O-3 maps, for risk and exposure assessment, is challenging. To overcome this issue, starting from a set of stations, a hybrid regression-interpolation approach was proposed. Annual surface O-3 metrics, O-3 human health metrics (number of exceedances of daily maximum 8-h values greater than 60 ppb and SOMO35) and O-3 vegetation impact metrics (AOT40 for vegetation and forests) were investigated at individual sites. Citizens are more exposed to high O-3 levels in rural areas than people living in the cities. The annual mean concentrations decreased by -0.12 ppb year(-1) at rural stations, and the significant reduction at 67% of stations, particularly during the warm season, in the number of episodic high O-3 concentrations (e.g. 98th percentile, -0.19 ppb year(-1)) can be associated with the substantial reductions in NOx and VOCs emissions in the EU-28 countries since the early 1990s Inversely, the O-3 background level is rising at 76% of urban sites (+0.14 ppb year(-1)), particularly during the cold period. This rise can be attributed to increases in imported O-3 by long-range transport and to a low O-3 titration by NO due to the reduction in local NOx emissions. The decrease in health-related and vegetation-relevant O-3 metrics, at almost all stations, is driven by decreases in regional photochemical O-3 formation and in peak O-3 concentrations. The short-term trends highlight that the threat to population and vegetation declined between 1999 and 2012 in France, demonstrating the success of European control strategies over the last 20 years. However, for all exposure metrics, the issue of non-attainment of the target value for O-3 persists in comparison with the objectives of air quality directives. The region at highest O-3 risk is the South-eastern France. This study contains new information on the i) spatial distribution of surface O-3 concentration, ii) exceedances and iii) trends to define more suitable standards for human health and environmental protection in France. (c) 2016 Elsevier Inc. All rights reserved.