The focus in earlier analyses of ground level ozone was on the peak O3 concentrations which occur under warm, sunny conditions, and thus largely occur in the UK from April to September. Policy actions to date across Europe have reduced the emissions of NOx and volatile organic compounds (VOCs) (ozone precursors). These emission controls have reduced peak ozone concentrations by typically 30 ppb in the UK, but over the last 20 years mean concentrations have been increasing in urban areas due to reductions in local depletion of O3 by NO, and in rural areas due to increases in the hemispheric background O3 concentration. The increases in background ozone concentrations and decreases in peak values are common to rural areas throughout the UK and therefore are potentially important for effects on sensitive vegetation across the country (RoTAP, 2012).