Are Changes in Ground-level Ozone Concentrations in line with emission changes?

Over the last 15-20 years European countries have implemented measures to reduce the emissions of SOx, NOx, NMVOC, NH3 and CO. The rationale is to reduce the sulphur and nitrogen fluxes at ecosystems as well as to reduce the exposure of humans to elevated ozone concentrations. This has resulted in a substantial European wide reduction of these emissions, albeit that timing and size of the emission reduction varies from country to country. Modeling studies have shown that in response to the emission changes the ozone concentrations in the upper part of the distribution (90-percentile and higher) should drop. An analysis of time series of ozone sites indicates that in the first part of the 1990s high ozone concentrations have decreased but that since 1996/7 the levels have stabilized. This lack of a downward trend over the last 10 years is seen at various regions in Europe. Comparison with time series of ozone concentrations from dispersion models might shed light on the causes of the discrepancies between observed and expected ozone trends.