Are we on the right track, or is the ozone problem getting worse?
This question cannot be answered unambiguously and the answer will depend on the status indicator used, which means what parameter is used to observe the evolution of the status.
For the protection of public health, the daily maximum 8-hour mean is used as a parameter. As a long term goal it is decreed that this parameter may not exceed 120 µg/m³ on a single day. As a stepping stone, since 2010 this threshold may only be exceeded 25 days per calendar year, averaged over 3 years.
With the aid of this parameter, a 'peak indicator', called the AOT60ppb-max8h can be defined as the excesses above 120 µg/m³ of the daily 8-hour mean summed for every day in a year. As a 'mean indicator', the average value of all ozone concentrations measured during one year can be used (annual mean).
When considering the peak indicator, from analysis of the ozone measurements, it becomes clear (because of the decreasing emissions of precursors in Europe) that the size and number of ozone peak concentrations decreases since the mid 1990's (when looking at comparable summers).
However, if we consider the mean indicator (i.e. annual mean ozone concentrations), an unfavorable increasing trend can be observed, which points to increasing background concentrations. The fact that these background levels do increase, can on the one hand be explained by the global increase of ozone precursors in the northern hemisphere and on the other hand by a decreased ozone decomposition because of already imported local NOx-reductions, which leads to an increase of ozone levels, at least for our region.