Observation-based trends in ambient ozone in the Czech Republic over the past two decades

Out of the 26 sites under review, seven have not recorded a significant decreasing trend in O3 in any of the considered statistics. The lack of trends in O3 at these seven sites is likely associated with changing time patterns in local NO and NO2 emissions: in particular, with the increasing ratio in NO2/NOx. There is an obvious geographical pattern in recorded O3 trends: most of the sites with no trend detected are situated in the North-western region of the CR with numerous energy-producing large emission sources, partly denitrified recently. Our results clearly indicated that, for O3 decrease, the ratio between individual NO and NO2 forms is critical, and that a simultaneous significant decrease in both NO and NO2 concentrations is not a sufficient prerequisite. Apart from changes in car fleet in urban areas or near motorways, this factor might be of particular relevance in formerly highly polluted areas, where emissions from large power plants recently substantially decreased.