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

Hůnová, Iva, Bäumelt, Vít
Atmospheric environment 2018 v.172 pp. 157-167
atmospheric chemistry, emissions, highways, monitoring, nitric oxide, nitrogen dioxide, ozone, power plants, statistics, summer, urban areas, Czech Republic
We present the trends in ambient ozone concentrations based on high quality data measured continuously at 26 long-term monitoring sites (9 urban, 17 rural including 10 mountain stations) in the Czech Republic in 1994–2015. We considered annual and summer medians, the 10ᵗʰ and 98ᵗʰ percentiles, maximum daily 8-h running mean concentrations and exposure index AOT40F. For all indicators taken into account except for the 10ᵗʰ percentile, our results showed a similar pattern with significant decreasing trends for about one half of the examined sites. We obtained similar results for all types of sites. The most pronounced decrease in O3 concentrations was recorded at mountain sites. Namely, at the Šerlich mountain site, with an overall decrease per year in annual median by 0.43 ppb, summer median by 1.17 ppb, maximal daily 8-h average by 0.45 ppb, the 10ᵗʰ percentile by 0.62 ppb. The peak concentrations indicated by the 98ᵗʰ percentile and AOT40F decreased most at urban site České Budějovice by 0.75 ppb and 0.84 ppb h per year, respectively. For sites exhibiting significant decreasing trends, an overall decrease per year in annual median was 0.22 ppb, in summer median 0.41 ppb, in the 10ᵗʰ percentile 0.23 ppb, in the 98ᵗʰ percentile 0.53 ppb, and in AOT40F 0.51 ppb h. A significant increasing trend was detected only in the 10ᵗʰ percentile at just three sites, with the highest increase of 0.19 ppb per year recorded at the rural site Sněžník. Moreover, a consistent decrease in limit value exceedances was detected, with by far the highest violation recorded in the meteorologically exceptional year of 2003.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.