RESUMEN
Nitrogen oxides (NOx) are important in atmospheric chemistry and have substantial environmental impacts. This study provides a detailed data analysis on long-term changes in ambient NOx levels within the framework of their emission pattern. We examined the trends in NOx, NO, NO2 and NO2/NOx ratio at 39 Czech sites representing different environments (urban, rural, mountain, industrial) in 1994-2016, i.e. a 23-year time series, using the non-parametric Man-Kendall test. The ambient air concentrations in NO, NO2 and NOx decreased significantly at most of the sites, as was assumed due to the substantial NOx emission decrease over the period under review. The largest decrease per year was detected at the urban site in capital Prague (decrease in the 98th percentile equal to 2.28â¯ppb, decrease in the annual median equal to 1.00â¯ppb). At some sites, however we observed a substantial equivocal temporal change in the NO2/NOx ratio. Whereas at some sites the NO2/NOx ratio was decreasing, at other sites the trend was increasing significantly. The highest increase per year in the annual median of the NO2/NOx ratio, equal to 0.0086 was detected at the urban Praha 1-nám-Rep. site; the highest increase per year in the 98th percentile, equal to 0.0126 was recorded at the rural Krupka site. Most sites (regardless of type) with generally increasing trends in the NO2/NOx ratio, are situated in the north-west portion of the Czech Republic The increasing NO2/NOx ratio detected at some sites is believed to implicate undesired changes in atmospheric chemistry, namely with respect to ambient O3 formation, promoting increasing O3 concentrations.