Nitrogen oxides (NOx) play an important role in air pollution phenomena such as acidification, eutrophication and photochemical smog. The main source of NOx within urban areas is use of fuel in traffic, combustion plants and power plants. Otherwise, nitrogen dioxide (NO2) concentrations are also determined by meteorological conditions. The amount of NOx is crucially influenced by ozone.

The indicator shows the frequency of exceeding the 1-hour limit concentration values of NO2 exceeding 200 μg/m3 and average annual NO2 concentrations (limit annual value is 40 μg/m3).

NO2 concentrations have specific daily and weekly cycle, caused by traffic jams and annual cycle, caused by weather conditions, which are more favourable in the summer. The highest NO2 concentrations are found at roadside locations. The use of catalytic converters in vehicles led to a reduction in emissions, but due to increased traffic not significant. Figure 17-1 clearly shows that in urban areas Slovenia has already achieved the required standards relating to exceedances of 1-hour limit values at individual monitoring sites, since 2001 there was none recorded. Average annual values of NO2 (Figure 17-2), as well, have been maintained below the limit value and individual monitoring sites show a slight downward trend. Reduction of NO2 concentrations affects favourably to human health, since long-term exposure to high concentration can lead to respiratory organ infections.

Analyses of NO2 concentration measurements in other European countries indicate a slightly downward trend in annual value since 1996.