IV.3 NITROGEN OXIDES

IV.3.1 Air pollution caused by nitrogen oxides in the year 2014

In the field of ambient air monitoring and evaluation, the term nitrogen oxides (NO_x) is used for the mixture of nitrogen moNO_xide (NO) and nitrogen dioxide (NO₂). The air pollution limit value for the protection of human health is set for NO₂ whereas the limit value for the protection of ecosystems and vegetation is set for NO_x.

Air pollution caused by nitrogen dioxide in the year 2015 with regard to the limit values for the protection of human health

Exceedances of annual ambient limit values for NO₂ occur only at a limited number of stations, namely those exposed to traffic in agglomerations and large cities. Of the total number of 93 localities at which NO₂ was monitored in 2015, the annual limit value of 40 μg.m^-3 was exceeded at 2.1 % (2 localities; Table XIII.8). Both stations (Prague 2-Legerova traffic hot spot and Prague 5-Smíchov) are classified as traffic urban localities. Exceedances of ambient limit values can be presumed to occur also occur at sites exposed to traffic where no monitoring is carried out. In 2015, 1-hour NO₂ concentrations exceeded the limit value of 200 μg.m^-3 at 3 stations (Table XIII.7). Nevertheless, neither of them reached the permissible number of 18 days with exceedances per calendar year. The Prague 2-Legerova (traffic hot spot) station registered 16 exceedances, the Plzeň-Slovany station registered 4 exceedances, and the Prague 5-Smíchov station registered one exceedance.

The higher levels of air pollution in towns and cities compared to out-of-town stations are caused mainly by traffic. The highest values of NO₂ concentrations are reached in Prague, Brno and Ostrava (Fig. IV.3.1). However, most of the territory of the However, in most of the territory of the Czech Republic (99.9 %), recorded annual average concentrations are below 26 μg.m^-3, i.e. the value of the lower assessment threshold. Higher NO₂ concentrations can occur also near local communications in settle- ments with intensive traffic and a dense local road network. Although the ambient limit value was exceeded at two traffic localities in Prague, these exceedances are not reflected in the map of annual average concentrations at the scale it is presented (Fig. IV.3.1) due to the fact that traffic stations have small radii of representation. In 2015, ambient concentrations of NO₂ exceeded the 1-hour value advised by the WHO for the protection of human health (200 μg.m^-3) at least once per 0,001 % of the territory of the Czech Republic.

Traffic localities measure higher NO₂ concentrations than other types of localities. In the April–September period, there was a marked general decrease of NO₂ concentrations at all localities (Fig. IV.3.4) The decrease was caused by a higher intensity of solar radiation (mainly of wavelength < 400 nm) in this season of the year, which results in photo- dissociation of NO₂ into NO and O (Warneck 2000). Under the right conditions, ground-level ozone is formed from the products of photodissociation, so concentrations of ground-level ozone are higher in the April–September period (Fig. IV.4.3). The large increase of the average concentration of NO₂ for traffic localities in August 2015 resulted from an exceptionally high monthly average concentration (73.5 μg.m^-3) at the locality Prague 2-Legerova (traffic hot spot).

Air pollution caused by nitrogen oxides in the year 2015 with regard to the limit values for the protection of ecosystems and vegetation

In 2015 the ambient limit for annual average concentrations of NO_x (30 μg.m^-3) was not exceeded at any of the 16 rural stations which provided sufficient data for the assessment (Table XIII.9 and Fig. IV.3.8). At 71 % of the stations (10 localities out of 14) which monitored NO_x in the years 2014 and 2015, the average annual NO_x concentration decreased as compared with the year 2014.

The map of the spatial distribution of annual average NO_x concentrations is constructed based on the combination of measurements and modelling. All stations measuring NO_x were used for the construction of the map of the field of NO_x. The abovementioned data on emissions from mobile sources in the Czech Republic were also taken into account. Higher NO_x concentrations can also occur near local communications in settlements with intensive traffic and a dense local transport network where concentrations are not measured. The spot symbols only highlight rural stations because the level of annual NO_x concentrations with regard to the limit value for the protection of ecosystems and vegetation is evaluated only for these localities (Fig. IV.3.3).

IV.3.2 The development of nitrogen oxides concentrations

A marked decrease of both annual average concentrations of NO₂ and NO_x as well as the 19th highest 1-hour concentrations of NO₂ occurred during the 90s.This was caused by a steep decrease of emissions in this period after Act No. 309/1991 Coll. came into force and after the consequent implementation of new technological measures aimed at reducing emissions. The change in the structure of industrial branches and the composition of the car fleet together with new types of fuels also had their influence. The course of year-to-year changes in concentrations of NO₂ and NO_x, but also of other pollutants, is significantly influenced by meteorological and dispersion conditions. The relatively steep downward trend from the 90s of the 20th century continued up to the year 2000.

Since this year, increases of both annual average concentrations and the 19^th highest hourly concentrations alternate with decreases of the same measures (Fig. IV.3.5). The most marked increases of concentrations were recorded in the years 2003, 2006 and 2010, probably as a result of unfavourable meteorological and dispersion conditions which occurred in these years. Since 2011, all monitored characteristics exhibit a gradually declining trend (Fig. IV.3.6). The average annual concentration decreased again in 2015, but there was a slight increase of the 19th greatest 1-hour concentration. This increase was caused by values measured at the locality Prague 4-Braník.

Throughout the monitored period (2000–2015), the highest values were reached in the Prague agglomeration and the O/K/F-M agglomeration. The reason are very high traffic loads in these areas, and in the case of the O/K/F-M agglomeration also industrial emissions. In Prague, for instance, traffic is currently the most significant source of nitrogen oxide emissions (ENVIS 2012). By contrast, the lowest levels of all agglomerations being monitored are reached in the agglomeration of Brno (Fig. V.1).

IV.3.3 Emissions of nitrogen oxides

Emissions of NO_x are formed during the combustion of fuels in dependence on the temperature of combustion, the content of nitrogen in fuels and the excess of combustion air. Emissions of NO_x are formed also during some chemical-technological processes (production of nitric acid, ammonia, fertilizers etc.). While in the combustion of fuels, the share of NO₂ in emissions of NO_x usually varies in the interval of up to 5 %, in some chemical-technological processes, the share of NO₂ can amount even to 100 % of NO_x emissions (Neužil 2012).

The greatest amount of NO_x emissions is generated by traffic. The sectors 1A3biii-Road transport: Heavy duty vehicles and buses, 1A3bi-Road transport: Passenger cars and 1A4cii-Off-road vehicles and other machinery from agriculture/forestry/fishing contributed to the total country-wide emissions of NO_x by 38.9 % in the year 2014 (Fig. IV.3.9). Sector 1A1a-Public electricity and heat production contributed by 32.0 % of NO_x emissions released into the air. The downward trend of NO_x emissions in the period 2007–2014 is connected primarily with renewal of the car fleet and the implementation of emission ceilings for NO_x emissions from sources in the sector of public electricity and heat production (Fig. IV.3.10).

The contributions of individual types of sources to total emissions differ depending on the particular mix of sources in the given area. Production of NO_x emissions is mainly concentrated along motorways, in large cities, and in the Ústí nad Labem region, the Central Bohemia region and the Moravia-Silesia region, where significant producers of energy are located (Fig. IV.3.11).

Tab. XIII.7 Stations with the highest values of the 19^th and maximum hourly concentrations of NO₂

Tab. XIII.8 Stations with the highest values of annual average concentrations of NO₂

Tab. XIII.9 Stations with the highest values of annual average of NO_x concentrations at rural stations

 

Fig. IV.3.1 Field of annual average concentration of NO₂, 2015

Fig. IV.3.2 Five-year average of annual average concentrations of NO₂, 2011–2015

Fig. IV.3.3 Field of annual average concentration of NO_x, 2015

Fig. IV.3.4 Annual course of average monthly concentrations of NO₂ (averages for the given type of station), 2015

Fig. IV.3.5 Trends of NO₂ and NO_x annual characteristics in the Czech Republic, 2000–2015

Fig. IV.3.6 Trends of selected characteristics of NO₂ and NO_x (index, year 2000 = 100), 2000–2015

Fig. IV.3.7 19^th highest hourly concentrations and annual average concentrations of NO₂ at selected stations, 2005–2015

Fig. IV.3.8 Annual average concentrations of NO_x at selected rural stations, 2005–2015

Fig. IV.3.9 Total emissions of NO_x sorted out by NFR sectors, 2014

Fig. IV.3.10 The development of NO_x total emissions, 2007–2014

Fig. IV.3.11 Nitrogen oxide emission density from 5x5 km squares, 2014