IV.3 NITROGEN OXIDES (NO_x)

IV.3.1 Air pollution caused by NO_x in the year 2013
 

Air pollution caused by NO₂ in the year 2013 with regard to the limit values for the protection of human health

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

The exceedances of annual limit values for NO₂ occur only in limited number of stations, and namely in the localities in agglomerations and large cities exposed to traffic. Of the total number of 90 localities in which NO₂ was monitored in 2013 the annual limit value was exceeded at 4.4 % of the stations (4 localities; Table XIII.8). All four stations are classified as traffic urban. It can be expected that the exceedances of the limit values can occur also at other sites exposed to traffic, where there is no measurement.

In 2013 the 1-hour NO₂ concentrations exceeded the limit value of 200 µg.m^-3 at 6 stations in total (Table XIII.7). Nevertheless none of them reached the permissible number of 18 days with exceedances per the calendar year (Prague 2-Legerova hot spot six exceedances, Prague 5-Smíchov two exceedances and at the stations Brno-Úvoz hot spot, Šunychl, Zlín-Svit and Ostrava-Mariánské Hory recorded one exceedance).

Higher levels of air pollution in the cities 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 CR (99.6 %) recorded the annual average concentration lower than 26 µg.m^-3, i.e. the value of the lower assessment threshold. Higher NO₂ concentrations can be measured also near local communications in settlements with intensive traffic and dense local road network. In 2013 the above-the-limit concentrations of NO₂ affected 0.2 % of population and 0.01 % of the area of the CR territory.

Figs. IV.3.3 and IV.3.4 show the graphs of the courses of daily and hourly concentrations of NO₂ in 2013 showing the evident limit value exceedances in selected localities.

Traffic localities measure higher NO₂ concentrations than other types of localities. In the period April–September there is a marked decrease of NO₂ concentrations in all localities (Fig. IV.3.2) The decrease is caused by higher intensity of solar radiation (mainly of wavelength < 400 nm) in this season of the year, which results in photodissociation of NO₂ to NO and O (Warneck 2000). Under favourable conditions ground-level ozone is formed from the products of photodissociation, and therefore concentrations of ground-level ozone are higher in the period April–September (Fig. IV.4.3).

Air pollution caused by NO_x in the year 2013 with regard to the limit values for the protection of ecosystems and vegetation

The number of localities classified as rural decreased from 25 in 2012 to 13 in 2013. None of the stations exceeded the limit value for the annual average concentration of NO_x (30 µg.m^-3) in 2013 (Table XIII.19 and Fig. IV.3.11). In 58 % of the stations (7 localities) which measured in the both years, the average annual NO_x concentration decreased as compared with the year 2012.

The construction of the map of the spatial distribution of annual average NO_x concentrations is based on the combination of measurement and modelling. All stations measuring NO_x were used for the construction of the map of the field of NO_x, and also the above mentioned data on emissions from mobile sources in the CR were regarded. Higher NO_x concentrations can occur also in the vicinity of local communications in the settlements with intensive traffic and dense local transport network, where there is no measurement of concentrations. The spot symbols highlight only the rural stations (Fig. IV.3.5) because the level of annual NO_x concentrations with regard to the limit value for the protection of ecosystems and vegetation is evaluated only in these localities.

This chapter is closed by the graphs of courses of 24-hour NO_x concentrations for the selected localities with higher annual averages of NO_x concentrations in 2013 (Fig. IV.3.6). Higher 24-hour concentrations of NO_x were recorded in the period from October to March. This was caused probably by deteriorated dispersion conditions, more frequent in winter periods, and also by increased NO_x emissions from local heating (Warneck 2000).

IV.3.2 The development of NO_x concentrations

During the 90s there occurred the marked decrease of both annual average concentrations of NO₂ and NO_x, and the 19th highest 1-hour concentrations of NO₂ (the development since 1996 is presented in Figs. IV.3.8 and IV.3.9). This was caused by a steep decrease of emissions in this period after the coming into force of the Act No. 309/1991 Coll., and the consequent implementation of new technological measures aimed at emission reduction. The change in the structure of industrial branches and the composition of the car fleet and the types of fuels had their influence as well. The course of the year-o-year changes in concentrations of NO₂ and NO_x, but also other pollutants, is significantly influenced also by meteorological and dispersion conditions. Relatively steep downward trend from the 90s of the 20th century continued up to the year 2000. Since then three episodes of the growth of the annual average concentrations as well as the 19th 1-hour concentration of NO₂ have occurred, and namely in the years 2003, 2006 and 2010. In all cases, these years had a marked occurrence of very poor meteorological and dispersion conditions, which probably caused the mentioned growth in concentrations. In 2006 the second highest value of the 19th highest 1-hour concentration was reached for the evaluated period. Generally, however, the presented characteristics for the whole evaluated period (1996–2013) have a gradually downward trend.

The highest values were reached within the whole monitored period (1996–2013) in the agglomeration of Prague and the agglomeration of Ostrava/Karviná/Frýdek-Místek (Fig. V.1). This is caused by very high traffic loads in these areas. For instance in Prague, traffic is currently the most significant source of nitrogen oxides emissions (ENVIS 2012). The lowest levels are reached by turns in the agglomeration of Brno and in the Ústí nad Labem region. It is necessary to stress that the agglomeration of Brno is represented in the evaluation by only one locality (Brno-Tuřany), classified as suburban background. This locality does not represent air pollution caused by NO₂ in the territory of Brno. The measurement results from other stations, which however were not involved in the trend due to insufficient amount of data necessary for the creation of the long-term time series, show clearly that NO₂ concentrations are higher in Brno.

IV.3.3 Emissions of NO_x

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 NO₂ share in emissions ranges usually in the interval 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 in traffic. The sectors of road freight transport with vehicles over 3.5 t, passenger cars and off-road vehicles and other machines used in agriculture and forestry contributed with 37.1 % to NO_x emissions in the year 2012 (Fig. IV.3.12). The sector of public electricity and heat production contributed with 35.6 % of NO_x emissions released in the air. The downward trend of NO_x emissions in the period 2007–2012 is connected primarily with the natural renewal of the car fleet and the implementation of emission ceilings for NO_x emissions from the sources in the sector of public electricity and heat production (Fig. IV.3.13).
 
Production of NO_x emissions is focused mainly along the highways, in big cities and in the Ústí nad Labem region, the Central Bohemia region and the Moravia-Silesia region in which significant producers of energy are seated (Fig. IV.3.14).

 

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.19 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 in 2013
 

Fig. IV.3.2 Annual course of average monthly concentrations of NO₂ (averages for the given type of station), 2013
 

Fig. IV.3.3 Stations with the highest hourly concentrations of NO₂ in 2013
 

Fig. IV.3.4 Stations with the highest exceedance of LV for annual concentrations of NO₂ in 2013
 

Fig. IV.3.5 Field of annual average concentration of NO_x in 2013
 

Fig. IV.3.6 24-hour concentrations at the stations with the highest annual NO_x concentrations in 2013
 

Fig. IV.3.7 19^th highest hourly concentrations and annual average concentrations of NO₂ in 2003–2013 at selected stations
 

Fig. IV.3.8 Trends of NO₂ and NO_x annual characteristics in the Czech Republic, 1996–2013
 

Fig. IV.3.9 Trends of selected characteristics of NO₂ and NO_x (index, year 1996 = 100), 1996–2013; (index, year 2000 = 100), 2000–2013
 

Fig. IV.3.10 Five-year average of annual average concentrations of NO₂, 2009–2013

Fig. IV.3.11 Annual average concentrations of NO_x and NO₂ in at selected rural stations
2003–2013
 

Fig. IV.3.12 Emissions of NO_x sorted out by NFR sectors, 2012
 

Fig. IV.3.13 The development of NO_x emissions, 2007–2012
 

Fig. IV.3.14 Nitrogen oxide emission density from 5x5 km squares, 2012