IV.3 NITROGEN OXIDES (NOx)
IV.3.1 Air pollution caused by NOx 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 (NOx) 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 NOx.
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 NOx 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 NOx (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 NOx concentration decreased as compared with the year
The construction of the map of the spatial distribution of annual average NOx concentrations is based on the combination of measurement and modelling. All stations measuring NOx were used for the construction of the map of the field of NOx, and also the above mentioned data on emissions from mobile sources in the CR were regarded. Higher NOx 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 NOx 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 NOx concentrations for the selected localities with higher annual averages of NOx concentrations in 2013 (Fig. IV.3.6). Higher 24-hour concentrations of NOx 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 NOx emissions from local heating (Warneck 2000).
IV.3.2 The development of NOx concentrations
During the 90s there occurred the marked decrease of both annual
average concentrations of NO2 and NOx, and the 19th highest
1-hour concentrations of NO2 (the development since 1996 is
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 NOx, 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 NOx
Emissions of NOx 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
NOx 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 NOx emissions (Neužil 2012).
The greatest amount of NOx 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 NOx emissions in the year 2012 (Fig. IV.3.12). The sector of public electricity and heat production contributed with 35.6 % of NOx emissions released in the air. The downward trend of NOx emissions in the period 2007–2012 is connected primarily with the natural renewal of the car fleet and the implementation of emission ceilings for NOx emissions from the sources in the sector of public electricity and heat production (Fig. IV.3.13).
Production of NOx 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).
Fig. IV.3.1 Field of annual average concentration of NO2
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 NOx
Fig. IV.3.6 24-hour concentrations at the stations with the
highest annual NOx concentrations in 2013
Fig. IV.3.7 19th highest hourly concentrations and
annual average concentrations of NO2 in 2003–2013 at
Fig. IV.3.8 Trends of NO2 and NOx
annual characteristics in the Czech Republic, 1996–2013
Fig. IV.3.9 Trends of selected characteristics of NO2
and NOx (index, year 1996 = 100), 1996–2013; (index,
year 2000 = 100), 2000–2013
Fig. IV.3.11 Annual average concentrations of NOx and
NO2 in at selected rural stations
Fig. IV.3.12 Emissions of NOx sorted out by NFR
Fig. IV.3.13 The development of NOx emissions,
Fig. IV.3.14 Nitrogen oxide emission density from 5x5 km squares, 2012