IV.4 GROUND-LEVEL OZONE (O₃)
 

IV.4.1 Air pollution caused by O₃ in the year 2013
 

Air pollution caused by O₃ in the year 2013 with regard to the limit values for the protection of human health

In 2013 ozone was measured in 63 localities out of which 19 % (12 localities) exceeded the limit value within the three-year period 2011–2013, or shorter (Table XIII.11). In comparison with the previous three-year period 2010–2012 the number of exceedances of the limit value 120 µg.m^-3 decreased in 35 % of localities (22 localities), 54 % (34 localities) recorded the increase and 11 % (7 localities) did not record any difference.

Consequently, in comparison with the previous three-year period 2010–2012, the situation has changed (Fig. IV.4.2). In the comparison of the evaluated three-year periods the important role is played by emission precursors and meteorological conditions, i.e. the intensity of solar radiation, temperature and precipitation or relative humidity (Blanchard et al. 2010; Ooka et al. 2011). However, the relation between the amount of emitted precursors and ground-level ozone concentrations is not linear. This non-linearity is caused by long-range transport of O₃ and its precursors and other factors, including the climate change, emissions of NMVOC from vegetation and forest fires (EEA 2013a). With regard to rather complicated atmospheric chemical processes during ozone formation and disintegration, its dependence on absolute amount and relative share of its precursors in atmosphere, connected also with long-range transport, and also on meteorological conditions, it is difficult to comment the year-to-year changes in more detail.

The traffic localities in the cities are the least loaded ones as ozone is degraded there through chemical reaction with NO (more details see in Chapter IV.4.3). It can be expected that ozone concentrations are below the limit value also in other cities with heavy traffic loads where, however, due to the absence of measurements, the probable decrease cannot be documented by the use of current methods of map construction. On the contrary, the highest concentrations are measured in rural background localities (Table XIII.11). Similar conclusions were reached also by Munir et al. (2012) who studied the influence of road transport on the concentrations of ground-level O₃. Their results show that the concentrations of ground-level O₃ measured in cities are even by 26 % lower than the concentrations measured in rural areas. At the same time 86 % of variability of O₃ concentrations between rural and urban localities was explained by the impact of traffic. The greatest positive impact on the decrease of O₃ in cities was recorded in bus transport.

The limit value for O₃ was exceeded in 2013 (the average for the three-year period 2011–2013) in 25.6 % of the territory of the CR with approx. 8.2 % of inhabitants (Fig. IV.4.1). In comparison with the previous year (the average for the period 2010–2012) the area of the affected territory increased by 9 % (from 16.6 %).

Table IV.4.1 presents the number of hours of the informative threshold value exceedance (180 µg.m^-3) for O₃ at selected AIM stations for the period of 2000–2013. More detailed evaluation related to the exceedance of the threshold value 180 µg.m^-3 is presented in Chapter VI. – Smog warning and regulatory system.

The annual course of average monthly concentrations of O₃ (max. 8-hour running average for the given month) is characterized by the increase of concentrations in spring and summer months (Fig. IV.4.3) due to favourable conditions for ozone formation, such as high intensity of solar radiation, high temperatures and low air humidity. Also in this case it is apparent that the highest maximum 8-hour running averages are recorded in rural localities where the most frequent exceedances of the limit value occur (Fig. IV.4.2). The seasonal trend of the annual courses of maximum daily 8-hour running averages of O₃ is more marked in the three subsequent years (2011–2013) in the most loaded localities (Fig. IV.4.4); among them the localities Štítná n. Vláří, Červená, Mikulov-Sedlec and others, are included in 2013 (Fig. IV.4.7).

Air pollution caused by O₃ in the year 2013 with regard to the limit values for the protection of ecosystems and vegetation

Of the total number of 34 rural and suburban stations for which the calculation of AOT40¹ exposure index is relevant according to the legislation, the ozone limit value for the protection of vegetation was exceeded only in one locality in 2013 (the average for the years 2009–2013), and namely Štítná n. Vláří with 19,861.8 µg.m^-3.h (Table XIII.20, Fig. IV.4.5). The second highest value of the exposure index for the recent five years was recorded in the locality Kuchařovice (Fig. IV.4.6). As compared with the previous evaluated period 2008–2012, the number of localities with exceedances decreased (from 15 % (5 localities) to 3 % (1 locality) of the total number 34 localities). The decrease of the value of the AOT40 exposure index for the year 2013 as compared with the year 2012 was recorded in 91 % of localities (30 localities), while the increase was recorded in 9 % of localities (3 localities); in one locality (Frýdlant údolí) the previous period was not evaluated.

IV.4.2 The development of O₃ concentrations

The trend of the 26^th highest daily maximum 8-hour running average of O₃ concentrations has been decreasing since 1996 (Fig. IV.4.8); there are however two years markedly beyond this trend, and namely the year 2003 (i.e. the average for the three-year period 2001–2003) and the year 2006 (i.e. the average for the period 2004–2006). In 2003 the 26^th highest value of the maximum 8-hour running average reached the highest level for the whole monitored period. The year 2003 was characterized by markedly above-the-average concentrations of ground-level ozone in the whole Europe (Sicard et al. 2011; Cristofanelli et al. 2007; Pires et al. 2012) with above-the-average temperatures in the summer period (EEA 2014a). The years 2003 and 2006 were characterized by favourable conditions for the formation of ground-level ozone. Up to the year 2008 there were several years with limit value (= 120 µg.m^-3) exceedances, in the following years the 26^th highest values of the maximum 8-hour average remained below the limit value. This evaluation shows apparently that higher values are reached usually in rural localities as against urban and suburban background localities (Fig. IV.4.8).

The downward trend of ground-level ozone concentrations was recorded not only in Europe (Sicard et al. 2013; EEA 2013a), but also in USA (Butler et al. 2011). In the years 1990–2010 the stations in Europe and USA recorded also the decrease of the differences between concentrations measured in rural localities and in urban localities (Paoletti et al. 2014). Simultaneously, these stations recorded the decrease of the maximum measured values. The mentioned decrease of O₃ concentrations is attributed i.a. to the emission reduction of its precursors, mainly NO_x, in the developed countries (Sicard et al. 2013).

Similarly, the development of AOT40 exposure index in the years 2009–2013 (the average for 5 years) has a downward trend in most localities (Fig. IV.4.10). It is apparent from the annual values of AOT40 exposure index that in the years 2009–2013 this trend was not as marked, however, it shows quite clearly the high level of the reached values of the exposure index in the given year (Fig. IV.4.11).

IV.4.3 Formation of ground-level ozone

Ground-level ozone (O₃) has no significant source of its own in the atmosphere. Ozone is the so called secondary substance produced during a number of very complicated non-linear photochemical reactions described in detail e.g. by Seinfeld and Pandis (2006). Ozone precursors include nitrogen oxides (NO_x) and non-methanic volatile organic compounds (NMVOC), at the global scale also methane (CH₄) and carbon moNO_xide (CO) take their part in the process. Photo-lysis of NO₂ by radiation of wavelength 280–430 nm is the principal reaction, during which NO and atomic oxygen are formed. During the reaction of atomic and molecular oxygen, in the presence of a catalyst, O₃ molecule is formed. Simultaneous titration of O₃ by NO results in the formation of NO₂ and O₂. If in this reaction O₃ is replaced with radicals, its concentrations in the atmosphere grow. Significant role in these reactions is played mainly by OH-radical.

NO_x are formed during all combustion processes. NMVOC are emitted from a whole number of anthropogenic sources (transport, manipulation with crude oil and its derivates, refineries, paint and solvent use etc.), but also natural sources (e.g. biogenic emissions from vegetation).

During the formation of O₃ from the precursors not only the absolute amount of precursors is important, but also their mutual proportion (Sillman et al. 1990; Fiala, Závodský 2003). In the areas with the regime limited by NO_x, characterized by relatively low concentrations of NO_x and high concentrations of VOC, O₃ concentrations grow with the growing concentrations of NO_x, while with the growing VOC concentrations they change only little. On the contrary, in the areas with the regime limited by VOC, O₃ concentrations decrease with the growing NO_x concentrations and they increase with the growing VOC concentrations. The areas with high NO_x/VOC proportion are typical polluted areas around the centres of big cities. The dependence of the formation of O₃ on the initial concentrations of VOC and NO_x are often depicted in the diagrams of ozone isopleths which show the maximum reached ozone concentrations as the function of initial concentration of NO_x and VOC (Moldanová 2009). The significant role in ozone formation is played not only by precursors but also by meteorological conditions (Colbeck, Mackenzie 1994). Ambient air pollution concentrations of O₃ grow with the increasing ultraviolet radiation and temperature, and, on the contrary, they decrease with the increasing relative humidity. High concentrations are usually related to long-lasting anticyclonic situations. In addition to the photochemical mechanism described above O₃ concentrations may occur also in episodes due to the penetration of stratospheric ozone to the troposphere and also during storms. Recently there is also the growing significance of the long-range transport of O₃ within the circulation in the northern hemisphere to Europe and North America from the source areas in south-east Asia. O₃ is removed from the atmosphere during the reaction with NO and by dry deposition.

 

Tab. IV.4.1 Number of hours of the ozone information threshold exceedance (180 μg.m^-3) per year at selected AIM stations, 2000–2013

Tab. XIII.11 Stations with the highest values of maximum daily 8-hour running average concentrations of ozone

Tab. XIII.20 Stations with the highest AOT40 values of ozone at rural and suburban stations

 

Fig. IV.4.1 Field of the 26^th highest maximum daily 8-hour running average of ground-level ozone concentration in three-year average, 2011–2013
 

Fig. IV.4.2 Numbers of exceedances of the tlimit value for the maximum daily 8-hour running average of surface ozone concentrations in three-year average, 2011–2013
 

Fig. IV.4.3 Annual course of average monthly concentrations of max. 8-hour running average of O₃ (averages for the given type of station), 2013
 

Fig. IV.4.4 Stations with the highest values of maximum daily 8-hour running average concentrations of ground-level ozone in 2011–2013
 

Fig. IV.4.5 Field of AOT40 exposure index values, average of 5 years, 2009–2013
 

Fig. IV.4.6 Stations with the highest exposure index AOT40 values in recent 5 years, 2009–2013
 

Fig. IV.4.7 26^th highest values of maximum daily 8-hour running average of ground-level ozone concentrations (three-year average) in at selected stations
 

Fig. IV.4.8 Trends of O₃ annual characteristics in the Czech Republic, 1996–2013
 

Fig. IV.4.9 Trends of selected characteristics of O₃ (index, year 1996 = 100), 1996 2013; (index, year 2000 = 100), 2000–2013
 

Fig. IV.4.10 Annual exposure index AOT40 values in 2009–2013 at selected stations
 

Fig. IV.4.11 Exposure index AOT40 values in 2003–2013 at selected stations, average of 5 years

---

¹For the evaluation of vegetation protection against ozone concentrations exceedances the national legislation uses, in compliance with the respective EU Directive, the AOT40 exposure index. AOT40 ccumulated exposure is calculated as the sum of the differences between hourly ozone concentration and the threshold level of 80 µg.m^-3 (= 40 ppb) for each hour when this threshold value was exceeded. Pursuant to the requirements of the Government Order No. 597/2006 Coll. AOT40 is calculated for the period of three months (May to July) measured between 8:00 and 20:00 Central European Time (= 7:00 and 19:00 UTC).