Ground-level ozone has been observed in the AIM monitoring network since 1992. In 2000, measurements were carried out at 61 stations in the Czech Republic; the measuring sites are indicated in Fig. 2-61. Ground-level ozone is a gas on which considerable experts� attention is focused in the context of its potential negative impact on all types of receptors, which fact is also reflected in the growing number of ozone monitoring stations in the Czech Republic over the past few years. On the other hand, it is to be noted that because of the relatively low spatial variability in ground-level ozone concentrations, its monitoring does not require as large a number of stations as other pollutants do (such as SO₂, NO_x and SPM).

Ozone is classified as a secondary pollutant in the atmosphere. No important sources of its emissions exist; rather, it is formed in the air as a result of a number of chemical reactions of its precursors – NO_x and volatile organic compounds, VOC – under the influence of solar radiation and O₂. Ozone concentration increases with rising temperature. Both during the night- and daytime, ozone molecules are decomposed in chemical reactions with reducing components of the air. The chemical reactions that lead to the formation of ozone result in annual ozone level profiles peaking in spring and summer, while the lowest ozone levels are observed in winter. Table 2-72 lists the annual course of concentrations and calculated annual statistics for ozone at certain stations in the Czech Republic. The values were determined by means of UV absorbance at all of these stations. Figs. 2-62 and 2-63 indicate annual courses of daily O₃ concentrations, separately for urban and rural stations, together with annual variations in NO and NO₂ daily concentrations, which contribute significantly to ozone formation and degradation. For the sake of clarity, the graphs do not cover the whole year but only the March 15 to October 15 period, i.e. the period in which higher ozone levels occur; winter is regarded as unimportant in this respect.

Ozone concentration goes up during daytime as the result of the mixing of the atmosphere�s lower layers with its upper layers in which ozone concentrations are higher, and also because of ozone formation through photochemical reactions. Diurnal profiles are more prominent at urban stations than at rural stations. This difference is caused in part by a faster destruction of ozone in urban air during the evening hours and at night, due to higher concentrations of nitrogen oxides. Ozone concentrations in rural areas at higher altitudes do not exhibit so prominent a diurnal course with large differences between daytime and night-time hours, and high O₃ concentrations may also persist during night-time.

The adopted ambient air pollution limit value for ozone is 160 μg.m^-3 (eight-hour mean; Federal Committee for the Environment�s Decree of 1 October 1991 attached to Act No. 309/1991, in its full amended reading in Act No. 211/1994). The alert thresholds laid down for issuing signals notifying of the occurrence of photochemical smog are 180 μg.m^-3 (hourly mean) to inform the population, and 360 μg.m^-3 (hourly mean) to warn the population (Decree 279/1993).

The warning alert threshold of 360 μg.m^-3 was not exceeded in 2000. The information alert threshold of 180 μg.m^-3 was exceeded in 259 cases at 32 stations in the course of the year. The maximum hour ozone concentration (257 μg.m^-3) was measured at Pardubice- Rosice station at 15.00 on 15 August 2000. The highest eight-hour average concentration, 219.6 μg.m^-3, was observed at the same station on 21 June 2000. In general, the eight-hour limit value was exceeded in 94 cases at 38 stations.

Table 2-73 illustrates that the hourly alert threshold was exceeded on three individual days in May, two three- and four-day-episodes were recorded in June, and the longest episode lasting 8 days was recorded in August. The maximum concentrations recorded ranged between 191–256 μg.m^-3 in June and 187–257 μg.m^-3 in August. Table 2-74 shows the exceedence of eight-hour ozone levels, and namely on 7.5., 17.5., 27.5., 3.–10.6., 19.–22.6., 24.7., 1.–2.8. and 13.–21.8.. The maximum concentrations ranged from 161 to 220 μg.m^-3.

There were more episodes and higher concentrations recorded as compared to 1999, and consequently more limit values exceedences with higher frequency.

Fig. 2-64 gives the number of hours in which the hourly alert threshold 180 μg.m^-3 for ozone was exceeded, Figs. 2-65 and 2-66 the number of stations at which the hourly alert threshold 180 μg.m^-3, resp. eight-hour limit value 160 μg.m^-3 were exceeded.

Fig. 2-67 gives the overview of maximum measured hourly and eight-hour ozone concentrations and the exceedence of hourly an eight-hour alert thresholds at selected stations in the Czech Republic.

Table 2-75 shows the long-term characteristics of concentrations and limit values exceedences for the whole period of ground-level ozone monitoring in the Czech Republic. It presents the overview of numbers of eight-hour limit value and hourly alert threshold exceedences at all stations for each year.

This Yearbook, like the previous ones, looks at ground-level ozone�s effects on forest ecosystems and farm crops. In line with the conclusions of the EEC�s Working Group on Effects, 13th session, Geneva, 29 June to 1 July 1994, the long-term critical ozone level expressed as cumulative exposure above the threshold concentration of 40 ppb has been evaluated. This exposure index is known as AOT40 (accumulated exposure over a threshold of 40 ppb) and is calculated as the sum of differences between hourly concentrations in ppb and 40 ppb for each hour in which the level rises over 40 ppb. The AOT40 index is applied when estimating ozone�s influence on farm crops and forest ecosystems during annual evaluations; over the past few years, it has been used quite extensively in Europe as borne out by the conclusions of the EMEP Workshop on the Control of Photochemical Oxidants over Europe, 24 to 27 October 1995, St. Gallen, Switzerland.

The previously used calculation of AOT40 [3] was replaced by a more detailed procedure [9]. Hence, the AOT40 for farm crops (AOT40C) is calculated for a three-month period for daytime hours defined as hours during which the global radiation equals or is greater than 50 W.m^-2. The AOT40C level for comparison with the set critical level should be determined as the highest moving three-month sum for the vegetation period. A level of 3 ppmh is considered to be the critical ozone level in terms of its influence on farm crops as well as on plant communities.

In order to protect forests in Europe, an interim critical AOT40 ozone level has been set, 10 ppmh. This AOT40F accumulated exposure is calculated for daytime hours (defined as the hours with global radiation greater than 50 W.m^-2) for a six-month period starting from 1 April. The critical level applies both for deciduous and coniferous tree species.

Fig. 2-68 indicates the field of AOT40 exposure index values for ozone for forests in the Czech Republic in 2000. The figure reveals that the critical level of 10 ppmh was exceeded on the prevailing part (99.8 %) of the total country�s area.

Fig. 2-69 shows the field of AOT40 accumulated exposure for farm crops in the Czech Republic in 1999. The figure reveals that 100 % of the country�s area were affected by exposure above the threshold level of 3 ppmh.

The AOT40 exposure index constitutes a useful tool for annual assessments of increased ozone level�s potential negative effects on vegetation. The field of exposure indices, be it for forests or farm crops, should be viewed as a very approximative expert estimate whose value might be influenced by cumulation of a potential systematic error.

Tab. 2-73 Exceedence of the hourly alert threshold for ozone (180 μg.m^-3) at all stations monitoring O₃ concentrations in the Czech Republic, 2000

Tab. 2-74 Exceedence of the 8-hour limit value for ozone (160 μg.m^-3) at all stations monitoring O₃ concentrations in the Czech Republic, 2000

Explanations:
ČR - The entire area of the Czech Republic
PH - Prague
SC - SCentral Bohemian Region
JC - South Bohemian Region
PL - Plzeň Region
KV - Karlovy Vary Region
UL - Ústí nad Labem Region
LIB - Liberec Region
KH - Hradec Králové Region
PA - Pardubice Region
VY - Vysočina Region
JM - JSouth Moravian Region
OL - Olomouc Region
ZL - Zlín Region
MSL - Moravian-Silesian Region

Tab. 2-75 Overview of the air quality limit exceedences for ground-level ozone for single stations and years for the entire monitoring period

Explanatory notes:
1h ... absolute frequency of exceedence of hourly alert threshold (180 μg.m^-3)
8h ... absolute frequency of exceedence of 8-hour limit value IH_8h (160 μg.m^-3)
italics ... not enough data for calculating the annual mean
– ... no records in the year under review