Air quality index is used mainly to inform the public on current air quality. It provides quick and simple orientation in measurement results, because no specific knowledge is required as for the users. The achieved class of air quality index is characterized by comprehensible description. The air quality index is calculated on the basis of measurements of pollutants� concentrations in the ambient air. The possible impact on human health is considered in the air quality index definition.

II.6.1 Air quality index definition

The calculation of air quality index used in the Czech Republic is based on the assessment of sulphur dioxide (SO₂), nitrogen dioxide (NO₂), carbon moNO_xide (CO,) ozone (O₃) and PM₁₀ suspended particles. Hourly measured concentrations are used to specify air quality index, with the exception of CO concentrations, where 8-hour average is used.

Six-scale definition of air quality index was proposed. The index value 1 represents very good air quality, index value 6 very poor air quality (tab. II.6.1).

The border concentration between index 4 and index 5 was defined on the basis of the valid air pollution limit values of individual pollutants. (In PM₁₀ particles and ozone concentrations, which have no relevant air pollution limit values, the border concentration between index 4 and index 5 was determined on the basis of statistical analysis of the relation between hourly concentrations and concentrations for a longer averaging period.)

II.6.2 Results of air quality assessment based on air quality index

Table II.6.2 presents the occurrence frequencies of individual classes of air quality index at selected measuring AIM stations in the Czech Republic. The assessment covers the stations with available results of SO₂, NO₂, ozone and PM₁₀ suspended particles measurements. (CO concentrations have practically no influence on the index result value in the conditions of the Czech Republic.) In most cases air quality was given index 2 or index 3 (good or satisfactory). The average frequency of the share of these two index classes at all assessed stations in 2005 represents 77 %. In localities in unpolluted areas the occurrence frequency of index 2 or 3 reached up to 90 % (Svratouch, Přebuz, Jeseník).

In the areas with higher air pollution loads, the decisive influence on determining the result air quality index has the concentration of PM₁₀ suspended particles. This is true in almost 100 % cases for index 5 and index 6 (poor and very poor air quality). There are sporadic cases when also NO₂ and ozone concentrations contribute to such high index value (however, the maximum number of cases per year at the respective station is only 0.1 %).

More cases of higher values of air quality index in polluted localities are reached during the cold period. At a number of stations (e.g. Karviná and Ostrava-Přívoz) this annual course is very indistinctive. In 2005 the average occurrence frequency of the two highest index classes (5 and 6) at all assessed stations reached 9 %. The most frequent occurrence was recorded at the following stations: Brno-střed (19.6 %), Karviná (22 %), Ostrava-Fifejdy (19.3 %), Ostrava-Přívoz (26.1 %) and Most (19.5 %). On the contrary, the stations Svratka and Krkonoše-Rýchory recorded zero occurrence of index 5 and index 6.

The measuring localities in unpolluted areas recorded the prevailing influence of ozone concentrations. Consequently, the annual course of air quality index at these stations reaches the maximum values during the summer period. The result values, however, are lower on the whole.

Tab. II.6.1 Air quality index definition

Tab. II.6.2 Occurrence frequencies of individual classes of air quality index at AIM measuring stations [%]