Ozone is a trivalent oxygen molecule (O3), which occurs both in the stratosphere, about 20-30 km above the ground (the so-called ozone layer) and in the lower troposphere, just below its surface. Stratospheric ozone protects us from harmful UV-C radiation, while tropospheric ozone is harmful to humans and vegetation.

Because ozone is a strong oxidant, getting into the respiratory tract of a person causes irritation and discomfort in breathing. Short-term exposure to high concentrations may cause inflammation of the respiratory tract. Prolonged or repeated exposure may aggravate symptoms. Exposure to ozone is particularly dangerous for people with respiratory disorders and for the ones with reduced body immunity, especially older people and children.

Ozone is a secondary pollution, which means that it is not emitted into the atmosphere, but is formed in the air by the reaction of nitrogen oxides (NO and NO2)and volatile organic compounds (VOCs) under the influence of the sun. These compounds are called precursors of ozone. Nitric oxide (NO) and nitrogen dioxide (NO2)are mainly emitted by wheel transport and industry, while the anthropogenic emission of volatile organic compounds is also associated with industrial processes. High concentrations of these substances allow the formation of ozone, so its highest concentrations are observed in urban and suburban areas.

Due to the nature of the ozone formation reaction, high concentrations occur in sunny and warm days with poor winds, with the highest concentrations in the afternoon. High concentrations of ozone in Poland occur in the period from March to September, most often in the high tide. Exceeding the permissible level of 120 μg / m3 8-hour rolling average) and public information (180 μg /m3,1 hour average) are most commonly observed in July and August. Due to the strong dependence on weather conditions, the frequency of high concentrations of ozone in the following years may vary considerably. Below is an example of a concentration chart for a city station (Wroclaw, Korzeniowskiego Street) in August 2015, when the public information threshold was exceeded.

Fig. Average 1-hour ozone concentrations and average hourly air temperature values, recorded on June 1-30, 2022, at the ZKiOA IGRR UWr Observatory

How do we determine exceedances of ozone standards? The determination of exceedances of standards for ozone concentrations is slightly different than for the concentrations of substances such as particulate matter (PM) or nitrogen oxides. In their case, exceedances of standards are referred to the daily or annual average (if the value of the average calculated for a given day or year exceeds the value defined in the standard, we talk about exceeding it.

In the case of ozone, exceeding the standards is defined slightly differently. According to the Chief Inspectorate of Environmental Protection, the reference value for the standard is described as:

The maximum eight-hour moving average value among such averages calculated from one-hour daily averages. Each 8-hour average calculated in this way is assigned to the day on which it ends. The first calculation period for each day is the period from 1700 CET of the previous day to 0100 CET of the given day. The last calculation period for each day is the period from 1600 CET to 2400 CET of that day.

Therefore, if the maximum value of the 8-hour average calculated in a given day exceeds 120 µg/m3, we say that the standard has been exceeded. For this reason, in the charts in which we want to show the relationship between the measured concentrations and the normative values, an additional line appears, presenting the values of the 8-hour moving average, calculated as above.

Exceedances of the WHO recommendations for ozone are determined in a similar manner, except that the recommended limit value is lower and amounts to 100 µg/m3.

The differences between the measurement results - 1-hour ozone concentrations, 8-hour averages and limit values for the norm and WHO and recommendation, for August 3-5, 2022, in Wrocław, are presented in the chart below.

Fig. Average 1-hour and 8-hour ozone concentrations, on the background of norm level value and WHO reccomendation level value, recorded on August 3-5, 2022, at the ZKiOA IGRR UWr Observatory

Elaborated by Kinga Wałaszek, update T. Sawiński