Springs and summers are the most favourable times for ozone to form. Sunny weather and suitable currents for long range transportation will accelerate the increasing of surface ozone. Very high surface ozone concentrations, however, are unlikely in Finland.
Ozone is found in two regions of the Earth's atmosphere – at the surface and in the upper regions of the atmosphere. Both types of ozone have the same chemical composition (O3). Concentrations are higher in rural areas than in city centres because ozone is consumed in reactions with other air pollutants. Surface ozone is not emitted directly into the air but it results from photochemical reactions in the presence of sunlight. Limit values for ozone concentrations can be exceeded in sunny and warm weather, when weather conditions are propitious for forming surface ozone. Also, suitable currents for long range transportation along with warm weather will accelerate the increasing of surface ozone. Finland also receives transported ozone in winds from other parts of Europe.
For example, in summer 2010, high surface ozone concentrations were observed when the weather was exceptional hot during July and August. Surface ozone values were over 140 µg/m3, which means that air quality was poor. High surface ozone concentrations are, however, very rare in Finland. Announcement level 180 µg/m3 (the level at which the public must be notified of health hazards) was exceeded the last time in May 2006 on the Virolahti background measurement station. The highest recorded exceedance levels in FMI's measurement stations have observed during years 1996, 2004 and 2006 in Finland. Springs and summers are the most favourable times for ozone to form, between noon and early evening.
FMI follows surface ozone concentrations at background stations. Local authorities have a responsibility to arrange air quality monitoring in urban areas. FMI is responsible for informing the public whenever the EU thresholds values 180 µg/m3 and 240 µg/m3, as an hourly average, are exceeded in some background stations maintained by FMI. If the threshold value 180 µg/ m3 as an hourly average has been exceeded, FMI informs the public in connection with daily weather forecasts. In this announcement, the magnitude of the observed exceeding figure is discussed, as well as the place this exceeding occurred and the estimation about the future development of the situation. The alarm threshold value is 240 µg/m3 as an hourly average, but so far such high concentrations have not been observed in Finland.