Surface ozone
Springs and summers are the most favourable times for ground-level ozone to form. Sunny weather or suitable currents for long range transportation can increase the concentrations of ground-level ozone. Very high surface ozone concentrations are not highly common in Finland.
Ozone is found in two layers of the Earth's atmosphere – at the surface and in the upper atmosphere. On both regions, ozone has the same chemical composition (O₃). In the upper atmosphere, ozone protects the Earth from the Sun's ultraviolet (UV) radiation. Surface ozone is not emitted directly into the air but it results from photochemical reactions in the presence of sunlight. Concentrations are higher in rural areas than in city centres because ozone is consumed in reactions with other air pollutants. Regulatory 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.
Surface ozone concentrations in Finland
High surface ozone concentrations can be observed when the weather is hot and dry for a long period of time, usually during July and August. Springs and summers are the most favourable times for ozone to form, between noon and early evening. High surface ozone concentrations are, however, relatively rare in Finland. Regulatory alert threshold level is 180 µg/m³ (the level at which the public must be notified of health hazards). It was exceeded the last time in May 2006 on FMI's Virolahti rural background measurement station. The highest recorded exceedance levels in FMI's measurement stations have observed during years 1996, 2004 and 2006 in Finland.
Activities in Finnish Meteorological Institute
FMI measures surface ozone concentrations in Finland's air quality background stations. Local authorities monitor the air quality in urban areas. FMI follows the risk for exceeding the EU information or alert thresholds values, 180 µg/m³ and 240 µg/m³ respectively, as an hourly average. If the threshold value 180 µg/ m³ as an hourly average is exceeded, FMI will make public notice about the situation. This announcement is then included in the daily weather forecasts, and it includes information on the magnitude of the observed exceeding concentrations, location where they were measured, as well as estimation about the future developments of the situation. The regulatory alarm threshold value is 240 µg/m³ as an hourly average, but so far such high concentrations have not been observed in Finland.
