During 7 to 9 January 2003 the air pollution in Oslo reached record levels. The reason was that cold air was building up at the surface in the capital city of Norway. The cold air persisted for days while much warmer air was moving above the heavy cold air layer. On 8 January before noon a strong low temperature inversion at about 100 to150 masl was observed. This inversion completely blocked the vertical dispersion of air pollution and acted as a lid over Oslo.
Measurement preformed by stations owned by the department of Public health in Oslo and Norwegian Public Roads Administration in Oslo, showed nitrogen dioxide concentrations (NO2) at levels that had not been observed since NILU started this kind of measurements in 1984.
The highest levels were observed in lower parts of the valley (Groruddalen), and in other areas with cold stationary air masses. The concentrations of NO2 at two sites in the valley; Alna and Løren was well above the Norwegian and EU limit value of 200 µg/m3. In these situations people with asthmatic problems or other lung diseases were advised against moving outdoors.
During the episode we also recorded high concentrations of small particles (less than 2,5 micrometer, PM2,5). The total particle level in Oslo can at times reach higher levels, especially with contribution from re-suspended road dust during dry conditions in the spring season.
The typical feature of the dust composition during this winter episode, however, was the unusual high level of the finest particles (PM2,5), which indicated that the main sources to the air pollution this time had been emissions from cars and wood-burning.