Emissions and dispersion of air pollutants

Air pollutants are harmful gaseous or particulate substances derived from human activities or from natural sources. The most important pollutants deteriorating the quality of urban air are thoracic particles (PM₁₀), fine particles (PM_2.5), nitrogen dioxide (NO₂), ozone (O₃), carbon monoxide (CO), sulfur dioxide (SO₂) and volatile organic compounds (VOC). At high concentrations these have harmful impacts on health and environment.

The main sources of air pollutant emissions in the Helsinki Metropolitan Area are transport, energy production and the use of small fireplaces. Pollutants also reach Finland from abroad by long-range transport.

Traffic emissions have the greatest impact on air quality in the Helsinki Metropolitan Area, as they are released near the ground and affect air at breathing height. Concentrations can elevate to especially high levels in busy street canyons lined by buildings, as there is minimal dispersion and dilution of pollutants. Emissions from energy production, by contrast, are released from high stacks and spread over a wide area, and therefore their impact on air quality is minimal.

Emissions are released to the lowest layer of the atmosphere, and they then mix with the surrounding air which dilutes the pollutants. Emissions may spread over large areas with moving air masses. During this transport stage pollutans may react with other compounds in the atmosphere forming new compounds (secondary pollutants). Pollutants leave the atmosphere when washed out by rain, as dry deposition on various surfaces, or by chemical transformation into other compounds.

Effects of seasons and weather conditions

Although the quality of air in the Helsinki Metropolitan Area is generally quite good, the concentrations of pollutants can occasionally rise to harmfully high levels. Emissions are large and conditions for mixing and dilution are unfavourable in winter, which is a peak time of year for concentrations of most pollutants. Traffic exhaust gases and smoke from wood burning, for example, can occasionally cause  high pollutant concentrations locally, when the weather is cold and calm. Street dust stirred up into the air from dry road surfaces deteriorates air quality especially in spring. Sunny weathers in spring and summer promote ozone formation, leading to long-range transport of ozone into the region at high concentrations. Smoke from forest fires and agricultural waste burning in fields in Eastern Europe can deteriorate air quality at times in spring and summer in certain years. Wet and windy weather tends to clear the air, meaning that the air is generally cleanest in autumn.

Further details of emissions and air quality are available in the following brochures
The Air You Breathe (pdf 1,6 Mb)