Clean air - Danish efforts
Organic
compounds
– and photochemical smog
Volatile hydrocarbons (Volatile Organic Compounds, VOC) are organic compounds which are of great significance to the impacts on health and the environment caused by air pollution. They can contribute to the formation of photochemical air pollution, give rise to odour nuisances or simply be hazardous. Usually a distinction is made between methane (CH4) and the other volatile hydrocarbons (NMVOC, nonmethane VOC). Methane is a greenhouse gas, but is chemically not particularly active. It therefore plays a minor role as direct pollution. Non-volatile hydrocarbons can, however, also affect health, as particles or adsorbed to the surface of particles.
Formation and emissions of hydrocarbons
VOC`s have many sources: evaporation of fuels, incomplete combustion, releases in industrial processes, use of organic solvents etc. Danish emissions have been largely constant at well over 200,000 tons per year from 1985 to 1990. It was then reduced to less than 15,000 tons per year in 2000, mainly because of the introduction of 3-way catalytic converters in petrol cars and reduced use of organic solvents. Both the EU's and the EEC's emission ceiling for 2010 is 85,000 tons per year, which - in view of the development - should be possible to achieve.
Ozone
Ozone (O3) is a reactive form of oxygen (O2), in which the molecules have three atoms instead of two. In relation to the environment, ozone plays two different roles: In the stratosphere it forms the so-called ozone layer that shields against short wave UVradiation. At low altitude, on the other hand, it is a contaminant that attacks respiratory systems, certain materials and vegetation.
Due to its reactivity ozone can influence the mucous membranes in the eye and in the respiratory system. Especially persons with respiratory diseases are sensitive and will at elevated levels experience an aggravated health condition.
Ozone load measured at forest stations
Ozone load on vegetation is measured as AOT (Accumulated exposure over a Threshold)
which is a product of the time in which the levels exceed a critical level, e.g. 40 ppb,
and this exceedance. The diagram shows AOT40 values measured at Ulborg and Frederiksborg
forest stations. It appears that Ulborg situated to the west is generally exposed to the
largest load, that it is frequently above the 10,000 ppb x hours a year, which is
considered a critical level. For many years this situation has remained fairly stable.
(ppb is parts per billion)
(Source: NERI)
Emissions of hydrocarbons
Danish emissions of hydrocarbons have been reduced by about 25% since 1985.
(Source: NERI)
As an air pollutant ozone is not a compound that is emitted, but a so-called secondary pollution - something that is formed from other, primary pollutants. The primary pollutants are nitrogen oxides and hydrocarbons that under the influence of sunlight react and form of series of so-called photochemical oxidants. Ozone is the most important.
In Southern Europe photochemical air pollution (photochemical smog) is often an urban phenomenon generated by car traffic. In the cities in Northern Europe, however, decomposition of ozone dominates as a consequence of emissions of NO from cars, and the levels are generally lower than in the countryside. Here the highest concentrations appear in the summer and at south easterly wind. The reason is that during highpressure episodes in Eastern and Central Europe high ozone concentrations build up and are transported to Denmark.
Consequently, it is not possible for Denmark to control ozone levels by national reductions of emissions. Ironically, the levels in Copenhagen are generally lowest on weekdays, when the traffic is most dense, and pollutants (especially nitrous oxide) consume ozone.
Volatile organic compounds can under the influence of sunlight react with nitrogen oxide and form ozone, the most important element of the so-called photochemical air pollution. Ozone is harmful for the respiratory system and for vegetation. In Danish cities the concentration is generally low, but in rural areas elevated concentrations result in loss of crop yield.
Economic evaluations of damage
In Denmark no detailed calculations have been made of the negative economic impact of ozone on the agricultural sector in the form of reduced crop yield. But an estimate of damages on wheat, grass and production forest is DKK 0.5 to 2 billion (Euro 65- 260 million) per year. This is in agreement with similar Swedish estimates. Such figures must, however, be considered with caution in a world with overproduction and economically subsidised agriculture. It has never been attempted to evaluate the economic impact on natural vegetation and health in Denmark.