Vegetationens inverkan på luftmiljön

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Sara Janhäll

Vegetation impacts the local air quality both positively and negatively. The positive effect is that air pollution concentration can be reduced by deposition on the vegetation surface and the air filtered. The total amount of air pollution is thus reduced. The negative effect is the ability of vegetation to act as windbreaks reducing the wind speed, which reduces the dilution of local emissions and can increase the levels in the surrounding area. In dense urban environments where dilution already without vegetation may be limited, such as in narrow street canyon with traffic emissions, this is a big risk. Vegetation can also be placed as a barrier between the emission sources (often major traffic routes) and the population, so that the transport of pollution from the source to the population may be limited, and the air is filtered on its way. This report describes the research in the area at present, and is mainly based on the scientific literature. It is at present difficult to describe vegetation in a way that simply relates to its effect on air quality. An example is the vegetation density can be measured in several ways, and best describes the effect either on wind movement or on deposit. The particle size of the pollution is of major importance to the processes, while chemical effects have been omitted in the description. Vegetation-curtains and different types of street canyon are studied with references to all the studies that is reported. The recommendations, explained in detail in the report, are: • place the vegetation near the source of air pollution, where the concentrations are high, increasing the ability to filter out pollution or redirect the polluted air, • ensure that the vegetation does not reduce the dilution of pollutants, for example avoiding close planting of dense trees in narrow and busy street canyons, where pollution is not easily diluted, • use vegetation barriers between transport and population, to introduce a deposit surface and control of the air movements, but preferably low hedges if dilution is limited at the site, • wall and roof vegetation in dense urban environments increases the deposition surface and has a limited restriction of ventilation compared to free-standing vegetation, • plan the vegetation from the start in view of all the effects of vegetation that can occur in a complex urban environment, and level different environmental effects against each other.The result has also been published scientifically in Janhäll (2015) “Review on urban vegetation and particle air pollution – deposition and dispersion” in the journal Atmospheric environment, 105, 130–137.

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