Urban Green Areas: Lots of Benefits, but some Drawbacks

摘要

From the viewpoint of urban ecology, especially from the urban climatology, urban green areas have an important implication on the local climate. The aim of the applied, planning-oriented urban climatology is to create a synergy effect between the positive recreational use of these green spaces and its positive consequences on local climate and air quality. During the daytime hours the shadowing of the vegetation and the effect of evapotranspiration causes a smoother thermal diurnal course in comparison to the sealed up hyperthermic surroundings. Ideally, an urban park area should be composed of meadows with scattered bushes and a loose stock of the trees. This ensures a suitable shadowing for the people, but also a sufficient solar radiation during the daytime hours. By implication local cold air could be formed at night. This local cold air induces a cold air flow and an air change between the cooler urban green area and the heated, sealed vicinity with a verifiable cooling effect on the air temperature near the ground. The coverage of the cold air flow indeed correlates with the diameter of the urban green area. So a cross-linkage of many small green spaces could result in decreasing the effect of the urban heat island, but also increase people's thermal comfort. By the way, urban green areas could also induce a positive effect on the local urban air quality. But attention has to be paid to the assortment of the tree species. Some trees have the ability to affect the local air quality negatively. More or less all tree species are emitting biogenic volatile organic compounds (BVOC's) in different concentration (e. g. isoprene and terpenes). These serve as precursors for the formation of tropospherical ozone near the ground. Especially within the urban green areas, which act as a resort for many people, wrong tree populations are contraindicated. Since this could lead to a significant exposure to noxious ozone. When many people visit an urban green space (clear and calm weather conditions with hot summery temperatures, low wind speed and a high solar radiation) the emission of BVOC's is highest, and accordingly ozone concentration is high as well. Therefore, an analysis and assessment of the urban green area air quality could help to make a statement about the recreational effect of these areas in dependence of the leading vegetation and for that matter for the exposure to ozone. Both try to analyze the influence of the BVOC's on the local formation of photo oxidation and furthermore the impact on the ozone near the ground. So using the calculation of short- and long-term air quality indices can describe the air hygienic situation. By the help of these the results can be used as a guidance of urban planning taking account of the influence of the biogenic emission as a function of the actual weather conditions.