Closing the Gap between Urban Climate and Urban Planning

Knowledge of local climate, the resulting air quality situation and their functional relationships are important aspects of environmental precaution in urban planning. The positive effect of urban green spaces is non-controversial. However, it is important to take into account that even green spaces like urban parks are quite capable of influencing the air quality of a site adversely. The applied, planning oriented urban climatology in this case represents the link between climatology and urban planning. Urban climatology can offer an extensive range of solutions that both appear in areas with microclimatic and air quality issues.

Urban climate analysis, climatological function maps and planning recommendation maps allow urban planning to address major climatic aspects within the municipal operational framework. Especially for the responsibles for urban and environmental planning it is now – more than ever – important to receive subject-specific responses to climatic and air quality problems in urban areas.

The phenomenon of "shrinking cities" offers a large number of vacant areas. These potentially free spaces can be integrated into future city planning by using urban climate knowledge. In urban areas the creation of green spaces is a useful strategy. Especially in this context arise different areas of activity for applied, planning-oriented urban climatology. These include, for example, the roadside vegetation and urban parks.

Basically roadside vegetation contributes to the improvement of local climate and also has an air-purifying function. Nevertheless, care should be taken of the air hygiene perspective in choosing the tree species, especially concerning the treetops. If trees with tight and closed treetops are planted on both sides of the road as an avenue, it has to be assumed that the "convergence" of the treetops reduces air exchange on the traffic route close to the ground. Then similar accumulations of pollutants such as within an urban canyon can be achieved, depending on the seasonal trend of the foliage.

Similarly, the value of inner-city green areas for regeneration purposes is indisputable and the positive local climatic effect on the surrounding area of urban parks is well-known as a function of size and design. This positive effect is not restricted to large spaces since it has been proven that even smaller parking areas have a cooling effect on the surroundings, thus ensuring a reduction in the thermal load. However, this is limited to the directly adjacent area.

Despite all local climatic advantages, there is some backlog demand regarding air hygiene. Some species emit different amounts of biogenic volatile organic compounds. The vegetation produces different organic substances that do not necessarily affect the growth and development of plants. These substances are also referred to as phytonutrients. One of these groups of substances are the isoprenoids. Decisive for the amount of the emission rates of the respective plants in terms of biogenic hydrocarbons are the meteorological conditions and how they act on the vegetation inventory.

Especially the emission of isoprene is determined by air temperature and the intensity of the photosynthetically active radiation. Accordingly, especially clear and calm weather conditions that are characterized by a high radiation intensity and, consequently, high air temperatures, are best suited to emit isoprene. This biogenic hydrocarbon has – compared to anthropogenic hydrocarbons – a relatively high ozone-generating potential. This appears because of the reaction rate with OH-radicals. This means that isoprene can be observed even at low concentration as quite serious precursor for the production of ground-level ozone.

Based on this a possible negative impact of emitting vegetation has to be considered on ozone formation in reverse. In particular, urban green spaces are – especially in the summer months – highly frequented by the general public. During sunny summer weather, however, it has to be expected that unfavorable planting (from the view of air hygiene) increases the emission of photochemical oxidants.

For urban planning, this means that environmental and urban climate issues need to be involved in the planning of public space in more detail than before. This includes different levels: At the level of area-based planning, the importance of small inner-city green spaces for the local climate of urban redevelopment especially in climatically strongly biased, neighborhoods has to be considered specifically. In the greening of road spaces planting concepts must be ensured by means of simulations of the growth of trees and possibly by modifying that no negative impact on air hygiene arises vertical ventilation of the street spaces. In principle, account must be taken to the emission of photochemical oxidants by certain tree species that have to be excluded from critical areas. These are issues that have hardly been taken into account in the “classical” city and open space planning.