With its location in a basin, the city of Aachen is at a climatic and air hygiene disadvantage compared to other cities, which has a negative impact on the level of air pollution, for example, and at times to a high and intolerable degree.
The city of Aachen is therefore called upon to improve air quality. 36 measures in the areas of traffic and energy were defined in the current update of the Aachen Clean Air Plan (2015). Such a plan had to be drawn up because the EU limit values for the air at the Aachen-Wilhelmstraße and Aachen-Adalbertsteinweg measuring stations have been regularly exceeded in recent years. As a result, in addition to the 36 measures, the city of Aachen also had to set up a low emission zone from February 1, 2016 on the instructions of the Cologne district government. The low emission zone is located in the center of the valley basin and extends to the Aachen outer ring road.
Air hygiene
The air hygiene situation in the city of Aachen is generally characterized by the fact that the EU-relevant air pollutant parameters particulate matter (PM10) and nitrogen dioxide (NO2) have high to critical daily and annual average values in some cases. In contrast, the short-term values for particulate matter (albeit varying locally), measured against the applicable limit values, are now being complied with.
Traffic-related pollutants
A differentiated development has been observed in recent years for air pollutants that are partly or predominantly caused by traffic. For the air pollutant nitrogen dioxide (NO2), significantly high immission levels are still being recorded on some streets, even though concentrations have fallen slightly at some measuring points compared to the first decade. In contrast, the trend for particulate matter is clearly pointing in the positive direction of reducing pollution to below the limit values.
In comparison to the linear air pollution levels, which are high in some cases along the traffic axes, the area-wide pollution levels with their differences can be described as moderate to low.
Spatially prominent industrial pollution hotspots, such as those found in the 1980s and 1990s (e.g. the Rothe Erde and Jülicher Strasse industrial areas), have lost their dominance thanks to successfully implemented emission reduction measures or production restrictions or production shutdowns.
Main traffic arteries
Due to the high exhaust emissions and the poor ventilation situation in the densely built-up valley basin, the areas of the main roads are particularly critical on a small scale. These main traffic arteries
- In the city center, especially the Alleenring, partly the Grabenring, Peterstraße and Theaterstraße,
- outside the ring of avenues, the main access roads close to the city center, Adalbertsteinweg to Rothe Erde, Jülicher Straße, Krefelder Straße, Zollernstraße, Roermonderstraße,
- also Josef-von-Görresstraße, Alt-Haarener-Straße and Von-Coels-Straße
have traffic-related immission concentrations that are within or even above the limit values. There is a clearly increased need for action here with regard to traffic planning and traffic control measures.
Spa areas
Particular attention must be paid to the air hygiene situation in the two Aachen spa areas of Burtscheid and Monheimsallee. These areas, located to the north and south-east of the city centre, are subject to considerably higher requirements than the general EU limit values for particulate matter nitrogen dioxide (North Rhine-Westphalia Health Resorts Act and quality standards for the designation of health resorts), so-called health resort guide values.
For a long time, compliance with these spa-specific requirements was not guaranteed, particularly during periods of low air exchange. Current air hygiene studies from 2016 in the two spa areas now show that even the particularly strict spa area guidelines can be met.
Today, the main emitter in the spa areas themselves and in the nearby spa fringe areas is motorized road traffic. By means of green planning measures and a solid fuel ordinance that has been in force since 2010, a significant reduction in domestic fuel emissions has also been achieved through the implementation of already known traffic reduction scenarios to ensure the sustainable quality of the spa areas. This is also associated with a reduction in excessive noise emissions in some areas.
Weather conditions
When the weather conditions are exchange-rich and neutral, Aachen's climatic and air-hygienic situation is comparable to that of other cities and relatively unproblematic. On a long-term average, this is the case on around 60 % of days per year. Strong exchange weather conditions mean for Aachen: Wind force 3 (Beaufort) with predominantly southerly to westerly wind directions. Sufficient air exchange is then guaranteed in the valley basin, which is particularly heavily polluted in terms of climate and air quality. Stream valleys and green corridors located along the main wind directions (for Aachen S, SW, W) play an important role as ventilation channels. These are, for example, the Kannegießerbach, Johannisbach, Gillesbach, Dorbach and Wildbach valleys. Neutral weather is defined as average exchange conditions in the lower atmospheric layer. This can occasionally lead to a climatically and air-hygienically problematic situation in the Aachen basin. However, this is not as often the case as with the following low exchange weather conditions.
As Aachen's city center is located in a basin, a climatically and air-hygienically problematic situation can arise, especially during stable high-pressure weather conditions (fair weather conditions with low winds and high solar radiation). Such weather conditions are generally characterized by a lack of or weak air currents in the lower atmosphere. The stale and polluted air of the city center cannot or can hardly be replaced by fresh air. This results in the formation of a larger "heat island" with air polluted by emissions.
Low-exchange weather conditions occur in the Aachen area on a long-term average of around 40% of the days in the year, particularly from December to March and in September/October. In such low-exchange weather conditions, the weak winds blow over a large area predominantly from north-easterly to south-easterly directions.
In this case in particular, the stream valleys and green corridors, whose orientation coincides with easterly wind directions, therefore fulfill an extremely important function as ventilation channels. They are therefore particularly worthy of protection. Examples include the Beverbachtal valley, the green corridor between Eilendorf and Brand, large sections of the Haarbachtal valley and the Wurmtal valley between Haaren-Ost and Europaplatz (e.g. large open spaces around Kalkofen estate).
Fresh and cold air supply
A special form of fresh air supply for Aachen is the evening and nightly inflow of cold air close to the ground into the valley basin. This occurs when there is no/low cloud cover and no/low ground currents in the lowest layer of the atmosphere. Cold air close to the ground forms at night, especially in larger open areas. This can slowly flow down into the Aachen basin when the frequently occurring inversion layer forms over the basin, thereby ensuring a positive exchange of air in large parts of the city center. These cold air currents are often the only way to transport fresh air into the city center during climatically and air-hygienically problematic weather conditions. The longer a high-pressure weather situation lasts and the associated inversion formation intensifies, the more important the influence of cold air becomes for urban air exchange. Although the cold air cannot usually displace the polluted city air completely or everywhere, it enables a mixing effect and thus reduces the summer heat and also the air pollution. Recent studies on the cold air situation in the entire area of the Aachen basin show the spatial effectiveness of the cold air flows from the surrounding stream valleys. However, there are also inner-city areas that cannot be reached by the cold air outflows. In these areas in particular, there are not only climatic but also air hygiene problems (central and eastern city center).
The Inde/Iterbach valley system outside the basin, which is also effective in terms of climate ecology, primarily supplies the Kornelimünster district with fresh air. Significant cold air production and drainage areas are also located in the area of the Senserbach in the west and the Amstelbach in the north of the urban area.
If cold air can no longer flow away and accumulates, cold air lakes can form in some areas, in which the immission load increases rapidly under unfavorable circumstances and thus contributes to a localized deterioration in air quality (extensive Soers, railroad embankment crossing in the Gillesbach valley, Beverbachtal, Wildbachtal and Johannisbachtal with cold air congestion zones).
In high-radiation, low-wind conditions in the highly sealed city center, small-scale vertical circulation during the midday and afternoon hours can also lead to effective local ventilation: when the city center is overheated, the air rises here and the resulting negative pressure at ground level causes an inflow of less polluted air from the less built-up / sealed surrounding area.
Green spaces
Large green spaces such as parks and cemeteries are of particular importance as potential cold air generation areas. For example, 50-100 m wide green spaces reduce the temperature by 3 to 4°C compared to neighboring buildings on hot, windless days. In conjunction with cold air-draining slopes or gullies, these areas can also contribute to the supply of fresh air to Aachen's city center.
Due to the enormous green space deficit, explained by a degree of sealing > 90 %, an effective fresh air supply is necessary there. All inner-city, larger and more or less open green spaces are therefore particularly climate-active and should not only be preserved, but also expanded due to climate adaptation requirements (future climate development with adverse effects). In addition, in densely built-up urban areas, general greening (e.g. with avenues, inner block, wall and roof greening) can also make an important contribution to improving the local urban climate, reducing climate-ecological impacts and thus increasing the quality of life in the immediate residential environment.