Aim of the research activity

The aim of the research project "Dispersion of Atmospheric Trace Elements in an Urban Area –taking the City of Graz as an example" is the investigation of the dispersion and conversion of atmospheric pollutants in an poor ventilated urban area under anticyclonic weather conditions. The Austrian city of Graz (340 m a.s.l.) is situated in the pre-alpine region south-east of the Alps. The city itself is located in a basin surrounded by hills of 100 – 400 m a.g.l.. In the vicinity (up to a distance of 20-40 km) the orography is dominated by the valley of the river Mur and mountains of more than 1500 m a.s.l.. Air quality levels during high pressure radiation conditions are determined by the quantity of emissions, by local wind systems, by temperature inversions combined with low turbulence and by chemical reactions. The frequent occurrence of local wind systems is typical for pre-alpine regions south of the Alps. In order to investigate the flow regimes and pollution dispersion two major measurement campaigns have been carried out. The concept of the measurements, the instrumentation and some results of the winter measurement campaign are described in the following chapters.

Activities during the year

Winter measurement campaign

The winter measurement campaign was intended to study a typical winter smog situation. Typically for the south-east of Austria are radiation weather conditions together with cold continental air leading to high emissions from domestic heating compared to emissions from other sources. The first anticyclonic weather episode during that winter started at January 9. The anticyclone in the 500 hPa ridge was very intensive and prevented fog even at the ground level in the investigated area. The episode ended with the advection of ground fog in the late evening of January 12. The measurements ended after 72 hours in the morning of January 13. During the three days of the field experiment, hourly vertical profiles of wind speed and direction, temperature and humidity were measured at four locations in the investigated area. In addition one meteorological tower, two SODARs, one eddy correlation measurement device and at least 10 standard devices at the ground were in operation. At the same time 2 DOAS systems, 1 DIAL LIDAR, 1 FTIR and 7 monitoring ground stations for the measurement of standard pollutants have been operated. The next picture shows the location of different monitoring points. The isolines show the altitude in a distance of 50 m. The highest elevation is at a height of 1450 m a.s.l.. The area includes the northern part of the Graz basin and the valley of the river Mur, which enters the basin from northwest. The Alps are located in the northwest of the area. The results of the winter campaign are available in a data bank using the netCDF-format.

Local Wind Systems

Very important for air quality levels during the prescribed meteorological situations are the local wind systems. They are determined by the orographical situation. Three typical local wind systems can be distinguished for the city of Graz (Almbauer et al., 1995) (Lazar, 1991):

a) Mountain-valley wind system: There exists a dominant mountain-valley wind system in the Mur valley northeast of Graz. The valley has length of approx. 190 km (north-east of Graz)

and has its origin in the inner Alps. The first 150 km are west-east oriented, the last 40 km north of Graz are mainly north-south directed. Several basins and side valleys enter the main valley. So the production of cold air leads to a mountain wind, which can be measured in the city 3 – 4 hours after sunset. The maximum wind speed can be expected in the early morning hours and the mountain wind can last until 3 –4 hours after sunrise. The mountain wind system is important for the north-westerly part of the city, where the wind is channelled through the narrow nozzle of the Mur valley near measuring point 8 in fig. 1 (Weinzöttl). During the day time depending on the season southerly winds occur, which can be described as valley winds. These winds are not really valley winds in the area of the city, as the valley is very wide. The wind is a more general flow in the direction to the Alps.

b) Katabatic winds from the ridges east of Graz: The small valleys in the east of the city produce katabatic winds, which occur soon after sunset. The valleys are of a typical length of 10 km and the ridges are approx. 100 m higher than the valley grounds. These winds are weaker than the other systems. They exist only due to the lower tempartures of the air, which let them flow underneath the other flow systems. During daytime the anabatic wind system, which can be expected, is often disturbed by the southerly winds.

c) Cold air in the basin: Cold stagnant air in the southern and western parts of the city are typically for the city of Graz under the prescribed weather conditions. This cold air flows very slowly from the southern part of the Graz basin to the city centre. The physical reason for this flow is under investigation. There are different effects which reinforce each other. One reason might be the production of cold air in the very flat basin of Graz, which spills cold air from south to the city centre. There might also be a small heat island effect. The major effect may stem from the mountain wind, which flows around the small mountains northwest of the city (fig. 1.: south and west of measuring point 8). This flow produces a low pressure area in the lee of this mountain, which sucks air from the south to the city. Concept of the measuring campaign

The concept of the measuring campaign was the measurement of all these three local wind systems and their consequences on the air quality. As the given description stems only from ground observations, the campaign should also give information about the vertical distribution of winds, temperature, humidity and pollution concentrations. All the results should be used together with the emission inventory of the area in order to validate the simulation of the pollution dispersion using the numerical model GRAMM (Graz Mesoscale Modell).

Principal Results

Associated with the clear sky conditions, night-time inversions and their slow break-up in the morning hours, the mountain-valley wind system, katabatic winds from the side valleys and a shallow mixed layer during daytime were observed. The period was rather mild with low levels of primary pollutants. The ozone measurements at ground stations and vertical ozone profiles from one tethered balloon show even some ozone production.

The mountain valley wind system of the river Mur and its impact on the flow field in the city is clearly documented by the next three figures: They show the temporal variation of temperature, wind speed and wind direction for station 14. Data is available for every hour over a height of app. 500 m. The first diagram shows the build up of a mixed layer during all three days (10 – 12 Jan) between 11:00 am and 4:00 pm (16:00). During the night an inversion layer with a crowing height can be measured. The flow direction near ground level during this time is from the south. The second diagram reflects the development of a low level jet, which starts after sunset (16:00) and stops in the morning hours.

For both nights the low level jet moves to higher altitudes during the night. The last diagram shows the wind direction, where black is used for northerly winds and white is used for southerly winds. For both nights (10 – 11 and 11 - 12 Jan) only during the first two hours after sunset the northerly wind is able to touch to the ground. Afterwards the northerly wind, which is a mountain wind coming from the Mur valley, flows over a flat cold air body, which enters from the south.

Ozone concentrations are closely related to the vertical distribution of the meteorological parameters. During the first two days, the ozone values in the lowest 150 to 200 m increase with a rate of about 5 ppb per hour between 11 and 15 UTC. The maximum values reach up to 20 ppb. During the rest of the days the ozone concentrations were down to less than 1.5 ppb. Daily mean values of ozone concentrations at the monitoring stations 2, 4 (at the city ground level) and 11 in a height of 210 m a.g.l. show a strong vertical layering. The temperature inversion and weak winds prevented an exchange of air masses (Baumann, 1998).

Main Conclusion

The shown data indicate that the winter campaign was well suited and caught all the expected phenomena. The existence of well developed local wind systems is just as documented as the fact that local emissions are mainly responsible for the air quality levels during the poor ventilated episode. Data is available on request in the netCDF format.

Aim for the coming year

The activity for the year 1999 will be the analyse of the summer measurement campaign and the numerical simulation of both periods.

Acknowlegdment

The project DATE Graz is funded by the Austrian research fund (Project-No.: TEC-P12168, TEC-P12169, TEC-P12170).

References