Transactions on Ecology and the Environment vol 8, 1996 WIT Press, ISSN

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1 Measurement of the contribution of a single plant to air pollution using the SF6-tracer technique H. Roetzer, J. Riesing Physics Department, Seibersdorf Research Centre, A-2444 Seibersdorf, Austria Abstract Many regional and local monitoring networks exist for controlling of air-quality. However the measured concentrations of pollutants are usually the sum of the contributions of a large number of sources of air pollution and therefore yield no information about individual plumes. The SF^-tracer method is used to investigate the dispersion of single plumes and to assess their contributions to overall air pollution without interference by other emitters. The exhaust gases are labelled by adding continuously minute quantities of the tracer gas. The tracer gas SF6 is a chemically stable, non toxic gas that can be detected to a very high sensitivity. Measurements of the tracer concentration in air samples indicate the dilution of the plume at the sampling points. A description of the applied techniques and three case studies are presented in the paper. For a large thermal power plant the contributions of the scrubbed flue-gases to ground level concentrations of air pollutants in the surroundings could be measured and turned out to be quite small. At a highway tunnel the dispersion of the exhausted air from the ventilation was measured to assess the impact on air quality in a neighbouring town. Measurements were carried out during stable weather conditions and the concentrations of pollutantsrisingnear to the limitvalues. At a sanitary landfill the dispersion of the emitted landfill-gas and the resulting concentrations of odorous substances in the surrounding communities were measured. By a comparison of the results with values of odour thresholds the odour impact of the landfill could be checked in this area although there existed other sources of odour too.

2 528 Air Pollution Monitoring, Simulation and Control 1 Introduction For controlling the quality of air, there exist many regional and local networks where the concentrations of pollutants are continuously monitored. However, the measured immission concentrations, especially in highly industrialized or densely populated regions, are usually the sum total of the contributions of a large number of air pollution sources, such as industrial facilities, craft trade enterprises, thermal power plants, road traffic and domestic heating. Moreover, considerable amounts are added by the long-range transport of pollutants. The measured values of pollutant concentrations in air therefore give no information about the dispersion of individual plumes and their contribution to these values. Yet for decisions concerning measures to improve air quality, well founded information is needed about the contributions of single sources of pollutants to the immission concentrations. By application of a tracer method, the dispersion of individual plumes can be measured without interferences from other sources of air pollution. The fluegases of the plant in question are labelled uniformly by adding a constant flow of tracer gas. The plume can then be detected selectively by measuring the tracer concentration in air. In the past various aerosols and gaseous tracers were used for this purpose, e.g. Sladefl]. Among all these substances sulfur hexafluoride (SF6) was found to be a nearly ideal tracer for a number of advantageous attributes, e.g. Dietz & Cote[2], Drivas & Shair[3], Binenboym & Gilath[4]. The Research Centre Seibersdorf has designed a mobile set of equipment to carry out tracer experiments for studying the environmental impact of single sources of air pollution. The method has been applied successfully to various emitters like industrial facilities, thermal power plants and sanitary landfills. 2 Measuring principle The principle of the tracer method is, to label exhaust gases by adding minute quantities of a tracer gas not found in nature or in other plumes. The tracer gas concentration is then measured in air samples collected at discrete positions downwind from the emission source. The measured concentrations indicate the dilution of the plume at the sampling points. The ratio immission/emission concentration for the tracer gas is applied to pollutants with known emission concentration in order to calculate the specific immission concentration for the plant in question. By a comparison of these results with the total concentration values recorded by air pollution networks, the contribution of a single emitter to the overall air pollution of the region can be assessed, Thereby it is assumed that during dispersion no loss of the pollutants occurs. Otherwise the results would represent upper limits of the contribution of the pollutants.

3 Air Pollution Monitoring, Simulation and Control 529 During all measurements meteorological data have to be recorded because the atmospheric dispersion of a plume depends on the actual meteorological conditions. 3 Tracer gas The tracer gas used is sulfur hexafloride (SFe). It is most suitable for this purpose because of the following advantages: SFe is a chemically inert and thermally stable gas which does not undergo chemical changes during atmospheric dispersion. SFe is non toxic, the TLV is 1000 ppm. SFe does not exist in nature. It's background concentration is less than 10"^ parts SFe per part air. SFe can be detected at very low concentrations of about 10"" parts SFe per part air, using gas chromatography and an electron capture detector. On account of this high detection sensitivity it is possible to effect tracer concentration measurements even at fairly large distances from the emission source (up to 100 km). SFe is commercially available at a reasonable price. The density of SFe is about 5,1 times that of air. No separation takes place because of the high dilution of the tracer gas in the exhaust gases and the subsequent turbulent atmospheric diffusion. 4 Execution of measurements 4.1 Injection of the tracer gas Sulfur hexafluoride with a purity of 99,8 % is used as tracer gas for the measurements. The gas is injected into the flow of flue-gases by using a thermal mass flow controller. The desired flow-rate of the tracer gas can be preselected and during dosing is kept constant to within 2 %, independent of temperature and pressure fluctuations. It is important to achieve a uniform mixing of tracer gas and exhaust gases before emission from the stack. The tracer gas flow-rate is monitored continuously in order to assure constant injection. 4.2 Equipment for SFe-analysis The measurements of the SFe ground-level concentrations are effected by using a gas chromatograph with an electron capture detector. The detection limit of the measuring device is 1,5x10"" parts SFe per part air. One air sample takes approximately 1,5 minutes to analyze. The gas chromatograph is calibrated with SFe-gas-standards at regular intervals. The whole measuring equipment is installed in a laboratory car. This ensures the mobility necessary to follow the trajectories of the plume emitted by the plant and reduces the time of transport for air samples collected in bags.

4 530 Air Pollution Monitoring, Simulation and Control An accuracy test of the tracer method has been carried out at an unit for acceptance tests of large blowers, e.g. Roetzer & Riesing[5]. The air-flow in this unit is measured with an accuracy of ±1 % by the help of a restrictor. This air-flow was in comparison also measured by the tracer dilution method. The maximum difference between the results of the two methods was 2 %. Therefore it can be concluded that the tracer method using the described equipment gives results with an accuracy similar to that of the restrictor method. 4.3 Collection of air samples Two methods of air sample collection are applied. Instantaneous SFegroundlevel concentrations are obtained by sucking in air from the vicinity of the laboratory car and analyzing the samples. These instantaneous values of SF6 concentration are taken while crossing the trajectories of the plume with the measuring car at different distances downwind from the emission source. Half-hour mean-value groundlevel concentrations are measured in air samples collected in bags. These samples are taken by programmable, automatic air sampling units which are placed at preselected locations covering the diffusion domain of the plant's plume. The start of air sample collection is triggered automatically at the preselected time. Each air sampling unit gives 8 consecutively collected samples. With a sampling interval of 30 minutes half hour mean values of SFe-groundlevel concentration are obtained. 5 Case studies of tracer measurements 5.1 Flue gases from a thermal power station The thermal power station at Duernrohr is located about 40 km in the west of Vienna in an area characterized by both agriculture and industry. According to public opinion the power station at Duernrohr causes the overwhelming portion of the air pollution in this region, despite the fact that the power station is equipped with highly efficient scrubbers for sulfur dioxide and nitrogen oxides as well as with electrostatic precipitators for the particles and with a 210 m high stack. Due to these environmental protection installations only relatively low contributions of the plant to the groundlevel concentrations could be expected, particularly with respect to the already existing significant background concentrations in the region. Altogether 17 plume dispersion measurements were carried out at various weather conditions and stability classes, e.g. Roetzer & Riesing[6]. The tracer experiment on January 30, 1986 is described as an example. On this day wind was blowing from SE at a mean wind speed of 5 m/sec, the prevailing stability class was neutral. The mean flue-gas emission rate was nrvh (STP) with an SOz-concentration of 100 mg/nf. Within the dispersion region of the plume more than 100 instantaneous groundlevel

5 Air Pollution Monitoring, Simulation and Control 531 concentrations were measured at different measuring points by crossing the plume's trajectory at 5 different distances from the emission source using the mobile laboratory. In addition at 7 fixed positions half hour mean values were recorded, 4 at each position. The measured maximum values for each of the 5 distributions are shown in the following table 1 which also includes the maximum values of groundlevel concentrations as obtained from dispersion calculations. For these calculations the Austrian standard Gaussian diffusion model[7] has been used. Table 1: Results of January 30,1986. Distance from emitter in m Max. measured instantaneous value in ng SOz/nf 2,21 2,96 2,49 2,18 1,54 Max. measured half hour mean value in ig SOz/nf 3,95 2,28 1,88 - Max. calculated value in o.g SOz/nf 0,5 4,2 4,0 3,4 2,3 As is apparent in the table the SCVconcentrations are quite low, the maximum being a half hour mean value of 3,95 ig SCVnv*. This value is less than 3 % of the authorized limit (150 jag SOz/nf). It is also small compared to the total SO%- concentrations of 24 to 55 ig SCVm*, which were recorded at the time of measurement in this area. Regarding the model calculations a satisfactory agreement with measured data can be seen in the table for this meteorological situation. Looking generally at the 17 dispersion experiments all half hour mean values and most of the instantaneous values were smaller than 10 % of the authorized limit (150 ig SOVnf). Nevertheless, the plume could be clearly detected at distances up to 30 km according to the high sensitivity and selectivity of the tracer method. 5.2 Exhaust gases from a highway tunnel The Plabutsch tunnel is a highway tunnel at Graz, capital of the province Styria (Austria). The tunnel with a length of 10 km protects the town from both noise and exhaust gases emitted by traffic. A ventilation system provides fresh air to the tunnel. The spent air polluted with traffic exhausts is released to the environment via 3 ventilation shafts which are situated in a hilly region. During winter time in the atmosphere of Graz very often longer periods of inversion conditions occur with the effect that pollutant concentrations exceed limit values. Complaints were expressed to local authorities that the exhausted air from Plabutsch tunnel ventilation is the main reason of high NOx concentrations in some districts of the town.

6 532 Air Pollution Monitoring, Simulation and Control Tracer technique was applied to measure the contribution of the tunnel exhausts to pollutant concentrations and to compare the values with the total concentrations obtained at the air monitoring stations, e.g. Roetzer & Donhoffer[8]. Five tracer experiments were carried out at the ventilation shaft,,buchkogel", which is situated on a hill therefore effecting an emission height of 200 m above the inhabitated area. The maximum flow rate of the exhausted air was 1, m3/h (STP) with a NO-concentration of 10 mg/m3. The tracer gas was injected into the blowers transporting the exhausted air. The experiments showed that in the case of continuous inversion conditions the contribution of the tunnel to the groundlevel NOx-concentrations was negligible. During the experiment on January 21, 1993, which is described as an example in the following, temperature inversion was decomposed around noon. In the afternoon when inversion was developing again, significant contributions from the tunnel could be measured in the districts adjoining to the ventilation shaft. The following table 2 shows the total NOx-concentrations and the contributions of the tunnel measured at the air monitoring station Graz SW: Table 2: NOx-contribution at station Graz SW Timeinterval NOx-total Ug/m3 NOx-tunnel Hg/m3 NOx-tunnel % 10:30 11: ,22 0,17 11:30 12: ,11 0,07 12:30 13: ,10 0,06 13:30 14:00 0,23 14:30 15: ,68 2,23 15:30 16: ,48 1,91 16:30 17: ,2 17,2 17:30 18: ,56 7,56 The table shows that in this case of the new formation of an inversion the contribution of the tunnel was rising up to 17 % of the total groundlevel concentration. 5.3 Odour impact of a sanitary landfill The sanitary landfill at Gasselsdorf is used for the disposal of solid waste from the district Judenburg (Austria). The landfill is situated in a valley running in a south-easterly direction. In this valley odorous substances are emitted both from a cellulose plant and from several farms. The licence to establish the landfill was given by the local authorities on condition that the odour impact in the area would not rise significantly. A gas collection system was installed therefore to reduce emissions from the landfill, in addition the surface is regularly covered with protecting foam. A dam with a height of 35 m was raised to protect the adjoining community located in the main wind direction.

7 Air Pollution Monitoring, Simulation and Control 533 For judging, if the additional odour impact from the landfill would be tolerable, the contributions of the landfill had to be assessed. As the frequently used olfactometric methods give no information about a single source of odour, tracer technique was applied to get selectively the odour impact of the landfill e.g. Roetzer, Muehldorf & Riesing[9]. The tracer experiments had to be carried out during the most unfavourable meteorological conditions with regard to the odour impact, i.e. when the atmosphere is only slightly mixed and the concentrations of pollutants increase. Local meteorological data showed that during high atmospheric pressure local air flows prevail. The direction of flow is upvalley (SE) during daytime and downvalley (NW) in the night. When the direction of flow is reversed, stagnations and a local increase of pollutant concentrations occur. Therefore the tracer experiments had to include the situations of flow reversion. Besides the tracer experiments the following measurement program was carried out to collect additional data. Monitoring of meteorological data at 6 significant points around the landfill. Qualitative and quantitative analysis of the gases emitted from the landfill. Olfactometric determination of odour in ambient air by field inspections. Dispersion experiments with smoke. Fifteen tracer experiments were conducted under different weather conditions. The tracer gas was released at ten points equally distributed on the landfill. The flow of tracer gas was the same at each point. Air samples were collected at 15 preselected positions in the surrounding communities. The tracer results were used to calculate the concentrations of N% and HzS, which had been selected as leading substances for odour nuisance. In the following the experiment on November 24, 1993 is described. The meteorological conditions on this day were characterized by a large high pressure area with center above Russia. The time of the measurements was 13:00 h - 20:30 h. Sampling was started every hour and lasted for 30 minutes. During thefirstpart of the experiment (13 h - 16 h) a slight wind was blowing from SE (upvalley), then the direction reversed to NW (downvalley). Ambient temperature dropped from -3 C to -T C during the experiment Figure 1 shows the ammonia concentrations caused by the landfill at two inhabitated places in the surroundings. Point 1 was situated at a distance of 800 m downvalley of the landfill but not behind the dam. There are two appartement houses at this place. Point 2 was situated at a distance of 700 m of the landfill at the opposite side of the valley. There is also a cluster of farm houses at this place.

8 534 Air Pollution Monitoring, Simulation and Control 1, E-13 16:30 17:30 Clock time Figure 1: NHg concentrations at points 1 and 2 - Nov. 24, 1993 The figure shows the ammonia concentrations determined at the points 1 and 2. In accordance with the registered wind data the values at point 1 are low for three hours and rise after the flow reversion at 16 h. The measured ammonia concentrations are the highest values obtained outside of the landfill during all experiments. At point 2 only small concentrations were measured. These results prove that there is only a very small impact of the landfill emissions at a right angle to the main flow direction on the opposite side of the valley. The following table 3 gives an overview of the maximum concentrations of NHs and RbS measured at the points 1 and 2 in comparison to odour thresholds taken from a data collection of Verschueren[10]. The table shows ranges resulting of the different values of odour thresholds measured by various authors. Table 3: Maximum concentrations and odour thresholds Substance NHa HzS Odour threshold 28 ppb - 42 ppm 0,46 ppb - 70 ppb Max. concentration at point 1 6,71 ppb 0,036 ppb Max. concentration at point 2 0,04 ppb 0,0002 ppb Although the values at point 1 were the highest of all experiments they were by far lower than the odour thresholds.

9 6 Concluding remarks Air Pollution Monitoring, Simulation and Control 535 The given examples prove that the tracer gas method is excellently suited to measure the atmospheric dispersion of a single plume without interferences from other emitters. The method can be used to assess the environmental impact of single sources of air pollution and therefore provides valuable informations to authorities and inhabitants. Furthermore, by comparing tracer results with model predictions, dispersion models can be evaluated and model parameters be adapted to local conditions. 7 References 1. Slade, D.H. Meteorology and Atomic Energy, TID-24190,USAEC, Dietz, R.N. & Cote, E.A. Tracing atmospheric pollutants by gas chromatographic determination of sulfur hexafluoride, Environ. Sci. andtechnol., 1973,7, Drivas, PJ. & Shair, F.H. A tracer study of pollutant transport and dispersion in the Los Angeles area, Atmosph. Environ., 1974,8, Binenboym, J. & Gilath, I. Use ofgaseous Tracers for Air Pollution Studies in Urban Areas, Israel Atomic Energy Commission, Roetzer, H. & Riesing, J. Air flow measurements by a tracer method in road tunnels with longitudinal ventilation, paper D4 in Proceedings of the 5th Int. Symp. on the Aerodynamics & Ventilation of Vehicle Tunnels, Lille, France, Roetzer, H., Riesing, J., Nentwich, A. & Szeless, A. Measurements of the contribution of a coal-fired power plant to the overall groundlevel concentration of air pollutants by using the SFe-tracer technique, in Environmental Meteorology (ed. K. Grefen & J. Loebel), pp , Proceedings of an Int. Symp. on Environm. Meteorology, Wuerzburg, Germany, 1987, Kluwer Academic Publishers, Dispersion of pollutants in the atmosphere - Calculation of ambient air concentrations and determination of stack heights, ONORM M 9444 (Austrian Standard Specifications), Wien, Roetzer, H., Donhoffer, D. & Semmelrock, G. Messung der Ausbreitung derabluftdes Plabutschtunnels, Report 10/94 of the Austrian Federal Ministry for Environment, Youth and Family, Vienna, 1994.

10 536 Air Pollution Monitoring, Simulation and Control 9. Roetzer, H., Muehldorf, V. & Riesing, J. Measurement of the Odour Impact of a Waste Deposit Using the Sp6-Tracer Method, Proceedings of the 2nd Int. Symposium on Environmental Contamination in Central and Eastern Europe, Budapest, Verschueren, K. Handbook of Environmental Data on Organic Chemicals, Van Nostrand Reinhold Company Inc., New York, 1983.