USE OF GASEOUS TRACEBi FOR AIR POLLUTION STUDIES IN URBAN AREAS J. BINENBOYM, I. GILATH, M. MEUZER, CH. GILATH. A. LEVIN, M. RINDSBERGER. and A. MANES Report to the National Council for Research and Development Israel Atomic Energy Commission and Israel Meteorological Service 19 7 5 USE UK GASEOUS 1.:ACERS FOR AIR POLLUTION STUDIES IN URBAN AREAS J. Binenboym and T. Gilath Inorganic and Physical Chemistry Dept. Soreq Nuclear Research Centre M. Meltzer, Ch. Gilath and A. Levin Isotope Applications De.pt. Poreq Nuclear Research Centre M. Rindsberger and A. Manes Israel Meteorological Service Beit Dagan Report to the National Council for Research and Development Soreq Nuclear Research Centre Israel Meteorological Service Israel Atomic Energy Commission Ministry of Transportation August, 1975 I.:ONTI:NTS 1 . INTRODUCTION 3 2. METHODOLOGY 6 3. MODELS FOR THE COMPUTATION OF DISPERSION OF POLLUTANTS EMITTED BY POWER PLANT STACKS 8 4. EXPERIMENTAL 11 4.1 Analytical 11 4.2 Tracer injection and sampling 13 4.3 Field experiments 14 5. RESULTS 16 6. ERROR ANALYSIS 18 7. DISCUSSION 21 8. CONCLUSIONS 28 APPENDIX 31 REFERENCES 33 TABLES 35 FIGURES 49 - 1 - AJ.SlKAi i A tracer technique, using the inert gas SF , was apolied to determine the dispersion of the stack el fluents frcm the Reading povcr plant, ur.i.., -.-' '), In the greater Tel-Avi\ ire,i. The contribution :--i the power plant to ground level concentratic .s of S0? was assessed by comparing the measured SO., values with t'nse calculated from the SF, dilution and the SO., emission concentration. It is shown that at some measuring stations the ground level SO, concentrations are greatly enhanced from sources other than the power plant. ''•y, ground level concer.t rat iont" computed using a t.aussian iLi'usion model '..ere found to bo lu'..cr than the measured values. 1. IN'IRODUCTLON- In urban areas there are a multitude ot sour'.fH emitting pollu- tants under different, conditions. The ground level concentration of air pollutants is determined by the contributions of the various sources, and the dispersion and possible disappearance of the pollu- tants during their travel from the point of emission to the point of interest. Efficient monitoring and control ot air pollution calls for prediction and measurement of each of the above factor? Gaseous tracers may he instrumental in achieving this goal. When several sources are polluting a given .area, it is of interest to establish the specific contribution of each source to the concen- trations measured. Thus one becomes interested in specifically labelling the pollutants emitted from a given source, i.e. tracing its pollutants. The interest in atmospheric tracer-::- started between World Wars I and II in connection with atmospheric dispersion studies Chemical warfare groups were among the first to become interested in tracer techniques for simulating the dispersion of chemical war agents. Later the construction of atomic energy installations (reactors, fuel repiocesslng plants, etc.) and the general interest in air pollution resulted in development of these techniques. The different tracer techniques in use until the mid-sixties have been reviewed by Slade . Oil fog, aerosols and fluorescent powders, rather than gases, were used most often in tracer studies. Generally the particle size was limited to about 10 microns and it was accepted that the dispersion of the particulate aerosol actually represented the dispersion of a gas. A considerable number of the diffusion and tranrport experiments performed with particulate tracers are reported by Slade^ . It is of interest to note that the great majority of dispersion coefficients and/or standard deviations used in connection with dispersion models today resulted from measurements performed with particulate tracers . Among the particulate tracers, inorganic fluorescent particles, such as zinc cadiuni sulfide, wore generally used in urban areas However, continuous use of this tracer is limited because ot its tosicity C) . Other problems connected with participate tracers are the loss by falLout and impaction, instability in the atmosphere (loss of fluorescent activity of small particles after exposure to sunlight), etc. The wide acceptance of particulate tracers was due to lack of a suitable gaseous tracer, measurable to high sensitivity and not occurring in nature. Gaseous air pollutants are transported by turbulent dispersion and therefore the molecular identities of the tracer and the traced ft s compound are of no importance, Kr was used as a tracer in disper- sion studies from ground sources at relatively short distances, i.e. less than 1 km . For distances greater than 1 km very high activities 41 would be involved. Ar produced in nuclear reactors was detected at downwind distances as far away as 6 kin . However, its use is possible only in connection with the stacks of nuclear reactors. Preliminary tests, "-I960,with halcgenated compounds such as freons and SF. proved their suitability as atmospheric tracers ' . SF, was found to be the most suitable tracer among thesa compounds. It was first suggested bv Collins et al. , and .later advocated by 8 9 10 Turk Et al.*" "*. demons et al.^ \ Hawkinr/ \ Dietz and Drivas and Sh.iir ' also used this tracer in their work. SF, is not found in nature and its main use is as an insulator b in very high voltage transformers. It is chemically inert, stable towards hydrolysis, oxidation and photolysis, and non-toxic, odorless <13) and colorless . It is not subject to fallout or other atmospheric removal processes ' . SF can be easily released into the atmos- phere at a well-controlled rate. It is inexpensive, costing about $ 7 (U.S.)/kg. The background level of SFft is negligible, i.e. below 10 cm SF-/cm air '. SF, can be detected to a very high sensitivity using a gas chromatograph equipped with an electron capture detector. Direct determination of SF during the early studies was limited to relatively lov sensitivity . It's detection involved preconcentration of the air samples to he analyzed either !y freeze-out or by adsorption and concentration on activated charcoal'' ' . At present however, it is possible to detect SF directly in concent rati< r. Lower than 10 ' cm SF,/cm air ' ' . A i rl.orrie chromatr'graphs designed for real-time studies of diffusion and plume transport have also been successfully developed ' '. !n conclusion, the develop- ment of very sensitive gas cliromatographic techniques foi determining traces of SF can be considered as a bieakthrouau in the use oi gaseous tracers for air pollution studi s. Thti aims o£ the present research were to work out a technique for tracing the stack effluents of power plants, over long distances. This involves working out the gas chrotnat.ograpnic technique for measurement of very low concentra- tions of SFr in the air. 6 to develop a tracer technique for identification of the contribu- tion of individual sources in the air pollution in an urban area; specifically to determine the contribution of the Sfl0 emitted by the Reading D power plant to the SO- concentration measured in the greater Tel-Aviv area. This is important since of all the S0~— emitting sources in the Tel-Aviv area only the Reading D power plant is provided with means for controlling the emission. tc gain information on the relation of measured tracer concentra- tions in an urban area to those predicted by models. - to assess the potential of the technique based on gaseous tracers for the determination of the pollutant disappearance rate. Reading D power plant refers to the whole Reading power station complex (i.e. units B+D). All units discharge effluents through a common stack. - b - 2. METHODOLOGY tracing the stack effluents of power plants requires essentially the injection ot the tracer into the stack, knowledge of the effluent ilow rate, temperature and composition, sampling of air at different locations and measuremer _•£ c . r.r er concentration in the samples. The tracer injection can u<- ^L...~,. instantaneous or continuous. Instantaneous injection results in a tracer puff, the concentra- tion of which has to be measured both in space and in time. Measuring . C dt (C i«* the concentration of the tracer as a function of time) o tr tr at a given location enables one to calculate the steady state concen- tration of any component of the effluent into which the tracer is injected . This calculation assumes however steady state conditions (i.e. no change at all in. wind speed and direction) which are almost impossible to achieve practically. This is the reason why only a few instantaneous injection experiments were performed in atmospheric dispersion studies . These fex^ experiments were aimed at obtaining standard deviations for instantaneous release conditions or for simulating accidental releases of pollutants. The continuous injection method requires that the injection be performed over a period sufficiently long so that fluctuations in wind direction and/or wind speed average out, yet short enough so that the same average wind direction and velocity are preserved during the entire injection and sampling time. It is common to perform the injection over a time period equal to 1-2 times the travel time from the source to the most distant sampling station . If the average wind velocity is about 5 m/sec and one is interested in working over distances of up to 10 km, injection time should be between 0.5 and 1 hour provided the average wind direction and speed does not change during this period of time. The majority of atmospheric dispersion parameters are reported in the lzLtidCure for 10 minute sampling (18i times . The correction of dispersion coefficients and/or concen- trations for sampling times different from 10 minute is made according - 7 - / 1 O \ to Cramer's power law ' , The concentration is inversely proportional to the sampling time raised to the power of 0.2.