METHOD FOR THE QUANTITATIVE DETERMINATION OF ELEMENTS FOR STUDYING THE DISTRIBUTION OF AIR POLLUTANTS IN THE SURROUNDINGS OF AN EMISSION SOURCE

Abstract: The object of the invention is a method for quantitatively determining elements for studying the distribution of air pollutants in the surroundings of an emission source. The method is characterized by the following steps: the sample to be examined is collected at a sampling plot from the plant cover surrounding the emission source, by taking the sample from a surface portion thereof, at that side of the sampling plot which faces the emission source; the sample is dried; the surface of the sample is made electrically conducting; the contents of the elements deposited on the surface of the sample are measured using an energy dispersive X-ray microanalyzer (EDX) connected to a scanning electron microscope (SEM).

Description: Method for the quantitative determination of elements for studying the distribution of air pollutants in the surroundings of an emission source The object of the present invention is a method for the quantitative determination of elements for studying the distribution of air pollutants in the surroundings of an emission source. Despite the rapid technological development in the field of environmental protection and the clear reduction in many emissions, the need to study the distribution of air pollutants grows continuously. New industrial activity creates new problems. Also the need to increase the knowledge of the state of the environment and to decrease emissions is still growing. In the field of air protection, the methods relating to the distribution of emissions are not yet very highly developed and they are partly under dispute and dependant on subjective factors. Thus there is clearly a need for developing very exact methods and a demand for such methods. Such are needed by productional plants which have to have an increasingly better knowledge of the distribution of their own emissions, on the one hand as a result of stricter legislation and on the other hand in order to safeguard the marketing of their products. They need accurately measured and undisputable information. Also the authorities monitoring the state of the environment often commission studies. The distribution of air pollutants has been studied earlier i.a. using a number of standard methods (SFS 5669, SFS 5781, SFS 5782, SFS 5783: determination of the sulphur content in the needles of coniferous trees using different methods; SFS 5670: lichen survey; 5671: chemical analysis of mosses; SFS 5672: the fluoride content of the needles of coniferous trees; SFS 5794: moss bag method). Also the surfaces of pine needles have been studied using a scanning electron microscope (SEM), whereby the object of the study has specifically been the pollution particles on the surface. The elemental composition of these particles has been studied earlier by using a energy dispersive X-ray microanalyzer (EDX) (Meinander, Finnish Meteorological Institute, Air-quality Department, Final Report to the Ministry of the Environment, 30 p., Helsinki 1993, Juhanoja & Meinander, Extended abstracts of the 46th Annual Meeting of the Scandinavian Society for Electron Microscopy, 1994). These studies were, however, limited to particles having a diameter in excess of 2 μm. EDX has also been used to measure the seasonal variations of Mg and S on the surface of Ramalina duriaei lichen in percentage proportions of 12 analyzed elements (Garty et dl., Arch. Environ. Contam. Toxicol. 24, 455-460 (1993)). This method is, however, not usable in many environmental studies because it gives information only of the relative proportions of the elements in a sample, but not of their absolute quantities. The aim of this invention was to develop a method which would make it possible to measure the total element contents of the whole surface studied, including those which were not detectable as particles in the normal SEM-range, that is also the non-particulate content. By means of the method it is possible to obtain much more precise information of the quantities of all pollutants, in contrast to earlier studies where only the element composition of separate particles have been studied. From the results obtained in the method according to the invention it is possible to obtain better information than before regarding i.a. the relative proportions between the emissions from a local emission source and those originating from elsewhere. By means of the method according to the invention such air pollutants are found, which cannot properly be measured by studying, for example, the total contents in needles or bark or the soil, because in these other methods the air pollutants studied form only a part of the total mass. The invention also makes it possible to retrospectively detect earlier emissions, and in monitoring studies i.a. in connection with air protection measures. The object of the invention is thus a method for the quantitative determination of elements in order to study the distribution of air pollutants in the surroundings of an emission source, the method comprising the following steps: - a sample to be examined is collected at a sampling plot from the plant cover surrounding the emission source, the sample being taken from a surface portion thereof, at that side of the sampling plot which faces the emission source, - the sample is dried, - the surface of the sample is made electrically conducting, - the contents of the elements deposited onto the surface of the sample are measured using an energy dispersive X-ray microanalyzer (EDX) connected to a scanning electron microscope (SEM). According to an advantageous embodiment of the invention, the contents of the elements are determined with respect to the sample surface area by using standard preparations of the said elements, especially a standard curve plotted from corresponding measurements using the standard, to which the analysis results obtained using the SEM/EDX are compared. Thus in the method according to the invention, the sample is taken from the vegetation or plant cover surrounding the emission source, such as a plant or a tree species, especially from a tree, from a surface portion thereof, to provide a sample of surface material. A sample taken from the surface of the tree, especially from the surface of pine bark or needles, or from lichen and algae, for example growing on the bark of a tree, are especially well suited for use as a natural surface material. According to an advantageous embodiment according to the invention, the surface samples are collected from sample plots which are situated on a study line extending away from the emission source. From each sample plot several, even tens of samples are collected at each sample plot, whereby for each sample a few parallell samples are taken in order to improve the reliability of the method. The samples are advantageously taken on a study line which extends away from the emission source in a direction corresponding to the main wind direction. Preferably the terrain between the sampling plot and the emission source is as obstacle free as possible, that is, it should not include a growth of trees, or other topographical obstacles which prevent the free flow of the air pollutants. According to the invention, a formula is developed from the average values of the sample contents measured at each sample plot as a function of the distance of the sample plot from the emission source. The regression equation corresponding to this formula and corresponding coefficient of determination (correlation coefficient) can be used to interpret the distribution of air pollutants on the studied line. Especially they can be used to measure the relative proportions of emissions originating from a local emission source and those originating from elsewhere. The method according to the invention thus comprises four major steps, namely - sampling - preparation of the samples - sample microscopy - sample measurement. 1. Sampling According to the most preferred embodiment of the invention, the sampling at a sample plot on the study line is done in the following way. The samples are taken from the trunk of a tree, such as a pine, at a suitable height, for example at a height of appr. 1.5 - 2 metres on the side of the tree facing the emission source. The terrain between the sample tree and the plant should be as obstacle free as possible, that is no terrain obstacles, growth of trees etc. As sample trees, preferably 60 to 80 year old pine trees from similar conditions are chosen, which have a bark structure which is as similar as possible. At each sample plot, samples are taken from a few trees using the tip of a knife, from each tree a few pieces of bark of a few centimeters in size. The samples can be thin, for example 0.5 to 3 mm. The samples are dried, for example in paper bags, at least for a week at room temperature protected from dust. 2. Sample preparation SEM-samples must not be handled with bare hands, as dirt and dust from the skin easily ruin both the sample and the microscope. Thus, in sample handling, preferably pincers or plastic gloves are used. The preparation of the samples preferably takes place in the following way. Double-sided tape is attached onto metallic sample supports. From each sample four pieces of appr. 1 cm 2 size are put on one sample support using a preparation knife. The pieces are carefully fastened to the tape by means of the pincers. The samples are stored in petri dishes. If attempts are made to study a biological sample as such with SEM, the sample becomes rapidly charged as the electron beam scans its surface and image formation is prevented. In order to conduct the charges away from the sample, its surface must be made to conduct electricity. This is done by vaporising onto the sample surface a thin layer of a electricity conducting material, for example carbon, or especially a gold-palladium mixture which is a good conductor. The vaporisation can take place using a per se known vacuum vaporizing apparatus, that is a so-called sputtering device. A gold source is bombarded with gas ions (argon), whereby gold atoms are released therefrom. The "gold atom cloud" generated forms an even thin gold layer on the surface of the sample. Ready sputtered samples can be stored in a petri dish for approximately a week. 3. Sample microscopy with SEM As electron source or gun a wolfram filament is used, wherefrom the released electrons are accelerated in an electrical field and the electron beam is focused (aimed) at the surface of the sample using magnetic lenses so that the diameter of the beam is as small as possible. Apertures are used in association with the lenses. By means of the electron beam, a number of signals are received from the sample indicative of its characteristics, including X-rays to be analyzed in the X-ray microanalysis. The easy and fast sample preparation technique in the SEM-analysis makes it possible to analyze a large number of samples and to interpret the results with the help of statistical tests. In this work SEM is used for general sample study and for choosing the measurement targets. The real measurement takes place with the X-ray microanalyzer connected to the SEM. 4. Measurement with the X-ray microanalyzer X-ray microanalysis is based on irradiating the material to be studied with an accelerated electron beam, whereby electrons are released from the inner shell (K) of the atom. Their place is taken by electrons from an outer shell (L or M). As the electrons on the outer shells have a higher energy than those on the K-shell, part of the energy of the electron transferring from the L- or M-shell is converted into X- rays, which energy is dependant on the orbital (electron shell) change taken place. This change and its generated energy is "type" specific for each element. A detector is mounted in the electron microscope chamber, which counts the amount of X-ray radiation on each energy level. It is graphically displayed so that the radiation from each element is shown as a peak in the spectrum in a place specific for the said element. The height of the peak can be used (taking into account the other factors) as a measure of how much of the element there is in the sample. The analysis can be made from a very small sample, whereby quantities of elements of even 10 "17 g are detected. The height of the spectral peak for each element is read using a scale attached to the monitor. A spreadsheet program (for example the Excel 5.0 program) is used for presenting the results. The average values for each sample plot are plotted for each element as a function of the distance from the plant as a (XY)-graph. For the graph, a trendline is generated with the program, for which a polynom of the 3. degree is generally best applicable, as well as a regression equation and coefficient of determination. By means of these it is possible to interpret the distribution of the air pollutants on the studied line. If necessary, the statistical significance of the differences between the various sample points are tested using variance analysis (for example the Statistix program). The following example illustrates the invention Example The applicability of the method has been tested in the surroundings of three different emission sources. The plants constituting the sources for the said emission can be described as follows: 1. An average sized Finnish industrial plant, which has caused substantial damage to its near surroundings with its air pollutants. In this case, attention has been focused primarily on the alkaline dust emissions from the plant. The test study extends for a distance of 1.3 km from the plant. In this area, the significance of other plants and of long-range transport as regards most emissions is negligible or small. 2. A larger Finnish metal industrial plant which has caused substantial damage to the near surroundings with its emissions. The study extends for a distance of 3 km from the plant and in this area, the said plant is almost the only emission source for the substances studied. 3. A large industrial complex in a neighbouring country, which has extensively polluted its surroundings. The study extends to appr. 100 km from the plant. A number of other plants are also situated in the area, which affect the test results. In all cases the new method has given very good results, which have been compared to studies made earlier with other methods. In the following the site no. 1 is presented in more detail. Here the contents of the seven elements Al, Ca, Fe, K, Mg, S and Si were studied on pine bark. The sample plots, 6 of them, situated on a study line, which extended northeast (the main wind direction) at the following distances from the plant (in brackets the number of the sample plot) 150 (1), 290 (2), 360 (3), 490 (4), 980 (5) and 1300 m (6). The size of the bark samples was 10-40 cm 2 . The samples were taken from the southwest side of the trunk, that is from the most exposed side of the tree, at a height of appr. 1.5-2 m. The sample trees were chosen from places where the terrain between the trees and the emission source was as obstacle free as possible. Also the presence and condition of the main species of epiphytic lichens and algae on the pine bark was studied in each sample plot, as bioindicators for these sample plots. The samples were air-dried in paper bags, then attached with double-sided tape to the surface of a support and coated with a gold-palladium-mixture (80%/20 , appr. 45 nm layer) using a JEOL Fine Coat Ion Sputter JFC-1100 coating unit. Gold was used in place of carbon in order to receive a better contrast. The samples were examined with a JEOL JSM scanning electron microscope using a JEOL Semaphore image recording system at 15 kV voltage. The analyses were performed directly in the SEM using a PGT Imix EDS microanalyzer. The voltage used in the energy dispersive analysis was 15 kV, and the X-ray collection time was 60 s. For each sample plot, 40 points were analyzed on four different bark samples (from two trees, two bark samples per tree, 10 points for each sample). For the X-ray collection, a 3000-fold magnification was used, which corresponded to a surface area of the size of appr. 450 μm 2 . With higher magnification, the role of individual particles became over-emphasized, whereas the measurement peaks in the analysis remained too small with a lesser magnification. It is, however, clear that when using different materials and conditions, it is possible to use also higher and lower magnification, such as a 100-5000-fold magnification. Also the number of analysis points per sample plot can vary, for example in the range 10-100. The calcium, magnesium, sulphur, silicon, aluminium, potassium and fluorine contents were measured in the following way. The peak for each element was measured using a scale of 0-5, with an accuracy of a quarter of a unit. In some cases, very high element contents resulted in the peak extending beyond the scale, in which cases a collection time of 40 s was used. The maximum elemental concent was in these cases 8 rather than 5. In order to standardize and calibrate the apparatuses and the measurements, standard preparations were used. In the case of calcium, for example, different concentrations of CaCl 2 -solutions were mixed with active carbon, water was evaporated and the C- CaCl 2 -mixture was spread on a plastic film. These preparations were used for standardization. A standard trendline was computed for the standard measurements (Fig. 1). The equation of this trendline was used to calculate the element content on the surface of the study material. The regression trendlines and corresponding regression equations and coefficients of determination were computed for the results using the Microsoft Excel-program, version 5.0, and the other statistical computations using the Statistix program, version 4.1. The following results were obtained for the different elements. Calcium The content of calcium on the pine bark surface in the vicinity of the emission source was very high and decreased rapidly when the distance increased, extending at a clearly measurable level up to 600-700 metres. The coefficient of determination for the regression equation was 0.99, which shows that the said emission source is completely responsible for the increased calcium levels on the study line. Very seldom does one reach such a high coefficient of determination in bioindicator studies of this type and the result shows that this method gives a very precise picture of the distribution of the calcium emissions. The coefficient of distribution can be mathematically computed from the formula y= -5E-09x 3 + lE-05x 2 - 0.0123x + 3.7643 (or y= -5 • 10 "9 x 3 + 1- 10 "5 x 2 - 0.0123x + 3.7643) (Fig.2) where y= the calcium content (mg/mm 2 ) on the pine bark surface and x= the distance (m) from the emission source. The distribution of calcium has been studied earlier in the same area using the best known methods. They have given in other respects similar results, but they have not been equally precise. Hereby the total pine needle mass and soil humus have been used as material, whereby the analysis results are affected by many other factors associated with the soil and needle composition in addition to the said emission source. In the needle studies, a value of 0.46 and for humus a value of 0.74 have been obtained for the coefficients of determination. The values are thus much lower than those of the method according to the invention, which is due to the fact that in the comparative methods the results are affected by many other factors. Magnesium The magnesium content on pine bark gives very similar results to calcium. The coefficient of determination is 0.95, which shows the strong effect of the local emission source. The magnesium contents are much smaller, as also the emissions are small. Sulphur Also the sulphur contents are clearly increased in the vicinity of the plant and decrease rapidly as the distance increases. The coefficient of determination is 0.83 (Fig. 3, where the y=axis denotes EDX-units). Therefrom it can be deduced that the results are affected also by other than the sulphur emissions from the plant: at a distance of appr. 10 km the greatest sulphur emission source in Finland is situated, in addition to which also long-range transport has an effect on the sulphur contents of the bark surface. By comparing the coefficients of determination for sulphur and calcium it is possible to estimate the relative proportions of the said emission source and other sources. The method which has generally been considered good, the measurement of the sulphur content of the pine needles, did not reveal any increased sulphur contents in the vicinity of this emission source. This is due to the fact that the sulphur content of the needles is mostly affected by emissions in the form of gaseous sulphur dioxide and in this case the sulphur is in the form of particulate calcium sulphate. Its sulphur can be determined and measured from the surface of both bark and needles with the new method. Silicon, aluminium and potassium The contents of silicon, aluminium and potassium on the surface of pine bark were also clearly increased in the vicinity of the emission source and decreased as a function of distance. The coefficients of determination were Si 0.95, Al 0.93 and K 0.99 (Figs. 4 and 5) and they showed that the local emission source had a very great effect on the concentration increase. With all of these, one obtained a clear rise in the polynom curve of the third degree up to the fifth sample plot, where the concentrations were higher than at the forth and sixth plot. The same applied to calcium. This was due to the fact that the said fifth sample plot was selected at the border of a sand-covered parking lot for 3-4 cars next to a woody road, which revealed the effect of some sand dust. Also this result shows that the new method is applicable to establish the effect of also small sources. Fluorine In addition, the fluorine content was measured from pine bark, which was clearly increased at the sample plot I and could also be detected at the second sample plot. 1.5 years prior to the sampling, the plant had experienced a singular calcium fluoride dust emission due to a storm, the effects of which could still be detected from the pine bark. This and other studies show that the method is applicable also in monitoring changes in emission levels and to studies, in which it is retrospectively possible to detect the distribution of individual emissions in the surrounding. In one sample plot on the study line also a PGT bulk sample analysis was carried out, using the ZAF method, whereby the following relative proportions of the following elements were obtained: Element % by weight Calcium 59.75 Silicon 14.06 Aluminium 10.98 Magnesium 3.24 Fluorine 11.97

Claims: Claims 1. Method for the quantitative determination of elements for studying the distribution of air pollutants in the surroundings of an emission source, characteri- zed by the following steps: - the sample to be examined is collected at a sampling plot from the plant cover surrounding the emission source, the sample being taken from a surface portion thereof, at that side of the sampling plot which faces the emission source, - the sample is dried, - the surface of the sample is made electrically conducting, - the contents of the elements deposited onto the surface of the sample are measured using an energy dispersive X-ray microanalyzer (EDX) connected to a scanning electron microscope (SEM). 2. The method according to claim 1, characterized in that for measuring the contents of the elements per surface area of the sample, standard preparations are used to which the measurement results obtained with SEM/EDX are compared. 3. The method according to claim 1 or 2, characterized in that the sample has been taken from the surface of a tree, especially from that of pine bark or needles. 4. The method according to any one of the preceding claims, characterized in that one or more of the elements Ca, S, Si, Al, Mg, K and F are measured. 5. The method according to any one of the preceding claims, characterized in that the sample is made electrically conducting by coating it with gold. 6. The method according to any one of the preceding claims, characterized in that a 100-5000-fold magnification is used for collecting the X-rays. 7. The method according to any one of the preceding claims, characterized in that for each sample plot, 10-100 sample points are analyzed. 8. The method according to any one of the preceding claims, characterized in that the samples are collected at sample plots on a study line extending in a direction away from the emission source, and that at each sample plot, several parallell samples are collected. 9. The method according to claim 8, characterized in that the study line extends away from the emission source in a direction corresponding to the main wind direction. 10. The method according to the claim 8 or 9, characterized in that a formula is developed for the average values of the sample contents measured at each sample plot as a function of the distance from the emission source, the corresponding regression equation and coefficient of determination of which being used for interpreting the distribution of air pollutants on the study line.

Inventors: HAAPALA HEIKKI, HAAPALA, HEIKKI
Applicants: HAAPALA HEIKKI, UNIV HELSINKI LICENSING, HAAPALA, HEIKKI, HELSINKI UNIVERSITY LICENSING LTD. OY
Application year: 1998
Main classification: G01N23
IPC codes: G01N23