COMPARISON OF MASS CONCENTRATIONS OF SO2 DETERMINED IN AIR BY TWO DIFFERENT METHODS VLADIMIRA VADJIC, MIRJANA GENTILIZZA and MIRJANA C A C K O V I ~
Institutefor MedicalResearch and OccupationalHealth, Universityof Zagreb, P. O. Box 291, 41000Zagreb, Yugoslavia IVAN ESKINJA
Department of Analytical Chemistry, Faculty of Technology, Universityof Zagreb, Yugoslavia
(Received July 1990) Abstract. Mass concentrations of sulphur dioxide were determined in parallel 24-hour samples of ambient air at a measuring site in Zagreb during a period of eight years. The methods used were the West-Gaeke's tetrachloromercurate-pararosaniline method (TCM) and the Standard British Method (SBM). Total results for the annual intervals of measuring were analyzed statistically and also discussed separately for winter and summer periods, in order to see whether they are influenced by seasonal rhythm.
I. Introduction As a product of fuel combustion sulphur dioxide is present with smoke in the air of each urban settlement. Therefore it is considered to be among the most widespread general air pollutants in polutated areas. In countries where atmospheric pollution is regularly monitored sulphur dioxide is established as the basic indicator of air pollution for a given area. The World Health Organization (WHO), within the framework of the GEMS project (Global Environment Monitoring System), organized the determination of sulphur dioxide in 50 countries at 175 measuring sites in the period from 1973 to 1980 (De Koning et al. 1986). The project is still in progress and measuring continues at the majority of measuring sites. Measuring sites are classified either as city centre (CC) or suburban (S) and in each category they are designated as residential (R), commercial (C) or industrial (I). Many methods have been reported in the literature for the determination of sulphur dioxide. However, problems occur when selecting the method, because either the methods of determination are not specific, or they are subject to interference by other materials during the determination, or else they require expensive equipment which is not available to every analytical laboratory. In Europe the Standard British Method (Measurement of Air Pollution 1957) is most commonly used, and results between countries are easily compared. As the results of the determination tend to be affected by acid, i.e. basic compounds present in the atmosphere have an effect in regions where these are present to a great degree, this method is not adequate and the application of specific methods is recommended. The Standard British Method for a typical urban area is generally satisfactory (Gentilizza et aL 1974). EnvironmentalMonitoring and Assessment 21: 19-26, 1992. 9 1992KluwerAcademicPublishers.PrintedintheNetherlands.
20
VLADIMIRA
VADJI(2 ET AL.
The spectrophotometric tetrachloromercurate-pararosaniline method (West and Gaeke 1956, Pate et al. 1967) is specific, sensitive and the most suitable method for laboratory investigation and calibration. In the case of determination in ambient air it is subject to interference (Gentilizza 1975, Gentilizza and Kukulj 1982) by some unidentified compounds which are occasionally present and which might cause instability in the samples. The interference of nitrogen oxides is entirely removed by adding sulphamic acid to the absorption solution immediately before the development of colour. In this work mass concentrations of sulphur dioxide in the ambient air determined by the Standard British Method and tetrachloromercurate method are compared with special reference to seasonal variations.
2. Experimental During an eight-year period, from 1975 to 1982, sulphur dioxide was determined in parallel 24-hour samples at a measuring site located in a suburban residential (SR) area. Samples were collected manually, by means of two standard non-automatic volumetric apparatuses placed side by side, daily, always at the same time. The sample volume amounted to approximate 2 m 3of air. There were no major air pollutants in the vicinity of the measuring site. The principle of the Standard British Method (SBM) is absorption of sulphur dioxide in a 1% solution of hydrogen peroxide during which sulphur dioxide oxidises to sulphuric acid. The latter is determined by titration with sodium tetraborate with BDH-4.5 indicator, which excludes the influence of CO2 from the air (BS-1747 1969, ISO-4219 1979, ISO--4220 1984, Selected Methods of Measuring Air Pollutants 1976). With the tetrachloromercurate-pararosaniline method (TCM) the sulphur dioxide is absorbed in tetrachloromercurate solution, yielding a stable complex of dichlorosulphitomercurate (HgCI2SO2)2-. The complex produces a coloured compound with pararosaniline hydrochloride and formaldehyde. The intensity of colour is proportional to the concentration of sulphur dioxide in the sample and is spectrophotometrically determined on a UNICAM SP-600 spectrophotometer at a wavelength of 560 nm (West and Gaeke 1956, ISO/DIS-6767 1983, ISO/TC-146/SC-3, 1979, D-2914-70-T 1974, No. L-229/40 1980). To prevent the interference of nitrogen dioxide sulphamic acid is added to the sample immediately before colour development (Pate et al. 1967). Daily samples were analysed once a week along with the standards. As very few laboratories are equipped with modern automatic instruments yielding accurate data, for the GEMS project the WHO recommend monitoring apparatuses which are available to a greater number of laboratories. Samples for the SBM method were collected continuously during the year, whereas TCM samples were only collected during the working week; there was no collection at weekends. For the statistical analysis, only the samples taken with the SBM method that had counterparts taken simultaneously with the TCM method on the same day were considered.
D E T E R M I N A T I O N OF SULPHUR D I O X I D E IN AIR
21
3. Results and Discussion Table I shows the mean annual mass concentrations of sulphur dioxide for the TCM and SBM methods over the period of eight years, together with the coefficients of correlation and levels of significance. The values determined by the SBM method are somewhat higher than those determined by the TCM method. A good correlation of results was obtained with a significance level of 0.01 for all years. Mass concentrations of sulphur dioxide, determined with the two methods in summer over the same period are given comparatively in Table II and those for the winter in Table III. All summer mean values determined by the SBM method were higher than those determined by the TCM method (Table II). For the first five years of measuring, from 1975 to 1979, a good correlation of results was obtained, with a level of significance 0.01. From 1980 to 1982 during the summer period of measuring the correlation of results was not significant. In the winter interval the mean values determined by the SBM method were higher than those determined by the TCM method for all the measuring periods, except for 1981 and 1982, when the values determined by the TCM method were slightly higher. In the winter measurement period the results showed a good correlation at the level of significance 0.01 for all the years. The histograms in Figure 1 show the movement of mean monthly values of mass concentrations of sulphur dioxide in the air, for each method separately, for all the eight years of measurement. Figure 2 shows the trend of a three-year moving average in sulphur dioxide concentration determined by the TCM and SBM methods. The trend is similar only in that the values determined by SBM are higher than those determined by TCM method. Many epidemiological investigations have been carried out to date on the acute and TABLE I Annual mass concentrations of SO 2 in air (~tg m -3) and correlation of results. TCM
SBM
Year
N
C
N
C
r
P
1975 1976 1977 1978 1979 1980 1981 1982
243 247 242 197 246 222 246 238
42 43 41 32 36 40 31 38
243 247 242 197 246 222 246 238
60 57 54 54 37 55 47 53
0.926 0.879 0.687 0.592 0.760 0.591 0.373 0.427
Institutefor MedicalResearch and OccupationalHealth, Universityof Zagreb, P. O. Box 291, 41000Zagreb, Yugoslavia IVAN ESKINJA
Department of Analytical Chemistry, Faculty of Technology, Universityof Zagreb, Yugoslavia
(Received July 1990) Abstract. Mass concentrations of sulphur dioxide were determined in parallel 24-hour samples of ambient air at a measuring site in Zagreb during a period of eight years. The methods used were the West-Gaeke's tetrachloromercurate-pararosaniline method (TCM) and the Standard British Method (SBM). Total results for the annual intervals of measuring were analyzed statistically and also discussed separately for winter and summer periods, in order to see whether they are influenced by seasonal rhythm.
I. Introduction As a product of fuel combustion sulphur dioxide is present with smoke in the air of each urban settlement. Therefore it is considered to be among the most widespread general air pollutants in polutated areas. In countries where atmospheric pollution is regularly monitored sulphur dioxide is established as the basic indicator of air pollution for a given area. The World Health Organization (WHO), within the framework of the GEMS project (Global Environment Monitoring System), organized the determination of sulphur dioxide in 50 countries at 175 measuring sites in the period from 1973 to 1980 (De Koning et al. 1986). The project is still in progress and measuring continues at the majority of measuring sites. Measuring sites are classified either as city centre (CC) or suburban (S) and in each category they are designated as residential (R), commercial (C) or industrial (I). Many methods have been reported in the literature for the determination of sulphur dioxide. However, problems occur when selecting the method, because either the methods of determination are not specific, or they are subject to interference by other materials during the determination, or else they require expensive equipment which is not available to every analytical laboratory. In Europe the Standard British Method (Measurement of Air Pollution 1957) is most commonly used, and results between countries are easily compared. As the results of the determination tend to be affected by acid, i.e. basic compounds present in the atmosphere have an effect in regions where these are present to a great degree, this method is not adequate and the application of specific methods is recommended. The Standard British Method for a typical urban area is generally satisfactory (Gentilizza et aL 1974). EnvironmentalMonitoring and Assessment 21: 19-26, 1992. 9 1992KluwerAcademicPublishers.PrintedintheNetherlands.
20
VLADIMIRA
VADJI(2 ET AL.
The spectrophotometric tetrachloromercurate-pararosaniline method (West and Gaeke 1956, Pate et al. 1967) is specific, sensitive and the most suitable method for laboratory investigation and calibration. In the case of determination in ambient air it is subject to interference (Gentilizza 1975, Gentilizza and Kukulj 1982) by some unidentified compounds which are occasionally present and which might cause instability in the samples. The interference of nitrogen oxides is entirely removed by adding sulphamic acid to the absorption solution immediately before the development of colour. In this work mass concentrations of sulphur dioxide in the ambient air determined by the Standard British Method and tetrachloromercurate method are compared with special reference to seasonal variations.
2. Experimental During an eight-year period, from 1975 to 1982, sulphur dioxide was determined in parallel 24-hour samples at a measuring site located in a suburban residential (SR) area. Samples were collected manually, by means of two standard non-automatic volumetric apparatuses placed side by side, daily, always at the same time. The sample volume amounted to approximate 2 m 3of air. There were no major air pollutants in the vicinity of the measuring site. The principle of the Standard British Method (SBM) is absorption of sulphur dioxide in a 1% solution of hydrogen peroxide during which sulphur dioxide oxidises to sulphuric acid. The latter is determined by titration with sodium tetraborate with BDH-4.5 indicator, which excludes the influence of CO2 from the air (BS-1747 1969, ISO-4219 1979, ISO--4220 1984, Selected Methods of Measuring Air Pollutants 1976). With the tetrachloromercurate-pararosaniline method (TCM) the sulphur dioxide is absorbed in tetrachloromercurate solution, yielding a stable complex of dichlorosulphitomercurate (HgCI2SO2)2-. The complex produces a coloured compound with pararosaniline hydrochloride and formaldehyde. The intensity of colour is proportional to the concentration of sulphur dioxide in the sample and is spectrophotometrically determined on a UNICAM SP-600 spectrophotometer at a wavelength of 560 nm (West and Gaeke 1956, ISO/DIS-6767 1983, ISO/TC-146/SC-3, 1979, D-2914-70-T 1974, No. L-229/40 1980). To prevent the interference of nitrogen dioxide sulphamic acid is added to the sample immediately before colour development (Pate et al. 1967). Daily samples were analysed once a week along with the standards. As very few laboratories are equipped with modern automatic instruments yielding accurate data, for the GEMS project the WHO recommend monitoring apparatuses which are available to a greater number of laboratories. Samples for the SBM method were collected continuously during the year, whereas TCM samples were only collected during the working week; there was no collection at weekends. For the statistical analysis, only the samples taken with the SBM method that had counterparts taken simultaneously with the TCM method on the same day were considered.
D E T E R M I N A T I O N OF SULPHUR D I O X I D E IN AIR
21
3. Results and Discussion Table I shows the mean annual mass concentrations of sulphur dioxide for the TCM and SBM methods over the period of eight years, together with the coefficients of correlation and levels of significance. The values determined by the SBM method are somewhat higher than those determined by the TCM method. A good correlation of results was obtained with a significance level of 0.01 for all years. Mass concentrations of sulphur dioxide, determined with the two methods in summer over the same period are given comparatively in Table II and those for the winter in Table III. All summer mean values determined by the SBM method were higher than those determined by the TCM method (Table II). For the first five years of measuring, from 1975 to 1979, a good correlation of results was obtained, with a level of significance 0.01. From 1980 to 1982 during the summer period of measuring the correlation of results was not significant. In the winter interval the mean values determined by the SBM method were higher than those determined by the TCM method for all the measuring periods, except for 1981 and 1982, when the values determined by the TCM method were slightly higher. In the winter measurement period the results showed a good correlation at the level of significance 0.01 for all the years. The histograms in Figure 1 show the movement of mean monthly values of mass concentrations of sulphur dioxide in the air, for each method separately, for all the eight years of measurement. Figure 2 shows the trend of a three-year moving average in sulphur dioxide concentration determined by the TCM and SBM methods. The trend is similar only in that the values determined by SBM are higher than those determined by TCM method. Many epidemiological investigations have been carried out to date on the acute and TABLE I Annual mass concentrations of SO 2 in air (~tg m -3) and correlation of results. TCM
SBM
Year
N
C
N
C
r
P
1975 1976 1977 1978 1979 1980 1981 1982
243 247 242 197 246 222 246 238
42 43 41 32 36 40 31 38
243 247 242 197 246 222 246 238
60 57 54 54 37 55 47 53
0.926 0.879 0.687 0.592 0.760 0.591 0.373 0.427
