2764
Anal. Chem. 1987, 59,2764-2766
Determination of Sulfur Dioxide in Ambient Air by a Computer-Controlled Thermodenuder System J a k o b Slanina, Menno P. Keuken,* a n d Carlo A. M. Schoonebeek
Netherlands Energy Research Foundation (ECN), 1755 ZG Petten, The Netherlands
An automatic themodenuder system for the measurement of SO, Is descrlbed. Alr Is sampled by Cu/CuO coated denuder tubes. When the denuders are heated to 800 OC, SO, Is llberated and detected by a standard monltor. For a sampllng tlme of 30 mln the llmlt of detectlon Is 0.1 and 0.015 pg/m3 SO, for a pulsed-fkrorescence and a flame-emlssion monltor, respectively. For both Instruments the response Is hear up to 100 pg/m3. Sulfurlc acid, sulfate aerosols, and organic sulfur compounds llke CS,, COS, and dhnethyl sulfMe do not interfere at concentrations up to 100 Mg/ma. Interference of H,S can be eHmlnated by a NaHSO,/Ag,SO, coated predenuder placed In the sample line. The measurement of SO, concentrattonsat background levels and the determination of dry deposltkn vekcltles by measuremenl of SO, gradients are obvious appllcatlons.
The determination of SOz at low concentrations can pose problems, even though SO2 is one of the compounds that is measured frequently and for a long period. The determination of SOzin low-polluted areas by means of instrumental methods is problematic, due to the low concentrations and various interferences. In our experience the detection limit of an optical technique such as UV photometry is about 3 pg/m3 SO2 The detection limit of flame-emission methods is lower, but here problems regarding calibration and selectivity are encountered (1,2). Filter packs, consisting of a Teflon filter to detain all sulfates in the form of aerosols followed by a coated filter that samples sulfur dioxide, can be applied for the measurement of low SOz concentrations, but only if the sample time is several hours (394). The measurement of the dry deposition velocity of SO2 by means of the gradient method is problematic as a precision of 5% relative or better is required. This precision is not attainable a t low ambient SOz concentrations with existing methodology (5). Recently a denuder system was described, consisting of two CuO coated denuders in series, that samples sulfuric acid and ammonium sulfates at temperatures of 120 and 220 OC, respectively (6). The detection limit of the method is less than 0.02 pg/m3 sulfuric acid for a sampling time of 60 min. Initial investigations already indicated that SO2 at ambient temperature is quantitatively retained by Cu/CuO denuders and that the method is able to measure very low concentrations. So it seemed worthwhile to apply automatic thermodenuder systems for the measurement of SOz in air. As it is possible to sample at three different heights by means of three denuder tubes operated in a parallel mode and to analyze the tubes sequentially by means of the same monitor, thermodenuder systems offer the possibility of acting as integrating sampling systems. This would be a nice feature for dry deposition measurements. In addition, the Cu/CuO denuder system has the advantage that the system can be used to measure several sulfur compounds simultaneously, dependent on the temperature of the
denuder tubes during the sampling period. EXPERIMENTAL SECTION Apparatus. Cu/CuO Denuder. A well-cleaned quartz tube (length 50 cm, i.d. 6 mm, 0.d. 8 mm) is fiied with a copper solution in 100 mL of prepared by dissolving 10 g of CU(NO,)~.~H,O acetone. The Cu/CuO denuder is then prepared in the same way as previously described (6). A more concentrated coating solution is used to obtain a thicker Cu/CuO coating. This is necessary to guarantee complete scavenging of SO2 at concentrations above 30 pg/m3. NaHS04/AgDenuder. A quartz tube (dimensions as above) is coated with a NaHS04/Ag2S04solution prepared by dissolving 2 g of Na2S04,1 g of NaHSO,, and 0.6 g of Ag2S04in 100 mL of demineralized water. Sample Valves. Three-way valves (Galtek) are constructed of Teflon (PFA). Ovens, oven control, flow control unit, computer, and interface are as previously described (6). Sulfur Analyzer. The flame-emission detector (Meloy SA 285) as used previously (6) can be applied in the laboratory but it is a rather delicate instrument for field experiments. So a pulsed-fluorescencedetector (Therm0 Electron 43W) was applied for this purpose. The detection limit of the pulsed-fluorescence detector is about 3 pg/m3 S. Filter Packs. Filters used were Whatman 41 (d = 50 mm) impregnated with K&03 solution (2.5 g of K2C03dissolved in a few milliliters of glycerine and diluted to 500 mL with demineralized water). Impingers. Impingers were filled with a mixture of 15 mL of 0.001 N NaOH and 100 pL of H202(3%). Procedure. For the measurement of SOz gradients, a configuration consisting of two sets of three denuders was used (see Figure 1). The first set of denuders (A) is sampled, flow 0.5 L/min, by means of three separate sampling lines of different heights. While set A is sampled, the second set of three denuders (B) is analyzed sequentially by heating to a temperature of 800 "C in a flow of 50 mL/min of nitrogen. The carrier gas stream containing the SOz is directed to the sulfur analyzer. As the sample time is 30 min and time of analysis per denuder is on the order of 7 min, the denuders are cooled to ambient temperature before the next sampling cycle. Choice of sampling mode or analyzing mode is controlled by three-way valves connecting either sample lines or nitrogen supply with the first or the second set of denuders. Details of cyclic heating of the denuders are as described for the determination of sulfuric acid and ammonium sulfates (6). The denuder system is calibrated by injection of either 14.5 or 35.0 pL from analyzed SOZ/Nzmixture into purified air carrier stream. Air was purified by means of a 0.2-pm Teflon filter (DFA 3001 FRP-Pall, Portsmouth, UK) and activated carbon, to get rid of SOz and gaseous organic sulfur compounds. For reference measurements a filterpack consisting of a 5-pm Teflon filter (Mitex-Millipore) followed by two KZCO3coated filters, the first for SO2 retention and the second as a backup, has been employed at a sample flow of 5 L/min for 2-20 h. The filters are leached by a mixture of 7 mL of water and 100 pL of hydrogen peroxide (3%); the sulfite is oxidized to sulfate and determined by ion chromatography. The detection limit (defined as 3 times the standard deviation of the results obtained by processing unloaded filters) for a sampling time of 2 h is 0.3 pg of
so2lrn3.
Impingers were also used for reference measurements. Test atmospheres are sampled at 3 L/min for 4-20 h by two impingers in series (to check breakthrough) and analyzed by ion chromatography. The results of the backup impinger did not exceed the
0003-2700/87/0359-2764$01.50/0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
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Table 11. Results of SO1 Measurements in Test Atmospheres by Denuders, Filterpacks, and Impingers in m/m9 automatic denuder system filterpack
SET
B
30.9 32.4 2.7 24.0 23.4 2.4 10.4 10.7 12.3 57.1 56.5 28.2
31.2 32.0 3.0 25.0 21.5 2.5 10.6 9.8 12.5 56.9 58.5 27.9
impinger 36.6 25.4 2.5 11.9 12.1
Table 111. Concentration of Organic Sulfur Compounds in Ambient Air in Relation to SO2 and Sulfate Aerosol Concentrations org S', PUMP
Figure 1. Denuder configuration for gradient measurements.
Table I. Adsorption Efficiency of SO2 Determined by Two Denuders in Series as a Function of Temperature and Relative Humidity denuder temp, "C
%
re1
first
second
humidity
35 a0 30 50 35 a0
50
65 65 22 22 5 5
a0 50 50 50 80
first denuder collection efficiency," % 99 a0 99
aa 67 53
" 100 X (SOz adsorbed in first denuder)/(SOzadsorbed in first denuder + second denuder). blank values of the impinger solution.
RESULTS AND DISCUSSION The efficiency of the Cu/CuO denuder was tested by sampling different test atmospheres of SOz by means of two denuders, placed in series. The efficiency depends on the temperatures of the denuders and the relative humidity of the sampled air. The results compiled in Table I show that the temperature of the denuders should not exceed 35 "C. At this temperature the efficiency of the denuder is quite acceptable, except a t relative humidities of 10% or less. In order to evaluate the proposed procedure, analyses of test atmospheres of SOz were performed by denuders, filterpacks, and impingers (see Table 11). Results of 63 comparisons between automated denuders and filterpacks were as follows: slope, 1.06; correlation coefficient, 0.99; intercept, -0.24 pg of SOz/m3. The possible interference of inorganic and some organic sulfur compounds was tested by laboratory experiments. Sulfuric acid, ammonium sulfates, and alkali sulfates do not interfere at concentrations of 100 pg/m3. The most abundant organic sulfur compounds in ambient air like CSz, COS, and dimethyl sulfide are retained for less than 1% at concentrations up to 200 pg/m3. However HzS, present in ambient air, is also retained by a Cu/CuO denuder and detected as SOz. The HzS interference was eliminated by a NaHS04/AgzS04 coated denuder, placed before the Cu/CuO denuder in the sample line. The efficiency of the predenuder to remove HzS
date
SOz," pg f m9
as SO4
aerosol sulfate: pg/m3 as SO4
20107f a5 25107fa5 2afoifa5 30f 07/85 14/os fa5 20foa fa5 23p afa5 03109fa5 05f 09fa5 06109185 08109fa5 09109fa5 27109fa5 26f 10f 85 27f 10f a5 29f 10fa5 30f 10fa5 31/10 fa5 02/11/85
5.0 13.9 3.1 1.7 3.7 1.3 7.3 3.3 4.1 0.9 3.8 6.6 12.3 22.1 2.0 2.3 2.9 18.4 6.4
1.6 1.4 0.3 0.2 1.3 0.9 1.6 1.3 1.4 0.8 1.0 1.4 2.4 3.6 2.0 1.9 1.9 3.7 1.4
2.7 10.6 3.1 1.8 3.2 1.5 2.6 2.0 2.2 1.0 1.9 5.3 22.0 19.3 5.2 1.6 9.6 19.9 1.2
pg f ms
"Sample system: filter pack (H2S04 and sulfate aerosol) + CufCuO denuder 35 "C (SO, + HzS) + Cu/CuO denuder 300 O C (org S). bSample system: PbOz denuder 35 "C (SO,) + active carbon denuder (org S) + Cu/CuO denuder 300 "C (H,SO, and sulfate aerosol). up to 50 pg/m3 was tested by sampling gas mixtures of HzS and SOz. Analysis of ambient air by direct measurements or with a NaHSO4/AgZSO4predenuder gives a correlation coefficient of 0.992, a slope of 0.96, and an intercept of -0.22, indicating systematic removal of a sulfur compound by the predenuder. Impinger measurements c o n f i i this compound to be HzS at a concentration on the order of 0.1-0.4 pg/m3. The concentration of gaseous organosulfur compounds is relatively high at the site of ECN, which is located about 200 m from the shore of the North Sea. Approximate measurements of the concentration of gaseous organic sulfur compounds were carried out by inserting a filter in one of the sample inlets of the denuder system and operating the two denuders in series at a temperature of 35 and 300 OC. Sulfates present in aerosol are retained on the fiiter and SOzplus HzS are scavenged by the first denuder. So the sulfur compounds collected in the second denuder indicate the presence of gaseous organic sulfur compounds. Some results are given in Table 111, indicating that concentrations of organic compounds could be of importance. It is fairly impossible by means of laboratory experiments to check the selectivity of the automatic denuder system for the interference of all organic sulfur compounds present in ambient air. So we decided
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 23, DECEMBER 1, 1987
Table IV. SO2 Concentrations in Ambient Air Measured by Filterpack and Automated Denuder System (June-September 1986) I
filterpack, rg/m3
denuder system, rcg/m3
filterpack, rg/m3
denuder system, rg/m3
3.6 7.7 4.3 6.5 5.8 1.4 0.9 3.1 22.2 6.9 13.0 18.7 13.8 11.1 6.7 1.0 15.4
4.0 7.6 3.7 7.0 5.0 1.2 0.8 3.3 20.4 7.3 10.5 16.0 13.1 7.9 6.5 1.2 15.6
6.0 2.2 15.5 30.7 4.9 0.5 3.4 2.4 2.7 21.9 22.2 17.7 10.5 5.3 10.8 18.4 9.9
5.2 2.6 18.4 29.5 5.4 0.8 3.1 2.4 2.7 21.5 23.3 17.8 10.8 5.1 11.0 19.6 10.8
in
0 a in
LL Y
1:
Table V. Precision Obtained with Three Identical SO2 Denuder Systems
Q
* c
mfciou
rimy
SAMPLING TIME CHART SPIED
30 MlN
S A 285
2 MMIMIN
DElEClOP SAMPLING TIME :hPUT SPEED
THERM0 E L Z I T O N L l k 10 Mlh
2 MYiMlN
concn, pg/m3
no. of observns
% RSD
Flgure 2. Monitor response for different SO2 concentrations.
2.1 3.5 7.5 13.6 18.4 42.0
8 10 14 6 8 26
5.0 3.6 2.2 2.3 3.0 2.2
Table VI. Results for Low SOz Concentrations (pg/ms)
to compare the results of ambient air measurements by filterpack and the automatic denuder system. The results of 3 4 measurements were as follows: slope, 0.98; correlation coefficient, 0.99; intercept, 0.07 pg of SOz/m3(see Table IV). As the fiterpack method is free from interferences by organic compounds, these results show that the interference of organic compounds by automated denuders for SOz detection is not important. The reproducibility of the method was tested in a configuration of two sets of three denuders each (see Figure 1). Results are given in Table V for the pulsed fluorescence detector. It is clear that a high reproducibility, necessary for dry deposition measurements by means of the gradient method, can be reached, even a t very low concentrations. At ambient concentrations a reproducibility of better than 3 % can be obtained. The detection limit of the automatic denuder system is dependent on the detector system that is chosen and the sampling time. The lowest detection limits are obtained by means of the flame-emission detector. The response at low concentrations of SOzis linear (see Figure 2). Detailed results are given in Table VI for a sampling time of 30 min. The accuracy and the precision of the method are quite acceptable under these conditions, even at concentrations below 0.3 %/m3 SOz. A linear response was obtained up to 100 pg/m3 for a sampling time of 30 min. It was not possible to prepare test atmospheres that contained less than 0.04 pg/m3 SO2. However the signal of this concentration from the flame-emission detector indicates that a detection limit (defined as the concentration that gives a signal 3 times the noise of the background) of 0.015 pg/m3 or less can be achieved. For field experiments the pulsed-fluorescence detector is used and a detection limit of 0.1 gg/m3 is obtained. A linear response
given
found (mean)
no. of observns
% RSD
24.5 3.2 0.90 0.23
23.4 2.9 0.82 0.24
5 17 9 9
2
1.5 6
13
was observed in the range of 0.1-100 gg/m3 SOzfor a sampling time of 30 min. The automatic denuder system is able to run during extended periods without any human assistance, as all functions including recalibration are performed automatically under computer control. The apparatus has functioned continuously as long as 1 month without malfunction. The lifetime of the denuders is at least 1 month and denuders can be easily exchanged.
CONCLUSION The automatic Cu/CuO denuder system is a suitable apparatus for the determination of SO2 in air when a very low detection limit and high reproducibility are necessary and 30-min time resolution is adequate. The measurement of SOz concentrations at background levels and the determination of dry deposition velocities by measuring SOz gradients are obvious applications. Rsgistry No. SO2, 7446-09-5; Cu, 7440-50-8; CuO, 1317-38-0; NaHSO,, 7681-38-1; Ag, 7440-22-4; HzS, 7783-06-4. LITERATURE CITED (1) Tanner, R. L.; D'OttavIo, T.; Garber, R.; Newman, L. Afmos . Environ . 1980, 74, 121-127. (2) Cobourn, W. G.; Husar, R. 6.; Husar, J. D. Afmos. Environ. 1978, 72, 89-98. (3) Johnson, D. A.; Atkins, D. H. F. Afmos. Environ. 1975, 9 , 825-829. (4) Slanlna. J. Proceedings of the Advanced Workshop on Architectural Conservatlon, Rome, 1984, In press. (5) Davis, C. S.; Wright, R. G. J . Geophys. Res., D : Afmos. 1985, 9 0 , 20091-20095. (6) Slanina. J.; Schoonebeek, C. A. M.;Kiockow, D.; Niessner, R. Anal. Chem. 1985, 57, 1955-1960.
RECEIVED for review March 6,1987. Accepted August 7,1987.
