Environ. Sci. Technol. 1991, 25, 531-535
(46) Cowan, C. E. Ph.D. Dissertation,University of Washington,
Seattle, WA, 1986. Received for review August 10, 1989. Revised manuscript received J u l y 19, 1990. Accepted October 10, 1990. This work was supported by the U S . Environmental Protection Agency, Athens
Environmental Research Laboratory, under a Related Services Agreement with the U.S. Department of Energy under Contract DE-ACO6-76RLO 1830, Interagency Agreement D W90059-01. However, this paper has not been subject to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.
Determination of Nitrogen Dioxide in Ambient Air by Use of a Passive Sampling Technique and Triethanolamine as Absorbent Dariusz Krochmal* and Ludwlk G6rski Institute of Inorganic Chemistry and Technology, Technical University of Cracow, ul. Warszawska 24, PL-31-155 Cracow, Poland
rn The effects of temperature, humidity, and storage on a diffusive sampler were tested by use of the AmayaSugiura method, modified previously. Several materials were used as carriers for triethanolamine in the sampler. The mass of NO2 absorbed in the sampler was determined spectrophotometrically as nitrite by using Saltzman solution. The collection efficiency of the sampler was lower than that calculated from Fick's law of diffusion due to significant contribution of liquid phase in the overall sampler diffusive resistance. This resulted in an increase of the mass of NO2 absorbed in the sampler by ca. 20% per 10 "C of temperature growth and by ca. 25% when the relative humidity rose from 0 to 100%. Dependence of concentration of TEA solution in the sampler on the relative humidity of the air was noted. The relative precision of the method characterized by RSD was 10%; the detection limit of NOz was 10 pg/m3 for a 24-h exposure.
Introduction Since Levaggi et al. (1) applied triethanolamine (TEA) for the quantitative trapping of NOz from the air stream, this substance has frequently been used as an absorbent in passive sampling methods of determination of NOz (2-7). Our earlier studies (8) on the Amaya-Sugiura method ( 5 )showed that parameters such as wind velocity and air temperature can seriously affect the accuracy of the method. While the face velocity effect was diminished to -20% by an appropriate change in the sampler design (91, the temperature effect remained unchanged in spite of modification of the method. In this work an effort was made to measure and explain the temperature and humidity effects as due to the application of TEA as the absorbent. As regards the modified sampler, it enables 24-h measurements of NOz concentrations in ambient air to be conducted at extremely low cost. The sampler is commerically available. The production cost of the sampler, which is reusable, is lower than 1U.S. dollar. On the basis of the modified method, a Polish Standard (10) concerning determination of nitrogen dioxide in ambient air using a passive sampling technique was established this year. Experimental Section Analytical Procedure. The sampler design and analytical procedure were described elsewhere (9). A photograph of the sampler is presented in Figure 1. In some tests 25-mm disks of glass microfiber filters (Whatman GF/C), cellulose filters (Whatpan 3), and different sorts of fiber materials produced by Slpkie Zaklady Przemyslu Lniarskiego "Lentex", Lubliniec, Poland (Catalog No. 0013-936X/91/0925-0531$02.50/0
46031 viscose fiber; Nos. 46012 and 46015 viscose polyester fiber) were used instead of nylon textile disks as triethanolamine (TEA) carriers. Before relative humidity controlled tests, the samplers were prepared following the usual procedure and then kept open together with blanks for 24 h in a desiccator containing a constant-humidity atmosphere (see below). Generation of Constant-Humidity Atmospheres. The following saturated, aqueous solutions of inorganic substances were placed inside desiccators at 20 "C to obtain the relative humidity given in parentheses (11): H3P04J/2H20(9%), CaClZ.6H2O(32%), Ca(NO3)y2H20 (53%), NH4C1+ KNOB(73%),NH4C1(79%), ZnS04.7H20 (90'70). For 0% RH, 5-A molecular sieves were applied. To evaluate the equilibrium curve between relative humidity and concentration of TEA, a series of measurements was carried out. For each value of relative humidity five samplers without caps were weighed to an accuracy of 1mg; three of them were treated in the usual way with 0.1 mL of 20% (m/m) aqueous TEA solution, weighted again, and placed inside an appropriate desiccator at 20 "C together with the remaining two samplers. After 24 and 72 h, the samplers were weighed again. The final concentration of TEA was calculated after subtracting the average mass of humidity absorbed by the samplers that had not been treated with TEA. Generation of Standard NOz Text Mixtures. Known concentrations of nitrogen dioxide were generated dynamically by using permeation devices. The system is shown schematically in Figure 2. A cleaned and dried air stream was pumped with a constant flow rate of 30 mL/min over a permeation device. The permeation device was held at 35 f 0.1 "C in a thermostat. Several different permeation devices of the permeation rate (determined gravimetrically) of nitrogen dioxide ranging from 0.2 to 0.5 pg/min were applied. An additional permeation device containing sulfur dioxide (permeation rate 1.2 pg/min) was used for studies on interference effects. The total amount of NO2 produced by a permeation device was diluted with another stream of purified air, whose flow rate was kept constant in the range of 5-10 L/min, depending on the desired NOz concentration. In relative humidity controlled experiments the diluting air was passed through a bubbler containing a constant-humidity solution (see above) to obtain a desired relative humidity. When the air was passed through a bubbler filled with distilled water, 100% RH (at a given temperature) was obtained. In the runs where the relative humidity was to be 0% , the air stream was additionally dried with 5-A molecular sieves. The zero air was passed through a container where three samplers were exposed in each run. These samplers were then used
0 1991 American Chemical Society
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Figun 1. View of assembled
sampler and panicuiar parts 01
the sampler
Figure 2. Schematic of flow system lor the exposure of samplers 1. air pump: 2. silica @bed: 3, reollte bed: 4. dust finer: 5. rotameter: 6. NO, permeation tube: 7. SO, permeation tube: 8. thermostat: 9. bubbler containing constanthumidity solution: 10, bhnk sampkrs: 11. samplers: 12. exposure chamber: 13. fritted bubbler lor NO, determination: 14. manometer: 15. !&as wet meter: 16. valve: 17. airconditioned cupboard.
as blanks during the analysis. The exposure chamber was constructed from a borosilicate glass tube (35-mm i.d., 250-mm length). The ends were closed with two polytetrafluoroethylene (Tarflen, Zaklady Azotowe, Tarnbw, Poland) disks with boreholes for input and output of the air stream. The connections between the tube and the disks were sealed with two pieces of rubber tubing. For decreasing the cross-sectional area of the chamher and, in conseqence, increasing the linear velocity of the air over the samplers, a cylindrical piece of Tarflen with three sockets for samplers was placed inside the chamber. For example, a t a mixture flow of the order of 5 L/min the linear velocity was 60 cm/s, Le., high enough to prevent the samplers from being subject to the starvation effect. Teflon tubing (i.d. 2 or 5 mm) was applied to minimize adsorption losses of nitrogen dioxide. Assuming that the adsorption losses in the exposure chamber and in the tubing were negligible, the 'true" concentration of NO, was calculated from the known mass of nitrogen dioxide emitted by the permeation device during the run and the total volume of the mixture (at the standard conditions, Le., a t 293 K and 101.3 kPa) measured with a wet gas meter. The validity of this assumption was confirmed by a series of measurements in which a manual 532 Environ.
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sampling procedure for nitrogen dioxide determination described by Levaggi et al. (12) was applied. A fritted bubbler having an over 95% absorption efficiency was applied to trap nitrogen dioxide. The measurements were carried out in pairs: in the first run the bubbler was connected directly to the vessel, where the permeation device was placed in the second run the bubbler was placed after the exposure chamber. In both ~ 1 1 the s whole amount of NO, emitted by the permeation device was passed through the bubbler. To assess the adsorption losses, a ratio of the masses of NO, absorbed as nitrite in both runs was calculated. Ten such pairs of measurements were conducted at different temperatures of the exposure chamber: -20, -10,2,10,20, and 30 "C. The losses of NO, assessed in the 10 tests were lower than 13% in all runs, being on the average 5%. No dependence on temperature of the exposure chamber was found. It should be noted that in the measurements that were carried out later to test the passive sampling method, the concentrations of nitrogen dioxide in the mixtures passed through the chamber were a t least 10 times lower and the periods of residence in the chamber were a t least 10 times shorter. Independently of this, while operating the NO, test mixtures generation system, the concentration of NO, in the exposure chamber was monitored by the same manual method. On the average, concentration calculated in that way was 5% higher than that measured in the same run by using the Levaggi method (22) as a reference method. The exposure chamber, the container with blank samplers, the air humidity unit, and the silica gel bed for zero air purifying were placed inside an air-conditioned cupboard. This made it possible to maintain a constant temperature in the range from -30 to +50 "C with an accuracy of 1 O C . Tightness of the test mixtures generation system was controlled before and after each run.
Results and Discussion Modification of Sampler. Among the materials tested as carriers for TEA in the collection element, two sorts of fiber materials containing polyester fiber have turned out to be useless for two reasons: strong adsorption of the dye formed during analysis of samplers and decomposition in TEA solutions. Products of the decomposition reacted
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,
,,,, ,,
102
,,
150
,,, , , , , 202
Concentration of NO2 , p g / d
. 252
Flgure 3. Response of the passive sampler vs concentration of NO,. Curves: 1, calculated from Fick's law of diffusion; 2, found for sampler with Whatman GF/C disk as TEA carrier: 3, found for sampler with textile disk as TEA carrier.
with the Saltzman solution during analysis, giving a fine crystalline precipitate that interfered with absorbance measurement. The remaining materials have proved to be resistant to TEA solution for periods as long as 6 months. A decline in the absorbance of standard solutions due to adsorption of the dye, measured as a difference between slopes of routine check graphs found for series of standard nitrite solutions with and without disks of a given material, was found to be less than 5% for Whatman CF/C and polyamide textile disks and less than 10% for Whatman 3 and viscose fiber material disks. The absorbance of solutions containing disks of these materials is stable over a period of at least 2 h. Disks of polyamide textile have been applied in the commercial version of the sampler because of their ability to stick to the bottom of the middle part of the sampler when treated with TEA solution. This simplifies the design of the sampler and the analytical procedure. Before being treated with TEA solution, the textile disks should be dried for 30 min at 105 OC to remove nitrous acid formed during storage as a product of absorption of NO, from air in the humidity contained in the textile. Disks that have not been treated in this way can give significant blank values. A similar procedure in the case of Whatman paper filter is recommended by Yanagisawa and Nishimura ( 4 ) . Storage Stability. No significant difference was noted in the performance of samplers with nylon textile disks that had been stored in an airtight container for a period as long as 6 months in comparison to samplers prepared just before exposure. Samplers can also be stored after exposure for at least 6 months. Sampling Rate. For determination of sampling rate of the sampler, the test atmosphere generation system was applied. The mass of nitrogen dioxide absorbed in the sampler as nitrite vs NOz concentration in the mixture generated in the test atmosphere system is shown in Figure 3. Two versions of the sampler were tested: one applying the nylon textile disks as a TEA carrier and the other with Whatman GF/C disks. The temperature of exposure was 20 "C, relative humidity less than 10%. The linearity of the response was confirmed also at higher concentrations of NO,, even as high as 1200 pg/m3. The dashed line represents the values calculated from Fick's law of diffusion (at 20 "C and providing that the face velocity is high enough to prevent the starvation effect). The most probable explanation why the experimentally found sampling rate is considerably lower than that calculated on the basis of diffusion in the gas phase is that
absorption of nitrogen dioxide by the TEA-coated collection element is not quick enough to collect 100% of the molecules that diffuse through the air gap inside the sampler to the surface of the collection element. This additional resistance in the mass transport may be caused by two factors: insufficient surface of the absorbent and limited absorption rate due to the viscosity of triethanolamine or low rate of the reaction between TEA and nitrogen dioxide (according to Palmes et al. (2),the fraction of NOz converted to nitrite in the case of diffusive samplers approaches 100%). The influence of the first factor seems to be very distinct when the two versions of the sampler are compared. The collection efficiency of the sampler with a Whatman GF/C disk applied as a TEA carrier is -50% higher than that of the sampler with a nylon textile disk. On the other hand, even application of a carrier having such a fine microporous structure as Whatman GF/C does not result in 100% absorption efficiency. Therefore, the contribution of the second factor mentioned above, i.e., liquid-phase resistance in the overall sampler resistance, cannot be neglected and the overall sampler uptake must be calculated including this factor. Such an equation, which contains the so-called "overall masstransfer coefficient" is used in chemical engineering for description of absorption processes. This equation was applied for a passive sampler by Yanagisawa and Nishimura ( 4 ) . As the sampling rate cannot be calculated theoretically, an empirical coefficient P has been introduced. The coefficient has been defined as the mass of nitrite determined in the sampler after 24-h exposure at 100 pg/m3 NOz (at 293 K and 101.3 kPa). Thus, the mean concentration c of nitrogen dioxide during the exposure, expressed in micrograms per cubic meter at 293 K and 101.3 kPa, is given by the formula mt c = 1.44 x 105P where m is the mass of nitrite determined in the sampler (pg) and t is the time of the exposure (min). The method is primarily designed for 24-h measurements in urban areas. It is possible, however, to determine accurately concentrations of N O p lower than 20 pg/m3 if a longer period of exposure is applied. The quantity of TEA in the sampler is large enough to trap (stoichiometrically) as much as 6 mg of NO,. Therefore, the sampler is expected to be applicable for monthly measurements. So far, positive results have been obtained for 1-week periods of exposure. Relative precision of the method, characterized by RSD, determined in field trials is 10%. No interference due to the presence of SO, at concentrations as high as 1 mg/m3 was noted in 12 pairs of measurements with/without SO, addition to the stream of the NO,-air mixture. Temperature a n d Humidity Effect. The influence of temperature and humidity on the determination of NO2 using two versions of the modified sampler (with textile disks and Whatman GF/C disks) was determined with the test atmosphere generation system. The results are shown in Figure 4. The dashed line represents values of P calculated from Fick's law of diffusion, providing that the diffusion coefficient D is proportional to PI2. As the concentration c, expressed in moles per cubic meter, is the overall sampler sampling rate is proportional to TI, dependent on T 1 / 2(2), providing that the mass transport is limited by diffusion in the gas phase. As can be seen from Figure 4, for the whole range of temperatures, the dependence of the sampler response on
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:
,
L - T - r d c
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Figure 4. Response of the passive sampler vs temperature. Curves: 1, calculated from Fick's law of diffusion; 2, found for sampler with Whatman GF/C disk as TEA carrier, RH C 10 % ; 3, found for sampler with textile disk as TEA carrier, RH = 100%; 4, found for sampler with textile disk as TEA carrier, RH < 10%.
temperature is significant. Similarly, as in the discussion on the sampling rate, this fact can be explained when the absorption step in the mass transport process is taken into consideration. Both the chemical reaction rate and the diffusion rate in the liquid phase are strongly temperature dependent. According to the Arrhenius equation, which is applicable to most of chemical reactions, the rate of chemical reaction is proportional to exp(const/ T ) , i.e., increases 2-3 times when temperature grows by 10 "C. As regards diffusion in liquids, the diffusion coefficient D is given by (13) D = - const T D where 1) is the viscosity of the liquid, decreasing proportionally to exp(const/ T ) . This strong dependence of D on temperature is not compensated, as in the gas phase, by concentration changes with temperature. Therefore, in both cases the process of NO, collection in TEA must be significantly temperature dependent. As the overall sampler uptake is partially limited by the absorption process, this dependency on temperature affects the overall sampler performance. The exponential best-fit equation for the sampler with nylon textile disk as TEA carrier, exposed a t RH < l o % , is P = 1.25 exp(O.O203T), where T i s temperature ("C). This means that in the range of temperatures from 0 to 30 "C the sampler uptake increases on the average by 2170 per 10 "C. This increase, expressed in relation to the average uptake, amounts to 11% in the case of the sampler with Whatman GF/C disks. A similar temperature effect on different diffusive samplers applying TEA as absorbent was reported by Kring et al. ( 3 ) ,by 15.5% per 10 "C, and by Matsumoto and Mizoguchi (14), by 18% per 10 "C. The same devices applied for SO, measurements showed no temperature effect. Although Palmes et al. (2)claimed insignificant temperature influence on their diffusive tube performance, studies carried out by Girman et al. (15) revealed a 15% growth in the collection efficiency a t 27 "C in comparison to that at 15 "C. In the other works cited in this paper and describing different passive samplers for NO, measurements ( 4 , 6, 7), the temperature effect has not been studied experimentally. In fact, the process is even more complicated for two reasons: first, triethanolamine solidifies at 21.2 "C; second, aqueous solutions of TEA used in passive samplers absorb or desorb water in such quantities that an equilibrium 534
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5':
Relative humidity
(2)
8'3
130
Figure 5. Equilibrium curve between concentration of triethanolamine in aqueous solution and relative humidity of air.
concentration, appropriate to relative humidity, is reached in a period of several hours. The liquid-solid transition temperature for TEA solutions of different concentrations is, obviously, situated between 0 and 21.2 "C. As Figure 4 shows, the uptake of samplers exposed a t RH = 100% is by 25% of the average value higher than those exposed a t RH < 10%. The best-fit linear equation found for values of P determined a t 20 " C for samplers exposed a t intermediate values of relative humidity and conditioned before the measurement a t the same RH is P = 1.84 + 0.0061RH (70).A comparison performed using samplers prepared following the usual procedure and samplers that were additionally dried showed no significant difference in their collection efficiency. This was because a t 20 "C the concentration of TEA in the absorbing solution in a period of several hours reached the equilibrium value, corresponding to the relative humidity. The equilibrium curve, concentration of TEA vs relative humidity, is given in Figure 5. An almost identical increase in the collection efficiency was observed by Boleij et al. (16) for Palmes diffusion tubes tested in humidity-controlled laboratory experiments. In the practical use of passive samplers, the humidity effect should be of lesser significance because the relative humidity of ambient air usually exceeds 40 % . Nevertheless, further work is necessary to establish the optimum concentration of TEA solution that could be affected by the relative humidity of the ambient air to the most limited extent. An intercomparison among ca. 70 laboratories is currently being conducted to determine the sampler uptake in field conditions. Registry No. TEA, 102-71-6; NOz, 10102-44-0.
Literature Cited (1) Levaggi, D. A.; Siu, W.; Feldstein, M. Enuiron. Sci. Technol. 1972, 6 , 250. (2) Palmes, E. D.; Gunnison, A. F.; DiMattio, J.; Tomczyk, C. Am. lnd. Hyg. Assoc. J . 1976, 37, 570. (3) Kring, E. V.; Lautenberger, W. J.; Baker, B. W.; Douglas, J. J.; Hoffman, R. A. Am. Ind. Hyg. Assoc. J. 1981,42,373. (4) Yangisawa, Y.; Nishimura, H. Enuiron. Int. 1982,8, 235. (5) Amaya, K.; Sugiura, K. Enuiron. Prot. Eng. 1983, 9, 5. (6) Cadoff, B. C.; Hodgeson, J. Anal. Chem. 1983,55, 2083. (7) Mulik, J. D.; Lewis, R. G.; McClenny, W. A. Anal. Chem. 1989, 61, 187. (8) Krochmal, D.; Rzemiiiski, M.; Gbrski, L. Chem. Anal. (Warsaw) 1987, 32, 581. (9) Krochmal, D.; Gbrski, L., submitted to Fresenius 2. Anal. Chem. (10) Polish Standard PN-89/2-04092/08; Wydawnictwa Normalizacyjne "Alfa": Warsaw, 1989 (in Polish). (11) Handbook on Physical Chemistry; Wydawnictwo Naukowo-Techniczne: Warsaw, 1974; pp 222-3 (in Polish).
(12) Levaggi, D. A,; Siu, W.; Feldstein, M. J.Air. Pollut. Control A s ~ o c .1973, 23, 30. (13) Barrow, G. M. Physical Chemistry, 3rd ed.; Paastwowe Wydawnictwo Naukowe: Warsaw, 1978; p 672 (in Polish). (14) Matsumoto, M.; Mizoguchi, T. Chem. Abstr. 1988, 109, 98000.
(15) Girman, J . R.; Hodgson, A. T.; Robinson, B. K.; Traynor, G. W. Lawrence Berkeley Laboratory Report LBL 16302,
Berkeley, CA, 1983 (after ref 16). (16) Boleij, J. S.; Lebret, E.; Hoek, F.; Noy, D.; Brunekreef, B. Atmos. Environ. 1986, 20, 597. Received for review June 19, 1990. Revised manuscript received September 28, 1990. Accepted October 10, 1990. This work is a part of central plans CPBP 01.17 and CPBR 11.4.
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