Environ. Sci. Technol. 1988, 22, 1463-1468
(8) King, E. J. Qualitative Analysis and Electrolytic Solutions; Harcourt: New York, 1959. (9) MacInnes,D. A. The Principles of Electrochemistry; Dover: New York, 1932. (10) Driscoll, C. T.; Newton, R. M. Environ. Sci. Technol. 1985, 19, 1018-1024. (11) Butler, J. N. Carbon Dioxide Equilibria and Their Applications; Addison Wesley: Reading, MA, 1982. (12) Schofield, C. L. Ambio 1976,5, 228-230. (13) Massachusetts Department of Environmental Quality Engineering Acid Rain and Related Air Pollutant Damage: A National and International Call for Action; Common-
United States; U.S. EPA, Office of Research and Development: Corvalis, OR, 1984. U.S. EPA The Acid Deposition Phenomenon and its Effects. Critical Assessment Review Papers, (Effects Sci-
ences);US. Government Printing Office: Washington, DC, 1984; Vol. 11, Chapter 4. National Academy of Science Acid Deposition: Long Term Trends; National Research Council: Washington, DC, 1986. Smith,R. A.; Alexander, R. B. “Evidencefor Acid Precipitation Induced Trends in Stream Chemistry of Hydrologic Bench-Mark Stations”;USGS Circular No. 910, 1982. Henriksen, A. Changes in Base Cation Concentration Due to Freshwater Acidification;Norwegian Institute for Water Research: Oslo, Norway, 1982. Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley: New York, 1981.
wealth of Massachusetts: Boston, MA, 1984. Received for review December 2, 1986. Revised manuscript received January 26, 1988. Accepted June 20, 1988.
Evaluation of an Annular DenuderIFilter Pack System To Collect Acidic Aerosols and Gases Petros Koutrakls,” Jack M. Wolfson, James L. Slater,+ Michael Brauer, and John D. Spengler
Harvard University, School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 021 15 Robert K. Stevens
U S . Environmental Protection Agency, Research Triangle Park, North Carolina 2771 1 Charles L. Stone
University Research Glassware, Carrboro, North Carolina 27510
rn A glass impactor was designed and evaluated along with an annular denuder/filter pack system. The glass impactor has a 50% aerodynamic cutoff of 2.1 pm a t a flow of 10 L m i d and allows a quantitative transfer of gases and fine particles to the annular denuder and filter pack components. Fine particle and gas concentrations, determined by using the glass impactor along with the annular denuderlfilter pack, were in good agreement with those obtained with colocated reference samplers. Measurements of SO2,“OB, and HN02 gases showed mean collection efficiencies of 0.993, 0.984, and 0.952, respectively, which compare well with predicted values. Additionally, it was found that artifact formation of nitrate and nitrite ions, representing about 5-10% of the concentrations of HNO, and HN02,occurs in the Na2C03-coatedannular denuder. Corrections for these artifacts were made with a second Na2COB-coatedannular denuder. The results of this pilot study suggest that the glass impactor/annular denuder/ filter pack sampling system is suitable for measuring acidic aerosols and gases.
Introduction During the last decade, diffusion denuders have been used in a variety of atomospheric monitoring studies to collect gaseous atmospheric pollutants (1-6). These previous studies have relied on the use of tubular denuder designs. Possanzini et al. (7) described the application of an annular denuder configuration that quantitatively collects reactive atmospheric gases 15-20 times more efficiently, per unit length, than the tubular denuders. Recently, Vossler et al. (8) evaluated a sampler (EPA system) consisting of a glass impactor followed by two annular denuders and a filter pack. The glass impaction plate is permanently attached to the inlet section. The ‘On leave from the University of Steubenville, Steubenville, OH. 0013-936X/88/0922-1463$01.50/0
impactor removes coarse particles, while gases and fine particles are quantitatively transferred into the annular denuder and filter pack components. Subsequently, HNO,, HN02, and SO2 vapors are trapped by a Na2C03-coatedannular denuder. A second Na2C03-coated annular denuder is used to determine artifact formation of nitrate and nitrite, to correct the apparent concentrations of HNO, and HN02 on the first denuder. The last component of the sampling system is a filter pack containing a Teflon filter followed by a nylon filter. The first filter collects fine particles, while the second traps HN03 originating from the dissociation of “,NO3 collected on the Teflon filter. Recently, we developed a substantially more versatile and convenient impactor design for an annular denuder/filter pack system, the Harvard-EPA annular denuder system (HEADS). In this system the impaction plate is decoupled from the inlet housing and placed onto the top of the first annular denuder. The impaction plate is a porous glass disk which is impregnated with mineral oil to minimize bounce-off of the collected coarse particles. After sampling, the impaction plate and its holder are removed and the first denuder is extracted with no interference from the coarse particles. The HEADS was evaluated in a pilot study in Boston, MA, during the summer of 1987. The results of this air sampling are presented and discussed in this paper. Design a n d Description o f C o m p o n e n t s
The sampling system, shown in Figure 1, consists of a borosilicate glass impactor, two glass annular denuders, and a FEP Teflon filter pack. As illustrated in Figure 2, the impactor consists of an entrance elutriator section containing the inlet tube followed by an acceleration jet and the impaction plate. The plate is mounted at the entrance to the first annular denuder. The elutriator section is 9.5 cm in length, with 1.1-cm i.d. The acceler-
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 12, 1988 1463
> FILTER PACK
I I
I
r
-
AIR O U T T O SAMPLE PUMP
-1 I
I
Table I. Comparison of Predicted and Experimental Collection Efficiencies for SOz, HNO,, and HNOa
J
gas SO2 HNOS "02
diff diff coeff coeff" ref 0.136 0.121 0.154
7
5 13
collectn effic exptl predicted 0.999 0.999 0.999
0.993 0.989 0.990
0.992 0.940
0.993 0.978 0.925
Diffusion coefficients are expressed in cm2 s-l.
IST DENUDER
-
TEFLON HOLDER IMPACTION PLATE INLET SECTION
AIR SAMPLE IN
Figure 1. Schematic of the glass impactor/annular denuder/filter pack system. SAMPLE FLOW
T
T
ticles. The distance between the end of the impactor acceleration jet and the impaction plate is maintained between 5.00 and 5.50 mm by a FEP Teflon-jacketed silicone rubber O-ring, which is placed between the entrance section and the first annular denuder inlet. The two pieces are connected with a coupling section, which has two molded Bakelite thermoplastic threaded sleeves separately attached to an aluminum holder. The impactor has been designed to have a theoretical 50% collection efficiency at -2.5 pm, for a flow of 10 L min-l (9). The design of the two annular denuders is similar to that of Vossler et al. (8). The first denuder has a length of 26.5 cm for the outer cylinder, and 21.5 cm for the inner cylinder. The outer diameter of the inner cylinder is 2.20 cm, and the thickness of the annulus is 0.10 cm. The second denuder has a length of 24.2 cm for the outer cylinder, with the other dimensions the same as for the first denuder. Predicted annular denuder collection efficiencies are calculated by using the following formula, based on the work of Possanzini et al. (7):
E = 1 - c/co
(1)
C/Co = 0.82 exp(-22.53Aa)
(2)
defining Aa
A I R SAMPLE
Figure 2. Glass impactor/denuder.
ation jet is 1.3 cm long, with an inner diameter of 0.300 f 0.005 cm. The impaction plate is a porous glass disk, with a nominal pore size of 10-16 pm, diameter of 1.10 cm, and thickness of 0.16 cm. The plate is mounted in a removable FEP Teflon holder, which is securely attached by friction fit into a cylindrical glass cavity fused to the entrance of the first annular denuder. The mounting for the plate is contoured to minimize turbulence, which could cause deposition of fine particles on the surfaces of the cavity. The plate may be removed from its Teflon holder for cleaning. When the holder is removed, the first denuder is extracted, with no interference from coarse par1464
Envlron. Scl. Technol., Vol. 22, No. 12, 1988
= (nDL/4F)(dl
+ dJ/(d2
- dJ
(3)
where E is the removal efficiency, C and Co are the gas concentrations at the exit and entrance of the denuder, respectively, D is the diffusion coefficient of the gas in air (in cm2 s-l), L is the length of the denuder (in cm), F is the flow (in cm3 s-l), and dl and d2are the inner and outer diameters of the annulus, respectively (in cm). Table I shows the predicted collection efficiencies for SO2,HN03, and HN02, calculated by using eq 1,for an annular denuder with the dimensions given above, at a flow of 10 L min-l. The first denuder is coated with Na2C03/glycerol to collect SO2, HNO,, and HN02. The second denuder is coated in the same way as the first and is used to measure artifact nitrate and nitrite for correction of the HN03 and HN02concentrations on the first denuder (see Results and Discussion). Following the second denuder is a FEP Teflon filter pack containing two filters and stainless steel support screens. The first filter is a PTFE Teflon membrane, with polyolefin ring, used to collect the fine particles for particle mass, sulfate, nitrate, and nitrite determinations, The second filter is a 47-mm-diameter glass-fiber and "02. filter, coated with Na2C03to trap "03 Sampling and Analysis Pilot air sampling experiments were conducted on the roof of the Harvard School of Public Health in downtown Boston, MA, during the summer of 1987. HEADS samplers, consisting of the improved glass impactor, two annular denuders, a filter pack, and a flow-controlled pump operating at 10 L min-l, were colocated with EPA systems,
40.0
Y= 0.9ox t 0.89
0.0
10.0
20.0
30.0
40.0
HI System
Flgure 3. Comparison of mass concentrations (in pg m-3) obtained from HEADS and H I sampling system.
used by Vossler et al. (8), and with Harvard impactors (IO). Sample duration varied between 1and 3 days depending on the observed air quality levels. The denuders were coated by using IO mL of 1% (w/v) Na2C03/1% (v/v) glycerol in a 1:l methanol/water solution. After being coated, the denuders were immediately dried with clean dry air and capped to protect them from acidic gases. A few drops of mineral oil were added to the porous glass impaction plate, with the excess carefully removed. The complete system was assembled and checked in the laboratory for leaks. The first filter in the filter pack was a 47-mm-diameter, 2-pm-pore PTFE Teflon membrane (Gelman Sciences). Teflon filters were weighed twice on a Cahn Model 21 electrobalance, after equilibration in a constant humidity and temperature room. The second filter (Millipore, glass fiber) was coated with 2% (w/v) Na2C03in 3:lO methanoljwater solution and dried in an acidic gas-free hood. The filter pack was assembled in the hood to protect the coated filter from acidic gases. After sampling, the denuders were extracted with 10 mL of ultrapure water (Millipore, Milli-Q water system). The extracts were stored at 5 "C and later analyzed for anions by ion chromatography (Dionex Model 4000i). The filter pack was opened in the acidic gas-free hood. The sodium carbonate coated filter was placed in a vial with 5 mL of ultrapure water and then sonicated for 15 min. The extract was analyzed for anions by ion chromatography. After equilibration, the Teflon filters were weighed twice on the electrobalance. They were then cut and placed inside a polycarbonate vial. Since the Teflon filter is hydrophobic, 0.100 mL of ethanol was added to wet the filter (11) before extraction and analysis in the same way as with the coated filter.
Results and Discussion Impactor Calibration. The improved glass impactor was designed to have a theoretical 50% collection efficiency at -2.5 pm, for a flow of 10 L m i d . Impactor calibration tests, conducted at the University of Minnesota according to the procedures of Marple and Rubow (12),found the experimental 50% aerodynamic particle cutoff point to be 2.1 pm. Measurement of Fine Particle Mass and Composition. The HEADS sampler was evaluated for collection of fine particle mass, as well as fine particle sulfate, nitrate, and nitrite ions. To investigate the collection performance of the HEADS sampler, the Harvard impactor (HI) system was used as a reference. The HI system has been designed and characterized to have a 50% aerodynamic particle
0.0
10.0
20.0
30.0
HI System Figure 4. Comparison of sulfate concentrations (in pg mm3)obtained from HEADS and H I sampling system.
cutoff of 2.5 pm at a flow rate of 4 L min-' (10). Figure 3 shows a comparison of fine particle mass concentrations determined by the HEADS and HI systems. Mass concentrations obtained from the HEADS samplers were consistently 10% lower than mass concentrations determined by the colocated HI samplers. This can be explained by the slightly lower aerodynamicparticle cutoff of the HEADS. In addition, comparisons between the HEADS and EPA system showed that the mass concentrations obtained from the EPA system were 10-30% higher than those determined by using the HEADS. An explanation for this result could be bounce-off of coarse particles from the impaction plate of the EPA system, which is not porous and could not be impregnated with mineral oil. In contrast, for the improved impactor, a porous glass impaction plate is used. The plate is coated with mineral oil to minimize bounce-off of the collected come particles. The bounce-off depends primarily on the total collected coarse particle mass. Since the coarse particle mass was not determined and also because the ratio of fine to coarse mass is not constant, we cannot determine a maximum collectable mass. Although a quantitative guideline for collection capacity was not determined, our results indicate that 24-h samples may be collected without significant coarse particle bounce-off. Colocated HEADS samplers in which impactors had elutriator lengths of 9.5 cm and 1.5 cm were compared to evaluate the effects of different elutriator lengths on the collection of fine particle mass. No differences in fine particle mass nor in the concentrations of ionic species were observed between the systems with different elutriator lengths. Next, we compared the concentrations of sulfate collected on the Teflon filter for both the HEADS and the HI system. The results, as shown in Figure 4, indicate excellent agreement. Even though this agreement was found, we also investigated the possible trapping of sulfate particles on the denuder surfaces. When samples were collected on denuders coated with glycerol, instead of the usual Na2C03/glycerolmixture, no sulfate was observed in the ion chromatographic analysis of extracts, indicating no loss of sulfate particles within the denuder series. Additionally, only trace amounts (
