Environ. Sci. Technol. 1991,25,519-524
Evaluation of an Enclosure Method for Measuring Emissions of Volatile Organic Compounds from Quiescent Liquid Surfaces Alex R. Gholson,+John R. Albritton, and R. K. M. Jayanty"
Center for Environmental Measurements, Research Triangle Institute, Research Triangle Park, North Carolina 27709 Joseph E. Knoll and M. Rodney Mldgett
Atmospheric Research and Exposure Assessment Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 277 11 An enclosure method for direct measurement of volatile organic emissions from quiescent liquid surfaces was investigated under simulated conditions in the laboratory and in the field a t two hazardous waste treatment facilities. In the laboratory study, accuracy and precision of the method was estimated under a variety of environmental and operational conditions including level of emission rates, emission composition, solar intensity, and enclosure sweep flow rate. Precision of the method was determined in the field studies a t two sites under a range of environmental conditions including winter versus summer, full sun versus shade, low emission rate versus high emission rate, and polar and nonpolar compounds. The results of the laboratory study show that precision for a single-component emission rate was 3.0% or less measured as pooled relative standard deviation with a sweep flow rate of 5 L/min or above. A consistent negative bias of approximately 50% was found, which was independent of emission rate, solar intensity, or sweep flow rate a t or above 5 L/ min. The bias was found to vary with compound and between single-compound and multicompound emissions. Precision in the field was found to be approximately 20% expressed as relative standard deviation.
Introduction Enclosure methods have been used to measure a variety of emissions from many different sources (1-6). During the treatment, storage, and disposal of hazardous waste, volatile organic compounds (VOCs) are released into the environment via the atmosphere. Under the 1984 Resource Conservation and Recovery Act (RCRA) Amendments, the U.S. Environmental Protection Agency (U.S. EPA) is required to take necessary measures to protect public health and the environment from these air emissions at hazardous waste treatment, storage, and disposal facilities (TSDFs). To support the regulatory process, analytical methods are required to measure or predict the emissions of VOCs from TSDFs, and the accuracy and precision of these methods should be well-characterized. The flux chamber method is an enclosure method that has been used to make direct air measurements of emissions from surface impoundments, land farms, landfills, and contaminated soils (7, 8). Emission measurements made with the flux chamber are providing a database for regulatory decision making, validating predictive air emission models, and assessing risk a t Superfund cleanup sites. The isolation flux chamber works by enclosing a representative area of the source surface. Then a controlled flow of pure inert sweep gas is added to the chamber, allowed to mix, and then released through the chamber's exits. The concentrations of the emitted compounds +Present address: National Council for Air and Stream Improvement Inc., Corvallis, OR 97339. 0013-936X/91/0925-0519$02.50/0
are measured in the exit gas. The emission rate of the emitted compound is given by where f is the sweep flow rate (L/min), A is the surface area enclosed (m2), Ci is the concentration of emitted compound (mg/L), and Eiis the emission rate of the emitted compound (mg min-' m-2). For the method to accurately measure the true emission rate, the chamber gas sampled must be well-mixed and the chamber must not disturb the surface in a way that might alter the natural emission rate. Mixing can easily be measured, but it is more difficult to determine if the chamber is affecting the natural emission from the surface. The classic theory for describing volatile emissions from a liquid surface is the two-film theory (9). It has been shown that for volatile compounds where Henry's law constant is greater than 1 x atm m3 mol-', the liquid film resistance is the controlling factor for emission (10). The environmental factors controlling the liquid film resistance or liquid mass-transfer coefficient have been previously studied (11-13) and found to be liquid turbulence and diffusion. Liquid turbulence and diffusion are controlled by surface wind velocity and surface temperature. Placing a chamber on a liquid surface can affect the surface wind velocity and temperature to some extent. Because the turbulent motion of a surface is a result of large-scale processes, the effect of the chamber may be minimal. Chamber insertion depth would be expected to effect the diffusion and mixing of compounds at the surface and should be kept at a minimum. Chamber sweep flow rate and inlet height could effect surface turbulence and vapor-phase concentration inside the chamber. Attenuation of solar radiation by the chamber may lower the surface temperature and decrease evaporation, or warming due to the greenhouse effect could cause an increase in the surface temperature. By choosing a clear chamber top and optimizing the sweep flow rate and enclosure duration time, surface temperature fluctuations are kept to a minimum. The sweep flow rate must be optimized to ensure good mixing and to prevent concentration buildup of volatile organics. If the sweep flow rate is too low, the vapor-phase concentration of organics will reach a point where gasphase resistance will control the emission process. In most environments, natural air turbulence and mixing will prevent the gas-phase concentration from reaching significant levels; therefore, a high sweep flow rate is needed to maintain a good concentration gradient across the liquid-gas interface. A series of laboratory studies were performed using a simulated surface impoundment simulator (SIS) to investigate the effect of the operational parameters and environmental conditions on the method accuracy and
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 3, 1991 519
Sample Tee Temperature Readout
Purge Vent Sample Inlet
Sample Outlet
Purge Gas Flow Control J
I Thermocouples
Flux Chamber
U
Zero Grade Air
Figure 1. Schematic of flux chamber and support equipment.
precision. T o evaluate the method in the field, precision studies were performed on four different liquid surfaces at two hazardous waste treatment facilities under various environmental conditions. Although environmental conditions and chemical composition and concentrations varied throughout the study, only their effect on the method was evaluated. No attempt was made to study the effect of these conditions on the actual emission rate or to predict emission rates from their measurements.
Experimental Section Flux Chamber Design. The flux chamber design used in this study is that described by Kienbusch and Ranum (14) with several minor modifications. The base of the chamber was a cylinder 0.41 m in diameter, 0.18 m long, and constructed of stainless steel. The chamber enclosed 0.13 m of liquid surface and contained a volume of 0.03 m3. The top of the cylinder was covered with a clear 3.2-mm-thick acrylic dome with a maximum height of 0.10 m. The dome was attached to the base with an aluminum flange and polytetrafluoroethylene (PTFE) gasket. Four equally spaced holes in the dome provided an inlet for sweep gas, an outlet for sample gas, wiring for temperature probes, and a vent for excess sweep gas. No mechanical impeller was used for mixing because several previous studies have shown that the impeller had no significant influence (6) or added a positive bias (14) to the emission rate. Complete mixing was demonstrated without an impeller in a similar flux chamber design (7). The sweep gas inlet tube skirted the base below the dome with gas exiting from four equally spaced holes pointing toward the center of the chamber. The height of the tube could be adjusted to study its effect on the measured emission rate. The sample exit line extended from 10 cm above the base opening to 2.5 cm from the top of the dome, and it was positioned so that it was equally distant from two sweep inlet holes and halfway between the center of the chamber and the chamber wall. The exit line was perforated with two columns of holes that faced perpendicularly to the flow of the sweep gas. For the laboratory study, the flux chamber was supported on the liquid surface by two handles on the outside of the chamber base. For the field studies, three 0.093-m2polyethylene foam floats were used to support the flux chamber. A removable aluminum collar was used to extend the floats 0.46 m from the chambers 520
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and was adjustable to set the depth of the chamber base in the liquid. Two thermocouples were placed inside the chamber supported by a piece of PTFE tubing so that one measured the bulk air temperature inside the chamber and the other measured the temperature a t the liquid surface inside the chamber. Figure 1 is a schematic of the flux chamber and the support equipment. The flux chamber was supplied with a source of nitrogen or hydrocarbon-free air for the sweep gas. The sweep gas was regulated and the flow rate was controlled. A Calibrated mass flowmeter was used to monitor the sweep flow rate in the laboratory studies, and a calibrated bubble flowmeter was used to check the sweep flow rate in the field. In the field, the sweep flow was supplied to the chambers with a 25-m length of 3.1-mm-0.d. fluorinated ethylene propylene (FEP) tubing. Shorter lengths of 6.2-mm PTFE tubing were used in the laboratory studies. The gas taken for sample analysis was drawn from the chamber with a diaphragm vacuum pump. A tee was placed before the pump to allow samples to be removed for analysis by either a syringe or an evacuated canister. The sample flow rate was controlled with a metering valve a t a rate of approximately 150 mL/min. A differential pressure gauge was used periodically to determine if any pressure buildup was occurring inside the chamber. A 25-m length of 3.1-mm-0.d. F E P tubing was used to transfer the sample to the pump in the field, and a shorter length of 6.2-mm-0.d. F E P tubing was used in the laboratory. The flux chamber temperature was monitored with type J thermocouples connected with 25 m of thermocouple cable to a multichannel monitor. For the laboratory studies, the thermocouples were connected directly to the temperature monitor. Surface Impoundment Simulator. The SIS consisted of a 0.83-m3 (220-gal) rectangular tank covered with an aluminum frame wrapped with clear 0.13-mm-thick FEP film. One end of the cover was open to the atmosphere through a series of honeycombed openings, and the other end was attached to a reducing duct connected to the inlet of a blower. Figure 2 is a diagram of the SIS. The tank surface was 1.2 m x 1.5 m (1.8 m2),and it had a maximum depth of 0.61 m and a minimum depth of 0.30 m. The tank was constructed of galvanized steel and supported with six legs resting on concrete blocks. The SIS cover was 0.51
Clear FEP Teflon Shell
A FluxChamber Sampling Poru
inlet
1 .51 m
i
~
~Aluminum ~ ~ m Transition Duct
SIS Sampling port6
I 0 0
A
Blower
sm .ril.-l
Flgure 2. Diagram of the surface impoundment simulator.
m high, 1.3 m wide, and 1.6 m long. Two 8-mm-0.d. pitot tubes were mounted in a 20-cm-diameter pipe leading from the cover to the inlet of the blower. A gas sampling port was placed just downstream from the pitot tubes. Relative to the SIS, a single flux chamber covered 7.3% of the SIS surface and occupied 3% of the air volume in the SIS. During the testing, the two flux chambers were placed side by side in a line parallel with the wind direction with the point half way between the chambers located in the center of the SIS. Organic compounds were added to water in the SIS in solutions that were denser than water so that amounts exceeding the solubility would remain at the bottom. An excess of l,l,l-trichloroethane was always present, and a window was placed a t the bottom of the tank to monitor the level of the organic layer. When toluene and 2-butanone were added for the three-component study, a mixture in l,l,l-trichloroethane was added through a tube submerged under the organic layer. This prevented an organic layer from forming on the surface and ensured a constant supply of organic material to the bulk solution. Normal tap water was used to fill the SIS tank to within 2.5 cm of the top. Sample Collection. Air from the flux chamber and SIS was collected as grab samples in a 50-mL glass syringe or integrated over 10 min in an evacuated 2-L stainless steel canister. Grab samples were collected at the beginning and the end of a 10-min period during some of the laboratory studies. All field samples were collected by using integrated canister sampling. A calibrated restrictor was used to maintain a constant flow rate of approximately 100 mL/min over a 10-min period into the evacuated canister. After sample collection, canisters were transported to the laboratory and pressurized to 200 KPa with pure nitrogen before analysis. Analysis. Gas samples collected from the flux chamber and the SIS were analyzed by gas chromatography (GC) with either a gas sampling valve or a preconcentration step. A six-port valve with a 2-mL stainless steel loop was used for most of the laboratory studies where sample concentrations were greater than 10 ppm. Either solid sorbent or cryogenic concentration was used to analyze samples with compound concentration less than 10 ppmV for the laboratory studies. All field samples were analyzed by using cryogenic concentration. The identities of the compounds in some of the field samples were confirmed by gas chromatography with mass spectrometric detection (GC/MS). A Perkin-Elmer 3920B GC with a flame ionization detector (FID) and an electron capture detector (ECD) were used for all gas analyses. A 3-m 1%SPlOOO on Carbopack glass packed column was used with an FID for the laboratory studies. A 30-m, DB624, 0.53-mm-i.d. capillary
column was used with the column effluent split to an FID and an ECD at a ratio of 5 1 for all the field measurements. In the field studies, the FID was used for quantification unless an interference was observed or the response was too small; then the ECD was used. An electronic integrator was used to record the peaks and calculate the area for all analyses. Calibration and quality control gas standards were prepared by flash evaporating a known amount of each compound into an evacuated stainless steel canister and diluting with clean nitrogen or air. At site two, where the waste composition was variable, an Bcomponent artificial gas standard (Scott Environmental) was used to calibrate the analyses. A second certified standard containing five compounds was used as a quality control check sample. A total organics concentration value was obtained for the measurements a t site two because of the large number and quantity of unidentified compounds. The response factor for toluene was applied to the FID area of each unidentified peak and the results were summed with identified compounds to obtain the value reported as total organics. Emission Measurement Procedure. The following procedure was used to measure emission rate with the chamber for all the studies. Before the chamber was placed on the surface, the sample and sweep lines were purged with the chamber suspended above the liquid surface. The chamber was lowered on the surface to a depth of approximately 1.3 cm with the sweep gas flowing and the time was recorded. After 20 min, sample collection was initiated. Either a grab sample was collected in a syringe or a 10-min-integrated canister sampler was started. After 30 min, sampling was stopped. Either a second grab sample was collected or the integrated canister sampler was stopped. During the 10-min sampling period, the temperature of the liquid surface inside the chamber, the liquid surface outside the chamber, and the ambient air outside the chamber were measured. Total emissions from the SIS were measured by collecting a sample (syringe or canister) from the sampling port after the blower had run for at least 10 min with the cover closed. Pilot readings were taken to measure the average velocity in the blower duct to calculate the flow rate through the SIS. Equation 1 was used to calculate the emission rate from the measured concentration for both the chamber and the SIS. Precision and Accuracy Study. The accuracy and the precision of the chamber method was determined under a variety of environmental conditions with the operational parameters set except for sweep flow rate. A total of 12 duplicate measurements were performed at a sweep flow rate of 5 L/min, 3 duplicate measurements at a sweep flow rate of 2 L/min and 3 duplicate measurements a t a sweep flow rate of 10 L/min during daylight hours with one compound. Three duplicate measurements were made a t night a t a sweep flow rate of 5 L/min with one compound. Nine duplicate measurements were made during daylight hours with a sweep flow rate of 5 L/min with three compounds. The set operational parameters were a sweep inlet height of 10 cm, chamber depth of 1.3 cm, and grab sampling timer of 20 and 30 min. The environmental conditions were allowed to fluctuate as outside temperatures changed, solar intensity varied, and liquidphase concentration of the volatile compound fluctuated. No attempt was made to correlate these changes with the emission rate. The goal was only to observe if the precision or accuracy of the method was affected. Environ. Sci. Technol., Vol. 25, No. 3, 1991 521
Table I. Precision and Accuracy Estimate for l,l,1 -Trichloroethane av emiss rate, mg % no. min-' m-2 RSD"
grouping 5 L/min, high emissn 5 L/min, low emissn 2 L/min 10 L/min 5 L/min, nighttime
9 3 3 3 3
10.1 0.51 3.84 9.36 10.4
3.0 2.9 4.1 1.4 1.5
Table 11. Precision and Accuracy Estimates for Three-Component Emission Measurement av bias, 70 (*go% CLb) -45 -67 -82 -49 -57
compound
f 6.4
f 17 f 9.6 f 8.3 f 21
"RSD is the pooled relative standard deviation. bCL is the confidence limit,.
The precision was estimated by calculating the pooled standard deviation for sets of duplicate measurements by use of the following equation.
where AXi is the difference between duplicate measurements and n is the number of duplicate measurements pooled. Accuracy of the method was estimated by comparing the average emission value for the duplicate chamber measurement with the emission measured from the SIS. SIS emission measurements were made before and after each set of duplicate chamber measurements. The average of the before and after SIS emission measurement was assumed to be the true emission rate for the chamber measurement. Accuracy is expressed as percent bias by using the following equation: % bias =
E,
- &Is
Es1s
x 100
(3)
where E, is the average emission measured in the chamber and EsIs is the average emission measured in the SIS. Field Evaluation. Method precision was evaluated in the field at two hazardous waste treatment facilities. The first site was an industrial waste water treatment facility in December and the second site was a waste stabilizer/ solidification facility in June. Two locations a t each site were selected, one with higher emission potential than the other. Each location at each site was measured twice, once at midday and once either in the evening or in the morning. Each measurement consisted of four chambers placed together on the surface and samples were collected a t the same time by using the sampling parameters. Environmental conditions varied from liquid temperatures of 15 "C and ambient temperatures of 6 "C at site one to a liquid surface temperature of 49 "C and an ambient temperature of 20 "C a t site two. Detailed description of each site and environmental conditions are reported elsewhere (15). The four chambers were placed on the liquid surface a t each location forming a square with 1.5 m on each side. The position was selected to avoid including nonhomogeneous surfaces such as oil layers and foam. Duplicate canister samples were collected from one chamber at each location and each of the duplicate samples was analyzed in duplicate. From the experimental design the individual variance due to sample collection and analysis can be determined by the NESTED procedure available with the Statistical Analytical System (SAS) software package. The overall precision was determined by calculating the sample standard deviation for the four replicate chamber measurements. 522
Environ. Sci. Technol., Vol. 25, No. 3, 1991
2-butanone l,l,l-trichloroethane toluene total organic content
range emissn av bias, no. of rate, mg precn, % replicates min-I m-2 % RSD" ( f90% CLb) 9 9
11.0-41.0 45.7-94.0
6.7 11
-68 f 3.1 -21 f 6.0
9 9
5.2-12.3 65.6-147
13 8.6
-38 f 4.4 -41 f 3.3
RSD is the pooled relative standard deviation. confidence limit.
bCL is the
Results and Discussion Laboratory Precision and Accuracy Study. A series of duplicate chamber measurements were made under both similar conditions and different conditions in the SIS. Table I lists the average emission rates found for groups of duplicate measurements, the pooled relative standard deviation, and the averaged percent bias for 21 sets of measurements grouped into 5 similar sets with only l,l,l-trichloroethane in the SIS. For the first nine sets of measurements made with a sweep flow rate of 5 L/min, high emission rates (between 7 and 15 mg min-' m-2) were found due to high temperatures and liquid-phase concentration of l,l,l-trichloroethane. Low emission rates (less than 0.6 mg min-' m-2) were found for the next three sets a t a sweep flow rate of 5 L/min as temperature and concentration decreased. Three additional measurement sets were made with a sweep flow rate of 2 and 10 L/min each. The last three measurement sets were made in the dark. These results indicate that the method's precision was very good under all conditions tested. There was a slight increase in the RSD for the 2 L/min measurement and a decrease for both the 10 L/min and the nighttime measurements. The flux chamber accuracy was not found to be as good as the precision. A consistent and negative bias was found. An increase in the bias was seen at sweep flow rates of 2 L/min and for the low emission rate. No improvement in the bias was found at the higher flow rate or at night, thus leaving no clue that would indicate a cause for the bias. Additional studies were made varying the wind velocity from 0.22 to 0.56 m/s in the SIS and monitoricg the change in solar intensity with a solarimeter (4.6-9.5 W/m2). No correlation in either wind velocity or solar intensity and bias was found for these ranges of conditions (15). To determine if multiple-component emissions affect the method's precision and accuracy, a three-component waste was generated in the SIS, and the precision and accuracy study was repeated. Table I1 gives the range of emission rates measured and accuracy estimates for the three-component study. The precision values were higher for the three-component study than for the single-component study. The accuracy varied with compound, but a consistent negative bias was found. The bias for 1,lJ-trichloroethane was significantly lower than in the singlecomponent mixture. The polar 2-butanone had the highest bias, and toluene's bias was -38%, which is substantially different from the +3070 average bias reported for toluene during a previous study (14). By addition of the emission rates measured for all three compounds, a total organic emission rate was calculated. The precision of this value was 8.670 and the average bias, -41%, was similar to that found for l,l,l-trichloroethane in the single-component study, which represents the total organic emission for that
Table 111. Emission Results from Site One Field Evaluation: Industrial Wastewater Treatment
compound dimethoxymethane chloroform toluene 2-heptanol aniline nitrobenzene
surface impoundment 02/17/87) 2:OO p.m. 4:OO p.m. emissn rate, emissn rate, mg min-I mg min-' m-2 (RSD)" m-2 (RSD)" 31.8 0.0471 0.0942 0.136 1.47 0.113
(3.0) (3.8) (4.9) (32) (37) (18)
25.3 0.0322 0.0533 0.101 1.18 0.100
(6.4) (5.4) (11) (12)
(20) (36)
open overflow channel (12/18/87) 12:30 p.m. 3:30 p.m. emissn rate, emissn rate, mg min-' mg min-' m-* (RSD)" m-2 (RSD)" 14.5 0.00219 0.00264 0.0458 3.59 0.132
(2.7) (49) (14) (0.3) (34) (20)
14.8 0.00252 0.00247 0.0426 2.68 0.115
(3.0) (7.3) (16) (6.4) (26) (9.0)
Only three measurements were averaged.
study. The results of the laboratory study show that emission rate can be precisely measured by the chamber method with little influence from environmental factors. It appears that the emission rate measured by the method is consistently low compared with the total SIS emission. The presence of the chamber appears to suppress the emission of the surface covered. When the SIS emission rate is measured with the chambers present, the emission is lowered by the percent bias found in the chamber times the fraction of the surface covered by the chamber. Changing the wind velocity in the SIS from 0.2 to 0.5 m/s had no effect of the bias. When the motion of the liquid surface with and without the chambers was observed, a large difference was noticed. With no chambers present, a clockwise circular motion was formed on the tank surface. The chambers disrupted the circular motion into smaller eddies with a stagnant region between the chambers and inside the chambers. It is consistent with the emission theory that the reduced surface turbulence would result in reduced emission, but quantifying the chamber's influence on the surface turbulence was not possible. It is not known if the chambers' effect on surface motion in the SIS can be extrapolated to motion on a large open surface. The two chambers cover 14% of the liquid surface and occupy 6% of the wind channel in the SIS. In real environments, the flux chamber would be expected to exert a much smaller impact on the surface turbulence. One could conclude that the bias found in this study is a worst case and more accurate measurements would be made under most field conditions. Field Evaluation of Precision. The results of the field evaluation at site one, the industrial waste water treatment plant, illustrate the strengths and limitations of the chamber method. The volatile compounds encountered were mostly oxygen- and nitrogen-containing organic compounds whose reactivities were not completely compatible with the analytical methods available. Technical problems associated with simultaneously measuring emissions with four flux chambers resulted in 3 of the 16 sets not being valid. Even with these problems precision was found to be less than 1070, measured as relative standard deviation (RSD), for 11 of the 24 individual precision determinations with the highest precision measurement being 49%. Table I11 lists the average emission rates calculated and the RSDs for the six target compounds. Large RSD values (>15%) were found for the more reactive compounds 2-heptanol, aniline, and nitrobenzene and for compounds whose emission rates were below 0.025 mg min-' m-2. The results of the evaluation a t site two, waste solidification/stabilization facility, were more consistent, with only 2 of the 16 chamber measurements being invalid. Precision estimates showed little variation between com-
Table IV. Emission Rates Measured at the Primary Runoff Tank at Site Two
compound acetone 2-propanol 1-butanol toluene o-xylene methylene chloride tetrachloroethylene" total organics
av emission rate, mg min-' m-2 4:20 p.m. 2:30 p.m. 0.484 (10) 0.641 (13) 1.36 (12) 0.0175 (9.4) 0.0031 (16) 0.0267 (14) 0.00085 (18) 3.32 (12)
0.480 (12) 0.686 (9.4) 1.04 (15) 0.0174 (17) 0.0031 (17) 0.0263 (16) 0.00082 (19) 3.22 (12)
" ECD was used to quantify this compound. Table V. Emission Rates Measured at the Nonhazardous Storage Tank at Site Two
compound benzene toluene m,p-xylene o-xylene decane limonene undecane l,l,l-trichloroethane* total organics
av emission rate, me min-' m-2 (RSD) 12:30 p.m.= 3:20p.m? 0.141 (5.5) 0.320 (7.9) 2.28 (11) 1.28 (11) 5.38 (14) 6.62 (15) 3.98 (16) 0.144 (2.8) 66.5 (14)
0.155 (5.2) 0.337 (8.9) 2.50 (1.2) 1.39 (7.6) 5.97 (5.3) 7.31 (5.1) 4.40 (5.2) 0.123 (8.2) 74.1 (5.4)
" Rates caculated by omitting one chamber suspected to be invalid. bECD was used to quantify this compound. pounds or over the range of emission rates measured. Tables IV and V list the average emission rates and RSDs for the two locations where emission measurements were made. RSDs varied from 9.4 to 19% for the runoff tank measurements. The nonhazardous tank RSDs varied from 1.2 to 16%. The lower RSDs found for the nonhazardous tank may be due to the higher emissions measured. The total organic emission rate for the nonhazardous tank averaged 70 mg min-l m-2 while the runoff tank total organic emission rates averaged 3.3 mg min-I m-2. Variation analysis for nested duplicate samples collected a t both sites one and two was performed using the NESTED software program available with the Statistical Analysis System software package. For all cases, the majority of the variation was attributable to the sample collection and storage. This was especially true for the reactive compounds. Table VI lists the results for the variation analysis for the runoff tank a t site two. The amount of variation due to the chambers alone can be calculated by subtracting the sum of the collection and analysis variance from the total variance. The chamber variance was found to vary Environ. Sci. Technol., Vol. 25, No. 3, 1991 523
Table VI. Results of Variation Analysis for the Primary Runoff Tank Emission Measurements at Site Two compound
total
acetone 1-propanol 1-butanol toluene o-xylene methylene chloride" tetrachloroethylene" total organics
13.1 14.1 14.7 15.1 18.0 15.5 19.0 14.7
" ECD results used
coeff of variatn, % sampling analysis 6.72 7.92 7.03 9.68 14.5 6.02 6.01 7.3
0.32 2.24 0.74 0.17 5.87 0.34 0.08 0.05
in analysis.
from 5 to 1370, which agrees with the variance found during the laboratory evaluation. Variance due to the instrumental analysis was generally less than 5% of the total variance. Conclusions and Recommendations The results of the laboratory and field evaluation of the chamber method indicate that liquid surface emission measurements can be made with good precision and that operational and environmental parameters have only a minor effect on the precision and accuracy of the method. A consistent compound-dependent negative bias ranging from 40 to 80% was found during the laboratory studies. It is believed that this bias is a worst case bias and that real measurements would be likely to be closer to the true emission rate. From the study, the following conclusions can be made. Precision of the 3% RSD can be obtained with the method under ideal conditions of a single compound, steady solar conditions, and emission rates above 0.5 mg min-' m-2. Sweep flow rates above 2 L/min should be used to prevent concentration buildup in the chamber adversely affecting both precision and accuracy. Sample storage, and collection variability, and technical problems such as monitoring proper depth of the chamber, limit precision in the field to 35% RSD for worst case scenarios, and less than 20% for most cases. Negative bias in the measurements is due to the chamber's influence on the surface turbulence in the SIS and may be less significant on a larger open surface. Several recommendations can be made regarding the use of the chamber method and future areas of research that are needed: The chamber method is a reliable method for measuring emission rates from liquid surfaces to evaluate control technology and to assess the relative potential of emission from different sources. Use of the method for generating data for emission inventories should be qualified until accuracy in the field is determined. Effects of the chamber on surface turbulence in the field should be evaluated to determine if laboratory bias found
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can be extrapolated to field studies. Further method development is needed to improve the measurement method especially for reactive volatile organic compounds. Further investigation of using chamber techniques for measuring emission from other sources such as landfills, contaminated soils, building materials, and protection barriers is warranted. Registry No. 2-Butanone, 78-93-3; l,l,l-trichloroethane, 7155-6; toluene, 108-88-3;dimethoxymethanol, 109-87-5;chloroform, 67-66-3; 2-heptanol, 543-49-7;aniline, 62-53-3; nitrobenzene, 9895-3; acetone, 67-64-1; 2-propanol, 67-63-0; 1-butanol, 71-36-3; o-xylene, 95-47-6;methylene chloride, 75-09-2; tetrachloroethylene, 127-18-4;benzene, 71-43-2; m-xylene, 108-38-3;p-xylene, 106-42-3; decane, 124-18-5; limonene, 138-86-3; undecane, 1120-21-4; 1propanol, 71-23-8.
Literature Cited (1) Zimmerman, P. In Proceedings of 1977 Environmental Protection Agency Emission Inventory/Factor Workshops, Raleigh, NC, 1977. (2) Adams, D. F.; Pack, M. R.; Bamesberger, W. L.; Sherrard, A. E. In Proceedings of 71st Annual Air Pollution Control Association Meeting, Houston, TX, 1978. (3) Hill, F. B.; Aneja, V. P.; Felder, R. M. J . Enuiron. Sci. Health 1978, A13, 199-225. (4) Matthias, A. D.; Blackman, A. M.; Brenner, J. M. J . E n uiron. Qual. 1980, 9, 251-256. (5) Johnson, C.; Richter, A.; Backlin, L.; Granat, L. A System for Measuring Fluxes of Trace Gases to and from Soil and Vegetation with a Chamber Technique. Report 1983-09-01; International Meteorological Institute in Stockholm, Anhenius Laboratory, 1983. (6) Jury, W. A.; Letey, J.; Collins, T. Soil Sci. SOC.Am. J . 1982, 46, 250-256. (7) Dupont, R. R. J. Air. Pollut. Control Assoc. 1985, 37, 168-176. (8) Eklund, B. M.; Balfour, W. D.; Schmidt, C. E. In Proceedings of the American Institute of Chemical Engineers National Meeting, Philadelphia, PA, 1984. (9) Liss, P. S.; Slater, P. G. Nature 1974, 247, 181-184. (10) Mackay, D.; Leinonen, P. J. Enuiron. Sci. Technol. 1978, 12, 553-558. (11) Mackay, D.; Yuen, A. T. K. Enuiron. Sci. Technol. 1983, 17, 211-217. (12) Berrafato, L. R. Air Emissions of Volatile Organics from a Simulated Hazardous Liquid Waste Lagoon. Ph.D. Thesis, University of Illinois, 1985. (13) Cohen, Y.; Cocchio, W.; Mackay, D. Enuiron. Sci. Technol. 1975, 9, 1178-1180. (14) Kienbush, M. R.; Ranum, D. Development and Validation of the Flux Chamber Method for Measuring Air Emission from Surface Impoundments. Radian Corp., Report for US. Environmental Protection Agency, Contract 68-02-3889, 1986. (15) Gholson, A. R.; Albritton, J. R.; Jayanty, R. K. M.; Knoll, J. E. Evaluation of the Flux Chamber Method for Measuring Volatile Organic Emissions from Surface Impoundments. Final Report for U.S.Environmental Protection Agency Contract 68-02-4550, 1988. ~~
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Receiued for review April 17, 1990. Revised manuscript received October 1, 1990. Accepted October 22, 2990.
