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A method for determining in-use efficiency of stage II vapor recovery systems
EnvironmentInternational,Vol.20, No. 2, pp. 201-207,1994 Copyright01994 ElsevierScienceLid Printedin theUSA.Allrightsreserved 0160-4120/94$6.00 +.00
Pergamon
A M E T H O D FOR D E T E R M I N I N G I N - U S E EFFICIENCY OF STAGE II VAPOR RECOVERY SYSTEMS David L. Macintosh, Dee A. Hull, Howard S. Brightman, Yukio Yanagisawa, and P. Barry Ryan Environmental Science and Engineering Program, Department of Environmental Health, Harvard University School of Public Health, Boston, MA 02115 U.S.A.
E1 9305-139 M (Received 26 May 1993; accepted 10 December 1993)
A practical methodology has been developed to evaluate the in-use efficiency of Stage II vapor recovery systems. While the theoretical efficiency of these vapor recovery systems may be measured in chamber studies, their true effectiveness can only be assessed in the field under normal operating conditions. VOC concentrations during automobile refueling were measured at a gasoline station with conventional pumps and a station equipped with a Stage II vapor recovery system. A dual VOC sampling train was developed in order to minimize measurement variability caused by environmental conditions. The results showed a significant difference between VOC emissions during refueling at the Stage II and conventional gasoline stations investigated. The estimated efficiency per refueling of the vapor recovery system at the Stage II station, relative to the conventional station, was between 81% and 93 %. Although gasoline spills during refueling have been documented as greater at typical Stage II stations as compared to conventional stations, the presented results indicate the magnitude of these spills is insufficient to substantially undermine the overall efficacy of these vapor recovery systems.
INTRODUCTION In the past decade, efforts at controlling hydrocarbon emissions associated with automobile refueling have been made in the United States and other countries in the form of Stage I and Stage II vapor recovery systems. Stage I refers to systems that collect gasoline vapors emitted when underground storage tanks, primarily those at gasoline retailers, are refilled from tanker trucks. Stage II refers to pump-based systems that collect gasoline vapors emitted during refueling of automobiles. Gasoline vapors contain hydro-
Mailing address: Harvard University School of Public Health, Department of Environmental Health, 665 Huntington Ave., Bldg. 1, R. 1310, Boston, MA 02115.
carbons that are precursors to formation of tropospheric ozone, a strong oxidant and human respiratory system irritant (Lippmann 1989). Ozone levels commonly exceed the World Health Organization air quality guideline (150-200 ~tg/m3., 1-h average) in major cities throughout the world including Beijing, Cairo, London, Mexico City, Sao Paulo, and Los Angeles (WHO and UNEP 1992). The effectiveness of a Stage II gasoline vapor recovery system in actual use can be examined using a newly developed sampling methodology. While the theoretical efficiency of vapor recovery systems can be accurately measured in large, closed-system conditions, such tests may not reflect the true in-use efficiency of these pollution control devices. The term in-use efficiency refers to the percent reduction in volatile organic compound (VOC) emissions per 201
202
refueling, achieved with Stage II systems relative to conventional systems, when operated by the general public under actual refueling conditions. The sampiing equipment and m e t h o d o l o g y e m p l o y e d in this study was inexpensive, reproducible, and able to readily distinguish between volatile organic compound (VOC) emission levels at two gasoline stations. Stage II vapor recovery systems have been reported as being 85-95% efficient at collecting refueling vapors (Austin and Rubenstein 1985). The EPA estimates that the actual in-use efficiency could be as low as 56% (EPA 1984), although a nominal effectiveness of 86% is used in the EPA estimates of VOC emission reductions that can be achieved by Stage II systems. The results of these studies are not available in the peer-reviewed literature; thus, the extent to which these measurements were performed under laboratory or field conditions, the degree of experimenter control, and the state of other potentially important experimental conditions is unclear. In this study, gasoline vapor samples were collected with minimal intervention in order to estimate the in-use efficiency of the Stage II vapor recovery system, that is, the efficiency when operated by lay persons in a normal situation rather than under highly controlled conditions. Although the generalizability of the presented results is limited due to the small number of stations investigated, they are at least a first approximation of the in-use efficiency of this particular pollution control method. These results should also be useful for formulating the design of any future, more comprehensive studies. MATERIALS AND METHODS
The gasoline stations chosen for this study were selected based on their relative proximity and the willingness of the respective owners to participate in the investigation. The Stage II station consisted of six self-service gasoline pumps outfitted with vapor balance Stage II vapor recovery nozzles. The conventional station consisted of three full-service pumps without vapor recovery nozzles or collars. The respective stations are located approximately 0.5 km from each other on Massachusetts Avenue in Arlington, Massachusetts. A preliminary visit was made to the Stage II station on 27 February 1992 in order to obtain information to aid in formulation of the sampling methodology and study design. The concentrations of VOCs emitted during refueling were measured by an HNU photoionization direct reading instrument (Model GP101) held between 2cm-10 cm from the filling nozzle. The survey indicated that VOC concentrations around the filling
D.L. Macintosh et al.
nozzle were not uniform and, in fact, the location of the VOC vapor plume was quite variable. Based on this information, a dual sampling device was constructed, consisting of two charcoal tube holders 13 cm apart, in order to minimize variability in our formal sampling results due to plume movement (Fig. 1). The sampler was held approximately 3 cm over the filling nozzle during each refueling. Held in this position, the two sampling tubes straddled the filling nozzle and gasoline tank aperture. The average concentration measured by the respective pairs was used in the quantitation of VOC emissions. The VOCs were collected on SKC Lot 120 charcoal tubes comprised of two separate sections, a front and back containing 100 g and 50 g of charcoal, respectively. Air was passed through the tubes at approximately 100 mL/min by battery-operated lowflow diaphragm pumps (MDA Accuhaler 808). The pumps were calibrated with a soap bubble meter in the laboratory prior to field use. Eight samples were collected at the conventional station over a 6-h period on 14 March 1992. Also, four samples were collected at the Stage II station over a 6-h period on 7 March 1992 and 9 March 1992. The timing of the study should not affect the relative effectiveness of the Stage II controls, even though fuel mixtures are known to vary with time of year. Two days were required at the Stage II station because of the lengthy sampling period required to yield quantifiable amounts of VOCs estimated from the results of the preliminary investigation. Three charcoal tubes were used per sample: two on the dual sampling device and the third held approximately 7 m from the gasoline filling point. The third tube was used to determine concurrent background VOC concentrations in order to account for any significant background fluctuations among samples. Each charcoal tube was connected to its own pump, which was operated during actual refueling periods only. The dual sampling tubes and background tube were maintained in the same location in between all refueling periods. Samples were collected over an approximately 150 L (40 gallon) filling period (4 - 6 cars) at the Stage II station and a 75 L (20 gallon) filling period (2 - 4 cars) at the conventional station in order to ensure collection of a measurable quantity of VOCs. In addition, one field blank was prepared on each sampling day. Details of the environmental conditions during the VOC sampling periods were recorded such as volume of gasoline dispensed per refueling, approximate wind speed (<4.5 m/s or >4.5 m/s), ambient outdoor temperature, and model year of each automobile being refueled.
Efficiency of Stage II vapor recovery systems
203
Charcoal ~
Charcoal
Tube 1
Tube 2 I
~
'
~
/
Gasoline Spout
Aperaulre 4
(Pumps Attached to Investigator's Belt) Fig. I. Schematic of the dual-sampling train developed to minimize variability in measurements of VOC refueling emissions.
Also, notes were made of any gasoline spillage during refueling to evaluate the overall efficiency per refueling of the Stage II system. The VOCs adsorbed onto the charcoal of the tubes were extracted with 1 mL of carbon disulfide (CS2). The front and back sections of each tube were analyzed separately in order to identify any sample breakthrough. The extracts were injected in 20 ~tL aliquots into a Shizmadzu GC-8A gas chromatograph (GC) equipped with a stainless steel column (length = 3.6 m, inner diameter = 0.32 cm, mesh size -- 90/100, packed with 10% Carbowax 20M). The analytic procedure used was a slightly modified version of NIOSH Method 1500 for hydrocarbons with boiling points between 36°C-126°C (NIOSH 1984). The column plateau temperature was raised to 220°C from the NIOSH-stipulated value of 150°C to account for the presence of gasoline hydrocarbon constituents with boiling points greater than 126°C (HEI 1988). Total VOCs in each tube were determined from the area under the entire chromatograph, as calculated by a Hewlett-Packard 3396A integrator and expressed as toluene, C6H5CH 3. The GC response was calibrated with standard solutions of toluene and carbon disulfide. Laboratory quality control was maintained by analyzing one laboratory blank per day and reinjecting samples at random intervals. Amounts of VOC emissions per gallon of gasoline dispensed (Mavg) were calculated from:
(C 1 V 1 ÷ C2 V2)
Mavg =
(1)
2 ELiterSaut °
where, Ci = the VOC concentration measured by each tube of a pair Vi = the volume of air and vapor passed through each tube Liters,,to = the number of liters of gasoline dispensed to each automobile comprising a given sample. In this manner, the VOC mass collected by the tubes comprising each pair was adjusted to account for any difference in air volumes sampled by the respective tubes. A bulk sample of 88-octane gasoline was collected from the conventionally-equipped station in a polypropylene container for laboratory analysis. The VOC concentration in the headspace of the sample container was measured by collecting 50 ~L of vapor at 25°C in a microsyringe, followed by direct injection into the GC. The results of this analysis were used as a qualitative check of the validity of the refueling vapor chromatographs and as a way to estimate VOC vapor concentrations in the fuel tank headspace of the automobiles included in this study.
204
D.L. Macintosh et al.
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Sampling Location Fig. 2. VOC concentrations measured during automobile refueling at the Stage/I and conventional station. The data from the Stage H station on 7 March 1992 and 9 March 1992 are shown on the left. Box plots of the Stage II and conventional station data are shown on the right.
RESULTS VOC concentrations at the Stage II gasoline station were significantly lower than those at the conventional station (Fig. 2). The mean concentrations from the Stage II and conventional stations were 4.5 g/m 3 (std. error = 0.95 g/m 3) and 41.6 g/m 3 (std. error = 4.46 g/m3). The means were determined to be significantly different by both the parametric Student's t-test (p<0.0002) and the nonparametric Wilcoxon Rank Sum test (p<0.0009). A nonparametric test of the difference between the mean concentrations measured from the two sampling periods at the Stage II station was also performed (7 March 1992 sample mean = 2.64 g/m3; 9 March 1992 sample mean = 6.36 g/m3). The difference was greater than zero at the 97% level of confidence. VOC concentrations measured by the two tubes comprising each of the 16 pairs were within a factor of two of each other (mean = 1.97, std. error = 0,08). For three pairs, the concentrations measured by each
tube were substantially different; tubes 24 and 25 by a factor of 5.0, tubes 37 and 38 by a factor of 3.2, and tubes 49 and 50 by a factor of 5.1. Wind speeds approached 9 m/s during the periods when those three pairs were used (9 March 1992 and 14 March 1992). On average, the ratio in VOC concentrations between the pairs at the Stage II and conventional stations were 1.9 (std. error = 1.1) and 2.2 (std. error = 1.05), respectively. The analytical limit of detection (LOD) and limit of quantitation (LOQ) were determined from variability in the GC response to the CS 2 used to desorb the back sections of the charcoal tubes. The standard deviation of 40 injections was 47 tolueneequivalent nanograms (ng). The LOD was defined as three times this number, or 141 ng, and the LOQ as ten times the standard deviation, or 470 ng, for each tube. All background VOC measurements were approximately equal to or below the LOQ and all refueling point samples were at least 10 times greater than
Efficiency of Stage II vapor recovery systems
205
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S a m p l i n g Location Fig. 3. VOC emissions per L of gasoline dispensed at the Stage II and conventional station. The data from the Stage II station on 7 March 1992 and 9 March 1992 are shown on the left. Box plots of the Stage II and conventional station data are shown on the right.
the LOQ. Since the background VOC masses were small in magnitude, they had a negligible effect on our concentration calculation. All laboratory and field blanks were below the LOD. The headspace VOC concentration from the bulk gasoline sample was determined to be 1.5 g/L at 25°C which compares favorably to the EPA emission factor of 1.39 g/L for refueling in Massachusetts in March (EPA 1991). VOC e m i s s i o n s per gallon d i s p e n s e d at the Stage II gasoline station were significantly lower than those at the conventional station as shown in Fig. 3. The mean Mavg from the Stage II and conventional stations were 20 g/L and 160 g/L. The means were determined to be significantly different by both the Student's t-test (p<0.0001) and the Wilcoxon Rank Sum test (p<0.0009). A nonparametric test of the difference between the mean VOC emissions per liter dispensed for the two sampling periods at the Stage II station was also performed (7 March 1992 sample mean = 11 g/gal, std. error = 3 g/gal; 9 March
1992 sample mean = 29 g/gal, std. error = 5 g/gal). The difference was greater than zero at the 96% level of confidence. The efficiency of the Stage II vapor recovery system per refueling was defined as the ratio of the VOC emissions per gallon dispensed at the Stage II station to that at the conventional station. Using the sample mean of the two Stage Ii sampling days and the sample mean of the single conventional station sampling day, the efficiency of the Stage II vapor recovery system was estimated to range between 81% and 93%. A more comprehensive definition of efficiency that includes fuel spillage will be discussed in the following section. DISCUSSION
The data collected from this study indicate that the vapor recovery system at the Stage II station is between 81% and 93 % efficient per refueling relative to the conventional station. These results compare
206
favorably to those of the California Air Resources Board which estimated an efficiency range between 80 and 92% based upon a series of state-wide inspections conducted in 1982 and 1983, and those of the EPA which are estimated as 86% and 92% under annual and semi-annual inspection scenarios, respectively (EPA 1991). The difference in VOC emissions per gallon of gasoline dispensed between the first and second sampiing days at the Stage II station can be attributed to environmental conditions. The wind speed was below 4.5 m/s on the first Stage II sampling day and greater than 4.5 m/s on the second. The wind direction was also more variable on day two than day one. Although statistically significant, the difference between the means from the two Stage II sampling periods was not substantial enough to affect the results as indicated by the highly significant difference between the means of the Stage II and conventional station data. The utility of using the average concentration between the paired collection tubes was investigated by comparing their respective results across samples, as given above. The general results indicate that refueling emissions from the two stations were equally influenced by environmental conditions and that the sampling method did not introduce significant bias. The outlier sample pairs were each collected on days when the wind speed was relatively high, suggesting that wind speed or direction does have the potential to bias samples taken from a single location. It is possible that a steady wind, in a single direction, with a clear path to the refueling point could direct the vapor plume toward one collection tube rather than the other and thus cause the discrepancy seen in each of these pairs. Similar observations have been reported by others (Mueller 1989). Quantifiable amounts of VOCs were found in the back sections of the paired sample tubes. This result generally would suggest sample breakthrough, although the complete absence of any higher-molecularweight compounds in the backs indicates equilibrium partitioning of the highly volatile low-molecularweight compounds during storage at room temperature. This conclusion was verified upon noting that the mass of lower-molecular-weight compounds in the two sections of the tube differed by a factor of 2, in direct correspondence with the mass of charcoal in each section. The overall efficiency of the Stage II vapor recovery system should be examined by taking account of gasoline spillage rates during refueling. Mueller (1989) measured gasoline spills over 2635 separate automobile refuelings. He found that spills occurred
D.L. Macintosh et al.
at 34% of the refuelings. The average gasoline spill rate was 0.87 g/L at Stage II stations and 0.53 g/L at conventional stations. Assuming that all spilled gasoline vaporizes to the atmosphere, it was considered whether the greater spill rate at Stage II stations substantially undermines the overall effectiveness of the vapor recovery system at reducing refueling emissions. To answer this question, the mass of VOCs emitted directly from refueling must be compared to that indirectly released from spills. The volume of vapor emitted during refueling was assumed to be equal to the volume of gasoline dispensed into the tank. This assumption was supported by the similar profile of the concentration and emissions per liter data which indicates that the VOCs measured were associated with the refueling process. The concentration of gasoline vapor in the headspace of the automobile tanks was approximated by the headspace concentration in the bulk gasoline sample (1.5 g/L) that was measured in the laboratory. Therefore, the mass of VOCs displaced from the tank at a conventional station and ambient temperature of approximately 25°C was estimated to be 1.5 g/L. The corresponding VOC release at a Stage II station based on our estimate of the average vapor collection efficiency was calculated as 0.2I g/L, 1.5 x (1-0.86). The overall efficiency (i.e., including spillage) of the Stage II system as determined from the above data was estimated to be 77% to 89%. The estimates of overall efficiency indicate that the larger spill rate at Stage II over conventional stations is insubstantial relative to the VOCs displaced directly from the tank during refueling. CONCLUSION
A practical methodology has been developed to evaluate the in-use efficiency of Stage II vapor recovery systems. The results of this study showed a significant difference between VOC emissions during refueling at the Stage II and conventional gasoline stations investigated. The estimated efficiency per refueling of the vapor recovery system at the Stage II station relative to the conventional station was between 77 and 89%. Although gasoline spills during refueling have been documented as greater at typical Stage II stations as compared to conventional stations, our results indicate the magnitude of these spills is insufficient to substantially undermine the overall efficacy of these vapor recovery systems.
Efficiency of Stage II vapor recovery systems
207
REFERENCES Austin, T.C.; Rubenstein, G.S. A comparison of refueling emissions control with onboard and Stage II systems. Paper 851204, Society of Automotive Engineers. Warrendale, PA: Soc. of Automotive Engineers; 1985 EPA (Environmental Protection Agency). Evaluation of air pollution regulatory strategies for gasoline marketing industry. EPA450/3-84-012b. Washington, D.C.: EPA; 1984. EPA (Environmental Protection Agency). Technical Guidance Stage II vapor recovery systems for control of vehicle refueling emissions during gasoline dispensing facilities, Volume 1. EPA 450/3-91-022a, Wasington, D.C.: EPA; 1991. HEI (Health Effects Institute). Gasoline vapor exposure and human cancer: evaluation of existing information and recom-
mendations for future research. Cambridge, MA: Health Effects Institute; 1988. Lippmann, M. Health effects and ozone; a critical review. J. Air Waste Manage. Assoc. 39: 672-691. 1989. Mueller, E.A. A survey and analysis of liquid gasoline released to the environment during vehicle refueling at service stations. API Publication No. 4498. Washington, D.C.: American Petroleum Institute; 1989. NIOSH (National Institute for Occupational Safety and Health). Analytic Methods. Cincinnati, OH; 1984:1500.1-1500.7 WHO and UNEP (World Health Organization and United Nations Environment Programme). Urban air pollution in megacities of the world. Oxford, UK: Blackwell Publishers; 1992.
Pergamon
A M E T H O D FOR D E T E R M I N I N G I N - U S E EFFICIENCY OF STAGE II VAPOR RECOVERY SYSTEMS David L. Macintosh, Dee A. Hull, Howard S. Brightman, Yukio Yanagisawa, and P. Barry Ryan Environmental Science and Engineering Program, Department of Environmental Health, Harvard University School of Public Health, Boston, MA 02115 U.S.A.
E1 9305-139 M (Received 26 May 1993; accepted 10 December 1993)
A practical methodology has been developed to evaluate the in-use efficiency of Stage II vapor recovery systems. While the theoretical efficiency of these vapor recovery systems may be measured in chamber studies, their true effectiveness can only be assessed in the field under normal operating conditions. VOC concentrations during automobile refueling were measured at a gasoline station with conventional pumps and a station equipped with a Stage II vapor recovery system. A dual VOC sampling train was developed in order to minimize measurement variability caused by environmental conditions. The results showed a significant difference between VOC emissions during refueling at the Stage II and conventional gasoline stations investigated. The estimated efficiency per refueling of the vapor recovery system at the Stage II station, relative to the conventional station, was between 81% and 93 %. Although gasoline spills during refueling have been documented as greater at typical Stage II stations as compared to conventional stations, the presented results indicate the magnitude of these spills is insufficient to substantially undermine the overall efficacy of these vapor recovery systems.
INTRODUCTION In the past decade, efforts at controlling hydrocarbon emissions associated with automobile refueling have been made in the United States and other countries in the form of Stage I and Stage II vapor recovery systems. Stage I refers to systems that collect gasoline vapors emitted when underground storage tanks, primarily those at gasoline retailers, are refilled from tanker trucks. Stage II refers to pump-based systems that collect gasoline vapors emitted during refueling of automobiles. Gasoline vapors contain hydro-
Mailing address: Harvard University School of Public Health, Department of Environmental Health, 665 Huntington Ave., Bldg. 1, R. 1310, Boston, MA 02115.
carbons that are precursors to formation of tropospheric ozone, a strong oxidant and human respiratory system irritant (Lippmann 1989). Ozone levels commonly exceed the World Health Organization air quality guideline (150-200 ~tg/m3., 1-h average) in major cities throughout the world including Beijing, Cairo, London, Mexico City, Sao Paulo, and Los Angeles (WHO and UNEP 1992). The effectiveness of a Stage II gasoline vapor recovery system in actual use can be examined using a newly developed sampling methodology. While the theoretical efficiency of vapor recovery systems can be accurately measured in large, closed-system conditions, such tests may not reflect the true in-use efficiency of these pollution control devices. The term in-use efficiency refers to the percent reduction in volatile organic compound (VOC) emissions per 201
202
refueling, achieved with Stage II systems relative to conventional systems, when operated by the general public under actual refueling conditions. The sampiing equipment and m e t h o d o l o g y e m p l o y e d in this study was inexpensive, reproducible, and able to readily distinguish between volatile organic compound (VOC) emission levels at two gasoline stations. Stage II vapor recovery systems have been reported as being 85-95% efficient at collecting refueling vapors (Austin and Rubenstein 1985). The EPA estimates that the actual in-use efficiency could be as low as 56% (EPA 1984), although a nominal effectiveness of 86% is used in the EPA estimates of VOC emission reductions that can be achieved by Stage II systems. The results of these studies are not available in the peer-reviewed literature; thus, the extent to which these measurements were performed under laboratory or field conditions, the degree of experimenter control, and the state of other potentially important experimental conditions is unclear. In this study, gasoline vapor samples were collected with minimal intervention in order to estimate the in-use efficiency of the Stage II vapor recovery system, that is, the efficiency when operated by lay persons in a normal situation rather than under highly controlled conditions. Although the generalizability of the presented results is limited due to the small number of stations investigated, they are at least a first approximation of the in-use efficiency of this particular pollution control method. These results should also be useful for formulating the design of any future, more comprehensive studies. MATERIALS AND METHODS
The gasoline stations chosen for this study were selected based on their relative proximity and the willingness of the respective owners to participate in the investigation. The Stage II station consisted of six self-service gasoline pumps outfitted with vapor balance Stage II vapor recovery nozzles. The conventional station consisted of three full-service pumps without vapor recovery nozzles or collars. The respective stations are located approximately 0.5 km from each other on Massachusetts Avenue in Arlington, Massachusetts. A preliminary visit was made to the Stage II station on 27 February 1992 in order to obtain information to aid in formulation of the sampling methodology and study design. The concentrations of VOCs emitted during refueling were measured by an HNU photoionization direct reading instrument (Model GP101) held between 2cm-10 cm from the filling nozzle. The survey indicated that VOC concentrations around the filling
D.L. Macintosh et al.
nozzle were not uniform and, in fact, the location of the VOC vapor plume was quite variable. Based on this information, a dual sampling device was constructed, consisting of two charcoal tube holders 13 cm apart, in order to minimize variability in our formal sampling results due to plume movement (Fig. 1). The sampler was held approximately 3 cm over the filling nozzle during each refueling. Held in this position, the two sampling tubes straddled the filling nozzle and gasoline tank aperture. The average concentration measured by the respective pairs was used in the quantitation of VOC emissions. The VOCs were collected on SKC Lot 120 charcoal tubes comprised of two separate sections, a front and back containing 100 g and 50 g of charcoal, respectively. Air was passed through the tubes at approximately 100 mL/min by battery-operated lowflow diaphragm pumps (MDA Accuhaler 808). The pumps were calibrated with a soap bubble meter in the laboratory prior to field use. Eight samples were collected at the conventional station over a 6-h period on 14 March 1992. Also, four samples were collected at the Stage II station over a 6-h period on 7 March 1992 and 9 March 1992. The timing of the study should not affect the relative effectiveness of the Stage II controls, even though fuel mixtures are known to vary with time of year. Two days were required at the Stage II station because of the lengthy sampling period required to yield quantifiable amounts of VOCs estimated from the results of the preliminary investigation. Three charcoal tubes were used per sample: two on the dual sampling device and the third held approximately 7 m from the gasoline filling point. The third tube was used to determine concurrent background VOC concentrations in order to account for any significant background fluctuations among samples. Each charcoal tube was connected to its own pump, which was operated during actual refueling periods only. The dual sampling tubes and background tube were maintained in the same location in between all refueling periods. Samples were collected over an approximately 150 L (40 gallon) filling period (4 - 6 cars) at the Stage II station and a 75 L (20 gallon) filling period (2 - 4 cars) at the conventional station in order to ensure collection of a measurable quantity of VOCs. In addition, one field blank was prepared on each sampling day. Details of the environmental conditions during the VOC sampling periods were recorded such as volume of gasoline dispensed per refueling, approximate wind speed (<4.5 m/s or >4.5 m/s), ambient outdoor temperature, and model year of each automobile being refueled.
Efficiency of Stage II vapor recovery systems
203
Charcoal ~
Charcoal
Tube 1
Tube 2 I
~
'
~
/
Gasoline Spout
Aperaulre 4
(Pumps Attached to Investigator's Belt) Fig. I. Schematic of the dual-sampling train developed to minimize variability in measurements of VOC refueling emissions.
Also, notes were made of any gasoline spillage during refueling to evaluate the overall efficiency per refueling of the Stage II system. The VOCs adsorbed onto the charcoal of the tubes were extracted with 1 mL of carbon disulfide (CS2). The front and back sections of each tube were analyzed separately in order to identify any sample breakthrough. The extracts were injected in 20 ~tL aliquots into a Shizmadzu GC-8A gas chromatograph (GC) equipped with a stainless steel column (length = 3.6 m, inner diameter = 0.32 cm, mesh size -- 90/100, packed with 10% Carbowax 20M). The analytic procedure used was a slightly modified version of NIOSH Method 1500 for hydrocarbons with boiling points between 36°C-126°C (NIOSH 1984). The column plateau temperature was raised to 220°C from the NIOSH-stipulated value of 150°C to account for the presence of gasoline hydrocarbon constituents with boiling points greater than 126°C (HEI 1988). Total VOCs in each tube were determined from the area under the entire chromatograph, as calculated by a Hewlett-Packard 3396A integrator and expressed as toluene, C6H5CH 3. The GC response was calibrated with standard solutions of toluene and carbon disulfide. Laboratory quality control was maintained by analyzing one laboratory blank per day and reinjecting samples at random intervals. Amounts of VOC emissions per gallon of gasoline dispensed (Mavg) were calculated from:
(C 1 V 1 ÷ C2 V2)
Mavg =
(1)
2 ELiterSaut °
where, Ci = the VOC concentration measured by each tube of a pair Vi = the volume of air and vapor passed through each tube Liters,,to = the number of liters of gasoline dispensed to each automobile comprising a given sample. In this manner, the VOC mass collected by the tubes comprising each pair was adjusted to account for any difference in air volumes sampled by the respective tubes. A bulk sample of 88-octane gasoline was collected from the conventionally-equipped station in a polypropylene container for laboratory analysis. The VOC concentration in the headspace of the sample container was measured by collecting 50 ~L of vapor at 25°C in a microsyringe, followed by direct injection into the GC. The results of this analysis were used as a qualitative check of the validity of the refueling vapor chromatographs and as a way to estimate VOC vapor concentrations in the fuel tank headspace of the automobiles included in this study.
204
D.L. Macintosh et al.
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Stage II Station
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Sampling Location Fig. 2. VOC concentrations measured during automobile refueling at the Stage/I and conventional station. The data from the Stage H station on 7 March 1992 and 9 March 1992 are shown on the left. Box plots of the Stage II and conventional station data are shown on the right.
RESULTS VOC concentrations at the Stage II gasoline station were significantly lower than those at the conventional station (Fig. 2). The mean concentrations from the Stage II and conventional stations were 4.5 g/m 3 (std. error = 0.95 g/m 3) and 41.6 g/m 3 (std. error = 4.46 g/m3). The means were determined to be significantly different by both the parametric Student's t-test (p<0.0002) and the nonparametric Wilcoxon Rank Sum test (p<0.0009). A nonparametric test of the difference between the mean concentrations measured from the two sampling periods at the Stage II station was also performed (7 March 1992 sample mean = 2.64 g/m3; 9 March 1992 sample mean = 6.36 g/m3). The difference was greater than zero at the 97% level of confidence. VOC concentrations measured by the two tubes comprising each of the 16 pairs were within a factor of two of each other (mean = 1.97, std. error = 0,08). For three pairs, the concentrations measured by each
tube were substantially different; tubes 24 and 25 by a factor of 5.0, tubes 37 and 38 by a factor of 3.2, and tubes 49 and 50 by a factor of 5.1. Wind speeds approached 9 m/s during the periods when those three pairs were used (9 March 1992 and 14 March 1992). On average, the ratio in VOC concentrations between the pairs at the Stage II and conventional stations were 1.9 (std. error = 1.1) and 2.2 (std. error = 1.05), respectively. The analytical limit of detection (LOD) and limit of quantitation (LOQ) were determined from variability in the GC response to the CS 2 used to desorb the back sections of the charcoal tubes. The standard deviation of 40 injections was 47 tolueneequivalent nanograms (ng). The LOD was defined as three times this number, or 141 ng, and the LOQ as ten times the standard deviation, or 470 ng, for each tube. All background VOC measurements were approximately equal to or below the LOQ and all refueling point samples were at least 10 times greater than
Efficiency of Stage II vapor recovery systems
205
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S
75th Percentile
2.50
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S a m p l i n g Location Fig. 3. VOC emissions per L of gasoline dispensed at the Stage II and conventional station. The data from the Stage II station on 7 March 1992 and 9 March 1992 are shown on the left. Box plots of the Stage II and conventional station data are shown on the right.
the LOQ. Since the background VOC masses were small in magnitude, they had a negligible effect on our concentration calculation. All laboratory and field blanks were below the LOD. The headspace VOC concentration from the bulk gasoline sample was determined to be 1.5 g/L at 25°C which compares favorably to the EPA emission factor of 1.39 g/L for refueling in Massachusetts in March (EPA 1991). VOC e m i s s i o n s per gallon d i s p e n s e d at the Stage II gasoline station were significantly lower than those at the conventional station as shown in Fig. 3. The mean Mavg from the Stage II and conventional stations were 20 g/L and 160 g/L. The means were determined to be significantly different by both the Student's t-test (p<0.0001) and the Wilcoxon Rank Sum test (p<0.0009). A nonparametric test of the difference between the mean VOC emissions per liter dispensed for the two sampling periods at the Stage II station was also performed (7 March 1992 sample mean = 11 g/gal, std. error = 3 g/gal; 9 March
1992 sample mean = 29 g/gal, std. error = 5 g/gal). The difference was greater than zero at the 96% level of confidence. The efficiency of the Stage II vapor recovery system per refueling was defined as the ratio of the VOC emissions per gallon dispensed at the Stage II station to that at the conventional station. Using the sample mean of the two Stage Ii sampling days and the sample mean of the single conventional station sampling day, the efficiency of the Stage II vapor recovery system was estimated to range between 81% and 93%. A more comprehensive definition of efficiency that includes fuel spillage will be discussed in the following section. DISCUSSION
The data collected from this study indicate that the vapor recovery system at the Stage II station is between 81% and 93 % efficient per refueling relative to the conventional station. These results compare
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favorably to those of the California Air Resources Board which estimated an efficiency range between 80 and 92% based upon a series of state-wide inspections conducted in 1982 and 1983, and those of the EPA which are estimated as 86% and 92% under annual and semi-annual inspection scenarios, respectively (EPA 1991). The difference in VOC emissions per gallon of gasoline dispensed between the first and second sampiing days at the Stage II station can be attributed to environmental conditions. The wind speed was below 4.5 m/s on the first Stage II sampling day and greater than 4.5 m/s on the second. The wind direction was also more variable on day two than day one. Although statistically significant, the difference between the means from the two Stage II sampling periods was not substantial enough to affect the results as indicated by the highly significant difference between the means of the Stage II and conventional station data. The utility of using the average concentration between the paired collection tubes was investigated by comparing their respective results across samples, as given above. The general results indicate that refueling emissions from the two stations were equally influenced by environmental conditions and that the sampling method did not introduce significant bias. The outlier sample pairs were each collected on days when the wind speed was relatively high, suggesting that wind speed or direction does have the potential to bias samples taken from a single location. It is possible that a steady wind, in a single direction, with a clear path to the refueling point could direct the vapor plume toward one collection tube rather than the other and thus cause the discrepancy seen in each of these pairs. Similar observations have been reported by others (Mueller 1989). Quantifiable amounts of VOCs were found in the back sections of the paired sample tubes. This result generally would suggest sample breakthrough, although the complete absence of any higher-molecularweight compounds in the backs indicates equilibrium partitioning of the highly volatile low-molecularweight compounds during storage at room temperature. This conclusion was verified upon noting that the mass of lower-molecular-weight compounds in the two sections of the tube differed by a factor of 2, in direct correspondence with the mass of charcoal in each section. The overall efficiency of the Stage II vapor recovery system should be examined by taking account of gasoline spillage rates during refueling. Mueller (1989) measured gasoline spills over 2635 separate automobile refuelings. He found that spills occurred
D.L. Macintosh et al.
at 34% of the refuelings. The average gasoline spill rate was 0.87 g/L at Stage II stations and 0.53 g/L at conventional stations. Assuming that all spilled gasoline vaporizes to the atmosphere, it was considered whether the greater spill rate at Stage II stations substantially undermines the overall effectiveness of the vapor recovery system at reducing refueling emissions. To answer this question, the mass of VOCs emitted directly from refueling must be compared to that indirectly released from spills. The volume of vapor emitted during refueling was assumed to be equal to the volume of gasoline dispensed into the tank. This assumption was supported by the similar profile of the concentration and emissions per liter data which indicates that the VOCs measured were associated with the refueling process. The concentration of gasoline vapor in the headspace of the automobile tanks was approximated by the headspace concentration in the bulk gasoline sample (1.5 g/L) that was measured in the laboratory. Therefore, the mass of VOCs displaced from the tank at a conventional station and ambient temperature of approximately 25°C was estimated to be 1.5 g/L. The corresponding VOC release at a Stage II station based on our estimate of the average vapor collection efficiency was calculated as 0.2I g/L, 1.5 x (1-0.86). The overall efficiency (i.e., including spillage) of the Stage II system as determined from the above data was estimated to be 77% to 89%. The estimates of overall efficiency indicate that the larger spill rate at Stage II over conventional stations is insubstantial relative to the VOCs displaced directly from the tank during refueling. CONCLUSION
A practical methodology has been developed to evaluate the in-use efficiency of Stage II vapor recovery systems. The results of this study showed a significant difference between VOC emissions during refueling at the Stage II and conventional gasoline stations investigated. The estimated efficiency per refueling of the vapor recovery system at the Stage II station relative to the conventional station was between 77 and 89%. Although gasoline spills during refueling have been documented as greater at typical Stage II stations as compared to conventional stations, our results indicate the magnitude of these spills is insufficient to substantially undermine the overall efficacy of these vapor recovery systems.
Efficiency of Stage II vapor recovery systems
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REFERENCES Austin, T.C.; Rubenstein, G.S. A comparison of refueling emissions control with onboard and Stage II systems. Paper 851204, Society of Automotive Engineers. Warrendale, PA: Soc. of Automotive Engineers; 1985 EPA (Environmental Protection Agency). Evaluation of air pollution regulatory strategies for gasoline marketing industry. EPA450/3-84-012b. Washington, D.C.: EPA; 1984. EPA (Environmental Protection Agency). Technical Guidance Stage II vapor recovery systems for control of vehicle refueling emissions during gasoline dispensing facilities, Volume 1. EPA 450/3-91-022a, Wasington, D.C.: EPA; 1991. HEI (Health Effects Institute). Gasoline vapor exposure and human cancer: evaluation of existing information and recom-
mendations for future research. Cambridge, MA: Health Effects Institute; 1988. Lippmann, M. Health effects and ozone; a critical review. J. Air Waste Manage. Assoc. 39: 672-691. 1989. Mueller, E.A. A survey and analysis of liquid gasoline released to the environment during vehicle refueling at service stations. API Publication No. 4498. Washington, D.C.: American Petroleum Institute; 1989. NIOSH (National Institute for Occupational Safety and Health). Analytic Methods. Cincinnati, OH; 1984:1500.1-1500.7 WHO and UNEP (World Health Organization and United Nations Environment Programme). Urban air pollution in megacities of the world. Oxford, UK: Blackwell Publishers; 1992.