Environ. Sci. Technol. 1997, 31, 3068-3073
Comparison of Stripping Coil and Condensate Techniques for the Collection of Gas-Phase Hydrogen Peroxide, with Applications of Condensate Collection in and off the Coast of North Carolina CINDY L. DEFOREST, ROBERT J. KIEBER, AND JOAN D. WILLEY* Department of Chemistry and Marine Science Program, University of North Carolina at Wilmington, Wilmington, North Carolina 28403-3297
Hydrogen peroxide concentrations obtained by a commonly used stripping coil method were compared with data obtained by the method of collection and analysis of atmospheric condensate. Good agreement was achieved between gas-phase hydrogen peroxide concentrations obtained by each method over the concentration range from 0.1 to 1.8 ppb; the average deviation between the analytical results and the mean of those results was 10%. The deviations between concentrations obtained by each method were random, suggesting no systematic differences. Because of this analytical agreement and the versatility of the condensate collection technique, this method was employed in several field applications. Gas-phase hydrogen peroxide concentrations were determined from condensate samples collected during the summers of 1994 and 1995 in and near Wilmington, NC. Concentrations were not statistically different (t-test, p < 0.05) in samples collected at the Wilmington reference site relative to a nearby salt marsh. Gas-phase hydrogen peroxide concentrations were lower at an automobile traffic-impacted site relative to this reference site. Midday net production rates at sea over the Gulf Stream and Sargasso Sea (110 + 55 ppt/h) were one-quarter of those on land (440 + 230 ppt/h) during comparable times, in agreement with prior stripping coil results.
Introduction Hydrogen peroxide (H2O2) has received a great deal of attention in atmospheric oxidant studies because of its central role in the conversion of sulfur dioxide to sulfuric acid in cloudwater and, to a lesser extent, in rainwater (1, 2). Although its importance to atmospheric oxidation processes is well documented, procedures for the collection and analysis of hydrogen peroxide, particularly in the gas phase, remain troublesome because it is a reactive species that exists at trace levels in the atmosphere. Many of the analytical techniques for the determination of gas-phase hydrogen peroxide involve trapping the analyte in aqueous solution and/or condensing atmospheric water vapor (3-7). There * To whom correspondence should be addressed. Telephone: (910) 962-3459; Fax: (910) 962-3013; e-mail:
[email protected].
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are several possible collection and analytical problems with these techniques, including the loss of H2O2 by reaction with O3, NOx, and SO2 during collection; artificial production of H2O2 from O3; and interference from particulate matter (810). Determining the degree to which these problems are present for a particular analysis is difficult because certified gas-phase H2O2 standards do not exist The only reliable way to verify the accuracy of analytical results in the absence of certified standards is by independent method comparison studies. Earlier intercomparison studies indicate that quantitative agreement of gaseous hydrogen peroxide values determined by different methods has not yet been achieved to better than 25% difference with systematic differences often occurring between analytical methods (9, 11-14). The reasons behind these discrepancies in analytical results are not clear. One way these differences in gas-phase hydrogen peroxide results may be better understood is by further intercomparison studies employing alternate analyses. To this end, a major objective of this study was to compare gasphase hydrogen peroxide data obtained by one of the most widely used collection and analysis techniques, the stripping coil (4), with the more versatile method of collection and analysis of atmospheric condensate (3), which was not included in the earlier intercomparisons (9, 11-13). In this comparison, both the method of collection [stripping coil (4), versus condensation (3)] and analysis of hydrogen peroxide differ [4-hydroxyphenylacetic acid with fluorescence analysis (4) versus scopoletin-based fluorescence decay (15, 16)]. The collection and analysis of atmospheric condensate was chosen for this study primarily because of its simplicity. Samples can be collected over short, well-defined time periods, which is useful in monitoring the effects of localized, small-scale influences (17). Furthermore, the apparatus and supplies are inexpensive, easy to clean, simple, mobile, and do not require power for operation, enhancing the versatility of the method. Because of this versatility, the second major objective of this research was to employ the condensate collection and analysis in several field applications where more conventional gas-phase peroxide analyses would be cumbersome.
Experimental Section Study Site. Comparison studies were conducted in an outdoor courtyard of DeLoach Hall located on the campus of the University of North Carolina at Wilmington (34°14′ N, 77°53′ W, 8 km from the Atlantic Ocean). This courtyard contained decorative vegetation (i.e., red tips, dogwood, azaleas, gardenias, forsythia, and Rose of Sharon) and provided close proximity to the laboratory, which was required for rapid analysis including calibration and preparation of standards. Ozone concentrations were monitored during the summers of 1994 and 1996 at a location 15 km north of this site (Castle Hayne, NC). During the summer of 1994, the 1 h maximum varied from 26 to 106 ppb, and in the summer of 1996, it varied from 24 to 103 ppb. The air quality standard of 125 ppb for a 1 h maximum was not exceeded during these sampling times (North Carolina Department of Environment, Health and Natural Resources, Division of Air Quality). Sample Collection and Analysis. The condensate collector used in this study consisted of a 20-L (38.5 cm height × 28 cm depth) polypropylene cylindrical tank (Fisher Scientific; Fairlawn, NJ) constructed to house six individual collecting stations (Figure 1). Each station consisted of a glass test tube (30 mm i.d., 35 mm o.d., × 30 cm) filled with
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1997 American Chemical Society
FIGURE 1. (a) External view of the condensate collector. (b) Crosssectional view of the condensate collector. ice above a polypropylene funnel leading into a 60-mL highdensity polyethylene (HDPE) sampling bottle. Holes were cut in the lid of the tank in order to suspend the test tubes, thus attaining minimal collector contact and sunlight exposure. Six slits (18 cm long × 1.5 cm wide) uniformly spaced on the perimeter of the tank allowed airflow through the collector. The slits were between 14 and 32 cm above the bottom of the collector. Holes cut in a circular piece of HDPE plastic held the bottles firmly in place in the dark in the bottom of the collector. The collector was placed on the ground for sampling or on a 5-cm spacer during collections at sea to insulate the collector from the hot deck of the ship. The height of the sampler could be easily altered for other studies. Condensation was collected for 1 h; usually between 3 and 6 mL was collected from each of the six condensers. Temperature and relative humidity were recorded at the beginning and end of sample collection. A volume of condensate sufficient for hydrogen peroxide analysis could be obtained from air with relative humidity as low as 41% (17). The H2O2 analysis was completed within 20 min after the end of collection. Between three and six replicates were collected for each condensation sampling. The average relative standard deviation (RSD) was 25% among three replicates of 14 collections. The average RSD was 32% for 69 samples, each with six replicates collected over a longer time period than the intercalibration study at the reference site. Virtually all of this uncertainty comes from sampling rather than analysis and probably reflects variability in hydrogen peroxide concentrations in ground level air. Similar condensate collectors gave RSD values of 4.4% and 7.0% for five replicates for the concentrations of formic and acetic acids at the same site in 1990 versus 2.5% and 4.2% analytical precision for these two acids, respectively (17). The condensate collector used in this study was similar in concept to the system used by Farmer and Dawson (3); however, their collector used vertical flat plates made of chromium-plated copper, with cooling provided by recirculating refrigerant rather than ice, and so required power for operation. Collection times were similar in both studies, approximately 1 h. Airflow modeling was performed for the Farmer and Dawson (3) collector but not for the one used in this study. Therefore, the simple collector described in this study is probably most appropriate for site comparisons over small distances. Condensate hydrogen peroxide was analyzed by a fluorescence decay technique involving the peroxidase-mediated oxidation of the fluorophore scopoletin by H2O2 in a phosphate-buffered (0.1 M) sample at pH 7 (15, 16). Each sample was analyzed at least three times. Calibration curves were obtained by recording the decrease in fluorescence upon addition of dilutions of hydrogen peroxide stock solution to the sample. The method has an analytical precision of 2% RSD with a detection limit of 2 × 10-9 M. Condensate samples
were in the concentration range from 0.5 to 10 µM and so were easily measured by this method. Laboratory air contamination studies indicated no increase in hydrogen peroxide levels in samples of deionized water left uncapped for up to 1 h, which is longer than the time frame of analysis. The conversion of aqueous H2O2 concentrations to gas-phase concentrations was performed as discussed in earlier condensate studies and is based upon achievement of the Henry’s law solubility of gas-phase hydrogen peroxide in the aqueous condensate, which works for highly soluble gases like hydrogen peroxide (3, 18). Concentrations are expressed as parts per billion by volume (ppb) in the gas phase. An experiment conducted to assess differences caused by storage in glass versus plastic sampling bottles on ambient hydrogen peroxide levels revealed no statistical differences for collections in glass versus plastic at three different sites over the concentration range encountered of 0.4 to 12.0 µM (t-test; t ) 1.67, n ) 34; r ) 0.842, p < 0.001). This agrees with a rainwater study conducted in this same location in which nine samples collected in both glass and plastic were found to have analytically equivalent hydrogen peroxide concentrations (19). The stripping coil method (4) involves the use of a scrubbing solution and monitored airflow through a stripping coil. The stripping coil intake was approximately 2 m away from the condensate collector during this comparative study. The scrubbing solution [5 mM potassium biphthalate (KHP) in 4 mM NaOH, pH approximately 6.0] is impelled by the air as it is pulled through a horizontal coil and forms a thin film on the glass, providing a large surface for gas exchange. Air and scrubbing solution are pumped from the collection coil into a vertical separator tube. The scrubbed air and stripping solution are pumped from the bottom of the separator through a series of coils, containing the necessary reagents (EDTA, formaldehyde, and KHP in NaOH, pH 6.0), into a fluorometric cell for automated analysis. The fluorescence reagent contained 4-hydroxyphenylacetic acid (POPHA) and peroxidase in a buffered solution. Results are reported as parts per billion by volume in the gas phase (ppb). The stripping coil method exhibited a relative standard deviation of 34% among the twelve 5-min readings in 14 1 h collections, with the collection apparatus located at 1 m above ground level. Some of this variability was probably due to actual changing concentrations during the hour of collection. The precision for this collection method at higher elevations and more remote locations is approximately 10% (14), which reflects sampling and analytical precision in a more homogeneous environment. Working standards for both methods were prepared immediately prior to calibration by serial dilution of a 30% H2O2 stock solution. All water used in the preparation of standards and stock solutions was obtained from a Milli-Q water purification system and further purified by distillation in 0.1% KMnO4. Both analytical methods yield hydroperoxide concentration, which is the sum of hydrogen peroxide and highly watersoluble organic peroxides (primarily methyl hydroperoxide and hydroxymethyl hydroperoxide in air) (20, 21). Catalase was not used in either analytical method; the validity of its use to determine organic peroxides is uncertain at this time. The sampling processes would also collect both hydrogen peroxide and highly water-soluble organic peroxides. Organic peroxides vary between 20 and 80% of the total gas-phase hydroperoxides in southeastern U.S. air (22) and between 17 and 98% in the marine troposphere, with organic peroxides becoming relatively more important at high latitudes where hydrogen peroxide concentrations are low (23). Additional research is needed to assess the relative importance of hydrogen peroxide versus organic peroxides in air and atmospheric waters. Because the proportion of organic peroxides was not established in this study and because hydrogen peroxide was used as a standard in both analytical
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FIGURE 2. Comparison of hydrogen peroxide concentrations obtained by the condensate method with those obtained by the stripping coil method, both reported as ppb (by volume) in the gas phase. The line drawn has a slope of 1 with the intercept through the origin, which would be obtained in the case of perfect agreement between results. The line is not a statistical fit to the data. Open squares represent statistically equivalent concentrations; the filled squares are not equivalent (t-test, p < 0.01). methods, results are reported as hydrogen peroxide concentrations in this study.
Results and Discussion Methods Comparison. A comparison of gas-phase hydrogen peroxide concentrations in ambient air samples collected by condensation versus stripping coil techniques between 13: 00 EST and 18:00 EST on six individual days between September 12, 1995, and October 3, 1995, revealed no statistical difference between hydrogen peroxide concentrations produced by the two methods in 12 out of the 14 analyses over the concentration range encountered (0.1-3.3 ppb) (ttest; comparison of individual data pairs, coil analyses n ) 12 readings per sample, condensate n ) 3 replicates per sample, p < 0.01; Figure 2). Temperatures ranged from 25° to 32 °C, and the relative humidity was between 45% and 75%. The line in Figure 2 is not a statistical fit to the data but rather represents a line with a slope of 1 that passes through the origin (which would be obtained in the case of perfect agreement). The regression equation for the data in Figure 2 is
coil ) 1.1 × cond - 0.08 for which r ) 0.918 and p < 0.001. A 10% average deviation between the analytical results and the mean of those results was observed. This compares well to deviations observed in earlier intercomparison studies of gas-phase hydrogen peroxide employing different analytical techniques where for ambient measurements the average deviation was 30% from the mean (11). Interfering aqueous-phase reactions are one of the major concerns with all gas-phase scavenging techniques, including the condensate and stripping coil methods used here. This was demonstrated in earlier intercomparison studies where systematic differences between gas-phase hydrogen peroxide concentrations were observed, resulting in consistently low or high values between methods when data were compared. Peroxide values determined by impinger bubbling, for example, were always higher relative to those obtained by the cold trap method (9). In a second study, ambient
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FIGURE 3. Gas-phase hydrogen peroxide concentrations (ppb) determined from condensation collection at a salt marsh site plotted versus concentrations determined at the reference location on the same days at 10:00 EST (open squares) and 14:00 EST (filled squares). The line has a slope of 1 with the intercept through the origin and is not a statistical fit to the data. hydrogen peroxide values determined with the stripping coil were 25% higher than mean values while diffusion scrubber concentrations were generally 25% lower (11). The randomness of the data presented in Figure 2 around the line of perfect agreement between results suggests that no large systematic interference or aqueous-phase reactions were present routinely in one analysis relative to the other. Some scatter in ground-level gas-phase hydrogen peroxide measurements is to be expected because of the reactivity of this gas and the variability of the surrounding atmosphere. Applications of Condensation Collection and Analysis. The second major objective of this research was to demonstrate the versatility of the condensate analysis in several field applications where more conventional techniques would be more difficult, costly, and/or cumbersome to use. The condensate collection apparatus is relatively inexpensive, easy to clean, simple, mobile, and does not require power for operation. These features facilitate field applications, such as simultaneous site comparison studies and shipboard analyses. Forty-one condensate samples were collected and analyzed between May and August 1994, and 106 samples were collected between May and September 1995, in Wilmington, NC. Thirty-two condensate samples were collected from the marine boundary layer offshore in May of 1994, November 1994, May 1995, and September 1995. Description of several applications follows. Site Comparisons. Because hydrogen peroxide can form and react quickly (hours), gas-phase concentrations may vary over small geographical regions. Single-site monitoring may therefore miss significant variations in concentration. Condensate sampling lends itself well to comparison of concentration variations among several sites. A site comparison study was conducted using simultaneous condensate collections on the UNC Wilmington campus and at a saltwater marsh located approximately 8 km to the east. The UNC Wilmington site is an open area of centipede grass, crab grass, and wire grass within a long leaf pine and turkey oak forest. This site is called the reference site because it is the site to which others are compared in this study. The marsh site was chosen to determine if conditions present in the salt marsh environment, i.e., increased water
FIGURE 4. Gas-phase hydrogen peroxide concentrations (ppb) determined from condensation collection every 3 h on July 18-19, 1995, at the reference and traffic sites. Error bars indicate the standard deviation based on between three and six replicates. The concentration means of the reference site samples are higher than at the traffic site for each time except the last one (t-test, p < 0.01).
FIGURE 5. Average hydrogen peroxide concentrations (ppb) determined on land (open bars) and at sea (filled bars). Samples are grouped by time of collection (EST) as follows: morning (4:00-11:00), mid-afternoon (11:00-17:00), and evening (17:00-22:00). n represents the number of collections performed during each time interval; error bars are standard deviations based upon n samples. vapor, increased potential for H2O2(g) deposition to the water surface, and possible reducing conditions due to production of H2S (24), affect gas-phase peroxide concentrations relative to a nearby location that is not under those same influences. Separate, identical, condensate collectors were set up at each location, and gas-phase concentrations of hydrogen peroxide were determined over several days, with collections beginning at 10:00 EST and at 14:00 EST at each location. The mean gas-phase H2O2 value found at the marsh site was statistically equivalent to the mean concentration observed at the reference site (t-test, t ) 0.739, n ) 18; Figure 3). The line drawn has a slope of 1 with the intercept at the origin and is not a statistical fit to the data. The scatter of the data around the line was again random, indicating no systematic trend of high or low values at one site relative to the other.
A similar site comparison of gas-phase peroxide concentrations over small geographical scales was conducted between the reference site and a nearby site heavily impacted by automotive traffic (estimated 1550 automobiles per hour). Separate identical condensate collectors were set up at each location, and gas-phase concentrations of hydrogen peroxide were determined every 3 h on July 18-19, 1995 (Figure 4). In contrast to the marsh comparison study, there was a systematic difference in the concentration of hydrogen peroxide concentrations at one site relative to the other with lower values in each case at the traffic-impacted site (except the final 6:00 reading, based upon data pair comparison with three replicates at each site; t-test, p < 0.01). Concentrations of nitrite, sulfite, and sulfate (but not nitrate) were higher in the condensate collected at the traffic site as compared with
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TABLE 1. Average Concentrations (µM) of Sulfate, Sulfite, Nitrate, and Nitrite at the Traffic and Reference Sites during Diurnal Cycle Conducted between July 18 and July 19, 1995a anion (µM) 2-]
[SO4 [SO32-] [NO3-] [NO2-]
reference
traffic
4.5 ( 1.1
