Environ. Sci. Technol. 1994, 28, 1180-1 185
Improved Nonenzymatic Method for the Determination of Gas-Phase Peroxides J. H. Lee' and I . N. Tang
Environmental Chemistry Division, Brookhaven National Laboratory, Upton, New York 1 1973 J. B. Weinstein-Lloyd' and E. B. Halpert
Chemistry/Physics Department, State University of New York, Old Westbury, New York 11568 We report an improved method for sampling and realtime determination of gaseous hydrogen peroxide, hydroxymethyl hydroperoxide (HMHP), and methyl hydroperoxide (MHP) in the atmosphere. The analytical method is based on the hydroxylation of benzoic acid by Fenton reagent [Fe(II) and H 2 0 ~ to 1 form the fluorescent product, hydroxybenzoic acid. Fluorescence intensity is enhanced by complexation with aluminum ion. A novel sampling device with a surfaceless intake permitting collection of gas-phase peroxides without inlet line losses is described. SO2 interference in the measurements has been fully characterized, and experimental conditions have been specified for minimizing such interference. Significant concentrations of organic peroxides were observed when the improved method was fielded during the Southern Oxidant Study/Southern Oxidant Research Program on Ozone Nonattainment (SOSISORP-ONA) in Atlanta, GA, in August 1992 and during the North Atlantic Regional Experiment (NARE) in Nova Scotia in August 1993.
Introduction The recent development of methods for the continuous monitoring of gas-phase hydrogen peroxide and organic peroxides has been spurred by the important role these species play in the chemistry of the atmosphere. Precipitation containing hydrogen peroxide is toxic to trees (1-3), and it has been suggested that organic peroxides, formed when ozone reacts with biogenic alkenes, are responsible for leaf disorders and death (4, 5 ) . The oxidation of SO2 by hydrogen peroxide in cloud and rain droplets is an important pathway to acid rain formation (6-9). Recent studies indicate that HMHP is even more soluble than Hz02 (IO),reacts rapidly with SO2 (IO),and may be as prevalent as HzO2 in the atmosphere under certain conditions (11, 12). This suggests the importance of field determination of atmospheric HMHP and other organic peroxides in addition to H2Oz. Atmospheric hydrogen peroxide is produced through a series of gas-phase free-radical reactions initiated by the photolysis of 0 3 at X < 320 nm to form O(lD), which produces OH radicals upon reaction with water vapor (13). Reaction of hydroxyl radicals with atmospheric hydrocarbons and/or CO ultimately produces HO2 and RO2 radicals, which disproportionate to give HzOz or ROOH and 0 2 . High concentrations of NO by converting HOz to OH and RO:, to OR may limit peroxide production (14). Gaseous peroxides partition into cloud- or rainwater, where
* Correspondence can be addressed to either author. + Present address: Department of Environmental Engineering, Manhattan College, Riverdale, NY 10471.
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they may oxidize SO2 and/or transition metal ions. They also increase the oxidizing capacity of the atmosphere through direct photolytic cleavage a t h < 360 nm to produce OH and OR radicals. Photolysis of atmospheric aldehydes and acids is an additional source of peroxy radicals (15). Photochemical models predict organic peroxides to be a principal product of the photooxidation of isoprene over the Amazon forest (16). Experiments have shown that organic peroxides are produced during the ozonolysis of biogenic alkenes (12, 17);the mechanism is thought to involve a reaction between water and the Criegee radical intermediate (18). Peroxides also can be produced in cloudwater by the aqueous-phase self-reactions of HO2 and RO2 radicals (19, 2O), although ROz concentrations and reaction rates are not well characterized. In the absence of convenient methods for the determination of concentrations of free-radical precursors to peroxides, theoreticians look to the accurate measurement of peroxide concentrations to validate models describing the behavior of atmospheric oxidants. Techniques for the determination of peroxides and results of field measurements have been summarized in several references (2123). Most recent measurements employ the enzymemediated reaction between H202 and p-hydroxyphenylacetic acid (pOHPAA) to form a fluorescent pOHPAA dimer (24). Peroxide speciation is accomplished by using two channels, one of which measures total peroxides while the other measures the residual peroxide after destroying hydrogen peroxide with the enzyme catalase. Because catalase also reacts with other peroxides, but a t a slower rate than with H202, catalase is added at a concentration high enough to destroy most of the NzOz, but low enough to prevent reaction with a significant fraction of other peroxides. These conditions are difficult to achieve under field conditions where ambient peroxide concentrations are constantly changing. Several groups have reported significant residual peroxide concentrations in the gas phase as well as in precipitation but were unable to identify the peroxides present (8, 25-27). Recently, progress in speciating peroxides has been accomplished using HPLC separation coupled with postcolumn derivatization with pOHPAA/horseradish peroxidase reagent and fluorescence detection (12,28,29). Small sample size makes this technique somewhat less sensitive than continuous flow methods, and the slow flow rate required for adequate resolution limits the number of samples which can be analyzed and, hence, time resolution. The results of the few HPLC field measurements to date indicate that the most common peroxides in the atmosphere are HzO2, MHP and HMHP, with smaller amounts of longer chain hydroperoxides (12,28). Other peroxides expected, such as ethyl hydroperoxide and peroxyacetic acid, have not yet been detected in ambient air. 0013-936X/94/0928-1180$04.~0/0
0 1994 American Chemical Society
SCRUBBING SOLUTION
J. + TOPUMP
AMBIENT+ AIR
FLUORESCENCE
Figure 1. Schematic diagram for one channel of the peroxlde measurement system.
In earlier publications, we described a method for peroxide determination that is comparable in sensitivity to the widely used pOHPAA method but is capable of speciating ambient peroxides (11, 21). This technique involves the formation of aqueous OH’ radical by the Fenton reaction 1 below and employs benzoic acid as a hydroxyl radical scavenger: Fe(I1) OH’
+ H,O,
-
+ C,H,COOH
OH’
-
+ OH- + Fe(II1)
(1)
‘C,H,(OH)COOH
(2)
In the presence of 0 2 and dissolved iron, the cyclohexadienyl radical formed in reaction 2 produces fluorescent hydroxybenzoic acid (OHBA) isomers (30, 31). Unlike the pOHPAA reagent, which is sensitive to all hydroperoxides, Fenton-OHBA reagent is expected to be specific for Hz02 because the reaction between Fe(I1) and ROOH should not generate a hydroxyl radical. The fluorescence of OHBA is weak at the pH necessary to carry out reactions 1and 2, but increases dramatically above pH 11. Earlier, we enhanced fluorescence by adding NaOH to the product stream; in this work, we describe the use of of a low pH Al(II1) solution for fluorescence enhancement, which makes the technique more convenient for field work. In most instruments, it has been conventional to conduct sampled air through Teflon tubing. Although this is an acceptable practice for many gases, earlier work demonstrated that substantial gaseous hydrogen peroxide can be lost when ambient air is drawn through a Teflon sampling line (32, 33). In addition to reducing ambient air concentration, decomposition on the walls of inlet tubing compromises the validity of using aqueous-phase standards to calibrate the system. Earlier work showed that a series of gas-phase standards scrubbed after passage through a Teflon sampling line exhibited different calibration curves depending on the length and condition of the line ( 3 2 )and frequently exhibited non-zero intercepts. Aqueous-phase standards, which did not pass through the inlet tubing, and thus were not subject to the same contaminants, would not provide accurate calibration under some conditions. Wall losses can be avoided by
mounting the scrubbing coil outdoors, thereby eliminating the need for air intake tubing. Without a sampling line, aqueous-phase standards introduced in place of scrubbing solution give reliable calibration data. A new air scrubbing system that eliminates sampling line losses and allows remote switching between zero and ambient air is illustrated in this paper. We also report the results of a series of experiments designed to characterize the potential for SO2 interference in peroxide determinations. Finally, the use of the new method is illustrated by presenting peroxide data obtained during the SOS/SORPONA Program in Atlanta, GA, in August 1992 and during the NARE Program in Yarmouth, Nova Scotia, in the summer of 1993.
Experimental Section
Sampling and Analysis of Peroxides. The schematic diagram of our sampling system, a modification of that designed by Lazrus et al. (341, is shown in Figure 1. We mounted the air scrubbing coil 1.5 m above the laboratory roof and pumped scrub solution past a 1.6-mm opening that had ambient air drawn in at a rate of 2 L/min. This arrangement, in which gaseous peroxide is scrubbed into the aqueous phase without inlet tubing, constitutes the “surfaceless intake sampler”. Aqueous peroxide standards were prepared from 3 76 HZOZthat had been titrated against standard KMn04. Although the sampler was mounted outside the laboratory, calibrations were convenient to perform by introducing standards in place of scrub solution. During calibrations, and for zero air measurements, a Teflon solenoid valve (Rainin Model 38083) switched the liquid stream to an alternate path where it scrubbed air which had been freed of peroxides by passage through a 15 cm hopcalite column (see Figure 1). Airflow rates were determined with a Singer/AMD gas meter. A 26-channel peristaltic pump (Perstorp Analytical) was used to control fluid flow, with pump tubing calibrated in situ. Fluorescence intensity was monitored with a McPherEnvlron. Scl. Technol., Vol. 28, No. 6, 1994
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- _ __ _
Table 1. Three-Channel System channel scrubbing solution fluorescence reagent enhancing reagent species detected"
1
2
3
low pH Fenton-OHBA Al(II1) HzOz -k 40% HMHP + 1 2 % MHP
high pH Fenton-OHBA Al(II1) HzOz + HMHP + 12% MHP
hlgh pH pQHPAA NaOH MzOz + HMHP -I60% MHP
See text. Table 2. Reagents and Flow Conditions for Peroxide Analysisa reagent pQHPAA
flow rate (mL/min)
composition
0.3
horseradish peroxidase, 17.5 mg/L (Sigma P8375) and p-hydroxyphenylacetic acid, 3.4 mM (Kodak) in pH 8.5 Tris buffer (Sigma reagent) Fenton-OHBA 0.3 FeS04,1.6 mM (Baker Analyzed reagent) benzoic acid, 4.0 mM (Baker Analyzed reagent) in pH 1.6 HzS04 (Ashland ACS reagent grade) high-pH scrub 0.5 1 mM pH 9 phosphate buffer (Fisher) and 1.23 mM HCHO (Mallinckrodt Analyzed reagent) low-pH scrub 0.5 1mM pH 5.3 phosphate buffer (Fisher) and 0.123 mM HCHO (Mallinckrodt Analyzed reagent) HzQz standards 0.5 Hz02 (Mallinckrodt Analyzed reagent) in high- or low-pH scrub; 3% HzOz was titrated against KMn04, which was standardized against Na2C2O4 NaOH 0.4 NaOH, 0.05 N (J.T.Baker Analyzed reagent) aluminum 0.4 AI(N03)3.9HzO, 6.0 mM (Baker Analyzed reagent) in 0.10 M acetate buffer (Mallinckrodt Analyzed reagent), pH 3.8 a All solutions were prepared from ultrapure water from a Barnstead water Dolishinp. svstem.
son HPLC fluorescence detector equipped with a single monochromator for excitation (Aex = 305 nm for FentonOHBA or A,, = 320 nm for pOHPAA) and a 390-nm cutoff filter for emitted light. For field measurements, data points were sampled at a frequency of 0.10 Hz using a Data Translations acquisition board and analyzed with the ASYSTANT software package. Three parallel channels, each as pictured in Figure 1, were used to speciate peroxides. The different scrubbing solutions, fluorescence reagents, and enhancing reagents are given in Table 1 and reagent compositions are summarized in Table 2. The scrubbing solution in channel 1is maintained at low pH, where H202, HMHP, and other peroxides are stable. Channels 2 and 3 have high pH scrubbing solutions, which rapidly hydrolyze HMHP to H202 (29,35). Channel 3 (the pOHPAA channel) gives a measure of total peroxides. The difference between channels 1 and 2 provides a measure of the amount of HMHP that hydrolyzed to H202. These can be subtracted from total hydroperoxides to give a measure of peroxides other than H202 and HMHP. Based on the determination of ambient peroxides by HPLC (12,29),we assume that the only other peroxide detected is MHP, which does not hydrolyze at high pH (29), and a correction factor to account for the low solubility of MHP is applied to the data ( 2 4 ) . HPLC Experiments. Equilibrium mixtures of H202 and HMHP were prepared by the reaction between H202 and HCHO (35). Dilute solutions of Hz02and MHP were prepared by cobalt-60 y-radiolysis of methane-saturated water. Details of the radiolysis experiments will be published separately. Aliquots of 50pL were injected onto a reversed-phase column (Inertsil 25 cm, 5 pm packing, Metachem Corp.), eluted with pH 3 sulfuric acid a t a flow rate of 0.3 mL/min, and detected by their fluorescence after postcolumn derivatization with pOHPAA or FentonOHBA reagents. Reagent compositions and fluorometer settings for the HPLC experiments were the same as in Table 2. SO2 Interference Studies. Using a gas-phase SOz standard, we characterized the extent of loss of signal from 1182
Environ. Scl. Technol., Vol. 28, No. 6, 1994
a 0.44pM aqueous H2Oz standard when various concentrations of SO2 were added to the airstream. The experimental setup was identical to that shown in Figure 1, with the addition of an SO2 inlet to the zero air line. Peroxide standards containing 1.23 mM formaldehyde were adjusted to pHs between 3 and 9 with H2S04 or dilute phosphate buffer. The SO2 source was a 9.78 (+2 % ) ppm standard in Nz (Scott Specialty Gas);SO2 was dynamically diluted with zero air to give concentrations up to 240 ppbv.
Results and Discussion Analytical Method. The replacement of NaOH by Al(II1) to enhance the fluorescence of OHBA is a substantial improvement for field measurements. In our original work, NaOH was used to raise the pH above 11, thereby shifting OHBA to its more highly fluorescent dianionic form (36). The precipitation of iron(II1) under these alkaline conditions necessitated frequent shut-downs in the field to clear clogged flow tubing. Another problem with high pH is suggested by the quenching of fulvic acid fluorescence by paramagnetic Fe(II1) salts (37). That Fe(II1) formed in our system quenches fluorescence is demonstrated by the maximum observed in a plot of fluorescence intensity vs Fe(I1) concentration (211, which occurs when the increase in rate of reaction 1begins to be offset by an increase in fluorescence quenching. These problems motivated a search for an alternative means of fluorescence enhancement that would operate at low pH. Complex formation between Al(II1) ion and hydroxynaphthanoic acid (HNA) is known to substantially increase the fluorescence of HNA at low pH (38). Al(II1) also enhances the fluorescence of fulvic acids at low pH, presumably through complex formation (37). We therefore investigated the use of Al(II1) as a potential fluorescenceenhancing agent for hydroxybenzoic acid and determined the conditions which yield maximum OHBA fluorescence for a fixed concentration of H202. The results are shown in Figure 2a, where the fluorescence signal is plotted against Al(II1) concentration for a fixed pH, and in Figure 2b, where the signal is plotted against buffer pH for a fixed
I
70 7
,z
60 -
[HzOzl (PM) 31 31 31
a
5
g
50 -
m
2
s
$
40 30 20
Table 3. HPLC Separation of HzOz and HMHP
-
[HCHO] (mM) 3.9 7.9 10.2
pOHPAA ratio of peak hts. (HMHP/HzOz)
Fenton ratio of peak hts. (HMHPIHzOz)
0.49 & 0.01 1.01 & 0.04 1.30& 0.01
0.21 f 0.01 0.39 f 0.01
0.49 f 0.01
-
1 0
2
4
6
10
8
[ AI(iIi) ] I rnM
b
3
4
5
6
PH
Figure 2. (a) Effect of AI(II1) concentration on intensity of OHBA fluorescence. Al(II1)-enhancing reagent contained 0.10 M acetate buffer at pH 3.5 and the indicated concentration of AI(NO&. (b) Effect of pH of AI(II1) solution on intensity of OHBA fluorescence. AI(II1)enhancing reagent contained 5.9 mM AI(II1) in 0.10 M acetate buffer at the indicated pH. The experimental apparatus is illustrated in Figure 1. Fenton-OHBA reagent contained 0.9 mM FeS04and 2.0 mM CBHSCOOH at pH 1.6. H202concentration was 8.8 X lo-’. Flow rates and fluorometer settings are as described in the text.
concentration of Al(II1). Based on the the substantial increase of OHBA fluorescence by Al(II1) illustrated here, we employ a fluorescence-enhancing reagent consisting of 6.0 mM Al(II1) in pH 3.8 acetate buffer (see Figure 1).The detection limit using Al(II1) enhancement, given as S/N = 3, is 1 X M, and previous problems associated with the formation of iron(II1) are eliminated. It was initially expected that only H202 would be measured in the low pH Fenton-OHBA channel, and because of the hydrolysis of HMHP to give H202, the high pH Fenton-OHBA channel would give the sum of H202 and HMHP. To test the hypothesis that channel 2 gives the sum of H202 and HMHP, we synthesized HMHP by the reaction between H202 and HCHO: H,02 + HCHO(aq) e HOOCH20H
(3)
In the presence of excess HCHO, HMHP reacts to form the dimer, bis(hydroxymethy1) hydroperoxide (BHMP): HOOCH,OH
+ HCHO(aq1 * HOCH,00CH20H
(4)
The rate of equilibration is pH-dependent, exhibiting both acid and base catalysis ( 3 5 ) . We carried out the reaction at pH 7, where it is complete within minutes, and then diluted the equilibrium mixture into pH 3 buffer, where the low concentration and pH prevent any change in composition. Several mixtures of H202 and HMHP with
composition determined using published equilibrium constants K 3 = 126 and K4 = 14 (10, 35) were separated by HPLC and analyzed alternately by Fenton-OHBA and pOHPAA reagents. Consistent with the conversion of HMHP to H202 by the high- pH pOHPAA reagent after separation on the column, a chromatogram for an equimolar mixture of HMHP and H202 shows equal peak heights. When the same equimolar mixture is analyzed with Fenton-OHBA reagent, a small HMHP peak is observed, although HMHP would not have hydrolyzed to H202 under the experimental conditions used (29). Ratios of peak heights analyzed by Fenton-OHBA and pOHPAA reagents are given in Table 3 for three different equilibrium mixtures of these peroxides. In each case, the ratio represents an average of 2-4 runs, and the observed ratios determined with pOHPAA reagent agree with those calculated from published equilibrium constants within 2 % . The data in Table 3 suggest that 40 f 2 % of the HMHP is observed in the low pH Fenton channel under our experimental conditions. The 40% correction factor was confirmed by analyzing H202/HMHP mixtures of known composition in the three-channel system for ambient air measurements. Similar experiments conducted with mixtures of H202 and MHP suggest that the Fenton channel shows a 20% response to MHP. Although the exact reasons for the Fenton channel response to organic peroxide are not yet established, HPLC experiments provide empirical correction factors, independent of peroxide concentration, which can be used to deduce reliable H202 and organic peroxide concentrations from the observed data. We are continuing to characterize this interference and to seek experimental conditions that will minimize the observed correction factor. SO2 Interference. Ambient SO2 concentrations range from a few pptvin rural areas to plumes of several hundred ppbv in urban areas. The rate coefficient for the reaction between SO2 and H202 depends on pH, increasing rapidly with increasing acidity (39, 4 0 ) . However, the higher solubilty of SO2 in alkaline solution counters this trend in ambient air (9). Loss of H202 by reaction with ambient SO2 co-collected in the scrubber may be avoided by adding HCHO to the scrubbing solution, thereby converting SO2 to hydroxymethanesulfonic acid, which is inert toward HzO2 (41,421. However, the rate of complex formation is also pH-dependent, becoming slower in more acidic solutions ( 4 3 , 4 4 ) . Our first field trial using the FentonOHBA method was conducted a t a rural site in Georgia, where the SO2 concentration remained below 2 ppbv (11). We saw no evidence of SO2 interference in this study in the low- or high-pH Fenton-OHBA channels, which were maintained a t pH 3 and 9, respectively, even though the scrubbing solution contained no HCHO. However, in a subsequent urban study, the pH 3 Fenton channel exhibited periods of substantial loss of signal, coinciding with the passage of SO2 plumes over the sampling site, despite the addition of 1.23mM HCHO. The pH 9 channel showed no evidence of SO2 interference. Environ. Sci. Technol., Vol. 28, No. 6, 1994
1183
4 r
,--
-______-
"
A
"
A
" h
I I
A
0
200
100
300
i
SO,lppbv Flgure 3. Effect of gas-phase SOn concentration on intensity of fluorescence from 3.6 ppbv H202. The experimental apparatus is illustrated in Figure 1, with the addition of a calibrated source of SOn in the air inlet line. Individual curves indicate pH of the stripping solution: (0),pH 3.0; (O), pH 4.0;(V), pH 5.3; (O), pH 9.0. Reagent composition and flow rates are given inthe text.
To quantify the extent of SO2 interference, we studied the loss of HzO2 by reaction with SO2 in our sampling system in the laboratory using aqueous HzO2 standards and gas-phase SO2 standards. As expected, the net effect of the factors described above leads to more extensive loss of H202 at lower pH. Figure 3 shows the fraction of fluorescence signal observed for a fixed H202 concentration in the presence of SO2 concentrations up to 240 ppbv when the scrubbing solution is maintained at pH 3, 4,S.3, and 9. The results show that the high-pH Fenton-OHBA and pOHPAA channels in our system, which use pH 9 air scrubbing solutions, are not susceptible to SO2 interference at ambient levels, while a substantial fraction of H202 collected in a pH 3 scrubbing solution is lost. Therefore, we sought to increase the pH to a value which minimized the loss of H202 but which also minimized hydrolysis of ambient HMHP in the scrubbing solution. Published values of the relevant rate coefficients suggest that, at pH 5.3, less than 2 % of HMHP would hydrolyze during its residence time in the scrubbing solution prior to analysis. Figure 3 indicates that S02-HCHO complexation is sufficiently fast at pH 5.3 that a negligible fraction of H202 will be lost to reaction with SO2 for SO2 concentrations below 40 ppbv. Field Measurements. The three-channel apparatus was deployed in Atlanta, GA, as part of the SOSISORPONA Program during August 1992. Although complete data are not yet available for this Program, ozone excursions (>I20 ppbv) occurred several times during our 2-week stay, and SO2 concentrations ranged from the detection limit of 5 ppbv to a maximum of 450 ppbv. Figure 4 illustrates peroxide measurements on one day during a high ozone episode. SO2 concentrations remained below 20 ppbv during the day. H202 concentration peaked in the late afternoon at 2.5 ppbv. HMHP comprised a significant portion of total peroxide for most of the day. MHP was not detectable for most of the day, but increased dramatically in the afternoon. Peroxide concentrations taken in Yarmouth, Nova Scotia, during the 1993 NARE Program are illustrated in Figure S. Total peroxides during this 2-day period reached as high as 2.5 ppbv, with comparable concentrations of H2Oz and MHP, both peaking in the late afternoon. HMHP was not detectable on this day. The rapid decrease 1184
Environ. Sci. Technoi., Vol. 28, No. 6,1994
,
I
L-J
- . - ~ - - - - L - - L
1000
1200
1500
3400
1800
2200
1000
TIME OF DAY
Flgure 4. Peroxide concentrations in the ambient atmosphere in Atlanta, GA, on August 3, 1992. Data points represent 5-min averages: (01, H201; (O),MHP; (-), HMHP. *
6i
12
>
P
a a
.
2 0.6
II F a c + z W
-
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-
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-
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i
-
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.
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z
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-
-O.ZI--LL-*--L----L. 1200
16:OO
'
2O:OO
'
'
24:OO
I
28:OO
'
'
32:OO
3
TIME OF DAY Figure 5. Peroxide concentrations in the ambient atmosphere in Yarmouth, Nova Scotia, on August 9, 1993. Data points represent 1-min averages: (0),H 2 0 2 ; (O),MHP; (-), HMHP.
in HzO2 concentration at 12:30 PM coincided with the sudden appearance of dense fog, which gradually burned off. MHP, which is less soluble than H202, was not removed as effectively by fog. Interpretation of the field data for these missions is being readied for publication.
Acknowledgments The authors acknowledge the help of L. Newman, D. Leahy, and the Georgia Department of Natural Resources for providing SO2 data, the information from Harold Berresheim on HPLC separations, and the financial support from the SOS/SORP-ONA and NARE Programs. J.W.-L. gratefully acknowledges support from Department of Energy Grant DE-FG02-91ER61206 and NSF Grant ATM-9112698. This work was carried out under the auspices of the United States Department of Energy under Contract DE-AC02-76CH00016.
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(2) Gaffney, J. S.; Senum, G. I. Gas-Liquid Chemistry of Natural Waters; 1984; Vol 1, pp. 5-1-5-7. (3) Kettrup, A. A. F.; Kincinski, H. G.; Masuch, G. Anal. Chem. 1991,63, 1047A-1056A. (4) Hellpointner, E.; Gab, S. Nature 1989, 337, 631-634. (5) Hewitt, C. N.; Kok, G. L.; Fall, R. Nature 1990,344,56-57. (6) Penkett, S. A.; Jones, B. M. R.; Brice, K. A,; Eggleton, A. E. J. Atmos. Environ. 1979, 13, 123-137. (7) Chandler, A. S.; Choularton, T. W.; Dollard, G. J.;Eggleton, A. E. J.; Gay, M. J.; Hill, T. A.; Jones, B. M. R.; Tyler, B. J.; Bandy, B. J.; Penkett, S. A. Nature 1988,336,562-565. (8) Heikes, B. G.; Kok, G. L.; Walega, J. G.; Lazrus, A. L. J. Geophys. Res. 1987,92, 915-931. (9) Schwartz, S. E. The Chemistry of Acid Rain; ACS Symposium Series 349; American Chemical Society: Washington, DC, 1987; pp 93-108. (10) Zhou, X.; Lee, Y.-N. J . Phys. Chem. 1992,96, 265-272. (11) Lee,J.H.;Leahy,D.F.;Tang,I.N.;Newman,L.J. Geophys. Res. 1993, 98, 2911-2915. (12) Hewitt, C. N.; Kok, G. L. J. Atmos. Chem. 1991,12, 181194. (13) Levy, H. Planet. Space Sci. 1973,21, 575-591. (14) Kleinman, L. I. J . Geophys. Res. 1986, 91, 10889-10904. (15) Bufalini, J. J.; Gray, B. W.; Brubaker, K. L. Environ. Sci. Technol. 1972, 6, 816-821. (16) Jacob, D. J.; Wofsy, S. C. J. Geophys. Res. 1988,93,14771486. (17) Simonaitis, R.; Olszyna, K. J.; Meagher, J. F. Geophys. Res. Lett. 1991, 18, 9-12. (18) Criegee, R. Angew. Chem. Int. Ed. Engl. 1975,14,745-753. (19) Bielski, B. H. J. Photochem. Photobiol. 1978,28,645-649. (20) Von Sonntag, C. The Chemical Basis of Radiation Biology; Taylor and Francis, Ltd.: London, 1987; pp 65-93. (21) Lee, J. H.; Tang, I. N.; Weinstein-Lloyd, J. B. Anal. Chem. 1990,62, 2381-2384. (22) Sakugawa, H.; Kaplan, I. R.; Tsai, W.; Cohen, Y. Environ. Sci. Technol. 1990,24, 1452-1462. (23) Gunz, D.; Hoffmann, M. R. Atmos. Environ. 1990,24,16011633. (24) Lazrus, A. L.; Kok, G. L.; Gitlin, S. N.; Lind, J. A.; McLaren, S. E. Anal. Chem. 1985,57,917-922. (25) Kelly, T. J.; Daum, P. H.; Schwartz, S. E. J . Geophys. Res. 1985, 90, 7861-7871.
(26) Kleindienst, T. E.; Shepson, P. B.; Hodges, D. N.; Nero, C. M.; Arnts, R. R.; Dasgupta, P. K.; Hwang, H.; Kok, G. L.; Lind, J. A.; Lazrus, A. L.; Mackay, G. I.; Mayne, L. K.; Schiff, H. I. Environ. Sci Technol. 1988, 22, 53-61. (27) Kok, G. L.; Walega, J. G.; Heikes, B. G.; Lind, J. A.; Lazrus, A. L. Aerosol Sci. Technol. 1990 12, 49-55. (28) Hellpointner, E.; Gab, S. Nature 1989, 337, 631-634. (29) Kurth, H.-H.; Gab, S.; Turner, W. V.; Kettrup, A. Anal. Chem. 1991,63, 2586-2589. (30) Klein, G. W.; Bhatia, K.; Madhaven, V.; Schuler, R. H. J. Phys. Chem. 1975, 79, 1767-1774. (31) Dorfman, L. M.; Taub, I. A,; Biihler, R. E. J. Chem. Phys. 1962,36, 3051-3061. (32) Lee, J. H.; Chen, Y.; Tang, I. N. Enuiron. Sci. Technol. 1991,25, 339-342. (33) Watkins, B. NOAA Aeronomy Laboratory, Denver, CO, private communication. (34) Lazrus, A. L.; Kok, G. L.; Lind, J. A.; Gitlin, S. N.; Heikes, B. G.; Shetter, R. E. Anal. Chem. 1986,58, 594-597. (35) Marklund, S. Acta Chem. Scand. 1971,25, 3517-3531. (36) Thommes, G. A,; Leininger, E. Anal. Chem. 1958,30,13611363. (37) Caboniss, S. E. Environ. Sci. Technol. 1992,26,1133-1139. (38) Kirkbright, G. F.; West, T. S.; Woodward, C. Anal. Chem. 1966,37, 137-143. (39) Lee, Y.-N.; Shen, J.; Klotz, P. J.; Schwartz, S. E.; Newman, L. J. Geophys. Res. 1986, 91, 13264-13274. (40) Martin, L. R.; Damschen, D. E. Atmos. Environ. 1981,15, 1615-1621. (41) Kok, G. L.; Gitlin, S. N.; Lazrus, A. L. J. Geophys. Res. 1986,91, 2801-2804. (42) Richards, L. W.; Anderson, J. A.; Blumenthal, D. L.; McDonald, J. A.; Kok, G.L.; Lazrus, A. L. Atmos. Environ. 1983,17,911-914. (43) Boyce, S. D.; Hoffman, M. R.; J . Phys. Chem. 1984, 88, 4740-4746. (44) Diester,U.; Neeb, R.; Helas, G.; Warneck, P. J . Phys. Chem. 1986,90, 3213-3217.
Received for review December27,1993. Accepted March 7,1994.' ~
Abstract published in Advance ACS Abstracts, April 1, 1994.
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