Envlron. Sci. Technol. 1991, 25, 791-800
(13) Proctor, C. J. Proceedings of the 82nd Annual Meeting of the AWMA, 1989, Anaheim CA, June 25-30,1989; Paper 89-80.4. (14) Samet, J. M.; Marbury, M. C.; Spengler, J. D. Am. Rev. Respir. Dis. 1989, 136, 1486-1508. (15) Altman, P. L.; Ditmer, D. S. Respiration and Circulation;
Federation of American Society for Experimental Biology: Bethesda, MD, 1971.
Received for review June 14,1990. Revised manuscript received October 10, 1990. Accepted: November 6, 1990.
Low-Intensity Radiolysis Study of Free-Radical Reactions in Cloudwater: H202Production and Destruction Judith Weinstein-Lloyd* and Stephen E. Schwartz
Environmental Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973
Reactions in cloudwater can be important pathways for chemical transformation of atmospheric trace gases. One such reaction is the oxidation of dissolved sulfur dioxide by hydrogen peroxide. Hz02is formed by the disproportionation of hydroperoxyl and superoxide radicals, 0 2 ( 3 . We report measurements of the rate of H 2 0 zproduction from 02(-1) radicals generated by low-intensity cobalt-60 radiolysis of synthetic cloudwater solutions and actual precipitation samples. Our results, employing 02(-1) production rates comparable to those expected upon transfer of H 0 2 from interstitial cloud air to cloudwater, confirm model predictions that H202production is frequently the major fate of 02(-1) radicals. However, there is evidence of significant reaction between S(IV) and 0, (-1): with a rate coefficient of (3 f 2) X lo4 a t pH 4.96. In addition, the presence of 1pM dissolved iron decreases the H202yield, principally because of destruction of H20zby Fe(I1). H
Introduction Despite the low fraction of the atmosphere occupied by clouds and the low fractional volume of clouds occupied by liquid water, reactions in cloudwater can be important pathways for chemical transformation of atmospheric trace gases (1-6). The component processes of such reactions are physical dissolution of the reactive gas in cloudwater, possible acid-base equilibria of the dissolved gas, and aqueous-phase reaction. Factors leading to the increased importance of aqueous-phase reaction relative to gas-phase reaction thus include high solubility of reagent gases, rapid aqueous-phase kinetics, and increased thermodynamic driving force for reaction due to low free energies of dissolved products. The products of these reactions may further react, may be returned to clear air as a gas or an aerosol particle, or may be deposited to the surface as precipitation. One of the most important cloudwater reactions is oxidation of dissolved sulfur dioxide, i.e., S(IV), by hydrogen
* Address: Chemistry/Physics Department, State University of
New York/College at Old Westbury, P.O.Box 210, Old Westbury, NY 11568. 0013-936X/91/0925-0791$02.50/0
peroxide, to form sulfuric acid (2,3,7).Hydrogen peroxide is quite soluble in cloudwater, and the aqueous-phase kinetics are sufficiently rapid that for conditions typical of the nonurban troposphere in eastern North America the reaction proceeds to completion on a time scale of the order of 10 min, depleting the reagent that was initially present a t lower concentration. Because in the atmospheric boundary layer in the vicinity of SOz emission sources this limiting reagent is often H202(8-IO), it is especially important to understand processes controlling H202formation in the atmosphere in order to describe acid deposition processes and to examine consequences of potential changes in SOz emission rates. Hydrogen peroxide is produced in the gas phase by combination of hydroperoxyl free radicals. HO, is present in daytime in rapid exchange with the more reactive but less abundant hydroxyl free radical. The principal source of these free radicals is UV photolysis of Os,yielding O(lD), which reacts with H,O(g) to form 20H. The principal alternative gas-phase free-radical sink is reaction of OH with NOz. In low-NO, atmospheres, reaction of OH with organics results in the formation of RO,; subsequent reactions of the latter may provide a significant radical sink. When liquid water is present, the potential exists also fol transfer of HO, into aqueous solution followed by aqueous-phase disproportionation of H02and/or its conjugate base, 02-(collectively denoted 02(-I)), to form H202and Oz,resulting in a greater overall yield of H202than when there is no liquid phase (11). These reactions are indicated schematically as
Alternatively, aqueous-phase reactions of 02(-1)may occur that do not yield H202or that result in the depletion of H202.
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 4, 1991 791
In order to describe the rates and yields of gas-aqueous reactions in clouds it is necessary to know, in addition to the concentrations of the reagents and the liquid water content, the pertinent mass-transport, solubility, and chemical kinetic properties of the reagent gases (12). Within the past several years, thermochemical data have become available that permit evaluation of Henry's law coefficients for OH and HOz ( 4 , 11, 13), and the mass accommodation coefficient of H 0 2 onto aqueous solution has been measured (14). These data, together with chemical kinetic data drawn largely from the radiation chemistry literature, have permitted development of models describing cloudwater free-radical chemistry generally and Hz02production in particular. Because of its relatively high abundance and aqueous solubility, as enhanced by the acid dissociation equilibrium, H 0 2 is the principal free radical transferring from the gas to aqueous phase ( 4 , 6). A remaining concern with models of free-radical chemistry is that of the applicability of present chemical kinetic schemes to such complex milieus. Cloudwater in industrialized regions such as the northeast United States typically contains, in addition to 10-4-10-3 M concentrations of the principal ionic species-H+, SO:-, NO3-, NH4+,Na+, C1--(8), 10-7-10-6 M or higher concentrations of transition metals-Fe, Cu, Mn-(15-17), 10-6-10-5 M HzOzor S(IV) (8, 18),and, if one may infer from precipitation composition, a suite of carboxylic acids, aldehydes, and carbohydrates at 10-6-10" M concentrations (19). Rate coefficients determined by direct optical absorption measurement in techniques such as pulse radiolysis have necessarily employed ultrapure solutions and micromolar radical concentrations (20). A t the low free-radical concentrations expected for cloudwater, however, pathways involving impurity reactions are favored over radicalradical pathways, which dominate a t high radical concentrations. Because of the reactivity of free radicals with many of the aforementioned species, evaluations of freeradical kinetics in cloudwater that ignore such pathways must be considered tentative. To address this concern, we have undertaken a study in which the rate of H z 0 2production from radiolytically generated 02(-I) radicals is monitored in synthetic cloudwater solutions and actual precipitation samples. Our results, employing Oz(-I) production rates comparable to those expected upon transfer of H 0 2 from interstitial cloud air to cloudwater, confirm model predictions that hydrogen peroxide production is frequently the major fate of 02(-1) radicals. However, there can be major departures from the Hz02yield that would be expected if H202were the only product of 02(-1)radicals. Specifically, iron-catalyzed disproportionation of Oz(-I), by maintaining a steady-state concentration of reduced transition metal, reduces the H 2 0 2yield and may in fact lead to H202decomposition. Calculations indicate that these conclusions are applicable also to reactions in clouds in the ambient atmosphere. Approach
Free radicals were generated in aqueous solutions placed within cylindrical @Coirradiation sources (21). Intensities for the three sources used, determined by ferrous sulfate dosimetry, were 1.05 X l O l s (source I), 1.62 X 1017(source TI), and 2.48 X 1015 eV L-l s-l (source 111) in December 1988. Dose rates at other times were adjusted by the "Co half-life of 5.27 years. y radiation produces the following species upon interaction with water (a good assumption for the dilute solutions employed): OH (2.75), e-aq(2.65), H (0.65), Hz (0.45), and H 2 0 z (0.70). Numbers in par792
Environ. Sci. Technol., Vol. 25, No. 4, 1991
entheses are the primary G values (22), which represent the yield of each species per 100 eV of absorbed radiation and are accurate to f 5 % . These products are rapidly converted to Oz(-I) by oxygen and formate by the following sequence of reactions: OH
+ HC02-
c02- + 02
HOz
-
HzO + C02-
- coz +
-
02-
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(2)
(3)
(6)
Under these conditions, the overall G value for 02(-I) is G[Oz(-I)] = GOH + Ge-Bq + G H = 6.05 and the 02(-1)production rates for the three sources are 106, 16.3, and 0.249 nM 8. These values are comparable to net rates of HOz transfer into cloudwater from the gas phase, for which modeling estimates of 0.1-30 nM s-l under representative ambient conditions are available ( 4 , 6,111. In the absence of other reactive species, 02(-I) radicals form hydrogen peroxide via reaction 7, where k , is a pH-
20z(-I)
2H+
HZOz
+02
(7)
dependent rate constant (20). HzOzyields can therefore be investigated under a variety of conditions by comparing the measured G value for Hz02production, G(H202),with the theoretical value expected for 100% yield of H 2 0 z formation from 02(-1), i.e., G(H202)= (1/z)G[02(-I)l + = 3.7 f 0.2. Here, GHzo2represents the primary 2;lytic yield of H202,Le., that portion of the hydrogen peroxide formed in spurs from the primary radicals before they have time to diffuse apart. Since H202produced in these experiments may be decomposed by oxidizing or reducing impurities, thus giving the appearance of a decreased yield for HzOzproduction, control experiments were conducted by adding an H 2 0 2 stock solution to samples with a syringe pump at a rate comparable to the H20zproduction rate in the radiolysis experiments. Experimental Section
Reagents for preparation of synthetic precipitation were of the purest grade available: Alfa Puratronic ",NO,, NaC1, (NH4)&30,, manganese(II), copper(II), and iron(II1) sulfate; GFS double-distilled sulfuric and nitric acids; Mallinckrodt AR hydrogen peroxide, sodium sulfite, and formaldehyde. Fresh water from a Millipore three-stage water purification system was used to prepare samples. Generally, solutions were irradiated within 2 h of preparation. Irradiations were carried out at room temperature. Rainwater for authentic precipitation studies was collected at ground level at Brookhaven National Laboratory, 100 km east of New York City. Sample pH was measured immediately; samples were refrigerated for later analysis and irradiation. Filtering precipitation through a 0.22-pm Teflon filter was found not to affect the radiolytic yield of HzOzin early studies and was discontinued. Concentrations of NH4+,NO3-, and S042-in precipitation were determined by ion chromatography. Cu, Mn, and Fe were determined by graphite furnace atomic absorption spectroscopy by the Mayo Clinic Laboratories, Rochester, MN. Sodium formate (Fisher certified) up to 2 mM was
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DoseiN,, krnol(1 OOeV) L" Flgure 1. Yield of H,02 in synthetic cloudwater as a function of radiation dose in kilorads (top axis) or in 100 eV L-' divided by Avogadro's constant N , (bottom axis). For sources I and 11, samples contained 1.0 mM NaOOCH, 8.3 pM ",NO, and 17.9 pM (",),SO4, with pH adjusted with H,SO, to 4.3. For source 111, samples contained 1.0 mM NaOOCH, 44 pM ",NO, 54 pM (",),SO, and 75 pM NaCI, with pH adjusted to 4.6. Solid line represents Gvalue of 3.7 H202molecules produced per 100 eV absorbed radiation energy.
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Results and Discussion Synthetic Samples. Figure 1 illustrates the buildup of H,O, as a function of absorbed dose for the three 6oCo sources in solutions containing the principal ionic constituents, H+, NH4+, NO3-, and SO,-. The solid line represents the theoretical H202yield of 3.7 molecules/ 100 eV. The close adherence to the theoretical value for the three sources shows that the measured H20zyield is independent of dose over 2 orders of magnitude and of dose rate over 3 orders of magnitude. Figure 2 illustrates the invariance of G(H202)at fixed dose for samples containing varying concentrations of H2S04,(NH4)2S04,",NO3, NaC1, HCHO, and Hz02. Again, there is close agreement between the experimentally measured yield and the theoretical yield, indicated by the dashed lines. These results establish the lack of any significant loss of free-radical precursors or product H 2 0 2 with principal cloudwater constituents. The findings are consistent with the known lack of reactivity of 02(-1)toward these species (20) and
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added to all samples prior to irradiation. Formate was recrystallized twice from Millipore water after an initial recrystallization from EDTA solution. All samples were saturated with ultrahigh-purity oxygen before irradiation. Hydrogen peroxide analysis was carried out by the ( p hydroxypheny1)acetic acid method, using a single-channel modification (23) of the method developed by Lazrus et al. (24). In order to minimize H 2 0 2decomposition, addition of reagents to form the stable fluorescent product was carried out immediately after irradiation. Syringe pump experiments (Harvard Apparatus, Inc., South Natick, MA) were conducted with all-glass syringes and Teflon tubing. Pump rates varied from 0.04 to 0.14 mL min-'. A Hach electrode, designed to analyze pH in solutions of low ionic strength, was used throughout this work.
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