Aromatic Carbonyl Compounds as Aqueous-Phase Photochemical

Environ. Sci. Technol. 1997, 31, 218-232

Aromatic Carbonyl Compounds as Aqueous-Phase Photochemical Sources of Hydrogen Peroxide in Acidic Sulfate Aerosols, Fogs, and Clouds. 1. Non-Phenolic Methoxybenzaldehydes and Methoxyacetophenones with Reductants (Phenols) C O R T A N A S T A S I O , †,‡ B R U C E C . F A U S T , * ,†,§ A N D C. JANAKIRAM RAO† Nicholas School of the Environment and Department of Chemistry, Environmental Chemistry Laboratory, Duke University, Durham, North Carolina 27708-0328

Non-phenolic aromatic carbonyl compounds (i.e., benzaldehydes and acetophenones not containing an -OH group on the aromatic ring) and various phenols are present in the atmosphere from the combustion of wood and other biomass and probably from the entrainment of terrestrial humic/fulvic substances present in wind-blown soil aerosol. Illumination (313 nm) of aqueous solutions containing a nonphenolic aromatic carbonyl and a given phenol (phenol itself or a substituted phenol): produces hydrogen peroxide (HOOH); destroys the phenol; and, in most cases, causes little or no destruction of the carbonyl. Little HOOH is photoformed from these chromophores without added phenol, but the quantum yield for HOOH photoformation (ΦHOOH,313) rapidly increases with increasing phenol concentration. These observations indicate that phenol serves as the terminal electron donor and that the non-phenolic aromatic carbonyl normally acts as a photocatalyst. The HOOH quantum yields of the aromatic carbonyls are strongly dependent upon pH; ΦHOOH,313 increases by up to 20-fold from pH 5.6 to pH 1.6, for non-phenolic methoxybenzaldehydes and methoxyacetophenones. HOOH photoformation is proposed to occur via oxidation of phenol by both protonated and unprotonated excited triplet states of the aromatic aldehyde or ketone, forming a ketyl radical (PhC•(OH)R), which reduces O2, thereby forming HOO• and regenerating the parent carbonyl. Calculated rates of HOOH photoformation in sunlight for methoxy-substituted benzaldehydes and acetophenones are generally greater than those of their unsubstituted analogs. At pH 3.7 calculated rates of HOOH photoproduction in sunlight (midday, equinox, Durham, NC) are typically 3-8 µM h-1, but range up to 34 µM h-1 for aqueous solutions containing 10 µM methoxysubstituted aldehyde or ketone with 30 µM phenol. Calculated rates at pH 1.6 are 2-3 times larger than those at pH 3.7. The combination of high photoreactivity and likely atmospheric prevalence for these chromophores suggests that they are important sources of HOOH in aqueous aerosols, fogs, and clouds.

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 1, 1997

Introduction Hydrogen peroxide (HOOH) is typically the most important oxidant of aqueous SO2 (SO2‚H2O, HSO3-, SO32-) in cloud, fog, and rain drops (1, 2). It is also a source of aqueousphase hydroxyl radicals (3-5). Therefore, knowledge of the mechanisms and rates of atmospheric aqueous-phase HOOH formation is crucial if one is to understand atmospheric processes such as the formation of sulfuric acid (and sulfate aerosols) and the oxidation of aqueous organic compounds. There are two major sources of HOOH to atmospheric water drops: gas-phase photochemistry coupled with gasto-drop transfer (6), and aqueous-phase photochemical reactions in atmospheric waters (7-17). While the gas-phase photoformation of HOO• and HOOH is comparatively well understood, relatively little is known about the chromophores and mechanisms that are responsible for the aqueous-phase photochemical formation of HOOH. Attention thus far has focused primarily on the photolysis of Fe(III)-oxalate complexes as possible aqueous-phase sources of HOOH (11, 1619). The potential roles of aromatic aldehydes and ketones in the aqueous-phase photoformation of HOOH have not been examined. Aromatic carbonyl compounds are widespread in the atmosphere and in atmospheric condensed phases (20, 21), such as aerosol particles (22), cloudwaters (23), and fogwaters (24, 25). The combustion of wood and other biomass is an important source for these carbonyls (as well as for phenols), which are thought to be derived primarily from the lignin component of the biomass (26-29). Lignin, an important component of vascular plants, is found most notably in woody stems (which are typically 20-30% lignin by dry weight), but it is also present in roots and foliage (30, 31). Other sources of aromatic carbonyls include volcanic ash (32) and the atmospheric oxidation of aromatic compounds (33, 34). In addition, terrestrial humic/fulvic substances contain aromatic carbonyl functionalities (35-37), and so organic matter present in windblown soil dust and plant detritus (38) may also be a substantial source of aromatic carbonyls to the atmosphere. With the exception of benzaldehyde (Figure 1A), there are few reports of speciated non-phenolic aromatic carbonyls (no -OH substituent) in the ambient atmosphere. However, available field data for a few speciated phenolic aromatic carbonyls (i.e., those containing an -OH group on the aromatic ring) indicate that these species can be present at micromolar concentrations in atmospheric waters (Figure 1A). There are comparable concentrations of phenolic and non-phenolic aromatic carbonyls in the smoke from biomass combustion (29). Hence, significant concentrations of non-phenolic aromatic carbonyls can be expected in aqueous aerosols, fogs, and clouds (Figure 1A). The atmospheric concentrations of aromatic carbonyl groups associated with humic/fulvic substances are largely unknown. However, as shown and explained in Figure 1A, estimated concentrations of terrestrial humic/fulvic aromatic carbonyls in atmospheric waters range up to a few hundred * To whom correspondence may be addressed. Present address: University of California at Los Angeles, School of Engineering and Applied Science, Department of Civil and Environmental Engineering, Environmental Chemistry Laboratory, Box 951593, 5732H Boelter Hall, Los Angeles, CA 90095-1593. Phone: 310-825-0494; fax: 310-2062222; e-mail: [email protected]. † Nicholas School of the Environment. ‡ Present address: Department of Land, Air & Water Resources, Hoagland Hall, University of California, Davis, CA 95616. § Department of Chemistry.

S0013-936X(96)00359-8 CCC: $14.00

 1996 American Chemical Society

B

A

FIGURE 1. (A) Concentrations of aromatic carbonyls in tropospheric rainwaters, cloudwaters, and fogwaters. Key: (b) measured benzaldehyde concentrations; (9) sum of measured concentrations of vanillin (4-hydroxy-3-methoxybenzaldehyde) and syringaldehyde (4-hydroxy-3,5dimethoxybenzaldehyde); (0 with ×) sum of vanillin and syringaldehyde concentrations, calculated based on measured aerosol concentrations of the carbonyls and assuming their complete dissolution into a fog with a liquid water content of 0.3 g m-3; (4) estimated concentrations of carbonyl functionalities in humic/fulvic substances. Concentrations of humic/fulvic carbonyls were estimated based on (i) measured dissolved/total organic carbon (DOC/TOC) concentrations; (ii) assuming 20% of the measured DOC mass (18% of the TOC mass) was humic/ fulvic substances [based on data from Likens et al. (95) where ∼20% of DOC (18% of TOC) was lignin/tannin substances]; and (iii) assuming a concentration of 3 × 10-3 mol of carbonyl groups/g of humic substance (based on soil fulvic acid data from Thurman; 37). References and notes: (a) ref 96; snow samples; (b) ref 95; estimated from measured lignin/tannin concentrations using assumptions noted above; (c and l) ref 23; (d and i) ref 12; (e) ref 25; (f) ref 24; (g and h) ref 22; molecular weight of syringaldehyde used for calculations; (j) ref 97; (k) Baier, 1975, as cited in ref 98; (m) ref 99; (n) ref 100. (B) Concentrations of phenolic compounds in tropospheric rainwaters, cloudwaters, and fogwaters. Key: (b) sum of measured concentrations of phenol and substituted phenols; (X) sum of concentrations of phenolic compounds, calculated based on measured aerosol concentrations of the phenols and assuming their complete dissolution into a fog with a liquid water content of 0.3 g m-3; (O) estimated concentrations of phenolic functionalities in humic/fulvic substances. Concentrations of humic/fulvic phenolics were estimated as described for humic/fulvic carbonyls in panel A, except a concentration of 2.2 × 10-3 mol of phenolic groups/g of humic substance was assumed (36; average value for six aquatic and terrestrial humic substances). References and notes: (a and h) ref 101; (b) ref 102; (c) ref 95; molecular weight of phenol used for calculations; (d) ref 103; it was assumed that the maximum and minimum concentrations for each class of phenol coincided; (e) ref 104; calculated mean concentration based on measured gas-phase concentrations and Henry’s law constants; (f) ref 95; estimated from measured lignin/tannin substances using assumptions noted above; (g) ref 105; (i and m) ref 12; (j) ref 106; (k) data of Tremp, 1987, cited in ref 99; (l) ref 25; (n) ref 97; (o) Baier, 1975, cited in ref 98; (p and s) ref 22; molecular weight of methylguaiacol used for calculations of guaiacols; (q) ref 23; (r) ref 99; (t) ref 100. micromolar. Since the degradation of lignin is postulated as one contributing mechanism for the formation of terrestrial humic/fulvic substances (39), it is not surprising that oxidation of these substances produces methoxy-substituted aromatic carbonyls similar to those present in lignin (35, 40-42). In addition, the photolysis of polymeric lignin and humic/fulvic type substances also produces methoxy-substituted aromatic carbonyl structures (43-45). Thus, there appear to be a number of means, including combustion, degradation, and photolysis, by which the methoxy-substituted aromatic carbonyl structures present in lignins are delivered to the troposphere. Phenol and substituted phenols, which can act as terminal electron donors (reductants) in the reduction of O2 (vide infra), have also been measured in atmospheric waters at micromolar concentrations (Figure 1B). Measured aerosol concentrations in areas impacted by residential wood burning (22) suggest that concentrations of phenol and substituted phenols in fogwater and aqueous aerosols may exceed 100 µM in such environments (Figure 1B). Sources of atmospheric phenolic compounds include the combustion of wood (27-29, 4649), coal (50), and other biomass (29, 51), and industrial processes (20). Terrestrial humic/fulvic substances may also be significant sources of phenolic -OH groups in atmospheric waters; estimated humic/fulvic phenolic -OH concentrations

range up to 200 µM for the most impacted fogwaters (Figure 1B). As noted for the aromatic carbonyl constituents, there are also several mechanisms, including combustion and degradation, by which the phenolic constituents of lignins can be delivered to the troposphere. Little information exists on the condensed-phase photoformation of HOOH from aromatic carbonyls, with or without added reductants. HOOH is photoformed from solutions of 0.5 M benzophenone in neat 2-propanol (52) and from citral in neat alcoholic solvents (53), but these studies have limited relevance to atmospheric water drops. Hence, as part of our investigation into aqueous-phase sources of HOOH, we have characterized the photochemistry of non-phenolic aromatic carbonyls (Figure 2), including methoxy-substituted benzaldehydes and acetophenones, in the presence of a reductant (a phenol). A future paper (54) will present studies of phenolic aromatic carbonyl compounds. The major objectives of this work were to (i) determine quantum yields for HOOH formation and carbonyl loss at 313 nm for different carbonyls with and without added phenol; (ii) investigate the reactivity of different potential atmospheric reductants (including phenols); (iii) determine the dependence of HOOH formation rates on pH and on the concentration of added reductant; (iv) postulate a mechanism for the HOOH photoformation from the carbonyl compounds

VOL. 31, NO. 1, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Structures of the 11 non-phenolic aromatic carbonyl compounds studied. studied; and (v) calculate sunlight rates of HOOH photoformation for these compounds.

Experimental Section Reagents. All reagents used in photolysis solutions were of the highest purity commercially available (reagent grade or better, generally g97% pure) and were used as received; they were stored in the dark and, when necessary, in a refrigerator and/or in a desiccator (12). Each compound was stored under argon after use. All organic compounds (e.g., chromophores and reductants) used in photolysis solutions were obtained from Aldrich, except for glyoxal (Fluka), 2-methoxyphenol and 4-methylbenzaldehyde (TCI America), and phenol (Sigma Ultra, >99.5%; Fe, Cu, and Mn 2.0, [H2SO4

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] + [Na2SO4 ] ) 0.010 M; (ii) for pH e 2.0, H2SO4 (occasionally HClO4) was added to achieve the desired pH, and Na2SO4 was not added. Calculated ionic strengths for these solutions were approximately 0.030 M for pH > 2.5 (H2SO4/Na2SO4); 0.024 M for pH ≈ 2.1 (H2SO4/Na2SO4); 0.037 M for pH ≈ 1.6 (H2SO4); 0.066 M for pH ≈ 1.4 (H2SO4); 0.21 M for pH ≈ 0.9 (H2SO4); 0.025 M for pH ≈ 1.6 (HClO4); and 0.050 M for pH ≈ 1.4 (HClO4). Analyses and Methods. Ultraviolet-visible absorption spectra (and single wavelength measurements at 313 nm) of the stock and photolysis solutions were recorded at room temperature (typically 20-25 °C) with a Shimadzu UV265 spectrophotometer (≈100 nm min-1 scan speed, 2-nm band width) using the appropriate diluent solution for baseline correction and as reference. A Cary 3E spectrophotometer was used (120 nm min-1, 2-nm band width) to determine the pH dependence of the carbonyl spectra (20 °C). Aromatic carbonyls and phenolic reductants were analyzed by HPLC with an LDC constaMetrics 3500 eluent pump; a 250 × 4.6 mm, 5 µm diameter bead, C-18 analytical column (Beckmann ODS Ultrasphere, or Supelco Supelcosil LC-18); and a Linear UVis 204 UV/VIS detector (12). Helium-degassed acetonitrile (Mallinckrodt)/Milli-Q water (usually 40%/60% v/v) was used as mobile phase. Hydrogen peroxide and organic peroxides were determined using the HPLC speciation technique of Kok et al. (58), which is able to measure HOOH and 11 C1-C3 organic peroxides. Further details on the use and absolute calibration of this technique for photochemical studies are given elsewhere (12, 13). A HOOH standard (typically 1.00-30.0 µM) was made for each experiment by serial dilution of a commercial 30% HOOH solution. Calibration curve data points were then made by adding small (e150 µL) aliquots of the HOOH standard to separate “snap-top” polyethylene vials containing un-illuminated photolysis solution to give a total volume of 1500 µL in each vial. pH was measured ((0.05) using an Orion 8103 (Ross) combination glass electrode, with NIST-traceable standard (H+ activity) buffers from Fisher (pH 1.00, 2.00, 3.00, and 5.00) and Baxter (4.00, 6.00, and 7.00). Photolysis Procedures. For each experiment, 23.0 mL of air-saturated photolysis solution was placed into a 5-cm fused-

quartz photolysis cell (Spectrocell R-3050-I FUV; modified to 70 mm overall height) equipped with a gas-tight Teflon-faced silicone septum in a Teflon screw cap. Samples were withdrawn from the cell through a 5-cm piece of Teflon tubing with a Kel-F luer hub that had been inserted through the septum (and kept just below the solution surface but above the light beam). Immediately prior to illumination, the absorption spectrum and (in single wavelength mode) absorbance at 313 nm of the photolysis solution were measured. The cell was then placed in the illumination system sample chamber and allowed to thermally equilibrate (in the dark) for at least 10 min prior to illumination. Samples were illuminated at 313 nm (20 ( 1 °C) with continuous stirring (55). The concentrations of HOOH and model compounds were determined by (i) periodically interrupting illumination with a manual mechanical shutter, (ii) removing an aliquot (∼250 µL) of photolyzed solution through the Teflon tubing into a gas-tight glass syringe (Hamilton 1725), and (iii) injecting 125 µL of solution into each of two HPLC systems for analyses of peroxides and carbonyl/reductant. A control for thermal reactions used a separate aliquot of the same solution under the same conditions but kept in the dark. A separate solution of 4 µM 2-nitrobenzaldehyde in air-saturated Milli-Q water was used as a chemical actinometer (55). Sample volumes were reduced by less than 10% during the course of illumination. The solar simulator system has been described previously (59). Data Analysis. Molar absorptivities (C,λ) for compounds reported here are based on the total concentration of the compound. Molar absorptivities were calculated from the average of several separate single-wavelength measurements for 313 nm (C,313) and from absorption spectra for other wavelengths. Initial rates of photochemical formation (HOOH) or loss (carbonyl or phenol), Ri,λ [mol L-1 s-1; where i ) HOOH, C (for carbonyl), or PHENOL; and λ ) 313 nm] were determined as follows. Values of RHOOH,λ were obtained from the slope of a linear-regression plot of [HOOH] versus illumination time. In ∼60% of the experiments at pH < 3.7 and in ∼10% of the experiments at pH g 3.7, the HOOH formation exhibited an apparent lag time, with the HOOH photoformation rate becoming constant after approximately 5 min (i.e., after ∼0.6 µM HOOH was present). This constant rate was used to calculate RHOOH,λ in these cases. The rate coefficient for destruction of carbonyl or reductant (e.g., phenol), ji,λ (s-1), was generally determined from the linear-regression slope of a plot of ln([i]/[i]0) versus illumination time, where [i] and [i]0 are the total concentrations of the compound at time t and time 0, respectively. The initial rate was then determined from Ri,λ ) ji,λ[i]0. For cases of low conversion ( di- and trimethoxybenzaldehydes. The photoproduction of HOOH is accompanied by little or no loss of parent carbonyl (Figure 7). The quantum efficiency for carbonyl destruction was statistically indistinguishable from zero for most of the non-phenolic aromatic carbonyls, both in the presence of 30 µM phenol (Table 5) and in the absence of phenol (data not shown). There is,


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TABLE 2. Quantum Yields of Hydrogen Peroxide at 313 nm (ΦHOOH,313) for Aqueous Solutions of Non-Phenolic Aromatic Carbonyls with Different Added Reductants, at pH ) 3.7a carbonyl (molar absorptivity at 313 nm) 4-methylbenzaldehyde (130 M-1 cm-1)

4-methoxybenzaldehyde (2000 M-1 cm-1)

3,4-Dimethoxybenzaldehyde (9300 M-1 cm-1)

reductantb

total [reductant] (µM)

103 × ΦHOOH,313

ΦHOOH,313EC,313c (L Einstein-1 cm-1)

none formate acetate ammonium formaldehyde glyoxal 2-propanol R-D-glucose toluene phenol 4-hydroxybenzoic acid 2,6-dimethoxyphenol 2-methoxyphenol 2-methylphenol 1,2-dihydroxybenzene none formate acetate phenol phenol 2-methoxyphenol 4-methylphenol 2,6-dimethoxyphenol none 2-propanol formate formaldehyde 4-hydroxybenzoic acid phenol

0 500 500 25 000 500 500 30 30 30 30 30 30 30 30 30 0 200 200 30 200 200 200 200 0 30 500 500 30 30

0.5 0.7 0.4 0.7d,e 0.4 0.0e 0.4 0.6 1.5 17 18 19 21 26 38 0.2 0.9 0.1 11 48f 29 54 79 0.02 0.03 0.04 0.04 2.1 3.6

0.06 0.09 0.05 0.09d,e 0.05 0.00e 0.05 0.07 0.19 2.1 2.3 2.5 2.8 3.4 4.9 0.3 1.8 0.1 23 110 58 110 160 0.2 0.3 0.4 0.4 19 34

a Photolysis conditions (except where noted): total concentration of carbonyl compound ) 10 µM, air-saturated aqueous solution, pH ) 3.7 (100 µM H2SO4), 100 µM Na2SO4, 313 nm, 20 °C. Structures for the carbonyl compounds are given in Figure 2. b Glyoxal, HC(OH)2C(OH)2H; formaldehyde, CH2(OH)2. Based on the reported hydration constants for glyoxal (120) and formaldehyde (121), these species will exist nearly exclusively (>99.9%) in their hydrated forms in aqueous solution. c A measure of photoreactivity potential. d 12.5 mM aqueous (NH4)2SO4 at pH 5.6. e Corrected for the minor amount of HOOH produced from direct photolysis of the reductant solution. f Interpolated value from linear regression of ΦHOOH,313-1 vs [phenol]-1 (eq 7).

TABLE 3. Linear Dependence of the HOOH Quantum Yield (ΦHOOH,313) on Phenol Concentration at Lower Phenol Concentrations and pH 3.7: Linear Regression of ΦHOOH,313 versus [Phenol]a

carbonyl

y-intercept

slope (M-1)

benzaldehyde acetophenone 4-methylbenzaldehyde 2-methoxybenzaldehyde 3-methoxybenzaldehyde 4-methoxybenzaldehyde 3,4-dimethoxybenzaldehyde 3,4,5-trimethoxybenzaldehyde 3,4,5-trimethoxyacetophenone furaldehyde methylbenzoquinone

(1 ( 1) × 10-4 (6 ( 3) × 10-4 (1.0 ( 0.7) × 10-3 (1.6 ( 0.2) × 10-3 (4 ( 2) × 10-3 (3 ( 1) × 10-4 (6 ( 7) × 10-5 (3.2 ( 0.8) × 10-4 (3 ( 1) × 10-4 (1.9 ( 0.9) × 10-4 (5 ( 5) × 10-3

g45 g680 630 ( 30 g590 g300 380 ( 4 120 ( 3 130 ( 3 180 ( 5 130 ( 4 ≈0

range of [phenol] (µM)b 0-100 (2) 0-30 (2) 0-30 (5) 0-30 (2) 0-30 (2) 0-30 (5) 0-30 (4) 0-30 (6) 0-30 (5) 0-30 (4) 0-30 (2)

a Structures of the carbonyl compounds are listed in Figure 2. Regression parameters were determined from a linear regression of the experimental data for each carbonyl fitted to: ΦHOOH,313 ) y-intercept + (slope)[phenol] (R 2 g 0.99 in all cases), where [phenol] is in molar units for the regression. Uncertainties are (1 standard error. For regressions based on two points, the slope represents a lower bound since insufficient data are available to demonstrate a linear dependence on [phenol]. Photolysis conditions: total concentration of carbonyl ) 10 µM carbonyl (1 µM for methylbenzoquinone) in air-saturated aqueous solution, pH ) 3.7 (100 µM H2SO4), 100 µM Na2SO4, 313 nm, 20 °C. b Value in parentheses is number of data points in the regression.

however, rapid destruction of phenol in these solutions, and the quantum efficiency for phenol destruction is linearly dependent upon phenol concentration (Figure 7). Based on these observations, it appears that the aromatic aldehyde or ketone acts as a “photocatalyst” and that phenol serves as the terminal source of electrons for reduction of O2 to HOO•/ •O -. Approximately 4-7 molecules of phenol are destroyed 2 for every molecule of HOOH produced (Table 5), suggesting that under these conditions the majority of the photodestroyed phenol leads to products other than HOOH.

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As noted previously, there are some exceptions to the behavior typically observed with the aromatic carbonyls studied. For 2-methoxybenzaldehyde there is significant production of HOOH in the absence of phenol (ΦHOOH,313C,313 ∼ 5); moreover, the parent carbonyl is destroyed, both with (30 µM) and without added phenol (ΦC,313 ) 0.014 ( 0.002 and 0.010 ( 0.002, respectively). Analogous behavior is observed with 3-methoxybenzaldehyde: ΦHOOH,313C,313 ∼ 1 in the absence of phenol and ΦC,313 ) 0.011 ( 0.001 with 0 or 30 µM added phenol. For methylbenzoquinone there is

TABLE 4. Linear Regressions of (ΦHOOH,313)-1 versus [Phenol]-1 for the Non-Phenolic Aromatic Carbonyls at pH 3.7a carbonyl

y-intercept

slope (M)

[phenol] range (µM)b

benzaldehyde acetophenone 4-methylbenzaldehyde 4-methoxybenzaldehydec 3,4-dimethoxybenzaldehyde 3,4,5-trimethoxybenzaldehyde 3,4,5-trimethoxyacetophenone furaldehyde

11 ( 4 7.7 ( 0.2 12 ( 1 9.2 ( 0.5 22 ( 10 55 ( 14 54 ( 6 56 ( 32

(2.2 ( 0.1) × 10-2 (1.2 ( 0.03) × 10-3 (1.2 ( 0.02) × 10-3 (2.3 ( 0.04) × 10-3 (7.6 ( 0.2) × 10-3 (5.6 ( 0.3) × 10-3 (3.8 ( 0.1) × 10-3 (6.0 ( 0.6) × 10-3

100-1000 (3) 30-1000 (3) 5-30 (4) 5-1000 (8) 10-30 (3) 10-30 (4) 10-30 (4) 10-30 (3)

a Structures of the carbonyl compounds are listed in Figure 2. Regression parameters were determined from a weighted linear regression of the experimental data for each carbonyl fitted to: (ΦHOOH,313)-1 ) y-intercept + (slope)[phenol]-1 (R 2 g 0.99 in all cases), where [phenol]-1 is in units of M-1 for the regression. In a given regression each data point was weighted by its phenol concentration in order to better estimate the y-intercept; estimates of slope were relatively unaffected by this weighting. Uncertainties are (1 standard error. Photolysis conditions: total concentration of carbonyl compound ) 10 µM, 30 µM phenol, air-saturated aqueous solution, pH ) 3.7 (100 µM H2SO4), 100 µM Na2SO4, 313 nm, 20 °C. b Range of phenol concentrations used in the linear regression. Number of data points in each regression is given in parentheses. c Data are plotted in Figure 6.

TABLE 5. Quantum Yields of HOOH (ΦHOOH,313) and Destruction Quantum Efficiencies of Non-Phenolic Carbonyl Chromophores (ΦC,313), with 30 µM Added Phenol at pH 3.7a carbonyl

103 × ΦHOOH,313b

ΦHOOH,313EC,313c (L Einstein-1 cm-1)

103 × ΦC,313d

ΦHOOH,313/ΦC,313e

ΦHOOH,313/ΦPHENOL,313f

benzaldehyde acetophenone 4-methylbenzaldehyde 2-methoxybenzaldehyde 3-methoxybenzaldehyde 4-methoxybenzaldehyde 3,4-dimethoxybenzaldehyde 3,4,5-trimethoxybenzaldehyde 3,4,5-trimethoxyacetophenone 2-furaldehyde methylbenzoquinone

1.4g 21 17 19 13 11 3.6 4.2 5.6 4.0 ≈0

0.13 1.9 2.2 59 33 23 33 23 20 1.3 ≈0

7(4 4(8 10 ( 2 10.7 ( 0.9 0.7 ( 0.8 0.04 ( 0.1 0.4 ( 0.3 0.2 ( 0.6 0.3 ( 0.2 930 ( 20

2-7 >1 1.7-2.5 1.1-1.3 >8 >20 6-50 >7 8-40 0.99) to plots of (ΦHOOH,313)-1 versus

[P]-1 over the entire range of phenol concentrations studied (Table 4 and Figure 6). Thus eq 7 correctly describes the


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FIGURE 9. Proposed mechanism for aqueous-phase HOOH photoformation from non-phenolic aromatic carbonyl compounds in the presence of a phenol, illustrated here for 3,4-dimethoxybenzaldehyde and phenol. Light absorption by the carbonyl yields an excited singlet state (1[ArCO]*), which undergoes intersystem crossing [k(ISC)] with unit efficiency to the excited triplet state (75, 111, 112). The excited-state triplet is present as protonated and unprotonated acid-base forms (the structure of the protonated triplet is based on calculations from ref 113). Both forms of the triplet can oxidize phenol [k(HT,P) and k(T,P)], by electron transfer and/or H-atom abstraction, to form a ketyl radical (71). The ketyl radical forms HOO• by reduction of O2: either through a concerted reaction; or by forming a ketylperoxyl radical (in dotted brackets; 114), which, by analogy to other r-hydroxy organic peroxyl radicals (115-118), could rapidly decay [k(ROO)] to form HOO•. In either case, the original carbonyl compound is regenerated. The formation of HOOH can occur by disproportionation of HOO•/•O2- [k(HOO)] (119), or by reactions with phenol [k(HOO,P)] (55). Note: k(ISC) ) kISC, k(F) ) kF, k(T,P) ) kT,P, k(HT,P) ) kHT,P, k(T,d) ) kT,d, and k(HT,d) ) kHT,d; HT and T denote processes for the protonated and unprotonated triplet state, respectively. observed dependence of ΦHOOH,313 on phenol concentration: a linear dependence at low phenol concentrations and a saturation behavior observed at high phenol concentrations. At lower phenol concentrations, where phenol is a minor sink for the excited-state carbonyl triplets, eq 7 simplifies (after taking the inverse) to

(

)

fT,PkT,PRT + fHT,PkHT,PRHT ΦHOOH,λ ) ΦISC[P] kT,dRT + kHT,dRHT

(8)

This aspect of the model also agrees well with the experimental observations, as evidenced by the excellent linear fits (R 2 g 0.99) for plots of ΦHOOH,313 versus [P] at low phenol concentrations (Figure 5 and Table 3). It should be noted that although there is good agreement between the experimental results and the model predictions of eqs 7 and 8, the model does not predict the lag time that was observed with some of the samples (see Experimental Section, Data Analysis). As can be seen from eq 8, at a given pH the observed HOOH quantum yield is the sum of contributions from the protonated (HT) and unprotonated triplets (T). Regression of the functional form of eq 8 (the “mechanistic model”) to the experimental data of Figure 8 gave good fits (R 2 g 0.95) as shown in Table 6. Thus, the proposed mechanism (Figure 9) also agrees well with the experimentally observed pH dependence of ΦHOOH,313 at low phenol concentrations. Evaluation of ΦHOOH,λ in eq 8 at the asymptotic limits RT )

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1 and RHT ) 1 yields the following, respectively:

(ΦHOOH,λ)T )

(ΦHOOH,λ)HT )

fT,PkT,PΦISC [P] kT,d

(9)

fHT,PkHT,PΦISC [P] kHT,d

(10)

where (ΦHOOH,λ)T [(ΦHOOH,λ)HT] is the HOOH quantum yield of the unprotonated (protonated) form of the carbonyl triplet. From these expressions, it can be seen that the net reactivities of the protonated and unprotonated triplet state of a given carbonyl each depend on the rate constant for reaction of the triplet with phenol to form the ketyl radical (fT,PkT,P, fHT,PkHT,P) and the lifetime of the excited triplet (1/kT,d, 1/kHT,d). As noted above, the protonated and unprotonated triplets exhibit vastly different net reactivities (Table 6). In addition to being consistent with the experimental evidence presented above, the proposed mechanism (Figure 9) is also supported by several reports that excited-state π-π* and n-π* carbonyl triplets react rapidly with phenols (k ∼ 108-109 M-1 s-1) to form ketyl radicals and phenoxyl radicals in high yields (70-73). The methoxy-substituted carbonyls studied here are all expected to have π-π* lowest triplet states (72, 73) since this state is stabilized, relative to the n-π* state, by polar solvents (e.g., water) and electron-donating substituents such as -OCH3 (74, 75).

TABLE 7. Calculated Rates of Equinox Sunlight Absorption and Equinox-Sunlight-Normalized Initial Rates of HOOH Formation in Aqueous Solutions Containing 10 µM Carbonyl and 30 µM Phenola

carbonyl

integrated rate of sunlight absorption (Einstein L-1 s-1 )b

benzaldehyde acetophenone 4-methylbenzaldehyde 2-methoxybenzaldehyde 3-methoxybenzaldehyde 4-methoxybenzaldehyde 3,4-dimethoxybenzaldehyde 3,4,5-trimethoxybenzaldehyde 3,4,5-trimethoxyacetophenone 2-furaldehyde methylbenzoquinone

6.1 × 10-9 5.4 × 10-9 7.7 × 10-9 5.0 × 10-7 2.2 × 10-7 6.7 × 10-8 6.1 × 10-7 4.0 × 10-7 2.3 × 10-7 2.5 × 10-8 1.8 × 10-7

equinox-normalized initial rate of HOOH formation (µM h-1)c pH 1.6 pH 3.7 0.24

17 18 13

0.03 0.41 0.47 34 10 2.8 7.9 6.0 4.5 0.35 ∼0d

a Photolysis conditions are given in Table 5 (pH 3.7) and Figure 8 (pH 1.6). Structures of the carbonyls are given in Figure 2. b Rate of sunlight absorption by a 10 µM aqueous solution of the carbonyl compound under clear sky conditions at midday of the equinox in Durham, NC (solar zenith angle ) 36°, eq 5). Phenol does not absorb at these wavelengths. c Equinox-sunlight-normalized initial rates of HOOH formation for each carbonyl were calculated (eq 4) from quantum yields determined at 313 nm (Table 5; pH 3.7 results) or from predicted quantum yields at 313 nm obtained from the regression fits in Figure 8 (pH 1.6 results). HOOH formation rates have an approximate relative standard deviation of (20%. d Result obtained using simulated sunlight illumination.

The oxidation of phenols by carbonyl triplets could occur either via (i) H-atom abstraction or (ii) electron transfer coupled with proton transfer from the phenoxyl radical cation or from solvent water. Reported H/D isotope effects (kH/kD) for the reactions between carbonyl triplets and several phenols (-H, p-OH, p-OCH3) in wet acetonitrile are ∼1.3 for benzophenone and 1.3-3.9 for p-methoxypropriophenone (71). Such small isotope effects do not strongly suggest that an H-atom abstraction mechanism was responsible for the observed reactivity, because isotope effects of up to 2.6 have been measured for electron transfer/proton transfer reactions of aqueous CH3OO• and reductants not containing an abstractable hydrogen (76). Thus, there is currently insufficient evidence to determine whether the oxidation of phenols by aromatic carbonyl triplets proceeds via H-atom abstraction or coupled electron/proton transfer. Based on published photochemical studies of aromatic carbonyls with transitionmetal chelates (77) and SO32-/HSO3- (78), it is probable that reduced transition metals [(e.g., Fe(II)] and SO32-/HSO3- could also reduce excited-state carbonyl triplets through electron transfer to form ketyl radicals. It is possible that HOOH photoproduction could occur through an oxylperoxyl biradical (PhC(O•)OO•), formed by addition of O2 to the carbonyl triplet (79). This biradical could oxidize phenol to form a ketylperoxyl radical which, in turn, could decay to HOO•/•O2- and the parent carbonyl (55). However, calculations indicate that the oxyperoxyl biradical is responsible for 107 s-1; 79); (ii) a maximum yield of 0.3 for the oxyperoxyl biradical from the interaction of O2 with the π-π* triplet excited state (based on reported yields for 1O2* of 0.8-1.0 for triplet π-π* states; 80); and (iii) the assumption that the rate constant for phenol oxidation is the same for the oxyperoxyl biradical and the carbonyl triplet state. The production of HOOH is not due to the oxidation of phenol by singlet (1∆g) molecular oxygen, 1O2*. If this was an important pathway, ΦHOOH,313 would be expected to increase with increasing pH, since the phenolate ion is a better reductant than phenol (81), and to exhibit an apparent pKa nearer to that of phenol (10.0; 81). In contrast, ΦHOOH,313 for the methoxy-substituted carbonyls increased with decreasing pH (Figure 8), and all of the apparent pKa values (1.7-3.9, Table 6) are far too low for phenol. Furthermore, if 1O2* was the dominant pathway for HOOH formation, then for a given

carbonyl with different phenolic reductants (PhOH ) phenol, ArOH ) substituted phenol) at the same concentration, a plot of (i) ΦHOOH,313(ArOH)/ΦHOOH,313(PhOH) versus (ii) the ratio of 1O2* rate constants k(1O2* + ArOH)/k(1O2* + PhOH) should be linear and have a slope of 1. However, this plot has a very poor linear relationship (R 2 ) 0.33) and a slope of 0.028 based on the ΦHOOH,313 values in Table 2 and on published rate constants (81). Calculated Rates of HOOH Formation and Carbonyl Loss in Sunlight. As shown in Table 7, calculated rates of sunlight absorption vary by a factor of ≈100. Table 7 also shows that calculated rates of sunlight absorption and calculated sunlight rates of HOOH photoproduction are highest for the methoxysubstituted carbonyls. At pH 3.7, HOOH photoformation rates for these compounds (10 µM carbonyl and 30 µM phenol) are typically 3-8 µM h-1. Calculated rates of HOOH formation at pH 1.6, where ΦHOOH,313 is near its maximum value for the three methoxy-substituted carbonyls tested, range from 13 to 18 µM h-1 for these chromophores. The two highest rates of HOOH photoformation at pH 3.7 are obtained for 2-methoxybenzaldehyde and 3-methoxybenzaldehyde (34 and 10 µM h-1, respectively; Table 7). However, these compounds will also undergo significant direct photolytic destruction in sunlight; calculated rates of carbonyl loss are 17 ( 3 and 8.4 ( 0.7 µM h-1, respectively. Based on measured values of ΦC,313 (Table 5), in the presence of phenol there will be no statistically significant direct photolytic destruction in sunlight for the other substituted benzaldehydes and acetophenones. As will be seen in a subsequent paper (54), there is good agreement between calculated sunlight HOOH formation rates and rates that were determined from solar-simulator illuminations. It should be noted that simple aliphatic monocarbonyls (formaldehyde, acetaldehyde, acetone) were found to be insignificant aqueous-phase sources of HOOH through direct sunlight photolysis (12). Implications for Atmospheric Chemistry. Based on the calculated HOOH production rates in sunlight (Table 7), illumination of aqueous-phase non-phenolic aromatic aldehydes and ketones in the presence of phenolic compounds is likely to be a significant source of hydrogen peroxide to cloud and fog drops and aqueous aerosol particles. In fact, equinox-normalized rates of peroxide photoproduction of up to 5.8 µM h-1 have been measured in winter fogwaters collected in Davis, CA (82), a region where high concentrations of aromatic carbonyls and phenols from wood combustion have been measured in winter fogs (25). As noted previously (13), based on modeling estimates, daytime sources of HOOH


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to continental cloud drops from gas-to-drop partitioning of HOOH and HOO• typically range from 1 to 10 µM h-1. The pH dependence of the HOOH photoformation is very important because large differences (up to 20-fold variation) in HOOH quantum yields are observed for a pH range (1.65.6) that is representative of many aqueous atmospheric condensed phases (Figure 8). The increase in HOOH photoformation rate with decreasing pH (Table 7) suggests that methoxy-substituted carbonyls will be even more significant photochemical sources of HOOH in acidified clouds and fogs and in acidic sulfate aerosols. This is significant since (i) the oxidation of aqueous forms of SO2 by HOOH is acid catalyzed down to pH ≈1-2 (1); (ii) the formation of H2SO4 from SO2 oxidation will accelerate HOOH photoformation from methoxy-substituted aromatic carbonyls; and (iii) in SO2 emission areas, HOOH is often the limiting reagent, relative to SO2, in the cloud-drop mediated oxidation of aqueous SO2 (13). It is significant that the methoxy-substituted benzaldehydes and acetophenones give the highest rates of HOOH photoformation and that phenolic compounds are efficient reductants, since these classes of compounds are major products from the combustion of the lignin component of biomass (22, 25, 27, 29, 47-49). In fact, an average of ∼2040% of aerosol organic carbon from wood-burning stoves and fireplaces is aromatic carbonyl and phenolic compounds (27). High concentrations of aromatic carbonyls and phenols have been measured in aerosols and fogs in areas impacted by winter residential wood burning (22, 25). Thus aqueousphase photoreactions of these compounds could also be significant in wood-smoke aerosol, based on (i) the findings reported here; (ii) the observations that wood-smoke particles take up significant amounts of water vapor (83); and (iii) the likelihood that small amounts of liquid water could significantly influence the protonation state of aromatic carbonyl triplets. Given that lignin is an important component of biomass and given the enormous magnitude of global biomass burning (an estimated 1012 kg of biomass carbon is burned annually; 84), it is likely that there are large inputs of aromatic carbonyls and phenolic compounds to the troposphere. In addition, biomass burning (including residential wood burning) occurs in all seasons, so there are continuous inputs of the resulting carbonyl and phenolic products into the atmosphere throughout the year. Furthermore, smoke particles from biomass burning are good cloud condensation nuclei (85), and smoke aerosol from biomass burning is efficiently scavenged by cloud drops (86). Hence, aerosol-associated aromatic carbonyl and phenolic compounds from these combustion sources will be efficiently delivered to atmospheric water drops. In fact, ultraviolet-visible absorption spectra of authentic cloudwaters and fogwaters (7, 12) commonly exhibit a (photobleachable) shoulder peak between 270 and 280 nm, the wavelength region where phenolic compounds typically exhibit absorption maxima (12, 87). Aromatic carbonyl and phenolic moieties are also present in lignin, (photo)degradation products of lignin, and terrestrial humic/fulvic substances (35, 36, 40, 42, 88-90). Thus, the incorporation of compounds derived from windblown soil dust (38, 91) and plant detritus into atmospheric waters and aerosols will serve as another source of aqueous-phase aromatic carbonyls and phenols. In this context, it is interesting to note that the mass fraction of organic carbon in wind-blown near-surface soil-dust aerosol ranges from 0.3 to 20% (38). It is not known how the photoreactivity of aromatic carbonyl structures are affected if they are part of a larger macromolecule (e.g., humic/fulvic substance). However, the observation that apparent triplet quantum yields range from 0.61 to 0.91 in aqueous solutions of soil fulvic acids between pH 1.5 and 8.5 (92) is consistent with the hypothesis that terrestrial humic/fulvic aromatic carbonyls could form HOOH in the same manner as the single-ring

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analogs studied here. Similar carbonyl and phenolic structural units are present in aquatic humic/fulvic substances (40), and it is possible that these units play important roles in the observed photoformation of HOOH in surface waters (93, 94). In models of aqueous aerosol and fog- and cloud-drop chemistry, the sources of aqueous-phase HOOH are generally considered to be gas-phase photochemical processes (followed by gas-to-drop transfer) and/or thermal reactions in the aqueous phase. However, the results presented here and the observed photoproduction of HOOH in authentic cloudwaters and fogwaters reported previously (7-10, 13-15, 17) demonstrate additional significant sources of HOOH to atmospheric water drops. Hence aqueous-phase photochemical reactions need to be included in future models of atmospheric chemistry in order to more accurately describe HOOH sources and reactions (e.g., S(IV) oxidation, hydroxyl radical production) in aqueous aerosols, fogs, and clouds. In addition, comprehensive measurements of speciated and total aromatic carbonyl and phenol concentrations in cloudwaters and fogwaters will allow the rates of aqueousphase HOOH photoformation from these chromophores to be estimated based on the calculated sunlight formation rates in Table 7. Given that terrestrial humic/fulvic substances may in some cases be the dominant sources of aromatic carbonyl and phenolic functionalities to atmospheric waters (Figure 1), it is also important to measure concentrations of these functional groups in atmospheric waters. Finally, measurements of intrinsic rate parameters and constants (fT,P, kT,P, fHT,P, kHT,P) and lifetimes (kT,d, kHT,d) of relevant triplet species would help to further characterize these systems.

Acknowledgments Funding for this research was supported by a grant from the Atmospheric Chemistry Program of the U.S. National Science Foundation and by the U.S. National Institutes of Health through the Duke University Integrated Toxicology Program. We would also like to thank G. L. Kok (NCAR) for assistance with construction of the HPLC-based peroxide speciation system and N. A. Porter (Duke University) for discussions on organic photochemistry. A portion of this work was presented to the Division of Environmental Chemistry at the 210th ACS National Meeting.

Supporting Information Available Eleven tables of values from the ultraviolet-visible absorption spectra of the aromatic carbonyls studied here (12 pages) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the Supporting Information from this paper or microfiche (105 × 148 mm, 24× reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St. NW, Washington, DC 20036. Full bibliographic citation (journal, title of article, names of authors, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $25.50 for photocopy ($27.50 foreign) or $12.00 for microfiche ($13.00 foreign), are required. Canadian residents should add 7% GST. Supporting information is available to subscribers electronically via the Internet at http://pubs. acs.org (WWW) and pubs.acs.org (Gopher). Author-Supplied Registry Numbers: Acetic acid (sodium salt), 127-09-3; acetophenone, 98-86-2; ammonium sulfate, 7783-20-2; benzaldehyde, 100-52-7; 1,2-dihydroxybenzene (catechol), 120-80-9; 3,4-dimethoxybenzaldehyde (veratraldehyde), 120-14-9; 2,6-dimethoxyphenol (syringol), 91-10-1; formic acid (sodium salt), 141-53-7; 2-furaldehyde, 98-01-1; R-D-glucose, 492-62-6; glyoxal, 107-22-2; 4-hydroxybenzoic acid, 99-96-7; 2-methoxybenzaldehyde, 135-02-4; 3-methoxybenzaldehyde, 591-31-1; 4-methoxybenzaldehyde, 123-11-5; 4-methylbenzaldehyde, 104-87-0; methyl-1,4-benzoquinone, 553-97-9; 2-methoxyphenol (guaiacol), 90-05-1; 2-meth-

ylphenol (o-cresol), 95-48-7; 4-methylphenol (p-cresol), 10644-5; phenol, 108-95-2; 3,4,5-trimethoxyacetophenone, 113686-3; 3,4,5-trimethoxybenzaldehyde, 86-81-7.

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Received for review April 22, 1996. Revised manuscript received September 5, 1996. Accepted September 9, 1996.X ES960359G X

Abstract published in Advance ACS Abstracts, November 15, 1996.

Aromatic Carbonyl Compounds as Aqueous-Phase Photochemical

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