Determination of o-phthalic acid in snow and its photochemical degradation by capillary gas chromatography coupled with flame ionization and mass spectrometric detection

Chemosphere 83 (2011) 1014–1019

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Determination of o-phthalic acid in snow and its photochemical degradation by capillary gas chromatography coupled with flame ionization and mass spectrometric detection Yuegang Zuo a,b,⇑, Kai Zhang a,b, Jinping Wu a,b, Bin Men a,b,c, Mengchang He c a

Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth and University of Massachusetts Graduate School of Marine Sciences and Technology, 285 Old Westport Road, North Dartmouth, MA 02747, USA University of Massachusetts Graduate School of Marine Sciences and Technology, 285 Old Westport Road, North Dartmouth, MA 02747, USA c State Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, Beijing 100875, China b

a r t i c l e

i n f o

Article history: Received 2 November 2010 Received in revised form 3 February 2011 Accepted 3 February 2011 Available online 3 March 2011 Keywords: Phthalic acid Iron Gas chromatography–mass spectrometry Snow Rain Photodegradation

a b s t r a c t Phthalic acid and its photochemical degradation has been determined in snow and rainwater samples collected during winters (2003–2010) in the Southeast of Massachusetts using capillary gas chromatography (GC) with flame ionization and mass spectrometric detection. Water samples were dried using a rotary evaporator and derivatized with a 14% BF3/methanol reagent before GC analysis. The developed method proved simple and accurate. Phthalic acid was found in snow samples collected in a concentration range of 7.22–76.5 nM. The photodegradation of phthalate was carried out under 300 nm UV light. The direct photodecomposition of the acid is slow (5% h1). However, the addition of dissolved Fe(III) species at 2.0 lM accelerated the light-induced degradation of phthalic acid by 3.5 times in the atmospheric water samples. Photodegradation rates of phthalic acid increases with decreasing pH value of water samples in the range of pH 2.8–4.5. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Dicarboxylic acids are a group of the most abundant organic compounds in the atmosphere. Due to their hygroscopic properties, dicarboxylic acids have been considered to be important cloud condensation nuclei and thus play a significant role in affecting the chemical and physical properties of atmospheric aerosols and water droplets, which influence the earth energy balance and climate. In the past few decades, phthalic acids have been the major industrial material used to produce plastic toys and bottles, adhesives, films, polymers, etc. (Staples et al., 1997). There are three phthalic acid isomers, o-phthalic, m-phthalic and p-phthalic acid. According to previous studies, phthalates are suspected as potent androgen receptor antagonists and could affect male reproductive systems (Sharp, 1998). In the 1980s the US Environmental Protection Agency (EPA) and several other countries classified the commonly occurring phthalates as priority pollutants and recommended maximum admissible concentration in water of 6 lg L1 for the di(2-ethylhexyl) phthalate (US Environmental Protection

⇑ Corresponding author at: Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth, USA. Tel.: +1 508 999 8959; fax: +1 508 999 9167. E-mail address: [email protected] (Y. Zuo). 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.02.008

Agency, 1991). As ubiquitous environmental contaminants, phthalic acids are detected not only in landfill leachate water, sewage sludge, surface water, and sediment but also in atmospheric aerosol and rainwater samples collected worldwide (Staples et al., 1997). Little information, however, is available about phthalate concentration levels in snow (Guo, 2008; Winterhalter et al., 2009). On the other hand, biodegradation of phthalates is believed to be an important source of phthalic acids existing in aquatic environment (Kleerebezem et al., 1999; Jonsson et al., 2003). Combustion of fossil fuel, biomass burning, as well as photodegradation of various aromatic hydrocarbons are the primary processes that influence the atmospheric concentration of phthalic acids (Chebbi and Carlier, 1996). Various studies have been conducted to examine the mechanisms, pathways, and conditions of biodegradation of phthalic acids. In aquatic environment phthalic acids can be aerobically degraded by microbes (Ribbons et al., 1983). Under anaerobic conditions, the fate of phthalic acids is determined by mineralization process. Photooxidation of phthalates in sunlit surface waters does not appear to represent an important transformation process. Based on the reported data, it is estimated that half-lives of diethyl phthalate (DEP) range from 2.4 to 12 years. A solution of butylbenzyl phthalate (BBP) exposed to sunlight for 28 d resulted in less than 5% degradation, which means the half-life of BBP is longer

Y. Zuo et al. / Chemosphere 83 (2011) 1014–1019

than 100 d if considering photolysis as the only degradation driving force (Chelsea et al., 1980; Wolf et al., 1980; Howard, 1991). In contrast to the minor role of photodegradation in natural surface water, photochemical processes may be much more important to the atmospheric fate of phthalic and other carboxylic acids ( Zuo and Hoigné, 1993, 1994; Zuo and Jones, 1996; Zuo et al., 2005; Deng et al., 2006). Photochemical reactions of phthalic acids could be initiated via either direct absorption of UV radiation by itself or through reactions with photochemically generated oxygencontaining transient species such as hydroxyl radicals. So far the photodegradation of phthalic acid has not been well understood; No study has been reported on the photodegradation of this compound in atmospheric condensed phases (Atkinson, 1988; Kelly et al., 1994). Though great efforts have been made to minimize these compounds in the effluent from plastic production factories, it is still necessary to investigate the fate of phthalic acids in the environment. The aims of this study are to examine o-phthalic acid in snow, which is a vital part of the cycling of phthalic acids in the atmosphere particularly during the winter, and the photochemical degradation of o-phthalic acid, including the effects of pH and concentration of Fe(III) on the degradation in snow water. 2. Experimental methods 2.1. Chemicals and samples Phthalic acid, diethyl phthalate (DEP), used as the internal standard, and ferric ammonium sulfate were purchased from Fisher Scientific (Fair Lawn, NJ). Boron Trifluoride (BF3) Methanol Solution (14% w/v in methanol) was purchased from EM Science (Gibbstown, NJ). Acetonitrile and n-hexane were obtained from Pharmco Products (Brookfield, NJ). Except where noted, all chemicals were analytical grade and were used as received. Snow samples were collected using either glass or aluminum containers from University of Massachusetts Dartmouth campus (North Dartmouth, MA; 41.63°N) during the winters of 2003– 2010. After collection, samples were stored in the dark at 4 °C for a few hours. Then the snow water was filtered through 0.45 lm Whatman nylon membrane filters and used for this study. Precautions have always been taken to minimize sample contamination. All sample containers, glassware and filtration devices were thoroughly cleaned with 0.1 M HCl solution and then finally with doubly distilled–deionized water. The blank chromatograms with the doubly distilled–deionized water have shown no phthalic acid peak. 2.2. Identification and quantification of phthalic acid in snow samples The pH of each sample was measured using a Corning M240 pH meter (Ciba Corning Diagnostics Limited, Sudbury, England) and then 2 L of each snow water sample was concentrated to about 5 mL by a rotary evaporator under a vacuum at room temperature. The concentrated sample was transferred to a 10 mL glass vial and further concentrated to almost dryness by the same rotary evaporator under a vacuum. A 14% BF3/methanol mixture (0.6 mL) was added into the glass vial. The sample and the reagent were heated at 100 °C for 40 min to form dimethyl phthalate. The ester of phthalic acid was extracted with 5 mL of n-hexane after adding 3 mL of pure water and 0.2 mL of acetonitrile. The hexane layer was further washed with 3 mL of distilled water twice and dried under a nitrogen stream, then dissolved in 50 lL of hexane with the internal standard, diethyl phthalate, for GC measurement. GC analysis was carried out on a Shimadzu GC-17A gas chromatograph equipped with a flame ionization detector (FID), a Shi-

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madzu AOC-20i GC auto-injector and a Gateway E-4200 computer that utilizes CLASS-VP Chromatography Data System Version 4.2 (Shimadzu Scientific Instruments, Columbia, MD, USA). Samples were separated on a 30 m  0.32 mm i.d., 1.00-lm film ECTM-5 capillary column (Alltech, Inc., Deerfield, IL, USA). The stationary phase was composed of 5% phenyl- and 95% dimethyl-polysiloxane mixture. Helium was employed as carrier gas and set at a linear velocity of 27 cm s1. N2 make-up gas, H2 and compressed air were used for the FID. The injector volume was 2 lL in splitless mode. The column temperature was programmed from 50 °C to 200 °C at a rate of 10 °C min1, with a final hold time of 2 min. The injector and detector temperature were maintained at 250 °C and 310 °C, respectively. The peak identification and purity verification was performed by employing a Hewlett–Packard (HP) model GC 5890 Series II gas chromatograph coupled with an HP 5971 series mass selective detector and an HP 7673 GC autosampler (Zuo et al., 2007). Samples were separated on a DB-5 fused silica capillary column (30 m  0.32 mm i.d., 0.25-lm film, J & W Scientific, Folsom, CA). The column temperature was programmed from 50 °C to 200 °C at 10 °C min1, then maintained for 2 min. Ultrahigh purity helium with an inline Alltech oxygen trap was used as carrier gas with a flow rate of 1.2 mL min1. The injector temperature was maintained at 280 °C and the injection volume was 1 lL in splitless mode. The interface temperature was held at 305 °C. Mass spectra were scanned from m/z 50 to 650 at a rate of 1.5 scans s1. Identification of phthalates was performed by comparing the retention time of the suspect peak with that of an authentic standard and its mass spectra. The quantification of dimethyl phthalate was based on the ratio of the peak area of dimethyl phthalate over the peak area of the internal standard, diethyl phthalate. 2.3. Fe(III)-catalyzed photodegradation of phthalic acid in aqueous solution The test solutions of phthalic acid were prepared by the dilution of a 10 mM stock solution to 10.0 lM. A 20 mM ferric ammonium sulfate solution was prepared and used as the catalyst. At pH = 3.70, with various Fe(III) concentrations (0.00 lM, 0.50 lM, 1.00 lM and 2.00 lM), photodegradation reactions of phthalic acid were conducted in quartz tubes (20 cm long and 2.5 cm i.d.) filled with 100 mL of the 10.0 lM phthalic acid solution. To study the effects of pH on Fe(III)-catalyzed photodegradation, three series of experiments were carried out at different pH (pH = 2.80, 3.70, and 4.50, adjusted with 0.10 M of HClO4 or 0.10 M of NaOH solutions) at an Fe(III) concentration of 2.0 lM, and phthalic acid solution of 10.0 lM. All the above described solutions were irradiated in a Rayonet photochemical chamber reactor (RTR-100, Southern N.E. Ultraviolet Co., Connecticut, USA) equipped with 16 high pressure mercury lamps, generating 300 nm UV light. For each test solution, at different irradiation times (0, 5, 10, 15, 30, 60, 90, and 120 min), 5.0 mL of the irradiated solution was pipetted out to monitor the concentration change of phthalic acid using the GC–MS method described. 2.4. Fe(III)-catalyzed photodegradation of o-phthalic acid in spiked snow water In order to examine the photodegradation of phthalic acid in the matrix of snow water, two sets of experiments were conducted. One set was carried out by irradiating 100 mL of snow water spiked with 100 lL of 10 mM phthalic acid standard solution without the addition of Fe(III) catalyst. The other set was carried out under the same conditions, except the addition of 2.0 lM Fe(III). The pH of the snow water was 4.22.

Y. Zuo et al. / Chemosphere 83 (2011) 1014–1019

k ¼ initial reduction rate=½FeðIIIÞ0 ;

and k ¼ 2:303DUk k Ik

So the intensity can be calculated by the following equation:

Ik ¼ k=2:303DUk k where D (cm) is the light pathlength, Uk is the quantum yield of the reaction at 300 nm, k (M1 cm1) is the molar absorptivity, and Ik (Einstein cm2 s1) is the intensity of light incident to the quartz tube sample. D = 1.0 cm, k = 300 nm, Uk = 0.14 and k = 2030 M1 cm1 (Faust and Hoigné, 1990). The light intensity Ik can be determined once k value is obtained by measuring the concentration change of Fe(II). 2.6. Determination of the quantum yield of Fe(III)-catalyzed photodegradation of phthalic acid Quantum yield reflects the efficiency by which the absorption of light leads to final products. The quantum yield (Uk) for the photochemical reduction of Fe(III) at 300 nm wavelength were obtained by irradiating a solution of Fe(III) (1.0 lM) and o-phthalic acid (10.0 lM) in the Rayonet photochemical chamber reactor installed with 16 high pressure mercury lamps. A UV-1601 spectrometer (Shimadzu) was used to measure the absorption of the solution at 300 nm at different time intervals. Ik was determined as described. Value of k can be calculated based on the concentration change of Fe(II). Therefore, if k is determined, quantum yield can be calculated by using the following equation:

Uk¼ ¼ k=2:303Dk Ik According to Beer’s law: Aphthalic acid+Fe(III) = kDCk = A/DC, where D = 1.0 cm and C = 1.0  105 M. 3. Results and discussion 3.1. Identification and quantification of phthalic acid in snow Phthalic acid contains two carboxylic function groups and its volatility is too low for direct GC analysis. In this study, a 14% BF3/methanol mixture was chosen as the derivatization reagent to convert the analyte into a volatile dimethyl phthalate ester. Prior to employing capillary GC for the determination of phthalic

a

Absorbance (mV)

It is necessary to measure the intensity of irradiating light for the calculation of quantum yield. By irradiating an Fe(III) solution, Fe(II) was formed as a product, which was analyzed spectrophotometrically. The irradiation was conducted in a Rayonet photochemical chamber reactor (RTR-100, Southern N.E. Ultraviolet Co., Connecticut, USA) equipped with high pressure mercury lamps, generating 300 nm UV light. The reagent solution used for Fe(II) analysis was prepared by mixing 2.00 mL of 0.25% (w/w) 1, 10-phenanthrolinium chloride solution, 3.00 mL of 63 mM acetic acid/0.10 M acetate buffer solution and 2.00 mL of 2.0 M NH4F solution in a 25-mL brown flask. The mixture was then transferred into the flask and diluted to 25 mL with distilled water. Fe(II) analysis was conducted by mixing 0.50 mL of the prepared reagent solution with 1.00 mL of irradiated Fe(III) solution at different time intervals. The concentration of Fe(II) produced was monitored using a UV-1601 spectrometer (Shimadzu). The initial concentration of the Fe(III) stock solution was standardized by reducing Fe(III) to Fe(II) with a solution of 1% (w/w) ascorbate, and measuring Fe(II) as described by Zuo et al. (2005). For constant illumination intensity, the photolysis of Fe(III) follows apparent first-order kinetics:

acid in snow water samples, the efficacy of the separation and detection of phthalate using GC/FID or GC/MS techniques was tested on a standard mixture of phthalic acid and diethyl phthalate as the internal standard. The instrument parameters, such as the temperature program and carrier gas flow rate, were optimized to obtain good resolution between phthalate esters in a short elution time. The results of this experiment are shown in Fig. 1a. Fig. 1b and c present the GC chromatograms for a typical snow and rain samples. Identification of phthalate in each sample was achieved by comparing the gas chromatographic retention times and mass spectra with the authentic standards. Fig. 2 shows a typical mass spectrum of dimethyl phthalate. The molecular ion is m/z 194 [M]+. The peak m/z 163 [M-31]+ indicated the loss of one –OCH3 group. Peak m/z 77 [M-118+1]+ was due to the cleavage of both –COOCH3 groups and the addition of H+ on the ring. Retention time of derivatized phthalate is 13.60 ± 0.10 min. The calibration curve for phthalic acid was linear over the concentration range tested: 0.00–300 lM. A typical curve followed the equation y = 14.0  –0.0125, with the square of the correlation coefficient being R2 = 0.998 (y is the ratio of peak area of standards to that of internal standard; x is the concentration). The analytical recovery of the described method was examined by adding known amounts of the standard into a snow water sample, and the percentage recoveries were found to be 96.5 ± 2.7%. The relative deviation of the developed method is less than 4%. In each sample, quantification of phthalic acid was conducted by relating the peak areas to calibration curve of phthalate standard solution. Table 1 lists the concentration of phthalic acid in 11 snow and a rain samples. Concentration of phthalic acid in snow samples was found to range from 7.22 to 76.5 nM, which are significantly higher than in those snow samples collected from remote areas (Narukawa et al., 2002; Winterhalter et al., 2009).

b

Absorbance (mV)

2.5. Determination of the volume-averaged incident light intensity, Ik

c

Absorbance (mV)

1016

60 50 40 30 20 10 0

Dimethyl Phthalate

I.S

9

11

13

15

17

Time (min) 25 I.S

20 Dimethyl Phthalate

15 10 5 0

9

11

13

15

17

Time (min) 60 50 40 30 20 10 0

Dimethyl Phthalate

I.S

9

11

13

15

17

Time (min) Fig. 1. (a) GC chromatogram for dimethyl phthalate and internal standard, (b) a typical GC chromatogram of a snow sample, and (c) a typical GC chromatogram of a rain sample.

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H+

163 31 O

77 C

O

CH3

C

O

CH3

149

O

133

H+

H+ Fig. 2. Mass spectrum and fragment analysis of dimethyl phthalate.

Table 1 Concentrations of phthalic acid in snow and rain samples. Snow samplesa

1

2

3

4

5

6

7

8

9

10

11

12 (rain)

pH Phthalic acid (nM)

6.22 10.1

5.85 76.5

3.80 7.46

4.87 7.22

5.45 16.6

4.99 12.0

5.12 7.22

4.88 18.7

4.55 11.4

5.22 30.7

5.01 31.3

5.30 43.9

a Snow samples No. 1–11 were collected on March 15, 2010; February 11, 2010; February 10, 2010; January 2, 2010; December 9, 2009; February 25, 2005; February 21, 2005; January 29, 2004; January 18, 2004; January 14, 2004; and December 19, 2003; respectively. Rain sample was collected on March 30, 2010.

3.2. Fe(III)-catalyzed photodegradation of phthalic acid and the proposed photodegradation mechanisms

This proposed mechanisms need to be confirmed by further experiments to identify the formation of HO(HOOCC6H3COOH).

Iron is one of most abundant transition metals observed in snow and rainwater with a reported concentration range of 1  106 to 1  104 M (Zuo et al., 2005). The iron hydroxide complexes are proved to be one of the precursors of hydroxyl radical in the Fe-catalyzed photochemical reactions (Faust and Hoigné, 1992; Zuo and Hoigné, 1992; Zuo, 2003.). Once hydroxyl radicals formed, due to their chemical reactivity, they would oxidize organic or inorganic compounds. Photodegradation of phthalic acid could be initiated by the reaction between phthalate ions (HOOCC6H4COO) and hydroxyl radicals, forming hydroxylphthalic radical as an intermediate, which could transform into monohydroxylphthalates. The monohydroxylphthalate formed can further react with hydroxyl radicals, generating dihydroxylphthalic acids or other intermediates, which would be further decomposed into CO2 and H2O.

3.3. Effects of Fe(III) concentration and pH on the photodegradation of phthalic acid

Step 1 FeðOHÞ2þ þ hm ! Fe2þ þ OH Step 2 OH þ HOOCC6 H4 COO ! HOðHOOCC6 H4 COOÞ Step 3 HOðHOOCC6 H4 COOÞ þ O2 HOðHOOCC6 H3 COOÞ þ HO2 Step 4 Fe2þ þ HO2 þ Hþ ! Fe3þ þ H2 O2

Four series of experiments were conducted to elucidate the effects of Fe(III) on the photodegradation of phthalic acid. The time courses in Fig. 3 show that about 10% and 18% of phthalic acid were decomposed after the test solution was irradiated with 300 nm UV light in the absence of Fe(III) for 60 min and 120 min, respectively. While in the presence of Fe(III) (0.5, 1.0, or 2.0 lM), half life of phthalic acid was no longer than 30 min. In the presence of 0.5 lM Fe(III), after 30 min irradiation only 31.3% of initial phthalic acid remained. When concentration of Fe(III) increased to 2.0 lM, more than 70% of phthalic acid was decomposed in 15 min irradiation. Fig. 4 presents the photodegradation at three different pH values. At pH 2.80 and 3.70, the degradation rates are slightly different but the time courses show similar kinetic behaviors. When the reaction was carried out at a higher pH (pH 4.5) the degradation rate was slower and half life for phthalate was much longer than those at pH 2.80 or 3.70, indicating that Fe(III)-catalyzed photodegradation of phthalic acid is pH dependent. These results are in good agreement with the previous findings that the formation of hydroxyl radical is pH dependent and Fe(III) could accelerate the formation of hydroxyl radicals (Zuo and Hoigné, 1992; Zuo, 2003).

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would be retarded when compared to the solution at pH 2.80 or 3.70. Last, the matrix of snow water was much more complex than that in distilled water. Species in the snow water might inhibit Fe(III)-catalyzed photochemical reactions. Consequently the phthalic acid would decompose in a less vigorous manner in the snow water. 3.5. Determination of the volume-averaged incident light intensity For constant illumination intensity, four 300 nm UV lamps were used. The photolysis of Fe(III) follows apparent first-order kinetics: Fig. 3. Effects of Fe(III) concentration on Fe(III)-catalyzed photodegradation of phthalic acid.

k ¼ initial reduction rate FeðIIIÞformation=½FeðIIIÞ0 ; k ¼ 2:303DUk k Ik where D is the light pathlength (cm), Uk is the quantum yield of the reaction at 300 nm, k is the molar absorptivity (M1 cm1), and Ik is the intensity of light incident to the quartz tube (Einstein cm2 s1). D = 1.0 cm, k = 300 nm, Uk = 0.14 and k = 2030 M1 cm1 (Faust and Hoigné, 1990). [Fe(III)]0 = 105 M Initial reduction rate = 0.00063 M s1.

Ik ¼ k=2:303DUk k = 0.096 M s1 3.6. Determination of the quantum yield of Fe(III) catalyzed photolysis of phthalic acid Fig. 4. Effects of pH on Fe(III)-catalyzed photodegradation of phthalic acid.

Sixteen 300 nm UV lamps were used for the quantum yield measurement.

3.4. Photodegradation of phthalic acid in snow water

Aphthalic acidþFeðIIIÞ ¼ 0:0101; A ¼ k DC; k ¼ A=DC

To simulate the photodegradation of phthalic acid in snow, spiked snow water with phthalic acid was irradiated. Experimental data indicate that Fe(III)-catalyzed photodegradation of phthalic acid was faster than that in the absence of Fe(III). The corresponding time courses of the reaction were presented in Fig. 5. Only 10% of phthalic acid was oxidized in the absence of Fe(III), after snow water was exposed to 300 nm UV light for 120 min, while the addition of 2.0 lM Fe(III) resulted in a decomposition of about 35% of phthalic acid. Without Fe(III) catalyst, degradation rates of phthalic acid were about the same in distilled water and snow water. Matrix effect was not obvious. However when catalyzed by 2.0 lM Fe(III), phthalic acid in distilled water decomposed much faster than it did in snow water. Three factors could contribute to the above difference. First, snow water contains many other organic compounds, which could slow down the decomposition of phthalic acid by consuming the OH radicals formed. Second, the pH of the sample solution affects the formation of OH radicals and thus the degradation of phthalic acid (see Fig. 4). The pH of snow water was around 4.22, at which the formation of OH radicals

where D = 1.0 cm and C = 1.0  105 M, k = 1010 M1 cm1, AFe(III) = 0.0028, A phthalic acid = 0.0000, k = (10–8.76)/600 = 0.0022 M s1 Ik = 0.384 M s1 (4  0.096 = 0.384 M s1), Uk= = k/2.303DkIk= 2.5  106. 4. Conclusions A GC–MS method was developed to identify and quantify ophthalic acid in snow. The organic compound was detected in the snow samples collected on the campus of University of Massachusetts, North Dartmouth, Massachusetts, the United States during winter time (2003–2010). The factors that affect photochemistry of phthalic acid and the kinetic characteristics of its photodegradation catalyzed by Fe(III) were also investigated. Compared with un-catalyzed photodegradation of phthalic acid, 2.0 lM of Fe(III) could significantly accelerate the photochemical processes under the conditions of atmospheric water. Kinetic data obtained in this study also show that pH could affect the photodegradation of phthalic acid significantly. Acknowledgments The authors would like to thank Drs. E. Ojadi, K. Lin, T. Su and Y. Yang for their contributions to this work. This work was partly supported by the NSF under Grants ATM 9984755 and OCE 0752033. References

Fig. 5. Photodegradation of phthalic acid in spiked snow water.

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