EPA/600/A-95/085 Heterogeneous Photoreaetion of Formaldehyde with Hydroxyl Radicals Daniel Driscoll Heterogeneous Chemistry and Aerosol Research Branch AREAL EPA RTP North Carolina 27711 ABSTRACT Atmospheric heterogeneous photoreactions occur between formaldehyde and hydroxyl radicals to produce formic acid. These photoreactions not only occur in clouds, but also in other tropospheric hydrometeors such as precipitation and dew droplets. Experiments were performed by irradiating Teflon tubes containing nanomole concentrations of formaldehyde and hydrogen peroxide. Following irradiation at a wavelength of 313 nanometers, the concentrations of formaldehyde and formate ion were analyzed using High Pressure liquid Chromatography and Ion Chromatography. Data are presented to illustrate how nitrate and sulfate anions effect the photo-oxidation of formaldehyde in the aqueous-phase, as well as the effects of these anions on the formate ion which is a product of this photo-oxidation. Formic acid and its precursor formaldehyde are soluble in the aqueous-phase of the atmosphere. However, inhibition of the atmospheric photo-oxidation reactions of these compounds coupled with evaporation of the clouds or dew droplets can result in recycling the compounds from the liquid-phase back to the gas-phase of the atmosphere. INTRODUCTION The literature contains little information regarding the heterogeneous chemistiy of atmospheric organic acids and their precursors. Grosjean1 reports that in the urban air of the California South Coast Air Basin, concentrations of formaldehyde range between 4-21 ppb, while formic acid concentrations varied from 1-13 ppb. Also Keene and Galloway2 determined that formic acid is responsible for 60% of the total acidity found in global non-urban precipitation. Emissions from anthropogenic activities as well as from natural sources combine to contribute formaldehyde to the gas-phase of the atmosphere. Formaldehyde is readily soluble in rain and cloud water. Figure 1 depicts the liquid-phase reactions of formaldehyde. After formaldehyde enters the aqueous-phase, hydrolysis converts the carbonyl compound to methanediol. This hydrate is stable in water solution. However, upon evaporation of the aqueous-phase, the hydrate reverts back to the volatile carbonyl form of the aldehyde. Virtually 100% of the formaldehyde present in aqueous solution exists as methanediol. This is not the case for other aldehydes. For example, only 58% of acetaldehyde present in aqueous solution exists in the hydrate form.3 In the presence of ultraviolet irradiation the hydroxyl radical abstracts a hydrogen atom from methanediol to form the hydrated formyl radical. The hydrate then reacts with dissolved oxygen to produce formic acid. Free radicals abstract a hydrogen atom from formic acid to produce formate, which then further reacts to carbon dioxide. EXPERIMENTAL A set of two FEP teflon tubes, each containing four milliliters, of solution was used in every ------- irradiation experiment. Both tubes contained three milliliters of an aqueous formaldehyde solution equivalent to 240 nanomoles. In addition, one tube also contained one milliliter of deionized water and was designated as the experimental control. The other contained one milliliter of hydrogen peroxide solution equivalent to 1000 nanomoles. Each irradiation experiment was conducted for time periods of from one to four hours. After irradiation, the samples were analyzed for formaldehyde and formate. Two additional four milliliter samples were also prepared prior to each irradiation experiment. These were similar in composition to the irradiated solutions, and were analyzed immediately for formaldehyde and formate. These results were used for the values of the irradiation control samples. A li-Cor spectroradiometer was used to measure the irradiance of the UVB lamp at a wavelength of 313 nanometers. The output of the lamp was then compared to the irradiance of ambient sunlight at the same wavelength. This analysis determined that solutions in the Teflon tubes were exposed to a radiant energy of 0.005 calorie/sq cm., while irradiance from the sun on a cloudless day is 0.004 calorie/sq cm. The contents of the tubes were maintained to within two degrees of ambient temperature by a stream of compressed air which impinged upon the exterior of the tubes during irradiation. The reagents used in these experiments were obtained from a commercial scientific supply source. The 37% formaldehyde and 3% hydrogen peroxide solutions were volumetrically titrated against standard iodine and potassium permanganate solutions respectively. Deionized water used in the experiment was obtained from a laboratory water filtration system which contained a five micron prefilter, activated carbon absorption, and deionization using a mixed bed ion exchange resin. ANALYTICAL All formaldehyde samples were prepared for analysis by reacting one milliliter of sample with one milliliter of a 2.4-Dinitrophenylhydrazine/Acetonitrile solution at ambient temperature for a period of ten minutes. The DNPH prepared from highly purified crystals, converts the formaldehyde to the corresponding hydrazone derivative. HPLC standards were prepared from pure crystals of formaldehyde hydrazone. Analysis were performed in the gradient mode using a Varian Model 5000 HPLC equipped with a Zorbax C18 reverse phase column. A Dionex UV detector operating at a wavelength of 360 nanometers was used for quantitation. Formate samples were analyzed on the Ion Chromatograph within one hour after collection to preclude losses in solution. Standards were prepared daily, and standard additions were performed to verify formate concentrations. Anion analysis was carried out in the isocratic mode using a Dionex Model 4000 Ion Chromatograph equipped with an IonPac AS-5 separator column, A conductivity detector was used for quantitation. discussion The lower line in Figure 2 shows the decrease in formaldehyde concentration resulting from the irradiation of solutions initially containing 240 nanomoles of formaldehyde and 1000 nanomoles of hydrogen peroxide. Formaldehyde continues to decrease in concentration up to three hours of irradiation. After that time the concentration stays relatively constant. Each data point on the upper line represents the results of two separate sets of experiments. Solutions in both sets contain formaldehyde and deionized water. In addition, samples in one set also contain 500 nanomoles of sulfate anion and the other solutions contain 500 nanomoles of nitrate anion. The anions were added as the sodium salts. The middle graph, like the one above it, also represents two separate sets of irradiation experiments. ------- Solutions in both sets were treated in the same manner as the ones just discussed. The difference, however, is that these solutions contain hydrogen peroxide. At a specified irradiation time, compare the differences in formaldehyde concentration between solutions in the lower two curves against concentrations depicted by the controls in the upper plot. At one hour irradiation, seventeen percent and twenty-four percent is photo-oxidized. At two hours irradiation, the removal is thirty percent and forty percent. Three hours irradiation removes thirty percent and fifty percent. After three hours irradiation, the formaldehyde removed is thirty percent and seventy percent. The inorganic anions are scavenging hydroxyl radicals causing a decrease in the removal of formaldehyde by photo-oxidation. The decrease is dependent upon the concentrations of hydrogen peroxide, formaldehyde, and inorganic anions. It demonstrates that the photo- oxidation of formaldehyde in the aqueous-phase is reduced by the presence of either sulfate or nitrate anions. The formyl radical, produced from the photo-oxidation of formaldehyde reacts with dissolved oxygen to produce formic acid. The formic acid is then photo-oxidized to formate ion. Aqueous solutions of formaldehyde and hydrogen peroxide were saturated with either air or oxygen. The solutions were then irradiated for periods of from one to four hours. As Figure 3 indicates, production of formate in air saturated aqueous solutions reaches a maximum concentration, then slowly diminishes as the available hydroxyl radicals are depleted, In oxygen saturated solutions, the initial formate concentration is slightly lower than that occurring in air saturated solutions. However, later on it achieves a greater maximum before decreasing back to a concentration similar to that obtained in air saturated solutions. McElroy and Waygood4 performed steady-state irradiation experiments with oxygen saturated aqueous solutions of hydrogen peroxide and formaldehyde. They too observed an initial inhibition of formate production followed by an increase in formate concentration. It appears that hydroxyl radicals first consume the dissolved oxygen present in solution before photo-oxidizing formic acid to produce formate ions. Figure 4 illustrates the effect that inorganic anions have upon formate ion production- The solutions in all three sets of experiments contain 240 nanomoles of formaldehyde and 1000 nanomoles of hydrogen peroxide. In addition, one set contains 500 nanomoles of nitrate anion and the other solutions contain 500 nanomoles of sulfate anion. Both anions were added as the sodium salt Within the irradiation interval between thirty minutes and two hours, formate production increases with length of irradiation. Also formate production is described by three separate rates of reaction. Several investigators conclude that nitrate and sulfate radicals are produced from the corresponding anions in aqueous solution. This occurs either by direct absorption of radiant energy or through reactions with hydroxyl radicals. Neta and Huie5 measured the reaction of nitrate, sulfate, and hydroxyl radicals with aqueous formic acid. They experimentally measured the rate constants < 1.0X10% 1.4X106, 1.5X1CP of these reactions to be respectively. After two hours irradiation, the formate production due to hydroxyl radicals decreases because of depletion of those radicals. However, formate production due to sulfate and nitrate radicals continues to increase. This increase is dependent upon the concentrations of anion and hydrogen peroxide. However radicals photolytically produced from inorganic anions result in an increase in formate ion production. CONCLUSION Formaldehyde is ubiquitous in the atmosphere. It is the precursor of formic acid which is responsible for a significant portion of the acidity found in global precipitation. Its unique ability to exist in the liquid-phase as a stable hydrate, would indicate that clouds, fogs, and dew are the ultimate sinks for removal of formaldehide from the atmosphere. However, if the ------- atmospheric aqueous-phase evaporates before hydrated formaldehyde is photo-oxidized, then the remaining hydrate reverts back to the volatile carbonyl form of the compound. Data indicates that in the aqueous-phase either sulfate or nitrate solutions reduce the concentration of available hydroxyl radicals, thereby inhibiting the photo-oxidation of formaldehyde. Sulfate and nitrate anions also increase the aqueous-phase formate ion concentration resulting from the photo-oxidation of formaldehyde. In so doing, the likelihood of recombination of formate and hydrogen ions to produce volatile formic acid is also increased. All of these conditions considered together indicate that the aqueous-phase of the atmosphere is not the ultimate sink for gas-phase formaldehyde. Furthermore, the atmospheric aqueous-phase may even function as a source for gas-phase formic acid. ------- H H— C + HOH < > H—C—OH j OH H H —C—OH + : OH > H— C—OH 313 nm OH OH H- C— OH „ I + 02 —» H02 + h— Q O OH \ OH /? H- C OH 313nm -* HCOO + H Figure 1. Reactions of formaldehyde in aqueous solution. 300 0 > 1 < EC o LL CO UJ —I O I> z < z 200 100 HCHO/H20/SO4 HCHO/H20/NO3 HCHO/H202/SO4 HCHO/H202/NO3 HCHO/H202 -1 r 112 3 HOURS OF IRRADIATION AT 313 NANOMETERS Figure 2. Effects of inorganic anions upon the photoreaction of formaldehyde. ------- OXYGEN SATURATED SOLUTION 60 - 50 - 40 - A IK S VI I K VI KI) SOLUTION 30 20 - 10 - HOURS OF IRRADIATION AT 313 NANOMETERS Figure 3. Effects of dissolved oxygen upon formate ion production. 70 HCH0/H202/S04 60 - HCHO/H202 50 - 40 - 30 - HCH0/H202/N03 20 - 10 - 0 2 4 HOURS OF IRRADIATION AT 313 NANOMETERS Figure 4. Effect of inorganic anions upon the production of formate ion. ------- References Grosjean, D.Environmental Science Technology 198216 254-262. Keene, W.C.; Galloway, J.N.; Holden, J.D., Journal of Geophysical Research 1983.28 5122-5130. Bell; Clunie. Transactions Faraday Society 1952 48 439. McElroy, J.W.; Waygood, S. J. Journal Chemical Society Transactions 1991.81 1513-1521. Neta, P.; Huie, R.E.; Journal Physical Chemistry 1986 jq 4644-4648. ------- Disclaimer The information in this document has been funded wholly by the United States Environmental Protection Agency. It has been subjected to Agency review and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. ------- Key Words Formaldehyde Formic Acid Hydroxyl Radicals Photo-oxidation Liquid Chromatography Ion Chromatography ------- TECHNICAL REPORT DATA 1. REPORT NO. EPA/600/A-95/085 2. 3 .R 4. TITLE MID SUBTITLE HETEROGENEOUS PHOTOREACTION OF FORMALDEHYDE WITH HYDROXYL RADICALS S.REPORT DATE 6.PERFORMING ORGANIZATION CODE 7. AUTHORCS) Daniel Driscoll 8.PERFORMING ORGANIZATION REPORT NO. 9. PERFORMING ORGANIZATION NAME AND ADDRESS Heterogeneous Chemistry and Aerosol Research Branch National Exposure Research Laboratory Research Triangle Park, NC 27711 10.PROGRAM ELEMENT NO. 11. CONTRACT/GRANT NO. 12. SPONSORING AGENCY NAME AND ADDRESS National Exposure Research Laboratory Office of Research and Development U.S. Environmental Protection Agency Research Triangle Park, NC 27711 13,TYPE OF REPORT AND PERIOD COVERED 14. SPONSORING AGENCY CODE 15. SUPPLEMENTARY NOTES 16. ABSTRACT Atmospheric heterogeneous photoreactions occur between formaldehyde and hydroxyl radicals to produce formic acid. These photoreactions not only occur in clouds, but also in other tropospheric hydrometeors such as precipitation and dew droplets. Experiments were performed by irradiating Teflon tubes containing nanomole concentrations of formaldehyde and hydrogen peroxide. Following irradiation at a wavelength of 313 nanometers, the concentrations of formaldehyde and formate ion were analyzed using High Pressure Liquid Chromatography and Ion Chromatography. Data are presented to illustrate how nitrate and sulfate anions effect the photo-oxidation of formaldehyde in the aqueous-phase, as well as the effects of these anions on the formate ion which is a product of this photo- oxidation. Formic acid and its precursor formaldehyde are soluble in the aqueous- phase of the atmosphere. However, inhibition of the atmospheric photo-oxidation reactions of these compounds coupled with evaporation of the clouds or dew droplets can result in recycling the compounds from the liquid-phase back to the gas-phase of the atmosphere. 17. KEY WORDS AND DOCUMENT ANALYSIS a. DESCRIPTORS b.IDENTIFIERS/ OPEN ENDED TERMS C.COSATI Formaldehyde Formic Acid Hydroxyl Radicals Photo-oxidation Liquid Chromatography Ion Chromatography ------- |