Atmospheric photolysis rate with hydroxyl

Although complex chemical transformations — mainly photochemical — take place in the atmosphere, many chemically stable compounds may be transported intact via the atmosphere and subsequently enter the aquatic and terrestrial environments in the form of precipitation. Although the whole issue of chemical reactions in the troposphere lies outside the scope of this account, some comments are given in Chapter 4, Section 4.1.2, and reference should be made to the comprehensive account of principles given by Finlay-son-Pitts and Pitts (1986). The persistence in the troposphere of xenobiotics — even those of moderate or low volatility — is determined by the rates of transformation processes. These involve reactions with hydroxyl radicals, nitrate radicals, and ozone, or direct photolysis. Reactions with hydroxyl radicals are generally the most important. Illustrative values are given for the rates of reaction (cm3 s 1 molecule1) with hydroxyl radicals, nitrate radicals, and ozone (Atkinson 1990). 

Vapor-phase solvents ean dissolve into water vapor, and be subjeet to hydrolysis reaetions and ultimately, preeipitation (wet deposition), depending on the solubility of the given solvent. The solvents may also be adsorbed by partieulate matter, and be subjeet to dry deposition. Lyman asserted that atmospherie residenee time eannot be direetly measured that it must be estimated using simple models of the atmosphere. Howard et al. ealeulated ranges in half-lives for various organie eompounds in the troposphere, and eonsidered reaetion rates with hydroxyl radieals, ozone, and by direet photolysis. 

The major fate mechanism of atmospheric 2-hexanone is photooxidation. This ketone is also degraded by direct photolysis (Calvert and Pitts 1966), but the reaction is estimated to be slow relative to reaction with hydroxyl radicals (Laity et al. 1973). The rate constant for the photochemically- induced transformation of 2-hexanone by hydroxyl radicals in the troposphere has been measured at 8.97x10 cm / molecule-sec (Atkinson et al. 1985). Using an average concentration of tropospheric hydroxyl radicals of 6x10 molecules/cm (Atkinson et al. 1985), the calculated atmospheric half-life of 2-hexanone is about 36 hours. However, the half-life may be shorter in polluted atmospheres with higher OH radical concentrations (MacLeod et al. 1984). Consequently, it appears that vapor-phase 2-hexanone is labile in the atmosphere. 

In addition to degradation by hydroxyl and nitrate radicals, all three cresol molecules absorb small amounts of W light with wavelengths above 290 nm (Sadtler Index 1960a, 1960b, 1966). Therefore, direct photolysis is also possible however, the photolysis rate is probably slow compared to the reaction with atmospheric radicals. 

Air half-life is a few hours in the sunlit troposphere ty, = 19 and 50 h by dry deposition and wet removal, respectively ty, = 12 d when reacts with NO3 radical by H-atom abstraction. (Howard 1989) photooxidation ty, = 7.13-71.3 h, based on measured rate constant for the vapor-phase reaction with hydroxyl radical in air (Atkinson 1985 quoted, Howard et al. 1991) ti/j = 1.26-6.0 h, based on photolysis half-life in air (Howard et al. 1991) atmospheric transformation lifetime was estimated to be 1 to 5 d (Kelly et al. 1994) calculated lifetimes of 1.2 d, 80 d and > 4.5 yr for reactions with OH radical, NO3 radical and O3, respectively (Atkinson 2000). 

According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere, bromotrichloromethane, which has a measured vapor pressure of 39mmHg at 25°C, is expected to exist solely as a vapor in the ambient atmosphere. Based on bromotrichloromethane s structural similarity to bromotrifluoromethane, it is expected to slowly degrade in the atmosphere by reaction with photochemically produced hydroxyl radicals the half-life for bromotrichloromethane s reaction in air is estimated to be greater than 44 years. Photolysis may occur based on bromotrichloromethane s structural similarity to other halogenated methane compounds but not at an environmentally relevant rate. 

PROBABLE FATE photolysis . C-Cl bond photolysis can occur, not important in aquatic organisms, photooxidation half-life in air 9,24-92.4 hrs, reported to photodegrade in water in spite of the lack of a photoreactive center oxidation-, not an important process hydrolysis . very slow, not important, first-order hydrolytic half-life 207 days, reaction with hydroxyl radicals in atmosphere has a half-life of 2.3 days volatilization may be an important process, however, information is contradictory, volatilization half-life from a model river 6 days, half-life from a model pond considering effects of adsorption 500 days, slow volatilization from water is expected with a rate dependent upon the rate of diffusion through air sorption important for transport to anaerobic sediments biological processes biodegradation is important occurs slowly in aerobic conditions, occurs quickly and extensively in anaerobic conditions... 

PROBABLE FATE photolysis-, no data for rate of photolysis in aquatic environment oxidation-, in aquatic systems not expected to be important fate, photooxidation in troposphere is probably the predominant fate hydrolysis expected to be slow, neutral aqueous hydrolysis half-life 25 °C >50 years, first-order hydrolysis half-life 37 years pH 7 volatUiz/ttion primary transport process, volatilization from soil will occur biological processes NA evaporation from water 25 °C of 1 ppm solution is 50% after 21 min and 90% after 102 min release to water primarily through evaporation (half-life days to weeks) rate of evaporation half-life from water 21 min photodegrades slowly by reaction with hydroxyl radicals, half-life 24-50 days in polluted atmosphere to a few days in unpolluted atmospheres will be removed in rain... 

Hydroxyl radicals will react with organic species within atmospheric droplets, as will any hydrogen peroxide that has not been consumed in oxidizing S(IV) to S(VI) [131]. Faust and Hoigne measured quantum efficiencies for the photolysis of Fe(OH) +, the dominant Fe(III)-hydroxy complex between pH 2.5 and pH 5 [111]. Sea salt water droplets rapidly acidify once ejected into the marine boundary layer, and have pH values within this range. Model calculations using the measured quantum yields of 0.14 0.04 at 313 nm and 0.017 0.003 at 360 nm, and absorption spectra, agree with the measured photolysis rate of... 

Methanesulfonic acid, dimethyl sulfoxide and dimethyl sulfone are potential intermediates in the gas phase oxidation of dimethylsulfide in the atmosphere. We nave measured the rate of reaction of MSA with OH in aqueous solution using laser flash photolysis of dilute hydrogen peroxide solutions as a source of hydroxyl radicals, and using competition kinetics with thiocyanate as the reference solute. The rate of the reaction k (OH + SCN ) was remeasured to be 9.60 1.12 x 109 M 1 s 1, in reasonable agreement with recent literature determinations. The rates of reaction of the hydroxyl radical with the organosulfur compounds were found to decrease in the order DMSO (k = 5.4 0.3 x 109 M-i s 1) > MSA (k = 4.7 0.9 x 107 M l S 1) > DMS02 (k = 2.7 . 15 x 107 M 1 s ). The implications of the rate constant for the fate of MSA in atmospheric water are discussed. 

Photolysis degraded photolytically on soil thin films, t,/2 = 13-57 d in artificial sunlight (Tomlin 1994). Oxidation photooxidation t,/2 = 4.2 h in air, based on an estimated rate constant for the vapor-phase reaction with photochemically produced hydroxyl radicals in the atmosphere (Atkinson 1985 quoted, Howard 1991). Hydrolysis neutral hydrolysis rate constant k < 1.5 x lO 5 h 1 with a calculated t,/2 > 700 d in neutral solution and with faster hydrolysis rates in acidic and basic solutions to be expected (Ellington et al. 1987, 1988 quoted, Howard 1991). 

PROBABLE FATE photolysis, no information available pertaining to the rate of photodissociation in aqueous environment, photodissociation to formyl chloride may occur in stratosphere, predominate fate process, if released to the atmosphere, is the reaction with photochemi-cally produced hydroxyl radicals with an estimated half-life of 40 days, less than 1% will eventually diffuse above the ozone layer where it will be destroyed by photolysis, direct photolysis is not important oxidation photooxidation in troposphere is the primary fate mechanism, photo-... 

PROBABLE FATE photolysis very little specific data, but photolysis may claim some of the dissolved compound, atmospheric and aquatic photolytic half-life 4.4-13 hrs, subject to near surface, direct photolysis with a half-life of 4.4 hrs, if released to air, it will be subject to direct photolysis, although adsorption may affect the rate, reaction with photochemically produced hydroxyl radicals gives an estimated half-life of gas phase crysene of 1.25 hrs oxidation chlonne and/or ozone in sufficient quantities may oxidize chrysene, photooxidation half-life in air 0.802-8.02 hrs hydrolysis not an important process volatilization probably too slow to compete with adsorption as a transport process, will not appreciably evaporate sorption adsorption onto suspended solids and sediment is the dominant transport process if released to soil or to water, expected to adsorb very strongly to the soil biological processes short-term bioaccumulation, metabolization and biodegradation are the principal fates... 

PROBABLE FATE photolysis the dissolved portion of the compound may undergo rapid photolysis to quinones, atmospheric and aqueous photolytic half-lives 6 hrs-32.6 days, may be subject to direct photolysis in the atmosphere, reaction with photochemically produced hydroxyl radicals has a half-life of 1.00 days oxidation rapid oxidation by chlorine and/or ozone may compete for dissolved DBA, photooxidation half-life in air 0.428-4.28 hrs hydrolysis not an important process volatilization probably too slow to be important, rate uncertain sorption strong adsorption by suspended solids, especially organic particulates, should be the principal transport process biological processes bioaccumulation is short-term, metabolization and microbial biodegradation are the principal fates... 

PROBABLE FATE photolysis, probably occurs slowly will react with photochemically produced hydroxyl radicals with a half-life of 31 days oxidation resistant to autooxidation by peroxy radical in water, oxidized by hydroxy radicals in atmosphere hydrolysis unimportant process first-order hydroxyl half-life >879 yrs volatilization volatilizes at a relatively rapid rate volatilization half-life <24 hr sorption probably absorbed by organic materials adsorption to sediment is a major environmental fate process biological processes bioaccumulates more than ehlorobenzene, too resistant to biodegradation to compete with volatilization will wash out in rain water... 

PROBABLE FATE photolysis . C-Cl bond photolysis is possible, could be important, may photolyze on the soil surface, when released to the atmosphere, it will react with photo-chemically produced hydroxyl radicals with an estimated half-life of 1.23 hr, adsorption onto atmospheric particles will increase this half-life oxidation , probably not important, photooxida-tion by u.v, light in aqueous medium 90-95°C, 25% CO2 formation 5.0 hr, 50% 9.5 hr, 75% 31.0 hr, oxidation rate constant 9.7x10 at pH 7, half-life 71.4 days hydrolysis , hydrolysis of sulfite group may be rapid, probably important above pH 7, hydrolyzed rapidly by alkalies, when released to water, hydrolytic half-life 37.5 and 187.3 days for pH 7 and 5.5 respectively, in the presence of ferric hydroxide, a higher rate of hydrolysis was observed at pH 7 and 20°C, in a solution of ferric oxide, hydrolysis half-life was 9.4 days volatilization could be important sorption sorption is an important process biological processes not important... 

---