Hydroxyl radical in atmosphere

Oxidation photooxidation t,/2 = 92 d in air, based on estimation for the vapor-phase reaction with hydroxyl radicals in atmosphere (Atkinson 1987 quoted, Howard 1991). 

The central role of hydroxyl radicals in atmospheric chemistry is well illustrated by examining the atmospheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcell Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth s surface. 

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, expected to oecur slowly oxidation no data available on aqueous oxidation, oxidized by hydroxyl radicals in atmosphere hydrolysis not important process first-order hydrolytic half-life >879 yrs volatilization volatilizes at a relatively rapid rate, half-life is about 10 hr volatilization from soil surfaces is expected to be a signifieant transport mechanism sorption sorbed by organic materials adsorption to sediment expected to be a major environmental fate process based on research in the Great Lakes area biological processes bioaccumulates more than chlorobenzene, biodegradation is not as significant as volatilization slightly persistent in water, half-life 2-20 days approximately 98.5% of 1,3-dichlorobenzene ends up in air 1% ends up in water the rest is divided equally between terrestrial soils and aquatic sediments. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

Air an atmospheric lifetime was estimated to be 13 h in summer daylight, based on the photooxidation rate constant of 1.04 x 10-n cm3 molecule-1 s-1 with hydroxyl radicals in air (Altshuller 1991). 

Endrin ketone may react with photochemically generated hydroxyl radicals in the atmosphere, with an estimated half-life of 1.5 days (SRC 1995a). Available estimated physical/chemical properties of endrin ketone indicate that this compound will not volatilize from water however, significant bioconcentration in aquatic organisms may occur. In soils and sediments, endrin ketone is predicted to be virtually immobile however, detection of endrin ketone in groundwater and leachate samples at some hazardous waste sites suggests limited mobility of endrin ketone in certain soils (HazDat 1996). No other information could be found in the available literature on the environmental fate of endrin ketone in water, sediment, or soil. 

Little information was found on the degradation of mirex in the atmosphere. Mirex is expected to be stable against photogenerated hydroxyl radicals in the atmosphere (Eisenreich et al. 1981). 

Apart from the economic significance of such loss there are potentially adverse effects on the environment arising from acidification of rain and soil. Ammonia may react with hydroxyl radicals in the atmosphere to produce NOx contributing to the acidification of rain (4). Wet and dry deposition of NH3/NH4+ inevitably contributes to soil acidification through their subsequent nitrification. This effect can be accentuated in woodland by absorption of aerosols containing NH4+ within the canopy followed by transport to the soil in stem flow (5). In more extreme cases, NH3 emission from feedlots, pig and poultry... 

Dibromoethane reacts with hydroxyl radicals in the atmosphere the half-life for the reaction has been estimated to be about 40 days (ERA 1987a). 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

Photolytic. Aldicarb vapor may react with hydroxyl radicals in the atmosphere. A half-life of 0.24 d was reported (Atkinson, 1987). 

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. 

Once sulfur dioxide has escaped into the atmosphere, it undergoes a series of reactions by which it is converted to sulfuric acid. Those reactions are somewhat complex and may follow at least three different courses. In the first of these reaction sequences, sulfur dioxide reacts with hydroxyl radicals in the atmosphere in the presence of some metallic catalyst (M) to form the bisulfite radical (HSO3 ) ... 

Crosley, D. R., Laser Fluorescence Detection of Atmospheric Hydroxyl Radicals, in Problems and Progress in Atmospheric Chemistry (J. R. Barker, Ed.), Advanced Series in Physical Chemistry (C.-Y. Ng, Ed.), Vol. 3, World Scientific, Singapore, 1995b. 

Except for occupational atmospheres, no information was found in the available literature on concentrations of HDI or HDI prepolymers in air, water, soil, or sediment. Because of the relatively rapid reaction of HDI with hydroxyl radicals in the atmosphere and its high reactivity with water, significant environmental concentrations of HDI are not expected to occur except near emission sources. 

HDI and HDI prepolymers can be released to the atmosphere during spray applications of polymer paints containing residual amounts (0.5-1.0%) of monomeric HDI (Alexandersson et al. 1987 Hulse 1984 Karol and Hauth 1982). These substances could also be released to the atmosphere from waste streams from sites of HDI or polymer production. No information is available in the Toxic Chemical Release Inventory database on the amoimt of HDI released to the atmosphere from facihties that produce or process HDI because this compound is not included under SARA, Title 111, and therefore, is not among the chemicals that facilities are required to report (EPA 1995). There is also a potential for atmospheric release of HDI from hazardous waste sites however, no information was found on detections of HDI in air at any NPL or other Superfund hazardous waste sites (1996). Beeause of the relatively rapid reaction of HDI with hydroxyl radicals in the atmosphere an possible hydrolysis (see Seetion 5.3.2.1), significant atmospheric concentrations are not expeeted to oeeur exeept near emission sourees. 

The hydroxyl radical is normally present only in low concentrations in the troposphere, as it reacts with further ozone to form the hydroperoxy radical HOO- which in turn gives hydrogen peroxide H202. Ozone, the hydroxyl radical, and hydrogen peroxide are the main oxidizing species in the troposphere, from the standpoint of environmental chemistry. The hydroxyl radical in particular performs an important function as a natural cleansing agent for the atmosphere.26 In elevated concentrations, however,... 

Oxidation atmospheric t/2 6.2 d for reaction with hydroxyl radicals in air (Howard 1997). 

Reaction of DEHP vapor with hydroxyl radicals in the atmosphere has been predicted, with an estimated half-life of about 6 hours using the Atmospheric Oxidation Program (Meylan and Howard 1993). The atmospheric half-life, however, is expected to be longer for DEHP adsorbed to atmospheric particulates. Based on the estimated half-life alone, extensive transport of DEHP would not be expected and concentrations in Antarctic snow would not be predicted. Nonetheless, DEHP appears to be present in urban and rural atmospheres (see Section 6.4), and its transport might be mainly in the sorbed state. Data confirming this degradation pathway have not been located. Direct photolysis and photooxidation are not likely to be important (Warns 1987). 

---