Stratospheric half-life

Thus, the Cl atom is a homogeneous catalyst it exists in the same phase as the reactants, speeds the reaction by allowing a different mechanism, and is regenerated. During its stratospheric half-life of about 2 years, each Cl atom speeds the breakdown of about 100,000 ozone molecules. 

Radiocarbon dating (43) has probably gained the widest general recognition (see Radioisotopes). Developed in the late 1940s, it depends on the formation of the radioactive isotope and its decay, with a half-life of 5730 yr. After forms in the upper stratosphere through nuclear reactions of... 

Typical precautions with trichloroethylene are summarized in Table 5.52. An important factor is that the vapours are much heavier than air they will therefore spread and may accumulate at low levels, particularly in undisturbed areas. Because of its volatility, releases to the environment usually reach the atmosphere. Here it reacts with hydroxyl or other radicals (estimated half-life for reaction with hydroxyl radicals is less than a week) and is not therefore expected to diffuse to the stratosphere to any significant extent. There is some evidence for both aerobic and anaerobic biodegradation of trichloroethylene. 

The half-life, f1/2, of a substance is the time needed for its concentration to fall to one-half its initial value. Knowing the half-lives of pollutants such as chlorofluoro-carbons allows us to assess their environmental impact. If their half-lives are short, they may not survive long enough to reach the stratosphere, where they can destroy ozone. Half-lives are also important in planning storage systems for radioactive materials, because the decay of radioactive nuclei is a first-order process. 

Atmospheric transport of hexachloroethane may occur, based on the stability of the compound in air (Class and Ballschmitter 1986 Singh et al. 1979). Hexachloroethane is expected to diffuse slowly into the stratosphere, with a half-life of about 30 years (Howard 1989). Deposition of hexachloroethane from air to water, plants, and soil has been reported (Cataldo et al. 1989). 

Tritium is also produced in ternary fission and by neutron-induced reactions with 6Li and 10B. Tritium is a very low energy (3 emitter with a half-life of 12.33 y. The global inventory of naturally produced tritium is 9.6 x 1017 Bq. Tritium is readily incorporated in water and is removed from the atmosphere by rain or snow. Its residence time in the stratosphere is 2-3 y after reaching the troposphere it is removed in 1-2 months. The natural concentration of 3H in streams and freshwater is 10 pCi/L. 

The long half-life of the two end products makes them especially dangerous. In an atmospheric nuclear explosion, the tertiary fission products are formed in the stratosphere and gradually come down to earth. Every spring about one-half to two-thirds of the fission products in the stratosphere come down and are eventually deposited by precipitation. Figure 11-6 gives a schematic outline of the pathways through which the fallout may reach us. 

The production rate of Be (half-life = 53 d) as a function of latitude and elevation by Lai and Peters (1967) is shown in Figure 9. Approximately one-third of the nuclide production rate is in the troposphere and two-thirds in the upper atmosphere (stratosphere and higher). This partitioning is valid for all radionuclides except where most is produced by secondary neutrons in the vicinity of the tropopause. 

The cosmogenic nuclide °Be (half-life = 1.5 Myr) is a logical candidate for dating deep-sea deposits. The production in the atmosphere is primarily in the stratosphere. Its entry into the troposphere from the stratosphere occurs primarily around 40-50° latitude where tropopausal... 

Most of the releases of carbonyl sulfide to the environment are to air, where it is believed to have a long residence time. The half-life of carbonyl sulfide in the atmosphere is estimated to be 2 years. It may be degraded in the atmosphere via a reaction with photochemically produced hydroxyl radicals or oxygen, direct photolysis, and other unknown processes related to the sulfur cycle. Sulfur dioxide, a greenhouse gas, is ultimately produced from these reactions. Carbonyl sulfide is relatively unreactive in the troposphere, but direct photolysis may occur in the stratosphere. Also, plants and soil microorganisms have been reported to remove carbonyl sulfide directly from the atmosphere. Plants are not expected to store carbonyl sulfide. 

Rasmussen et al. 1983). 1,1,1 -Trichloroethane removed by rain water would be expected to re-volatilize rapidly to the atmosphere. Because of its long half-life of 4 years in the atmosphere (see Section 5.3.2.1), tropospheric 1,1,1-trichloroethane will be transported to the stratosphere, where it will participate in the destruction of the ozone layer. It will also undergo long-distance transport from its sources of emissions to other remote and rural sites. This is confirmed by the detection of this synthetic chemical in forest areas of Northern and Southern Europe and in remote sites (Ciccioli et al. 1993). 

In fact, a detailed calculation of the CF3O2NO2 half life and its dependence with altitude and latitude shows a maximum half life at aroxmd 15 km. This maximum reaches values as long as 220 days for mid-latitudes allowing the compound to be transported though the atmosphere. Therefore, CF3O2NO2 could be transported either through convective stream to the stratosphere, or to remote tropospheric places where it can dissociate again. 

Most of the released 1,1,2,2-tetrachloroethane enters the atmosphere where it is extremely stable (half-life > 2 years). Some of the chemicals will eventually diffuse into the stratosphere where it will rapidly photodegrade. There is evidence that 1,1,2,2-tetrachloroethane slowly biodegrades. A product of biodegradation under anaerobic conditions is 1,1,2-trichloroethane, a chemical which is resistant to further biodegradation. Under alkaline conditions, 1,1,2,2-tetrachloroethane may be expected to hydrolyze. When disposed of on soil, part of the 1,1,2,2-tetrachloroethane may leach... 

PROBABLE FATE photolysis could be important, photooxidation half-life in water 54.1-541 days, direct photolysis in the stratosphere may occur, but is insignificant in the troposphere, reaction with photochemically produced hydroxyl radicals yields a half-life of 1.45 yrs oxidation atmospheric photooxidation by hydroxyl radicals to COBT2 is relatively rapid hydrolysis too slow to be important, first-order hydrolytic half-life 687 yrs volatilization volatilization has been demonstrated, could be an important transport process, volatilization from moist soil surfaces expected to occur sorption no information is available biological processes slight potential for bioaccumulation/metabolization is known to occur in some organisms other reactionsAnteractions possibly produced by halogen reaction... 

PROBABLE FATE photolysis could be important in aqueous environment, in the stratosphere, photodissociation occurs to eventually form phosgene as the principal product oxidation no information available, in troposphere it exhibits an extremely slow rate of reaction with hydroxyl radicals, photooxidation half-life in air 1.8-18.3 yrs hydrolysis first-order hydrolytic half-life 7000 yrs based on a rate constant of 4.8xl0 mol s pH 7 and 25°C vola-... 

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 information is lacking, probably unimportant, appreciable photodissociation may occur in stratosphere, photooxidation half-life in air 61.3-613 days oxidation information lacking, probably unimportant, in troposphere oxidation by hydroxyl radicals for formyl chloride and other products is an important fate hydrolysis slow hydrolysis, unimportant in comparison to volatilization, first-order hydrolytic half-life 292 days at pH 7 volatilization volatilization to the atmosphere is rapid and is a major transport process for removal of methyl chloride, evaporation from water 25°C of 1 ppm solution 50% after 27 min, 90% after 91 min volatilization half-life in a typical river 2-1 hr sorption no data is available, sorption onto sediments and suspended particulates probably unimportant biological processes data is lacking, biodegradation and bioaccumulation are not expected to be important fates... 

PROBABLE FATE photolysis-, information lacking, photodissociation to chloroacetyl chloride in stratosphere is predicted oxidation-, photooxidation in troposphere may be the predominant fate, photooxidation in aquatic environments probably occurs at a slow rate hydrolysis-. unimportant compared to volatilization volatilization due to high vapor pressure, volatilization to the atmosphere should be the major transport process, if released in water, will be removed by volatilization with a half-life of 6-9 days, 5-8 days, and 23-32 hr, in a typical pond, lake, or river respectively, will be removed quickly by volatilization if released on land biological processes data is lacking, bioaccumulation not expected, biodegradation may be possible evaporation from water 25°C of 1 ppm solution 50% after 22 min, 90% after 109 min. 

PROBABLE FATE photolysis-, direct photolysis is not significant, photodissociation in stratosphere to chloroacetyl chloride oxidation photooxidation in water expected to be slow primarily removed in air by photooxidation degraded in atmosphere by reaction with hydroxyl radicals, half-life of 1 month and 1.9% loss/12 hr sunlit day products of photooxidation CO and HCl oxidation half-life 1.5 weeks-4 months hydrolysis not significant first-order hydrolytic half-life 1.1 yr volatilization high vapor pressure causes rapid volatilization, major transport process, half-life 30 min 25°C evaporation primary removal from water half-life from 1 ppm solution 25°C, still air, and an avg. depth of 6.5 cm 28 min., evaporation from water 25 °C of 1 ppm solution 50% after 29 min. and 90% after 96 min. 

PROBABLE FATE photolysis , aquatic photodissociation is precluded by volatilization, tropospheric photooxidation precludes stratospheric photodissociation, C-Cl bond can photolyze slowly oxidation tropospheric photooxidation by hydroxyl radicals is rapid and yields a variety of products the half-life due to photooxidation is 11 hrs in relatively clean air and <2 hrs in polluted air in water, photooxidation is unimportant hydrolysis to slow to be significant very... 

PROBABLE FATE photolysis, photodissociation in stratosphere may be important, not important in aquatic environment, photooxidation half-life in air >7.3->73 yrs photooxidation by U.V. in aqueous medium 90-95 C time for the formation of C02 (% theoretical) 25% 25.2 hr, 50% 93.7 hr, 75% 172.0 hr oxidation not an important process hydrolysis probably unimportant volatilization some volatilization occurs, importance as a fate mechanism is unknown, measured half-life for evaporation from 1 ppm aqueous solution 25 C, still air, and an average depth of 6.5 cm 40.7 min, should volatilize slowly from dry soil surfaces, if released to water, volatilization appears to be the most dominant removal mechanism (half-life 15 hrs from a model river) sorption no data available, moderate to slight adsorption to suspended solids and sediments may occur biological processes high Log Kow indicates possibility of bioaccumulation... 

PROBABLE FATE photolysis volatilized methyl bromide should photodissociate above the ocean layer, probably not significant in aquatic systems, reaction with photochemi-cally produced hydroxyl radicals has a half-life from 0.29-1.6 yrs, direct photolysis is the dominant fate in the stratosphere, but is not expected to be important in the troposphere oxidation atmospheric photooxidation by hydroxyl radicals releases inorganic bromide which is carried... 

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