Tropospheric half-life

No data were located regarding the transformation and degradation of hexachlorobutadiene in air. Based on the monitoring data, the tropospheric half-life of hexachlorobutadiene was estimated by one author to be 1.6 years in the northern hemisphere (Class and Ballschmiter 1987). However, analogy to structurally similar compounds such as tetrachloroethylene indicates that the half-life of hexachlorobutadiene may be as short as 60 days, predominantly due to reactions with photochemically produced hydroxyl radicals and ozone (Atkinson 1987 Atkinson and Carter 1984). Oxidation constants of <10 and 6 (m hr) were estimated for reactions with singlet oxygen and peroxy radicals, respectively (Mabey et al. 1982). 

Air t1/2 = 6 h with a steady-state concn of tropospheric ozone of 2 x 10-9 M in clean air (Butkovic et al. 1983) t/2 = 2.01-20.1 h, based on photooxidation half-life in air (Howard et al. 1991) calculated atmospheric lifetime of 11 h based on gas-phase OH reactions (Brubaker Hites 1998). Surface water computed near-surface of a water body, tl/2 = 8.4 h for direct photochemical transformation at latitude 40°N, midday, midsummer with tl/2 = 59 d (no sediment-water partitioning), t,/2 = 69 d (with sediment-water partitioning) on direct photolysis in a 5-m deep inland water body (Zepp Schlotzhauer 1979) t,/2 = 0.44 s in presence of 10 M ozone at pH 7 (Butkovic et al. 1983) calculated t,/2 = 59 d under sunlight for summer at 40°N latitude (Mill Mabey 1985) t,/2 = 3-25 h, based on aqueous photolysis half-life (Howard et al. 1991) ... 

The rate constants for the reaction of l,2-dibromo-3-chloropropane with ozone and OH radicals in the atmosphere at 296 K are <5.4 x 10 ° and 4.4 x lO cm /molecule-sec (Tuazon et al., 1986). The smaller rate constant for the reaction with ozone indicates that the reaction with ozone is not an important atmospheric loss of l,2-dibromo-3-chloropropane. The calculated photolytic half-life and tropospheric lifetime for the reaction with OH radicals in the atmosphere are 36 and 55 d, respectively. The compound l-bromo-3-chloropropan-2-one was tentatively identified as a product of the reaction of l,2-dibromo-3-chloropropane with OH radicals. 

Tuazon et al. (1984a) investigated the atmospheric reactions of TV-nitrosodimethylamine and dimethylnitramine in an environmental chamber utilizing in situ long-path Fourier transform infared spectroscopy. They irradiated an ozone-rich atmosphere containing A-nitrosodimethyl-amine. Photolysis products identified include dimethylnitramine, nitromethane, formaldehyde, carbon monoxide, nitrogen dioxide, nitrogen pentoxide, and nitric acid. The rate constants for the reaction of fV-nitrosodimethylamine with OH radicals and ozone relative to methyl ether were 3.0 X 10 and <1 x 10 ° cmVmolecule-sec, respectively. The estimated atmospheric half-life of A-nitrosodimethylamine in the troposphere is approximately 5 min. 

Photolytic. Irradiation of vinyl chloride in the presence of nitrogen dioxide for 160 min produced formic acid, HCl, carbon monoxide, formaldehyde, ozone, and trace amounts of formyl chloride and nitric acid. In the presence of ozone, however, vinyl chloride photooxidized to carbon monoxide, formaldehyde, formic acid, and small amounts of HCl (Gay et al, 1976). Reported photooxidation products in the troposphere include hydrogen chloride and/or formyl chloride (U.S. EPA, 1985). In the presence of moisture, formyl chloride will decompose to carbon monoxide and HCl (Morrison and Boyd, 1971). Vinyl chloride reacts rapidly with OH radicals in the atmosphere. Based on a reaction rate of 6.6 x lO" cmVmolecule-sec, the estimated half-life for this reaction at 299 K is 1.5 d (Perry et al., 1977). Vinyl chloride reacts also with ozone and NO3 in the gas-phase. Sanhueza et al. (1976) reported a rate constant of 6.5 x 10 cmVmolecule-sec for the reaction with OH radicals in air at 295 K. Atkinson et al. (1988) reported a rate constant of 4.45 X 10cmVmolecule-sec for the reaction with NO3 radicals in air at 298 K. 

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. 

The UV absorption spectrum of gaseous trifluoromethyl peroxynitrate (1) shows a continuous decrease of intensity from 185 nm to 340 nm. The reported absorption cross section (a/10 ° cm ) ranges from 370 at 190 nm, to 1.0 at 290 nm and to 0.014 at 340 nm. Using published solar flux data in the troposphere at sea level, and assuming a unity quantum yield, a half-life time of about 1 month can be estimated for compound 1, which makes it a potentially effective carrier of pollutants from industrial zones to remote unpolluted sites . ... 

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. 

Radiocarbon. Radiocarbon (14C) is unstable, with a half-life of 5730yr, and decays by emission of an electron to form 14N. It is continuously produced in the upper atmosphere by interactions of high-energy cosmic rays with the upper atmosphere. The 14C is oxidized to 14C02 within a few weeks and is then mixed into the troposphere (the lower, well-mixed part of the atmosphere), where it is taken up by plants during photosynthesis and exchanges with the surface waters of the ocean. 

It is likely that both the above mentioned effects - larger particle sizes and increase in airborne activity with height - contributed to the high values of W. At later times, the airborne activity from Chernobyl was mainly submicrometre in size and had equilibrated with the accumulation mode of natural nuclei. Over the period 10-90 d from the emission, 137Cs disappeared from the atmosphere with a half-life of 6 d, or mean life of 9 d (Fig. 2.8). The mass of air in the troposphere is 9000 kg per m2 of the earth s surface, and the average daily rainfall in the northern hemisphere is 3.1 mm. Using these data, it can be deduced that the washout ratio of 137Cs was... 

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). 

Air half-life in the atmosphere was estimated to be 1.96 d (Howard 1989) tropospheric lifetime of 1 d, calculated based on reactions principally with OH radical on March 21 at 43°N (Bunce 1991). 

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. 

Of the isotopes listed in Table 2 the study of the cosmogenic isotopes of phosphorus (half-life = 14.3 d) and (half-life = 25.3 d) provides insights into the residence time of aerosols in upper troposphere air because of the relatively short half-lives of the nuclides. Waser and Bacon (1995) measured the concentrations of P and P in precipitation at Bermuda over three seasons. They concluded that, with the average activity ratio of P to P of 0.96 and a production activity ratio of 0.7, the average residence time of aerosols in the upper troposphere was 40 d. This increase in the... 

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. 

Styrene is a liquid and will partition to the atmosphere when released to the environment due to its volatility. In the atmosphere, styrene is rapidly eliminated due to its reaction with hydroxyl radicals (7h half-life) or tropospheric ozone (10 h half-life). Water does not provide a significant sink for styrene due to its low water solubility (300 mg 1 ), rapid volatilization from water to air (half-life of 1-3 h), and biodegradation (15 days half-life). In soil, styrene rapidly volatilizes from the surface (Imin half-life) but more slowly from deeper strata. Styrene... 

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