Lifetime tropospheric

Rate constants (cm molec s ) Tropospheric Lifetimes (days) ... 

Air calculated lifetimes x = 4.6 h due to reaction with 03 in 24-h period, x = 4.6 h with OH radical during daytime, and x = 20 min for N03 radical during nighttime for clean atmosphere x = 1.4 h for reaction with 03 in 24-h period, x = 2.3 h with OH radical during daytime, and x = 2 min with N03 radical during nighttime in moderately polluted atmosphere (Atkinson et al.1984a, Winer et al. 1984) calculated atmospheric lifetimes of 5.6 h, 5.1 h and 11 min for reaction with 03, OH and N03 radicals respectively for clean tropospheric conditions at room temp. (Atkinson et al. 1986) calculated tropospheric lifetimes of 3.4 h, 4.6 h and 2.0 h due to reactions with OH radical, 03 and N03 radical respectively at room temp. (Corchnoy Atkinson 1990). 

Oxidation rate constant k, for gas-phase second order rate constants, kOH for reaction with OH radical, kNC,3 with N03 radical and k(), with 03, or as indicated data at other temperatures see original reference kOH = (7.49 0.39) x 10 11 cm3 molecule-1 s-1 at (298 2) K with a calculated tropospheric lifetime ranging from 1.9 to 2.4 h using a global tropospheric 12-h daytime average OH radical concentration of 2.0 x 10s molecule cm-3 (relative rate method, Phousongphouang Arey 2002)... 

Air calculated tropospheric lifetime ranging from 1.9 to 2.4 h for dimethylnaphthalenes using a global tropospheric 12-h daytime average OH radical concentration of 2.0 x 106 molecule cm-3 for the reaction with OH radical (Phousongphouang Arey 2002). 

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. 

Nimitz, J.S. and Skaggs, S.R. Estimating tropospheric lifetimes and ozone depletion potentials of one- and two-carbon hydrofluorocarbons and hydrochlorofluorocarbons, fJnvzron. Sci. TechnoL, 26(4) 739-744, 1992. 

Two points should be made about such calculations of tropospheric lifetimes. First, they are valid only for the specified reaction if there are other competing loss processes such as photolysis, the actual overall lifetime will be shortened accordingly. On the other hand, for a species such as CH4, which does not photolyze or react significantly with other atmospheric species such as O. or N03, t,, ]4 is indeed close to the overall lifetime of CH4. 

In general, for a compound to reach the upper troposphere in sufficient concentrations to impact the chemistry, it must not react rapidly in the lower troposphere. However, more reactive compounds can be rapidly transported (on the time scale of a few minutes) from the surface to the upper troposphere through convective events (e.g., thunderstorms) (e.g., Gidel, 1983 Chatfield and Crutzen, 1984 Chatfield and Alkezweeny, 1990 Pickering et al., 1992 Wang et al., 1995 Kley et al., 1996 Mahlman, 1997 Kley, 1997 Jaegle et al., 1998a Talbot et al., 1998). As a result, some compounds with relatively short tropospheric lifetimes can be carried into the upper troposphere and act as free radical sources. 

Table 8.17 summarizes the rate constants and estimated tropospheric lifetimes of some of these sulfur compounds with respect to reaction with OH or NO-,. The assumed concentrations of these oxidants chosen for the calculations are those characteristic of more remote regions, which are major sources of reduced sulfur compounds such as dimethyl sulfide (DMS). It is seen that OH is expected to be the most important sink for these compounds and that NO, may also be important, for example, for DMS oxidation (see also Chapter 6.J). 

While there are a variety of other chlorinated organics such as methylchloroform (CH3CC13) that are emitted, these have relatively short tropospheric lifetimes because they have an abstractable hydrogen atom (e.g., see WMO, 1995). For example, while the stratospheric lifetime of methylchloroform is estimated to be 34 7 years (Volk et al., 1997), its overall atmospheric lifetime is only 5-6 years, primarily due to the removal by OH in the troposphere (toii 6.6 years), with a much smaller contribution from uptake by the ocean (roi i an 85 years) (WMO, 1995). 

Tropospheric lifetime estimated to be 0.5-3 days by De Bruyn et al. (1992, 1995) based on measurements of uptake by water surfaces see also George et al. (1994a,b). 

CF3C(0)C1 from the oxidation of HCFC-123 is rapidly taken up into cloudwater (Table 13.8). However, it also photolyzes (Rattigan et al., 1993 Wallington, 1994a WMO, 1995), with an estimated tropospheric lifetime of 33 days assuming a quantum yield for dissociation of unity (Rattigan et al., 1993) ... 

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