Photolysis atmospheric lifetimes

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) ... 

Air t,/2 = 2.02-20.2 h, based on estimated sunlight photolysis half-life in water (Howard et al. 1991) calculated atmospheric lifetime of 26 h based on gas-phase OH reactions (Brubaker Hites 1998). 

TABLE 10.36 Calculated Atmospheric Lifetimes of Selected PAHs and Nitro-PAHs Due to Gas-Phase Reactions with the OH Radical, the N03 Radical, and Ozone and from Photolysis (from Arey, 1998a)... 

Orkin, V. L., R. E. Huie, and M. J. Kurylo, Atmospheric Lifetimes of HFC-143a and HFC-245fa Flash Photolysis Resonance Fluorescence Measurements of the OH Reaction Rate Constants, J. Phys. Chem., 100, 8907-8912 (1996). 

In addition to photolysis (Chapter 15) and chemical reactions (see the next section), wet and dry deposition also can remove gas- and particle-phase chemical compounds from the troposphere (Eisenreich et al., 1981 Bidleman, 1988). Thus to completely characterize the atmospheric loss processes and overall lifetime of a chemical, we must understand its atmospheric lifetime due to dry and/or wet deposition. Wet deposition refers to the removal of the chemical (or particle-associated chemical) from the atmosphere by precipitation of rain, fog, or snow to earth s surface). Dry deposition refers to the removal of the chemical or particle-assodated chemical from the atmosphere to the Earth s surface by diffusion and / or sedimentation. 

Oxidation rate constant k, for gas-phase second order rate constants, koH for reaction with OH radical, kND3 with N03 radical and kQ3 with 03 or as indicated, data at other temperatures see reference rate constant for the reaction with OH- k = (2.5 0.3) x H)6 M-1 s-1 was measured in 66.7% dioxane-water at 35.7°C (Hine et al. 1956 quoted, Roberts et al. 1992) koH = 1.11 x 10 13 cm3 molecule-1 s-1 with atmospheric lifetime x = 0.43 yr at 298 K, measured range 277-377 K (flash photolysis resonance fluorescence and discharge flow electron paramagnetic resonance, Orkin et al. 1997)... 

Particulate-bound CDDs are removed by wet or dry deposition with an atmospheric lifetime 10 days (Atkinson 1991) and, to a lesser extent, by photolysis. Miller et al. (1987) measured photolysis of 2,3,7,8-TCDD sorbed onto small-diameter fly ash particulates suspended in air. The results indicated that fly ash confers photostability to the adsorbed 2,3,7,8-TCDD. The authors reported little (8%) to no loss of 2,3,7,8-TCDD on the fly ash samples after 40 hours of illumination in simulated sunlight. 

The direct photolysis of compounds such as HONO, 03, HCHO, and N02 in the tropospheric gas phase is a very important source of reactive species, which are then involved in the transformation of organic compounds. Additionally, some organic molecules including organic pollutants undergo photolysis as a significant or even the main process of removal from the atmosphere. It is for instance the case for nitronaphthalenes, the atmospheric lifetime of which can be as low as a couple of hours because of direct photolysis [11, 12]. 

Tetrafluoromethane and other perfluoroalkanes require radiation of wavelength <100 nm to cause photodissociation of the very strong C—F bond. These species must therefore diffuse upwards to altitudes beyond 100 km before photolysis occurs, and consequently they have atmospheric lifetimes of thousands of years51. 

Kinetic studies of the reaction of 1-bromopropane with OH radicals have been previously performed [27,30,32,36,37]. Donaghy et al. [30], using the relative rate technique, found a rate constant of (11.8 3.0) x 10 cm molecule s while using C-C6H12 as the reference compound. The obtained atmospheric lifetime was 11-16 days. Teton et al. [32] via pulsed laser photolysis followed... 

Icqjj = 18.0 X 10" cm molecule" S" by relative rate method Icqjj = 16.7 x lO cm molecule s by pulse laser photolysis-laser induced fluorescence and atmospheric lifetime calculated to be 16 h at 298 2 K measured range 263-372 K (Moriarty et al. 2003)... 

The straight chain C3-C9 aldehydes all have similar photolysis rates and values for (peff in die range 0.20-0.30. For these compounds the atmospheric lifetimes for photolysis by sunlight are longer than the lifetimes for reaction with OH radicals. In contrast, the a-branched aldehydes possess significantly higher values for (petr and their photolysis lifetimes are shorter than those for reaction with OH radicals. Interestingly, the (pea values for the... 

A likely source of active iodine is provided by the photolysis of methyl iodide (CH3f) and perhaps of other iodocarbons. As the atmospheric lifetime of CH3f is relatively short (a few days), the tropospheric abundance of this compound is generally lower than 10 pptv (Moyers and Duce, 1972 Singh et al., 1983 Atlas et al., 1993) although local maxima are found over the productive regions of the ocean (Oram and Penkett, 1994). If iodine atoms are released above the tropopause, they react with ozone to form the iodine monoxide radical... 

PAN acts as a reservoir species for both CH3C(0)02 radicals and NO. Because of this, the atmospheric lifetime of PAN is important if its lifetime is relatively long, PAN can act as an effective reservoir for NO. Potential atmospheric removal processes for PAN include thermal decomposition (reaction 4 above), UV photolysis, and OH reaction. PAN is not highly water-soluble it is more soluble than NO or N02 but considerably less soluble than HNO3. Thus, wet deposition is a minor removal process. Dry deposition is also unimportant. The PAN-OH rate constant is <3 x 10 14 cm3 molecule-1 s 1, and OH reaction is not an effective removal process. PAN absorbs UV radiation up to 350 nm (Libuda and Zabel 1995 Talukdar et al. 1995). Thus, thermal decomposition and photolysis are the principal removal processes for PAN. 

The primary atmospheric removal processes for halocarbons are photolysis and reaction with tropospheric hydroxyl radicals (OH). For the fully halogenated CFCs and halons, photolysis is the only important sink and their atmospheric lifetimes are dependent on their absorption cross-sections, the solar flux, and the surface to stratosphere transport time. As a general rule, the greater the number of Cl, Br, or I atoms on any one carbon atom, the larger the cross-section and the shorter the lifetime. For example, the lifetimes of CCIF3 (CFC-13), CCI2F2... 

The thermal decomposition rate constants of PAN is given by the lUPAC Subcommittee Report Vol. II as (298 K) = 3.8 x 10 " s (Atkinson et al. 2006), and the atmospheric lifetime of PAN is calculated as 43 min at 298 K. Thus, PAN is lost by the thermal decomposition and is not transported in a long range in the lower troposphere. However, the lifetime is much longer in the upper troposphere where the temperature is low, and it is transported in a long range as a NOx reservoir, and serves as a slow regeneration source of NOx subject to the reaction with OH or photolysis. 

Photolysis is an important atmospheric loss mechanism for aldehydes and ketones. The lifetimes with respect to photolysis of oxygenated compounds are estimated in chapter DC. The atmospheric lifetimes of the alcohols, hydroperoxides, ethers, esters. 

---