Atmospheric lifetimes, estimation

COS is the most stable of the atmospheric sulphur gases with an atmospheric lifetime estimated in years (Hewitt and Davison, 1988). It is therefore well mixed throughout the... 

Pollutants have various atmospheric residence times, with reactive gases and large aerosols being rapidly removed from air. In the London air pollution episode of December 1952, the residence time for sulfur dioxide was estimated to be five hours daily emissions of an estimated 2,000 tons of sulfur dioxide were balanced by scavenging by fog droplets, which were rapidly deposited. Most relatively inert gases remain in the atmosphere for extended periods. Sulfur hexafluoride, used extensively in the electric power industiy as an insulator in power breakers because of its inertness, has an estimated atmospheric lifetime of 3,200 years. 

For polychlorinated biphenyls (PCBs), rate constants were highly dependent on the number of chlorine atoms, and calculated atmospheric lifetimes varied from 2 d for 3-chlorobiphenyl to 34 d for 236-25 pentachlorobiphenyl (Anderson and Hites 1996). It was estimated that loss by hydroxy-lation in the atmosphere was a primary process for the removal of PCBs from the environment. It was later shown that the products were chlorinated benzoic acids produced by initial reaction with a hydroxyl radical at the 1-position followed by transannular dioxygenation at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998). Reactions of hydroxyl radicals with polychlorinated dibenzo[l,4]dioxins and dibenzofurans also play an important role for their removal from the atmosphere (Brubaker and Hites 1997). The gas phase and the particulate phase are in equilibrium, and the results show that gas-phase reactions with hydroxyl radicals are important for the... 

Extensive research has been conducted into the atmospheric chemistry of organic chemicals because of air quality concerns. Recently, Atkinson and coworkers (1984, 1985, 1987, 1988, 1989, 1990, 1991), Altshuller (1980, 1991) and Sabljic and Glisten (1990) have reviewed the photochemistry of many organic chemicals of environmental interest for their gas phase reactions with hydroxyl radicals (OH), ozone (03) and nitrate radicals (N03) and have provided detailed information on reaction rate constants and experimental conditions, which allowed the estimation of atmospheric lifetimes. Klopffer (1991) has estimated the atmospheric lifetimes for the reaction with OH radicals to range from 1 hour to 130 years, based on these reaction rate constants and an assumed constant concentration of OH... 

Air photooxidation reaction rate constant of 6.30 x 10-12 cm3 molecule-1 s-1 with hydroxyl radicals and an estimated atmospheric lifetime of 22 h during summer daylight (Altshuller 1991). 

Air atmospheric lifetime was estimated to be 5.5 h, based on the reaction rate constant k = 3.14 x 10-11 cm3 molecule-1 s-1 with OH radicals during summer daylight in the gas phase (Altshuller 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 3 shows the atmospheric lifetime for eleven PAH with respect to gas-phase reaction with OH and NO3 radicals, O3 and N2O5. This was calculated from the estimated and calculated rate constants. It is evident that most of the nitroarenes formed under ambient atmospheric conditions were produced by reaction of PAH with OH. The PAH reaction with NO3 radical was also considered as an important step because it resulted in the formation of nitroarenes from the N2O5 reaction with gas-phase PAH. 

X 10 cmVmolecule-sec, respectively (Kurylo and Knable, 1984). The estimated lifetime of acetonitrile in the atmosphere is estimated to range from 6 to 17 months (Arijs and Brasseur, 1986). 

Photolytic. The following rate constants were reported for the reaction of hexane and OH radicals in the atmosphere 7.15 x lO cmVmolecule-sec (Atkinson, 1990). Photooxidation reaction rate constants of 7.19 x lO and 1.36 x 10 cmVmolecule-sec were reported for the reaction of heptane with OH and NO3, respectively (Sabljic and Giisten, 1990). Based on a photooxidation rate constant 7.15 x lO cmVmolecule-sec for heptane and OH radicals, the estimated atmospheric lifetime is 19 h in summer sunlight (Altshuller, 1991). 

The estimated atmospheric lifetimes for the reaction of 2-methyl-1,3-butadiene with ozone, OH, and NO3 radicals are 28.3, 2.9, and 0.083 h, respectively (Atkinson and Carter, 1984). Chemical/Physical. Slowly oxidizes and polymerizes in air (Huntress and Mulliken, 1941). 

Atkinson, R. Estimation of OH radical reaction rate constants and atmospheric lifetimes for polychlorinated biphenyls, dibenzo-/rdioxins, and dibenzofmans. Environ. Sci Technol, 21(3) 305-307, 1987a. 

Roberts, J. M., and R. W. Fajer, UV Absorption Cross Sections of Organic Nitrates of Potential Atmospheric Importance and Estimation of Atmospheric Lifetimes, Enciron. Sci. Technol., 23, 945-951 (1989). 

TABLE 6.23 Room Temperature Rate Constants and Estimated Atmospheric Lifetimes for the Gas-Phase Reactions of Some Alkyl Amines and Amides with OH and 03b... 

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). 

---