Lifetimes atmospheric degradation, 238, Table

In addition to OH radicals, unsaturated bonds are reactive towards O3 and NO3 radicals and reaction with these species is an important atmospheric degradation mechanism for unsaturated compounds. Table 4 lists rate constants for the reactions of 03 and NO3 radicals with selected alkenes and acetylene. To place such rate constants into perspective we need to consider the typical ambient atmospheric concentrations of O3 and NO3 radicals. Typical ozone concentrations in pristine environments are 20-40 ppb while concentrations in the range 100-200 ppb are experienced in polluted air. The ambient concentration of NO3 is limited by the availability of NO sources. In remote marine environments the NO levels are extremely low (a few ppt) and NO3 radicals do not play an important role in atmospheric chemistry. In continental and urban areas the NO levels are much higher (up to several hundred ppb in polluted urban areas) and NO3 radicals can build up to 5-100 ppt at night (N03 radicals are photolyzed rapidly and are not present in appreciable amounts during the day). For the purposes of the present discussion we have calculated the atmospheric lifetimes of selected unsaturated compounds in Table 4 in the presence of 100 ppb (2.5 x 1012 cm 3) of O3 and 10 ppt (2.5 x 108 cnr3) of NO3. Lifetimes in other environments can be evaluated by appropriate scaling of the data in Table 4. As seen from Table 4, the more reactive unsaturated compounds have lifetimes with respect to reaction with O3 and NO3 radicals of only a few minutes ... 

The database regarding the gas-phase degradation of the acid anhydrides is limited to only a pair of Cl-atom rate coefficient measurements, as is summarized in table VI-H-1. Gas-phase destruction of these species is likely controlled by reaction with OH, for which no data are presently available. However, on the basis of estimates from structure-activity relationships (Kwok and Atkinson, 1995 Kwok et al 1996b), these rate coefficients are likely quite small. For example, rate coefficients of 12, 6.5, and 0.9 (all in units of 10 cm molecule" s ) can be estimated for formic anhydride [HC(0)OCHO], formic acetic anhydride [HC(0)0C(0)CH3], and acetic anhydride [CH3C(0)0C(0)CH3], respectively, implying lifetimes for reaction of OH of about 10, 18, and 130 days for these species, respectively. Heterogeneous processes (e.g., hydrolysis to acetic acid on aerosol) seem likely to be important in the atmospheric removal of the anhydrides. 

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