Ozone lifetimes, 244, Table

Table 1 lA presents tabulations of the safety of important refrigerants, but this list does not include aU available refrigerants. Table 11-5 summarizes a limited list of comparative hazards to life of refrigerant gas and vapor. The current more applicable refrigerants from the m or manufacturers of the CFC and HCFC refrigerants and their azeotropes/ blends/mrxtures are included, but the list excludes the pure hydrocarbons such as propane, chlorinated hydrocarbons such as methyl chloride and others, inorganics, ammonia, carbon dioxide, etc. See Table 11-6. The CFC compounds have a longer and more serious ozone depletion potential than the HCFC compounds, because these decompose at a much lower atmospheric level and have relatively short atmospheric lifetimes therefore, they do less damage to the ozone layer. Table 11-7 summarizes alternate refrigerants of the same classes as discussed previously. Table 11-8 correlates DuPont s SUVA refrigerant numbers to the corresponding ASHRAE numbers.

Table I. Trace gas rate constants and lifetimes for reaction with ozone, hydroxyl radical, and nitrate radical. Lifetimes are based upon [O3]=40ppb [HO ]=1.0x10 molecules cm (daytime) [NO3 ]=10ppt (nighttime).

HO oxidation of CO is much faster than the reaction with methane, resulting in a mean CO lifetime of about two months, but considerably slower than reaction with the majority of the nonmethane hydrocarbons. Table I gives representative removal rates for a number of atmospheric organic compounds their atmospheric lifetimes are the reciprocals of these removal rates (see Equation E4, below). The reaction sequence R31, R13, R14, R15 constitutes one of many tropospheric chain reactions that use CO or hydrocarbons as fuel in the production of tropospheric ozone. These four reactions (if not diverted through other pathways) produce the net reaction... 

The first thing that stands out in Table 6.2 is that the OH-CH4 rate constant, 6.2 X 10 15 cm3 molecule 1 s-1, is much smaller than those for the higher alkanes, a factor of 40 below that for ethane. This relatively slow reaction between OH and CH4 is the reason that the focus is on non-methane hydrocarbons (NMHC) in terms of ozone control in urban areas. Thus, even at a typical peak OH concentration of 5 X 106 molecules cm 3, the calculated lifetime of CH4 at 298 K is 373 days, far too long to play a significant role on urban and even regional scales. Clearly, however, this reaction is important in the global troposphere (see Chapter 14.B.2b). 

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)... 

TABLE 13.3 Atmospheric Lifetimes and Steady-State Ozone Depletion Potentials (ODP) Predicted Using Either a Two-Dimensional Model or a Semiempirical Method b... 

For further detailed information see refs 6, 76-81, 183 and 184. For MAK values of CFCs, etc., see Table 3. Most of the compounds listed above show considerable cardiac cffccts.7,, 0 J2 Carcinogenic and mutagenic potentials are reported in refs 93 and 94. b ODP = ozone depletion potential relative to Rll, calculated over their full lifetime.75 For global w arming potentials, see Table 13. 

Table I. Trace Gas Rate Constants and Lifetimes for Reaction with Ozone, Hydroxyl Radical, and Nitrate Radical...

Table 3. Base background scenarios and subsonic aircraft NOx scenarios used in the global model studies. These scenarios are used to study ozone increases, non linearity in ozone productions from aircraft emissions and the impact on methane lifetime and methane concentrations for future aircraft emissions.

TABLE 2. Atmospheric lifetime, ozone depletion potential (ODP) and global warming potential (GWP) of CFCs, HCFCs, HFCs and halons... 

The Ta <— So transition moments to particular spin sublevels for the three lowest triplet states of the ozone molecule, 3B2,3 A2 and 3B, were calculated by the MCQR method in ref. [70] using CASSCF wave functions. Table 7 recapitulates results for electric dipole radiative activity of different S-T transitions in ozone [70]. The type of information gained form this kind of spin-orbit response calculations are viz. transition electric dipole moments and oscillator strengths for each spin sublevel T , their polarization directions (7), radiative lifetimes (r ) and excitation energies (En). The most prominent features of the Chappuis band are reproduced in calculations, which simulate the photodynamics of ozone visible absorption [78, 79]. Because the CM (M2) state cannot be responsible for the Wulf bands, the only other candidates ought to... 

Hydroxyl radical, OH, is the principal atmospheric oxidant for a vast array of organic and inorganic compounds in the atmosphere. In addition to being the primary oxidant of non-methane hydrocarbons (representative examples of these secondary reactions are given in Table 6), OH radical controls the rate of CO and CH4 oxidation. Furthermore, the OH reaction with ozone also limits the destruction of O3 in the troposphere, it also determines the lifetime of CH3CI, CHsBr, and a wide range of HCFC s, and it controls the rate of NO to HNO3 conversion. Concentration profiles for hydroxyl radical in the atmosphere are shown in Fig. 2. 

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 ... 

At steady-state, as we noted before the downward revision of a (O2) causes the ozone depletion — AOs to diminish, (Table 6) no doubt due to the smaller peak value of ClY as a result of the shorter lifetime of the chlorof luorocarbons. 

Table 1.7 Atmospheric lifetimes (years), global-warming potential (GWP), and ozone-depleting potential (ODP) of different fluorochemicals. The global warming potential of a material is the integrated radiative forcing over 100 years after release of 1 kg divided by the integrated radiative forcing over the same period from release of 1 kg carbon dioxide [29, 31, 32a],...

Pedersen and Sehested (2002) showed that the aqueous-phase reaction of isoprene with ozone was insignificant for the processing of isoprene in the atmosphere. They estimated the overall and individual lifetimes of isoprene due to reactions with ozone and the hydroxyl radical, at 25 "C and typical in-cloud conditions. The results (Table 3) indicate that clouds generally should not contribute much to the processing of isoprene in the atmosphere. Only in the aqueous phase, were the lifetimes of isoprene due to reactions with ozone and with OH radicals comparable. Similar conclusions were drawn for methyl vinyl ketone, while for methacrolein the clouds could reduce the overall atmospheric lifetime by 50 %. 

The reactions with ozone, H02, CH302, and other R02 radicals cause NO to be rapidly reverted back to N02 so that steady-state conditions are set up for NO. The lifetimes of H02 and CH302 are quite sensitive to this stationary NO concentration. The tropospheric background level of N02 is of the order of 30 pptv in marine air, and higher over the continents. Measurements of NO in the free troposphere indicate daytime mixing ratios of about 10 pptv or less (see Table 9-12). In rural continental air, values of... 

Table 6-1. Hydrocarbon Reactivities Rate Coefficients (at 298 K) for Reactions with OH Radicals and Ozone The Corresponding Rates for h( OH) = 5 x 105, n (O,) = 6.5 x 10" molecules/ cm1-, And the Associated Lifetimes of the Hydrocarbons in the Troposphere...

Spatial scales characteristic of various atmospheric chemical phenomena are given in Table 1.1. Many of the phenomena in Table 1.1 overlap for example, there is more or less of a continuum between (1) urban and regional air pollution, (2) the aerosol haze associated with regional air pollution and aerosol-climate interactions, (3) greenhouse gas increases and stratospheric ozone depletion, and (4) tropospheric oxidative capacity and stratospheric ozone depletion. The lifetime of a species is the average time that a molecule of that species resides in the atmosphere before removal (chemical transformation to another species counts as removal). Atmospheric lifetimes vary from less than a second for... 

Table 1 Atmospheric lifetime, ozone depletion and global warming potentials of the major atmospheric halocarbons ...

Table 1.7 Lifetimes (in years) of Some Common VOCs in Presence of Selected Reactive Species (OH radicals and Ozone) in Atmosphere [41, 42]...

Calvert et al. (2008) and the present authors have used the cross section data tabulated in table IX-C-11 and the effective quantum yield of CHF2CHO photodissociation (0.30) reported by Sellevag et al. (2005) to calculate the photolysis frequencies as a function of solar zenith angle for a cloudless day within the lower troposphere (ozone column = 350 DU) see figure IX-C-31. These data suggest a photochemical lifetime for an overhead Sun of about 6 h. 

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