Rate constants atmospheric reactions

Reaction 2-6 is sufficiently fast to be important in the atmosphere. For a carbon monoxide concentration of 5 ppm, the average lifetime of a hydroxyl radical is about 0.01 s (see Reaction 2-6 other reactions may decrease the lifetime even further). Reaction 2-7 is a three-body recombination and is known to be fast at atmospheric pressures. The rate constant for Reaction 2-8 is not well established, although several experimental studies support its occurrence. On the basis of the most recently reported value for the rate constant of Reaction 2-8, which is an indirect determination, the average lifetime of a hydroperoxy radical is about 2 s for a nitric oxide concentration of 0.05 ppm. Reaction 2-8 is the pivotal reaction for this cycle, and it deserves more direct experimental study. 

In Table 1 (pp. 251-254), IM rate constants for reaction systems that have been measured at both atmospheric pressure and in the HP or LP range are listed. Also provided are the expected IM collision rate constants calculated from either Langevin or ADO theory. (Note that the rate constants of several IM reactions that have been studied at atmospheric pressure" are not included in Table I because these systems have not been studied in the LP or HP ranges.) In general, it is noted that pressure-related differences in these data sets are not usually large. Where significant differences are noted, the suspected causes have been previously discussed in Section IIB. These include the reactions of Hcj and Ne with NO , for which pressure-enhanced reaction rates have been attributed to the onset of a termolecular collision mechanism at atmospheric pressure and the reactions of Atj with NO and Cl with CHjBr , for which pressure-enhanced rate constants have been attributed to the approach of the high-pressure limit of kinetic behavior for these reaction systems. 

The importance of OH radicals in atmospheric chemistry is the basis of another reactivity scale for organics that do not photolyze in actinic radiation (Darnall et al., 1976 Wu et al., 1976). This scale is based on the fact that, for most hydrocarbons, attack by OH is responsible for the majority of the hydrocarbon consumption, and this process leads to the free radicals (H02, R02) that oxidize NO to N02, which then leads to 03 formation. Even for alkenes, which react with 03 at significant rates, consumption by OH still predominates in the early portion of the irradiation before 03 has formed. It has therefore been suggested that the rate constant for reaction between OH and the... 

The incremental reactivity of a VOC is the product of two fundamental factors, its kinetic reactivity and its mechanistic reactivity. The former reflects its rate of reaction, particularly with the OH radical, which, as we have seen, with some important exceptions (ozonolysis and photolysis of certain VOCs) initiates most atmospheric oxidations. Table 16.8, for example, also shows the rate constants for reaction of CO and the individual VOC with OH at 298 K. For many compounds, e.g., propene vs ethane, the faster the initial attack of OH on the VOC, the greater the IR. However, the second factor, reflecting the oxidation mechanism, can be determining in some cases as, for example, discussed earlier for benzaldehyde. For a detailed discussion of the factors affecting kinetic and mechanistic reactivities, based on environmental chamber measurements combined with modeling, see Carter et al. (1995) and Carter (1995). 

Photooxidation atmospheric ty, = 364.4-3644 h, based on estimated rate constant for reaction with OH radical and aqueous L, = 66.0-3480 h, based on reaction rate constants with OH and ROj- radicals with phenol class (Howard et al. 1991) ... 

Estimate an atmospheric half-life of acrolein. The second-order rate constant for reaction with OH- is approximately 2 X ICO11 cm3/(molecule sec) and the second-order rate constant for reaction with 03 is 4 X ICR13 cm3/(molecule sec). Assume [OH-] is approximately 106 molecules per cubic centimeter and the 03 partial pressure is ICR7 atm. 

The role of FNO in atmospheric chemistry is related to the destruction of stratospheric ozone.244 246 The reaction F + NO has been extensively studied using different experimental techniques, including mass spectrometry, chemiluminescence, ESR, IR, and UV spectroscopy.241,247 251 The pressure dependence of the kinetics of FNO formation was recently studied by Pagsberg et al.25i in order to obtain the fall-off curve and the high- and low-pressure limiting rate constants. The reaction was initiated by pulse radiolysis of a SF6/NO gas mixture. In the presence of NO, the decay of the formed... 

The reported measured rate constant for reaction of hydrazine with atmospheric hydroxyl (OH) radicals producing ammonia and nitrogen gas was 6.lx 10 cm molecule s (Harris et al. 1979). The rate constant for 1,1-dimethylhydrazine was not measured since the chemical decomposed rapidly in the test system, but the value was estimated at 5 /10 cm molecule s . Assuming an average OH radical concentration of about 10 molecLile/cm . the tropospheric half-lives ofboth chemicals due to reaction with OH were estimated to be about 3 hours. The half-lives are expected to range from less than 1 hour in polluted urban air to 3-6 hours in less polluted atmospheres (Tuazon et al. 1981). 

Many of the rate constants for reactions important in atmospheric chemistry are surveyed periodically b> a group organized through the Jet Propulsion Laboratory (JPL), Pasadena, CA. The latest report is that of De More et al. (1997) Evaluation No. 12, JPL Publication 97-4. Recommended values of rate constants, absorption cross sections, and quantum yields appear in these reports. 

Because of the ozone depletion that occurs by photolysis, the NO, break-even concentration at which net O, production occurs is somewhat larger than the value based just on the ratio of the rate constants of reactions 5.47 and 5.25. The approximate crossover point for NO, between O destruction and production is usually considered to be at about 30 ppt. Ozone mixing ratios in the planetary boundary layer over the remote Pacific Ocean are only about 5 to 6 ppb NO, levels are about 10 ppt. Thus, this region of the atmosphere is probably below the crossover point. 

Reaction (25) represents collisional relaxation of the excited oxygen atom. While halogenated hydrocarbons form too small a fraction of the available collision partners to be of atmospheric consequence for the relaxation channel, this pathway must be considered for laboratory experiments relying on 0( D) or 0( P) vs. time profiles to deduce the rate constant of reaction (24). In reaction (26), a sizable fraction of the 190 kJ mol excess energy of 0( D) is transferred to internal excitation of the HCFC, which then can dissociate to products. Between reactions (24) and (26) a range of products is possible, including OH + R, CIO -I- R, and 0( P) -I- HCl -f chlorofluoroalkene. 

In analogy with reactions (48) and (49) the reactions of FC(0)02 and FC(0)0 with O3 also form a potential ozone destruction cycle. Preliminary data for the FC(0)02 -I- O3 reaction indicate an upper limit of k < 1.5 x 10 cm s at 298 K [87]. The same data place an upper limit oik <3 x 10 cm s for the FC(0)0 -I- O3 reaction, consistent with the recently reported value of k<6 x 10 cm s [83]. These rate constants are roughly four orders of magnitude smaller than those for the reaction between FC(0)0, and NO. The much slower rate constants for reaction with ozone as opposed to NO imply that FC(0)0, will be rapidly removed from the atmosphere and contribute little to ozone destruction. 

Nevertheless, the choice of either strategy in many respects is dependant on the quantity and quality of the available information, on the reactivity of species and the rate constants of reactions. Due to the availability of reliable quantitative data for the reactions, with participation of possible species of a reaction system, recently it became possible to apply successfully a deductive approach for modeling the reactions on combustion [21-27], cracking [28,29], atmospheric processes [30-36] and others. 

Here, the quantities within the brackets [ ] represent concentration of each species, and the defined above is called a reaction rate constant. The reaction rate constant of a bimolecular reaction has the dimensimi of (concentration) (time) . In atmospheric chemistry, the concentration of gaseous species is expressed in general by the number density of molecules, molecules cm so that the unit of the rate constant of a bimolecular reaction is commonly expressed as cm molecule s . ... 

Under the atmospheric conditions, reaction (5.27) is in the intermediate region between the low-pressure and high-pressure limit, and the pressure dependence has to be calculated by using Eq. (5.7). The lUPAC subcommittee report Vol. II proposed the approximate equation for the overall rate constants combining reaction (5.26) and (5.27),... 

---