The essential elements of tropospheric chemistry have been presented in Sections 6.1-6.9. The remainder of this chapter is devoted to an in-depth treatment of the tropospheric chemistry of individual classes of organic species. For many readers, for example, those in a first course in atmospheric chemistry, it may not be necessary to proceed beyond this point in this chapter. All problems at the end of the chapter can be answered based on the material in Sections 6.1-6.9.
There are a number of excellent reviews of tropospheric chemistry that contain more details on mechanisms than can be included here.
General tropospheric chemistry: Atkinson (1989, 1990, 1994) Atkinson and Arey (2003b) Alkenes
Atkinson et al. (2000) Aromatic s
Calvert et al. (2002) Oxygenerated organic compounds
Mellouki et al. (2003) Biogenic organics
Atkinson and Arey (2003a)
Under tropospheric conditions, alkanes react with OH and N03 radicals; the latter process generally is of minor (< 10%) importance as an atmospheric loss process under daytime conditions. Both reactions proceed via H-atom abstraction from C—H bonds5
to produce the alkyl radical, R. Any H atom in the alkane is susceptible to OH attack. Generally, the OH radical will tend to abstract the most weakly bound hydrogen atom in the molecule. The overall rate constant reflects the number of available hydrogen atoms and the strengths of the C—H bonds for each of these. Correlations have been developed for calculating the OH rate constant of alkanes that account for the number of primary (—CH3), secondary, (—CH2—), and tertiary (> CH) hydrogen atoms in the molecule (Atkinson 1987, 1994). Hydroxyl attack on a tertiary hydrogen atom is generally faster than that on a secondary H atom and is the slowest for primary H atoms. For propane, CH3CH2CH3, for example, these structure-activity correlations predict that 70% of the OH reaction occurs by H-atom abstraction from the secondary carbon atom (—CH2—) and 30% from the —CH3 groups.
5In the remaining sections of this chapter we will explicitly denote free radicals with a dot (■) in order to identify more clearly radicals in the mechanisms.
As with the methyl radical, the resulting alkyl (R) radical reacts rapidly, and exclusively, with 02 under atmospheric conditions to yield an alkyl peroxy radical (R02) [see the comprehensive reviews of the chemistry of R02 radicals by Lightfoot et al. (1992) and Wallington et al. (1992)]:
These alkyl peroxy radicals can be classed as primary, secondary, or tertiary depending on the availability of H atoms: RCH200 (primary); RR'CHOO- (secondary); RR'R"COO-(tertiary). The alkyl radical-02 addition occurs with a room-temperature rate constant of > 10~12 cm3 molecule"1 s 1 at atmospheric pressure. Given the high concentration of 02, the R + 02 reaction can be considered as instantaneous relative to other reactions occurring such as those that form R in the first place. Henceforth, the formation of an alkyl radical can be considered to be equivalent to the formation of an alkyl peroxy radical.
Under tropospheric conditions, these alkyl peroxy (R02) radicals react with NO, via two pathways:
For alkyl peroxy radicals, reaction a can form the corresponding alkoxy (RO) radical together with N02, or the corresponding alkyl nitrate, reaction b, with the yield of the alkyl nitrate increasing with increasing pressure and with decreasing temperature. For secondary alkyl peroxy radicals at 298 K and 760 torr total pressure, the alkyl nitrate yields increase monotonically from <0.014 for a C2 alkane up to ~ 0.33 for a C8 alkane (Atkinson 1990). It has been shown that CH30N02 and C2H50N02 may have a substantial natural source in the oceans (Chuck et al. 2002).
Alkyl peroxy radicals react with N02 by combination to yield the peroxynitrates,
Limiting high pressure rate constants for > C2 alkyl peroxy radicals are identical to that for the C2H502- radical: k = 9 x 10 12 cm3 molecule"1 1, independent of temperature over the range 250 to 350 K.
Alkyl peroxy radicals also react with H02 radicals
or with other R02 radicals. The self-reaction of R02- and R02- proceeds by the three pathways
RiR2CHOH + R^CO + 02 RiR2CHOOCHR!R2 + o2
Pathway b is not accessible for tertiary R02 radicals, and pathway c is expected to be of negligible importance.
Alkoxy (RO) radicals are formed in the reaction of alkyl peroxy (R02) radicals with NO. Subsequent reactions of alkoxy radicals determine to a large extent the products resulting from the atmospheric oxidation of VOCs (Orlando et al. 2003). Alkoxy radicals react under tropospheric conditions via a variety of processes: unimolecular decomposition, unimolecular isomerization, or reaction with 02. Alkoxy radicals with fewer than five carbon atoms are too short to undergo isomerization; for these the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical-02 reaction involves abstraction of a hydrogen atom by 02 to produce an H02 radical and a carbonyl species:
Rate constants for the CH30- + 02 and C2H50- + O reactions are given in Table B.l. For primary (RCH20 ) and secondary (R |R2CHO ) alkoxy radicals formed from the alkanes (Atkinson 1994),6
£(RCH20- + 02) = 6.0 x 10"14 exp(—550/r) cm3 molecule"1 s~' = 9.5 x 10"15 at 298 K ¿(RiR2CHO- + 02) = 1.5 x 10"14exp(—200/7") cm3 molecule"1 s"1 = 8 x 10-15 at 298 K
Tertiary alkoxy radicals are not expected to react with 02 because of the absence of a readily available hydrogen atom.
Unimolecular decomposition, on the other hand, produces an alkyl radical and a carbonyl:
Atkinson (1994) presents a correlation that allows one to determine the relative importance of 02 reaction and decomposition for a particular alkoxy radical. Generally, reaction with 02 is the preferred path for primary alkoxy radicals that have C-atom chains of two or fewer C atoms in length attached to the carbonyl group.
To illustrate alkoxy radical isomerization, let us consider the OH reaction of n-pentane. The n-pentane-OH reaction proceeds as follows to produce the 2-pentoxy radical:
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