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RH -|- OH* —ó R* -Ü H20 RH + NO3 R - + HNO3
R* -f 02 -b M —> RO2 • -|- M
CHEMISTRY OF NONMETHANE ORGANIC COMPOUNDS IN THE TROPOSPHERE 265
These alkyl peroxy radicals can be classed as primary, secondary, or tertiary depending on the availability of H atoms: RCH200-(pnmary); 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 will 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,
R02 • + NO —> RO • + N02 (5.59a)
For alkyl peroxy radicals, reaction 5.59a can form the corresponding alkoxy (RO) radical together with N02, or the corresponding alkyl nitrate, reaction 5.59b, with the yield of the alkyl nitrate increasing with increasing pressure and with decreasing temperature. For secondary alkyl peroxy radicals at 298 Ê 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). The rate constant for the CH302- + NO reaction is (see Table B.l): k5M = 4.2 X 10~12exp( 180/Ã) = 7.7 X 10-12 cm3 molecule-1 s-1 at 298 K. Rate constants for higher (>C2) alkyl peroxy radicals with NO are taken as (Atkinson, 1994): k55g = 4.9 X 10~12 exp( 180/Ã) = 8.9 X 10"12 cm3 molecule-1 s-1 at 298 K.
Alkyl peroxy radicals react with N02 by combination to yield the peroxynitrates (recall reaction 5.41),
R02 • +N02 + M R00N02 + M (5.60)
Limiting high pressure rate constants for >C2 alkyl peroxy radicals are identical to that for the C2Hs02- radical: k5M = 9 X 10-12 cm3 molecule-1 s-1, independent of temperature over the range 250 to 350 K.
Alkyl peroxy radicals also react with H02 radicals,
R02 +H02- -*R00H + 02 (5.61)
or with other R02 radicals. The self-reaction of R02 • and R02 • proceeds by the three pathways
R,R2CH02- + R,R2CH02- 2 R,R2CHO- + 02 (5.62a)
R,R2CHOH + R,R2CO + 02 (5.62b) R,R2CHOOCHRiR2 + 02 (5.62c)
Pathway 5.62b is not accessible for tertiary R02 radicals, and pathway 5.62c is expected to be of negligible importance. Under urban conditions, and indeed possibly for much of the lower troposphere in anthropogenically influenced continental regions, reaction with NO is the dominant reaction pathway for R02 radicals.
Alkoxy (RO ) radicals are formed in the reaction of alkyl peroxy (R02 ) radicals with NO, reaction 5.59a. Subsequent reactions of alkoxy radicals determine to a large extent the
266 CHEMISTRY OF THE TROPOSPHERE
products resulting from the atmospheric oxidation of VOCs. Alkoxy radicals react under tropospheric conditions via a variety of processes: unimolecular decomposition, unimole-cular 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 CH3 Î • -I- 02 and C2H5 Î • + 02 reactions are given in Table Â. 1. For primary (RCH2O) and secondary (R1R2CHO) alkoxy radicals formed from the alkanes (Atkinson, 1994),7
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 Ñ-atom chains of two or fewer Ñ 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:
RO- + 02 -* R'CHO + H02-
rch2o- R- + HCHO RRjCHO- -> R +RjCHO RRiR2CO- R- + RiC(0)R2
ÑÍ3ÑÍ2ÑÍ2ÑÍ2ÑÍç + OH • CH3CHCH2CH2CH3 + H20
CH3CHCH2CH2CH3 + NO
CH3CHCH2CH2CH3 + no2
7Note a slight difference in the value of the preexponential factor between that in Table Â. 1 and that recommended by Atkinson (1994).
CHEMISTRY OF NONMETHANE ORGANIC COMPOUNDS IN THE TROPOSPHERE 267
The 2-pentoxy radical can then react with 02, decompose, or isomerize:
CH3CHO + CH3CH2CH2
CH3- + CH3CH2CH2CHO
CH3C(0)CH2CH2CH3 + H02 •
Rate constants for alkoxy radical isomerizations can be combined with rate constants for alkoxy radical decomposition and reaction with 02 to predict the relative importance of the three pathways (Atkinson, 1994). Alkoxy radicals can also react with NO and N02, but under ambient tropospheric conditions these reactions are generally of negligible importance.