Health Assessment Document for Diesel Emissions - NSCEP | US ...

1 For the simplest alkoxy radicals, reaction with 0 2 (pathway a) is predominant. For larger
.2 radicals, however, isomerization (c) and decomposition (b) may become significant, the relative
3 importance of these two pathways depending on the structure of the radical. In all three cases,
'4 free radicals are produced, which then will carry on the chain reactions. The first-generation
5 products include aldehydes, ketones, and alkyl nitrates, which can react further under
6 atmospheric conditions.
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8 2.4.2.1.4. Reactions of alkene. Lower molecular weight alkenes, such as ethylene, propylene,
9 and isomeric butenes, are present in exhaust from gasoline and (although in lower amount) diesel
10 engines (see Table 2-2). Gas-phase alkenes are removed from the troposphere by reaction with
11 · OH radicals, N0 3 radicals, and 0 3 (Finlayson-Pitts and Pitts, 1986; Atkinson, 1988; Atkinson
12 and Carter, 1984). Reactions with OH radicals are rapid (see Table 2-13) and proceed by OH
13 radical addition to the double bond.
14 Because the OR-olefin adduct is essentially an alkyl radical, it reacts further in a manner
t5 similar to alkyl radicals formed from the reaction of alkanes with OH radicals (e.g., by the
16 addition of 0 2 followed by reaction with NO). For example, for radical (a):
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23 Table 2-3). Similar to the OH radical reaction, this reaction proceeds through N03 radical
24 addition to the double bond, followed by reaction with 0 2 (Finlayson-Pitts and Pitts, 1986;
25 Atkinson, 1988). Carbonyl compounds are formed as major products, but minor products
26 (possibly dinitrates) are not well defined (Atkinson, 1988).
27 The reactions of alkenes with ozone compete with the daytime OH radical reaction (Table
28 2-3). These reactions proceed by addition to the double bond, followed by rapid decomposition
29 of so-called "ozonide" or "molozonide" into a carbonyl compound and an energy-rich biradical:
2/1/98 2-33 DRAFT--DO NOT CITE OR QUOTE