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Time (min)O atom rateO 3 rateExperimentally
determined
ratePropene loss rate (ppb min–1)FIGURE 3 Experimentally observed rates of propene loss and
calculated loss rates due to its reaction with O 3 and O atoms. From
Finlayson-Pitts and Pitts (1986).an acetaldehyde molecule have been formed, and the hydroxyl
radical that initiated the reaction sequence has been re-formed.
This mechanism shows the importance of the hydroxyl radi-
cal in explaining the excess removal rate of propene observed
in smog-chamber studies. In addition, it provides a clue about
how NO is converted to NO 2 in the atmosphere.
Hydroxyl radicals are present in the atmosphere at very
low concentrations. Since the hydroxyl radical is reformed in
the atmospheric photooxidation of hydrocarbons, it effectively
acts as a catalyst for the oxidation of hydrocarbons. Figure 4
illustrates the role of the hydroxyl radical in initiating a chain
of reactions that oxidize hydrocarbons, forming peroxy radi-
cals that can oxidize NO to NO 2 and re-form hydroxyl radicals.
The NO 2 can photolyze, leading to the formation of ozone.PAN FormationAcetaldehyde may react with hydroxyl radicals, forming
the peroxyacetyl radical (CH 3 C(O)O 2 ) under atmospheric
conditions:CH 3 CHO OH → CH 3 CO H 2 O (10)
CH 3 CO O 2 → CH 3 C(O)O 2 (11)The peroxyacetyl radical may react with NO:CH 3 C(O)O 2 NO → CH 3 C(O)O NO 2 (12)
CH 3 C(O)O O 2 → CH 3 O 2 CO 2 (13)oxidizing NO to NO 2 and producing a methylperoxy radi-
cal. The methylperoxy radical can oxidize another NO to
NO 2 , forming a HO 2 (hydroperoxy) radical and a molecule
of formaldehyde:CH 3 O 2 NO → CH 3 O NO 2 (14)
CH 3 O O 2 → HCHO HO 2 (15)Alternatively, the peroxyacetyl radical may react with NO 2
to form peroxyacetyl nitrate (CH 3 C(O)O 2 NO 2 , or PAN):CH 3 C(O)O 2 NO 2 ↔ CH 3 C(O)O 2 NO 2 (16)Which reaction occurs with the peroxyacetyl radical depends
on the relative concentrations of NO and NO 2 present.
PAN, like ozone, is a member of the class of compounds
known as photochemical oxidants. PAN is responsible for
much of the plant damage associated with photochemical-
oxidant problems, and it is an eye irritant. More recent mea-
surements of PAN throughout the troposphere have shown
that PAN is ubiquitous. The only significant removal process
for PAN in the lower troposphere is, as a result of its ther-
mal decomposition, the reverse of reaction (16). This thermal
decomposition of PAN is both temperature- and pressure-
dependent. The lifetime for PAN ranges from about 30 min-
utes at 298 K to several months under conditions of the upper
troposphere (Seinfeld and Pandis, 1998). In the upper tropo-
sphere, PAN is relatively stable and acts as an important res-
ervoir for NO x. Singh et al. (1994) have found that PAN is the
single most abundant reactive nitrogen-containing compoundIn both cases the unpaired electron is on the end oxygen in
the peroxy group (in parentheses). These peroxy radicals
react like all other alkylperoxy or hydroperoxy radicals
under atmospheric conditions, to oxidize NO to NO 2 :CH 3 CH(O 2 )CH 2 OH NO →
CH 3 CH(O)CH 2 OH NO 2 (6a)
CH 3 CHOHCH 2 (O 2 ) NO →
CH 3 CHOHCH 2 (O) NO 2 (6b)The resulting oxy radicals are then expected to dissociate toCH 3 CH(O)CH 2 OH → CH 3 CHO CH 2 OH (7a)
CH 3 CHOHCH 2 (O) → CH 3 CHOH CH 2 O (7b)Forming CH 3 CHO (acetaldehyde or ethanal) and a new, one-
carbon radical (7a) and HCHO (formaldehyde or methanal)
and a new, two-carbon radical (7b). These new radicals are
expected to react with O 2 to form the appropriate aldehyde
and a hydroperoxy radical, which can oxidize NO to NO 2.CH 2 OH O 2 → HCHO HO 2 (8a)
CH 3 CHOH O 2 → CH 3 CHO HO 2 (8b)
HO 2 NO → OH NO 2 (9)So far in this hydrocarbon oxidation process, two NO molecules
have been oxidized to two NO 2 molecules, a formaldehyde andC001_008_r03.indd 123C001_008_r03.indd 123 11/18/2005 10:15:20 AM11/18/2005 10:15:20 AM