Page 209 - Fundamentals of Air Pollution

III. Gas-Phase Chemical Reaction Pathways 175

1. Ozone Reaction with Propene
A schematic diagram of the O 3 reaction with propene (Fig. 12-5) is based
on the work of Atkinson and Lloyd (9). The molozonide formed by addition
of ozone to the double bond decomposes to form an aldehyde and an
energy-rich (£) biradical. In the case of propene, two sets of products are
formed. Along the pathway on the right, approximately 40% of the biradi-
cals (HtHOO):j: form a thermalized biradical (£H 2OO).* The remainder
undergo rearrangement to form energy-rich acetic acid (HCOOH)|, which
subsequently decomposes to form H 2O, CO, CO 2, H 2, H, and HCO 2 radi-
cals with percentages assigned to each pathway. The larger biradical
(CH 3CHOO)| follows a slightly different pathway. Approximately 40%
forms a thermalized biradical (CH 3CHOO). Of the remaining 60%, a por-
tion decomposes to CH 4 and CO 2 and two additional energy-rich species
(CH 3COOH)t and (CHOOCH 3)J. These two unstable species decompose
as shown to form CH 3, OH, H, HCO, CH 3O, CO, and CO 2.
Alkyl radicals, R, react very rapidly with O 2 to form alkylperoxy radicals.
H reacts to form the hydroperoxy radical HO 2. Alkoxy radicals, RO, react
with O 2 to form HO 2 and R'CHO, where R' contains one less carbon. This
formation of an aldehyde from an alkoxy radical ultimately leads to the
process of hydrocarbon chain shortening or clipping upon subsequent reac-
tion of the aldehyde. This aldehyde can undergo photodecomposition form-
ing R, H, and CO; or, after OH attack, forming CH(O)OO, the peroxyacyl
radical,

2. Hydroxyl Radical Addition to Propene
As shown in Fig. 12-6, hydroxyl radicals primarily add to either of the
carbon atoms which form the double bond. The remaining carbon atom
has an unpaired electron which combines with molecular oxygen, forming
an RO 2 radical. There are two types of RO 2 radicals labeled C 3OHO 2 in Fig.
12-6. Each of these RO 2 radicals reacts with NO to form NO 2, and an alkoxy
radical reacts with O 2 to form formaldehyde, acetaldehyde, and HO 2.

3. Aldehyde Photolysis and Reactions
Aldehydes undergo two primary reactions: photolysis and reaction with
OH radicals. These reactions lead to formation of CO, H, and R radicals.

4. Radical Reactions with Nitric Oxide and Nitrogen Dioxide
Alkylperoxy (RO 2) and peroxyacyl (RC(O)OO) radicals react with NO to
form NO 2. The alkylperoxy radicals (RO 2) react with NO 2 to form pernitric
acid-type compounds, which decompose thermally as the temperature in-
creases. The peroxyacyl radical reacts with NO 2 to form PAN-type com-
pounds, which also decompose thermally.
* The dots represent unpaired electrons.