Encyclopedia of Environmental Science and Engineering, Volume I and II

124 ATMOSPHERIC CHEMISTRY

in the free troposphere. Talukdar^ et al. (1995) have found that

photolysis of PAN can compete with thermal decomposition

for the destruction of PAN at altitudes above about 5 km. The

reaction of the hydroxyl radical with PAN is less important

than thermal decomposition and photolysis throughout the

troposphere.

The oxidation of hydrocarbons does not stop with the

formation of aldehydes or even the formation of CO. It can

proceed all the way to CO 2 and H 2 O. CO can also react with

hydroxyl radicals to form CO 2 :

OH  CO → H  CO 2 (17)

H  O 2  M → HO 2  M (18)

The chain of reactions can proceed, oxidizing hydrocarbons,

converting NO to NO 2 , and re-forming hydroxyl radicals

until some chain-terminating reaction occurs. The following

are the more important chain-terminating reactions:

HO 2  HO 2 → H 2 O 2  O 2 (19)

RO 2  HO 2 → ROOH  O 2 (20)

OH  NO 2  M → HNO 3  M (21)

These reactions remove the chain-carrying hydroxyl or

peroxy radicals, forming relatively stable products. Thus, the

chain oxidation of the hydrocarbons and conversion of NO to

NO 2 are slowed.

Radical Sources

This sequence of hydrocarbon oxidation reactions describes

processes that can lead to the rapid conversion of NO to NO 2.

The NO 2 thus formed can react by (1) and (2) to form O 3. In

order for these processes to occur, an initial source of hydroxyl

radicals is required. An important source of OH in the nonur-

ban atmosphere is the photolysis of O 3 to produce an electroni-

cally excited oxygen atom (designated as O(^1 D)):

O 3  h (   320 nm) → O(^1 D)  O 2 (22)

The excited oxygen atom can either be quenched to form

a ground-state oxygen atom or react with water vapor (or

any other hydrogen-containing compound) to form hydroxyl

radicals:

O(^1 D)  H 2 O → 2OH (23)

Other possible sources of hydroxyl radicals include the pho-

tolysis of nitrous acid (HONO), hydrogen peroxide (H 2 O 2 ),

and organic peroxides (ROOH):

HONO  h (   390 nm) → OH  NO (24)

H 2 O 2  h (   360 nm) → 2OH (25)

The atmospheric concentration of HONO is sufficiently low

and photolysis sufficiently fast that HONO photolysis can only

act as a radical source, in the very early morning, from HONO

that builds up overnight. The photolysis of H 2 O 2 and ROOH

can be significant contributors to radical production, depend-

ing on the quantities of these species present in the atmosphere.

Another source of radicals that can form OH radicals includes

the photolysis of aldehydes, such as formaldehyde (HCHO):

HCOC  h (   340 nm) → H  HCO (26)

HCO  O 2 → HO 2  CO (27)

forming HO 2 radicals in (27) and from H atoms by reac-

tion (18). These HO 2 radicals can react with NO by reaction

(9) to form OH. The relative importance of these different

NO 2

RH + OH

R ́

CO

HO 2

NO

O 2

RO

NO 2

NO

RO 2

O 2

+ R ́CHO

hυ

FIGURE 4 Schematic diagram illustrating the role of the hydroxyl-radical-initiated oxidation of hydrocarbons in the

conversion of NO to NO 2.

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