MECHANISMS OF SMOG FORMATION
13.5. MECHANISMS OF SMOG FORMATION
Here are discussed some of the primary aspects of photochemical smog formation. For more details the reader is referred to books on atmospheric chemistry 6 and atmospheric chemistry and physics. 7 Since the exact chemistry of photochemical smog formation is very complex, many of the reactions are given as plausible illustrative examples rather than proven mechanisms.
The kind of behavior summarized in Figure 13.4 contains several apparent anomalies which puzzled scientists for many years. The first of these was the rapid increase in NO 2 concentration and decrease in NO concentration under conditions where it was known that photodissociation of NO 2 to O and NO was occurring. Furthermore, it could be shown that the disappearance of alkenes and other hydrocarbons was much more rapid than could be explained by their relatively slow
reactions with O 3 and O. These anomalies are now explained by chain reactions involving the interconversion of NO and NO 2 , the oxidation of hydrocarbons, and the generation of reactive intermediates, particularly hydroxyl radical (HO • ). Figure 13.5 shows the overall reaction scheme for smog formation, which is based upon the photochemically initiated reactions that occur in an atmosphere con- taining nitrogen oxides, reactive hydrocarbons, and oxygen. The time variations in
levels of hydrocarbons, ozone, NO, and NO 2 are explained by the following overall reactions:
Solar energy input
Absorption of solar energy
by NO 2 produces NO
and atomic oxygen, O.
NO reacts with
NO
O 3 or RO 2 .
to produce NO 2 .
Atomic oxygen, HO . and O 3
O react with hydrocarbons 3 O reacts with
O 3 to produce highly reactive
hydrocarbon free radicals. ozone, O 3
O 2 , yielding
Hydrocarbon free radicals NO 2 Hydrocarbon free radicals
react further with species such as NO 2 to produce PAN, aldehydes, and other smog components.
NO
Figure 13.5. Generalized scheme for the formation of photochemical smog.
1. Primary photochemical reaction producing oxygen atoms: NO 2 + h ν ( λ < 420 nm) → NO + O
2. Reactions involving oxygen species (M is an energy-absorbing third body):
(13.5.2) O 3 + NO → NO 2 + O 2 (13.5.3)
Because the latter reaction is rapid, the concentration of O 3 remains low
until that of NO falls to a low value. Automotive emissions of NO tend to
keep O 3 concentrations low along freeways. 3.Production of organic free radicals from hydrocarbons, RH: O + RH → R • + other products
(13.5.4) O 3 + RH → R • + and/or other products
(13.5.5) (R • is a free radical which may or may not contain oxygen.)
4.Chain propagation, branching, and termination by a variety of reactions such as the following:
NO + ROO • → NO 2 + and/or other products
(13.5.7) The latter kind of reaction is the most common chain-terminating process in smog
NO 2 + R • → products (for example, PAN)
because NO 2 is a stable free radical present at high concentrations. Chains may terminate also by reaction of free radicals with NO or by reaction of two R • radicals, although the latter is uncommon because of the relatively low concentrations of radicals compared to molecular species. Chain termination by radical sorption on a particle surface is also possible and may contribute to aerosol particle growth.
A large number of specific reactions are involved in the overall scheme for the formation of photochemical smog. The formation of atomic oxygen by a primary photochemical reaction (Reaction 13.5.1) leads to several reactions involving oxygen and nitrogen oxide species:
(13.5.9) O + NO 2 → NO + O 2
O + NO + M → NO 2 + M
(13.5.10) O 3 + NO → NO 2 + O 2 (13.5.11)
O + NO 2 + M → NO 3 + M
(13.5.12) O 3 + NO 2 → NO 3 + O 2 (13.5.13)
There are a number of significant atmospheric reactions involving nitrogen oxides, water, nitrous acid, and nitric acid:
(13.5.14) N 2 O 5 → NO 3 + NO 2
NO 3 + NO 2 → N 2 O 5
NO 3 + NO → 2NO 2 (13.5.16) N 2 O 5 + H 2 O → 2HNO 3 (13.5.17)
(This reaction is slow in the gas phase but may be fast on surfaces.) Very reactive HO • radicals can be formed by the reaction of excited atomic oxygen
with water, O* + H 2 O → 2HO •
(13.5.18) by photodissociation of hydrogen peroxide,
(13.5.19) or by the photolysis of nitrous acid, HNO 2 + h ν → HO • + NO
H 2 O 2 + h ν ( λ < 350 nm) → 2HO •
(13.5.20) Among the inorganic species with which the hydroxyl radical reacts are oxides of
nitrogen, HO • + NO 2 → HNO 3
(13.5.22) and carbon monoxide,
HO • + NO + M → HNO 2 + M
(13.5.23) The last reaction is significant in that it is responsible for the disappearance of much
CO + HO • + O 2 → CO 2 + HOO •
atmospheric CO (see Section 11.3) and because it produces the hydroperoxyl radical HOO • . One of the major inorganic reactions of the hydroperoxyl radical is the oxidation of NO:
HOO • + NO → HO • + NO 2 (13.5.24) For purely inorganic systems, kinetic calculations and experimental measure-
ments cannot explain the rapid transformation of NO to NO 2 that occurs in an atmosphere undergoing photochemical smog formation and predict that the concentration of NO 2 should remain very low. However, in the presence of reactive hydrocarbons, NO 2 accumulates very rapidly by a reaction process beginning with its photodissociation! It may be concluded, therefore, that the organic compounds form species which react with NO directly rather than with NO 2 .
A number of chain reactions have been shown to result in the general type of species behavior shown in Figure 13.4 . When alkane hydrocarbons, RH, react with O, O 3 , or HO • radical,
RH + O + O 2 → ROO • + HO •
(13.5.26) reactive oxygenated organic radicals, ROO • , are produced. Alkenes are much more
RH + HO • + O 2 → ROO • + H 2 O
reactive, undergoing reactions with hydroxyl radical,
RR rapid
products
R Radical adduct
(where R may be one of a number of hydrocarbon moieties or an H atom) with oxygen atoms,
R Biradical
or with ozone:
R Primary ozonide
Aromatic hydrocarbons, Ar-H, may also react with O and HO • . Addition reactions of aromatics with HO • are favored. The product of the reaction of benzene with HO • is phenol, as shown by the following reaction sequence:
(13.5.31) In the case of alkyl benzenes, such as toluene, the hydroxyl radical attack may occur
+ O 2 + HOO•
on the alkyl group, leading to reaction sequences such as those of alkanes. Aldehydes react with HO • ,
RCH + HO . +O 2 → R C OO . + H 2 O
CO + HO . + O _3 2 → CO 2 + HOO . 2 + H 2 O
H and undergo photochemical reactions:
RCH + h ν + 2O 2 → ROO . + CO + HOO .
C O + h ν + 2O 2 → CO + 2HOO .
H Hydroxyl radical (HO • ), which reacts with some hydrocarbons at rates that are
almost diffusion-controlled, is the predominant reactant in early stages of smog formation. Significant contributions are made by hydroperoxyl radical (HOO • ) and
O 3 after smog formation is well underway. One of the most important reaction sequences in the smog-formation process
begins with the abstraction by HO • of a hydrogen atom from a hydrocarbon and
leads to the oxidation of NO to NO 2 as follows:
(13.5.36) The alkyl radical, R • , reacts with O 2 to produce a peroxyl radical, ROO • : R • + O 2 → ROO •
RH + HO • → R • + H 2 O
(13.5.37) This strongly oxidizing species very effectively oxidizes NO to NO 2 , ROO • + NO → RO • + NO 2 (13.5.38) thus explaining the once-puzzling rapid conversion of NO to NO 2 in an atmosphere
in which the latter is undergoing photodissociation. The alkoxyl radical product, RO • , is not so stable as ROO • . In cases where the oxygen atom is attached to a carbon atom that is also bonded to H, a carbonyl compound is likely to be formed by the following type of reaction:
H 3 CO• + O 2 → HCH + HOO• (13.5.39) The rapid production of photosensitive carbonyl compounds from alkoxyl radicals is
an important stimulant for further atmospheric photochemical reactions. In the absence of extractable hydrogen, cleavage of a radical containing the carbonyl group
occurs:
H 3 C C O• → H 3 C• + CO 2 (13.5.40) Another reaction that can lead to the oxidation of NO is of the following type:
O R C OO• + NO + O 2 → ROO• + NO 2 + CO 2 (13.5.41)
Peroxyacyl nitrates (PAN) are highly significant air pollutants formed by an addition reaction with NO 2 : O
RC OO• + NO 2 → R C OO NO 2 (13.5.42) When R is the methyl group, the product is peroxyacetyl nitrate, mentioned in
Section 13.4. Although it is thermally unstable, peroxyacetyl nitrate does not undergo photolysis rapidly, reacts only slowly with HO • radical, and has a low water solubility. Therefore, the major pathway by which it is lost from the atmosphere is thermal decomposition, the opposite of Reaction 13.5.42.
Alkyl nitrates and alkyl nitrites may be formed by the reaction of alkoxyl radicals (RO • ) with nitrogen dioxide and nitric oxide, respectively:
RO • + NO 2 → RONO 2 (13.5.43) RO • + NO → RONO
(13.5.44) Addition reactions with NO 2 such as these are important in terminating the reaction
chains involved in smog formation. Since NO 2 is involved both in the chain initiation step (Reaction 13.5.1) and the chain termination step, moderate reductions in NO x emissions alone may not curtail smog formation and in some circumstances may even increase it.
As shown in Reaction 13.5.39, the reaction of oxygen with alkoxyl radicals produces hydroperoxyl radical. Peroxyl radicals can react with one another to produce reactive hydrogen peroxide, alkoxyl radicals, and hydroxyl radicals:
HOO • + HOO • → H 2 O 2 + O 2 (13.5.45) HOO • + ROO • → RO • + HO • + O 2 (13.5.46) ROO • + ROO • → 2RO • + O 2 (13.5.47)
Nitrate Radical
First observed in the troposphere in 1980, nitrate radical, NO 3 , is now recog- nized as being an important atmospheric chemical species, especially at night. 8 This First observed in the troposphere in 1980, nitrate radical, NO 3 , is now recog- nized as being an important atmospheric chemical species, especially at night. 8 This
NO 2 + O 3 → NO 3 + O 2
and exists in equilibrium with NO 2 :
NO 2 + NO 3 + M ←→ N 2 O 5 + M (energy-absorbing third body) (13.5.49) Levels of NO 3 remain low during daylight, typically with a lifetime at noon of only
about 5 seconds, because of the following two dissociation reactions: NO 3 + h ν ( λ < 700 nm) → NO + O 2 (13.5.50)
(13.5.51) However, at night the levels of NO
NO 3 + h ν ( λ < 580 nm) → NO 2 + O
3 typically reach values of around 8 × 10 molecules cm- 3 compared to only about 1 10 × 6 × molecules × cm- 3 for hydroxyl
radical. Although the hydroxyl radical reacts 10 to 1000 times faster than NO 3 , the much higher concentration of the latter means that it is responsible for much of the atmospheric chemistry that occurs at night. The nitrate radical adds across the double bonds in alkenes leading to the formation of reactive radical species that participate in smog formation.
Photolyzable Compounds in the Atmosphere
It may be useful at this time to review the types of compounds capable of undergoing photolysis in the troposphere and thus initiating chain reaction. Under most tropospheric conditions, the most important of these is NO 2 :
NO 2 + h ν ( λ < 420 nm) → NO + O (13.5.1) In relatively polluted atmospheres, the next most important photodissociation
reaction is that of carbonyl compounds, particularly formaldehyde:
• CH 2 O + h ν ( λ < 335 nm) → H • + HCO
(13.5.52) Hydrogen peroxide photodissociates to produce two hydroxyl radicals: HOOH + h ν ( λ < 350 nm) → 2HO •
(13.5.53) Finally, organic peroxides may be formed and subsequently dissociate by the
following reactions, starting with a peroxyl radical:
H 3 COO • + HOO • → H 3 COOH + O 2 (13.5.54)
H 3 COOH + h ν ( λ < 350 nm) → H 3 CO • + HO • (13.5.55)
It should be noted that each of the last three photochemical reactions gives rise to two free radical species per photon absorbed. Ozone undergoes photochemical dissociation to produce excited oxygen atoms at wavelengths less than 315 nm.
These atoms may react with H 2 O to produce hydroxyl radicals.