In atmospheres of urban centres, under conditions of relatively low humidity, plenty of sunshine, a large amount of exhaust emissions from motor vehicles and moderate to low wind speeds, photochemical processes lead to a secondary pollution situation commonly termed “photochemical smog’. A large number of compounds and reactions have been characterized in the urban air where such smog situation occurs. The chemistry of this photochemical smog condition is extremely complex. Major photochemical processes associated with this condition have been discussed below.
1. Nitrogen oxide pseudo-equilibrium
The oxides of nitrogen, particularly NO and NO2 are at the root of photochemical smog problem. Oxidation of atmospheric nitrogen during high temperature combustion processes (particularly in motor vehicles) results in formation of NO which is further oxidized to NO2:
(a) O + N2 —-> NO + N
N + O2 —-> NO + O
N2 + O2 —-> 2NO
(b) 2NO + O2 ——> 2NO2
R1 = k1[NO]2[O2] where R1 and R2 are reaction rates and k1 and k2 are the rate
NO + O3 —-> NO2 + O2
R2 = k2[NO][ O3]
The reaction of NO with oxygen at the concentrations found even in the polluted air is very slow, therefore, NO2 is mainly produced by oxidation of NO by ozone. In the polluted air, typical early morning concentrations of ozone and NO are 40 ppb and 80 ppb respectively. Values for k1 and k2 are 1.93 x 10-38 cm6s-1 and 1.8 x 10-14 cm3s-1 for ozone and NO respectively. From these values the calculation shows that R1 = 4.6 x 10-5 cm-3 s-1 and R2 = 3.8 x 1010 cm-3 s-1. This confirms the far greater importance of the oxidation of No by ozone.
The NO2 produced in this way can be photodissociated back to NO. Thus a sequence of reactions describing its destruction and regeneration can be given:
NO2 + hv ( O(3P) + NO
O(3P) + O2 + M ——> O3 + M
O3 + hv (300-330 nm) ——> O2 + O(1D)
O3 + HO2 ——> 2O2 + OH ( R = 1.1 x 10-14)
O3 + NO —–> NO2 + O2 (R = 2.3 x 10-12)
In a volume of air in steady-state where production and destruction rates of NO2 are equal and where oxidation of NO by oxygen is assumed to be unimportant, the reaction rate may be written as:
k2[NO][O3] = J[NO2]
where J is effective first-order rate constant for photodissociation. The equation may be rearranged as:
J/k2 = [NO][O3]/[NO2]
where the term on right-hand side may be ignored as a pseudo-equilibrium constant relating the partial pressures of NO, NO2 and O3. The value of J will varies with change in intensity of sunlight throughout the day. However, measurements have shown that overall the equality implied in this equation holds in the polluted atmosphere. During first half of the day radiation intensity increases which means J will increase and during this period increasing amounts of ozone and NO would be expected. Since both these are produced by destruction of NO2, the amount of ozone should approximately equal the amount of NO.
Measurements from polluted atmospheres show that neither of the above predictions are borne out. The level of NO rises in the early morning but level of ozone rises much more slowly. Further, the fact that the level of NO2 falls by mid-day is even more in contrast to the theoretical prediction. A possible explanation for these observations is that the observed rises and falls in the concentrations of pollutants are merely functions of the pattern of generation and dispersion in the atmosphere.
2. Role of organic molecules in smog
Under constant illumination the rise in the level of ozone indicates a decreasing NO:NO2 ratio in the pseudo-equilibrium. For the latter to happen, another source of oxidant is needed because above described sequence of reactions does not result in any overall production of ozone. However, ozone production in polluted atmosphere may be explained by following scheme.
As in unpolluted atmosphere, oxidation in polluted atmosphere also occurs through reactions in which hydroxyl radical plays a key role. The hydroxyl radical attacks a veriety of pollutants in the urban air resulting in formation of free radicals like methyl radical (CH3), acetyl (CH3CO) and atomic hydrogen (H) which may become involved in subsequent reactions which oxidize to NO to NO2 and regenerate hydroxyl radical at the same time.
(a) Alkanes in smog: Presence of alkanes such as methane in polluted air provides a way in which NO can be oxidized to NO2 without consuming ozone. For example, methane may be oxidized by OH radical to produce methyl radical which further undergoes a series of reactions:
CH4 + OH —-> H2O + CH3 (R = 2.4 x 10-12)
CH3 + O2 +M —-> CH3O2 + M
CH3O2 + NO —-> CH3O + NO2 (R = 7.0 x 10-12)
CH3O + O2 —–> HCHO + HO2 (R = 5.0 x 10-13)
HCHO = hv ( 2 H + CO
CO + OH —–> CO2 + H (R = 1.35 x 10-13)
HO2 + NO —–> NO2 + OH (R = 4.3 x 10-12)
HO2 radical can also react photochemically or with ozone, atomic hydrogen or atomic oxygen to regenerate OH radical. HCHO can photodissociate into atomic hydrogen or react with oxygen to give the HO2 radical and CO.
The above reactions can be summed up and show the importance of methane in generating NO2 in photochemical smog:
CH4 + 2 O2 + 2 NO —-> H2O + HCHO + 2 NO2
This indicates net oxidation of NO in a manner that has not used ozone, therefore, it is different from pseudo-equilibrium situation.
(b) Aldehydes in smog: Aldehydes also provide effective ways of oxidizing NO to NO2. For example, acetaldehyde is attacked by OH radical producing acetyl radical which undergoes following subsequent reactions:
CH3CO + O2 —–> CH3COO2
CH3COO2 + NO —–> NO2 + CH3CO2
CH3CO2 ——> CH3 + CO2
Methyl radical produced is oxidized as described above. There are analogous reactions for higher aldehydes.
(C) Atomic hydrogen in smog: The atomic hydrogen produced by attack of OH on CO or photodissociation of HCHO can react with HO2 radical to produce two OH radicals that can initiate further attack on organic compounds in air.
OH + CO —–> CO2 + H
H + HO2 —–> OH + OH
Atomic hydrogen can also form HO2 radical which can oxidize NO to NO2:
H + O2 + M —–> HO2 + M
HO2 + NO —–> NO2 + OH
In general, hydrocarbons present in the polluted urban air promote the oxidation of NO to NO2 by reactions of the types described above. The NO2 is subsequently photolysed to produce NO for reoxidation and increasing amount of ozone.
NO2 + hv ( O(3P) + NO
O(3P) + O2 + M —–> O3 + M
Though there are losses in the above described scheme, the built-up of ozone throughout the day can thus be well explained.
3. Other products in photochemical smog
A number of other features of photochemical smog can also be explained by photochemical mechanism described above.
1. Formation of PAN: Peroxyacetylnitrate (CH3COO2NO2) or PAN is a major eye-irritant found characteristically in photochemical smog. The peroxyacetyl radical (CH3COO2) produced by attack of acetyl radical on oxygen can combine with NO2 to form PAN:
CH3COO2 + NO2 ——> CH3COO2NO2
PAN is the principal member of a group of rather similar nitrated compounds which includes higher peroxyalkyl compounds such as peroxypropionyl nitrate which has also been detected in low concentrations in photochemical smog. There is also much current interest in the natural production of compounds like PAN.
2. Formation of N2O5 : NO2 is oxidized by ozone to NO3 which subsequently reacts with NO2 to form N2O5. The NO3 may also react with NO to produce more NO2.
NO2 + O3 —-> NO3 + O2
NO3 + NO2 ——> N2O5
NO3 + NO ——-> 2NO2
3. Formation of nitric acid: The OH radical formed in the smog reacts with NO to form HNO2 and with NO2 to produce HNO3. There may be reaction between NO and NO2 to form HNO2.
NO + OH —-> HNO2
NO2 + OH + M—–> HNO3 + M
NO + NO2 + H2O ——> 2HNO2
HNO2 undergoes photodissociation to produce NO and provide a source of OH radicals.
HNO2 + hv ( NO + OH
4. Formation of hydrogen peroxide: Formaldehyde in polluted air is an important source of atomic hydrogen and hence OH and HO2 radicals:
HCHO + hv ( 2H + CO
H + O2 + M ——> HO2 + M
HO2 + NO —–> NO2 + OH
The OH and HO2 radicals may produce H2O2:
OH + OH + M —–> H2O2 + M
HO2 + HO2 ——–> H2O2 + O2 (R = 3.8 x 10-14)
5. Oxidation of sulfur dioxide: Sulfur dioxide can be oxidized under photochemical conditions but the S-O bond is very strong. So the sulfur dioxide can not undergo photodissociation as in the familiar case of NO2. The oxidation of SO2 involves OH radical:
OH + SO2 ——> HSO3
HSO3 + O2 —–> HSO5 or,
HSO5 ——-> HO2 + SO3 HSO3 + O2 ——> HO2 + SO3
SO3 + H2O —–> H2SO4
There is increasing evidence that the two middle reactions occur as a single reaction.
4. Degradation of larger organic molecules
Larger organic molecules (other than methane and acetaldehyde) are also split up in photochemical smog.
(a) Alkanes: Degradation of large alkane molecules (e.g. butane) starts with attack by OH radical:
OH + CH3CH2CH2CH3 ——-> H2O + CH3CH2CH2CH2
O2 + CH3CH2CH2CH2 ——-> CH3CH2CH2CH2O2
CH3CH2CH2CH2O2 + NO ——> CH3CH2CH2CH2O + NO2
CH3CH2CH2CH2O + O2 ——-> CH3CH2CH2CHO + HO2
CH3CH2CH2CHO + hv ——–> CH3CH2CH2 + HCHO
(b) Alkenes: Large alkane molecules may be degraded by being attacked by ozone, atomic oxygen (O(3P) or OH radical. Attack by OH radical predominates in polluted atmosphere. A typical reaction scheme may be illustrated using butane as example:
OH + CH3CH=CHCH3 ——–> CH3CHOHCHCH3
O2 + CH3CHOHCHCH3 ——-> CH3CHOHCH(O2)CH3
NO + CH3CHOHCH(O2)CH3 ——-> CH3CHOH + CH3CHOH + NO2
The process goes on and on.
The above reaction schemes show degradation of larger organic molecules into smaller ones resulting in greater predominance of low molecular weight compounds in typical urban atmosphere with exhaust fumes of automobile.
5. Heterogeneous reactions in photochemical smog
Gas-phase photochemical reactions may lead to formation of aerosols in polluted urban atmosphere and these give rise to visual obscurity associated with smog condition. High opacity of smog gives an exaggerated impression of the amount of particulate material present yet it is estimated that as little as 5% of pollutants present in photochemical smog could be converted into suspended particulate materials. Various heterogeneous reactions could occur on the surface of these particles or in cloud or rain droplets associated with smog. The material forming condensed phase of smog may consist of both inorganic and organic substances.
(i) Inorganic substances: These include metal oxides and the salts of acids produced within urban air. The acids (particularly sulfuric and nitric acids) are usually present in association with solid particles or more probably as droplets due to their high affinity for water. The latter can react rapidly with atmospheric ammonia. The ammonium sulphate and ammonium nitrate produced are important aerosols that are main causes for the reduction of visibility that accompanies photochemical smog.
(ii) Organic solids: Relatively little is known of the reaction pathways that produce organic particulate materials in the polluted urban air. Nitrogen has been detected in rather unusual reduced oxidation states on particles in photochemical smog. This nitrogen is thought to be present as nitriles, amines or amides bound onto the surface of soot particles. By denoting the soot surface as S, the process may be written as:
S-OH + NH3 —-> S-ONH4
(a phenolic hydroxy ammonium complex)
S-ONH4 —–> S-ONH2 + H2O (at higher temperature)
S-COOH + NH3 —–> S-COONH4
S-COONH4 —–> S-COONH2 + H2O (at higher temperature)
S-COONH2 ——> S-CN + H2O
Most thoroughly studied heterogeneous reaction in the atmosphere Is the oxidation of sulfur dioxide in atmospheric liquid droplets by the ozone, hydrogen peroxide or oxygen in the presence of a transition metal ion catalyst. This oxidation reaction has been discussed earlier and may proceed much faster in polluted urban atmospheres than in unpolluted atmospheres because the concentrations of oxidants (H2O2 or O3) and metal ion catalysts may be much higher. Metal ions may, in particular, be leached from particulates that are added into the air through anthropogenic activities. Leaching of metals from ash may be particularly significant in their surface concentrations being enriched. High amounts of soluble metal ions have been observed in association with fly ashes from the combustion of refuge derived fuels. A further mechanism for increasing the rate of oxidation involves dissolution of materials such as calcium oxide which are present in high concentrations in coal fly ash making the droplet alkaline:
CaO + H2O —–> Ca2+ + 2OH-
This allows dissolution of larger amounts of sulfur dioxide and thus increases the rate of catalytic oxidation. Alternatively, dissolution of ammonia from a polluted atmosphere will also increase the pH and enhance both the dissolution and oxidation of sulfur dioxide.
Oxidation of sulfur dioxide may also occur via absorption of gas onto solid surfaces followed by subsequent oxidation. However, the surface area of particulate material even in polluted atmosphere is quite small and, therefore, such mechanism requires some method of ‘cleaning’ the surface in order to make oxidation process significant. If particulates are wet, this mechanism may be effective since water would ‘clean’ the surface of particulate material.
In the atmosphere, changes in the size and/or composition of particles also occur. These include leaching of particulate material by water, oxidation or reduction of particles. Zinc vapour from copper smelters condenses to form highly angular and crystalline zinc oxide crystals in the atmosphere. These are gradually degraded, then rounded and now acquire a carbonaceous coating. Slowly zinc oxide core decomposes and particle ends up as a carbonaceous pseudomorph with little or no zinc. Possibly, carbonaceous particles are formed by reduction of zinc oxide following deposition of hydrocarbons onto the surface of the particle.