Photochemical smog is a brownish haze caused by the action of the sun’s visible and ultraviolet radiation on the primary pollutants from the combustion of fossil fuels – unburned hydrocarbons, carbon particulates, carbon monoxide and nitrogen oxides (NOx).
A series of reactions produce secondary pollutants including ground-level ozone, nitric acid, peroxyacetylnitrate and hydrogen peroxide. As you would imagine, it is a serious respiratory health hazard in many cities.
The need for visible and uv radiation (ℎ𝜈) tells us that the first step is essentially an initiation step where bonds are photolysed. Homolytic fission leaves us with exceptionally reactive free radicals in the lower troposphere that catalyse the formation of ozone, peroxyacetylnitrate (PAN) and other secondary pollutants.
Nitrogen(II) oxide (NO•) is produced in when nitrogen from the air burns in the very high temperatures of a car engine. It reacts with oxygen molecules in the air to form nitrogen(IV) oxide (NO2•).
2NO• + O2 ⇾ 2NO2•
NO2• is readily photolysed to form nitrogen(II) oxide and an oxygen atom.
NO2• + ℎ𝜈 ⇾ NO• + O
The oxygen atoms react with oxygen molecules to form ozone.
O + O2 ⇾ O3
Under normal conditions nitrogen(II) oxide, NO•, reacts with the ozone formed to produce oxygen and nitrogen(IV) oxide, NO2•, is regenerated. Ozone levels do not build substantially at ground level.
NO• + O3 ⇾ O2 + NO2•
We can think of NO and NO2 as catalysts, catalysing the breakdown of ozone in a series of steps. Both nitrogen oxides are being created and used up in cycle of reactions.
However, if there are significant concentrations of unburned hydrocarbons present in the atmosphere (from the incomplete combustion of fossil fuels or evaporation from fuel tanks or industrial pollution), then we have a problem. These molecules are collectively called volatile organic compounds (VOC). The most reactive VOC contain a C=C bond which allows them to easily add free radicals (you can check out a mechanism for this here).
Volatile organic compounds react with hydroxide (also known as hydroxyl) radicals present in the atmosphere to create water and a VOC radical (R•).
‘R’ commonly stands for any alkyl group in chemistry shorthand e.g. R-H could be CH3-H, C2H5-H or something considerable more complicated!
R-H + •OH ⇾ R• + H2O
The VOC radical (R•) reacts with oxygen molecules to form a peroxy radical (R-O-O•), which rapidly reacts with NO•.
R• + O2 ⇾ RO2•
RO2• + NO ⇾ RO• + NO2
The result is that there is no longer much NO• present to react with ozone. The catalyst has been removed and ozone levels rise rapidly.
Further reactions between VOC and NO• form peroxyacetylnitrate (PAN) which is a carrier of NO2 and causes eye irritation and damages plants in its own right.
R-H + NO• + O2 ⇾ NO2• + PAN
So what is the solution?
We need to prevent (or at least reduce) the concentrations of all primary pollutants in the troposphere, particularly in towns and cities, especially NOx.
Unburned hydrocarbons, carbon monoxide and nitrogen oxide are all removed by the use of a catalytic converter in petrol engines. CxHy and CO are oxidised to CO2 and H2O, and nitrogen oxides, NOx, are reduced to N2.
2CO(g) + O2(g) ⇾ 2CO2(g)
C7H16(g) + 11O2(g) ⇾ 7CO2(g) + 8H2O(g)
2CO(g) + 2NO(g) ⇾ N2(g) + 2CO2(g)
Diesel engines operate at higher temperatures (more nitrogen in the air is oxidised to NOx) and the air-fuel mix is leaner so there is a greater proportion of air in the mixture compared with the air-fuel mix in a petrol engine.
An oxidation catalyst is able to deal with the unburnt hydrocarbons and carbon monoxide in the same way as detailed above, but reduction catalysts are ineffective because of the greater concentration of oxygen in the exhaust gases.
We need another way to deal with the NOx in diesel exhaust gases.
Modern diesel vehicles have an exhaust-gas recirculation (EGR) system that recycles some of the exhaust gas back into the combustion chamber where they are mixed with fresh air. The result is less oxygen, more water vapour and a lowering of the temperature of the gases all of which result in lower NOx emissions.
However, we can only lower the temperature of the gases so far if we don’t want to reduce the fuel efficiency and increase the amount of carbon particulates in the exhaust fumes.
In addition to ERG systems, diesel engines may also use a catalyst to temporarily ‘trap’ NOx gases. Every so often the amount of fuel in the air-fuel mixture being combusted is briefly increased so that the exhaust gases contain proportionally less oxygen and more unburned hydrocarbons. The trapped NOx in the catalyst reacts with hydrocarbons in the exhaust to produce N2, H2O and CO2.
Alternatively, a process called selective catalytic reduction (SCR) passes NOx over a catalyst and then uses ammonia (stored in the form of urea) as a reducing agent.
4NO(g) + 4NH3(g) + O2(g) ⇾ 4N2(g) + 6H2O(g)
Practice questions
- What is the difference between a primary and a secondary pollutant?
- Using relevant equations, describe how nitrogen(II) oxide, carbon monoxide, unburned hydrocarbons and sulphur dioxide are all present in untreated exhaust gases.
- Explain why the three-way catalytic converter used to reduce nitrogen oxide emissions in a petrol engine will not work for a diesel engine.
- Volatile organic compounds react with hydroxide radicals present in the atmosphere to create water and a VOC radical (R•) according to the following equation:
R-H + •OH ⇾ R• + H2O
(a) Using curly arrows, suggest a mechanism for this reaction.
(b) Is this reaction initiation, propagation or termination? Explain your answer.
- Unburnt hydrocarbons from vehicle emissions contribute to the formation of ozone, O3, at ground level. They interfere with the natural tropospheric reactions in which nitrogen oxides, NO and NO2, react in a series of steps to breakdown ozone.
NO2 +hv ⇾ NO + O equation 1
NO +O3 ⇾ NO2 + O2 equation 2
(a) Combine these two equations to show how ozone is broken down.
(b) The unburnt hydrocarbons react with hydroxyl radicals, OH, to form volatile organic compounds which are alkyl radicals. These alkyl radicals react with O2 and NO in a series of steps to form NO2. Explain how this leads to a build up of ozone with reference to the equations above.
Answers
- Primary pollutants are released directly into the atmosphere from the combustion of fossil fuels. Secondary pollutants are formed from primary pollutants in a series of reactions catalysed by sunlight.
- Nitrogen(II) oxide is formed from the combustion of nitrogen in the air at the very high temperatures of petrol and diesel engines: N2 + O2 ⇾ 2NO
Carbon monoxide is the result of the incomplete combustion of hydrocarbons: C7H16 + 7.5O2 ⇾ 7CO + 8H2O
Unburned hydrocarbons are simply not combusted.
Fossil fuels contain some sulfur (even after desulfurisation) which burns to form sulfur dioxide: S + O2 ⇾ SO2
- The high temperature of a diesel engine means more NOx are produced and the excess oxygen in the exhaust gases means reduction catalysts as found in a three-way catalytic converter are ineffective – NOx cannot be reduced to N2.
4. (a)
(b) this is a propagation reaction as both reactants and products contain a highly reactive radical with an unpaired electron.
5. (a)
(b) NO is key to catalysing the breakdown of ozone as seen in equation 2 so if it is removed by reactions with VOCs / alkyl radicals / unburnt hydrocarbons, this reaction won’t happen and ozone is not removed.