Which one among the following are the primary reactants of photochemical smog ?
- (a)Sulphur Hexafluoride and Soot
- (b)Carbon Monoxide and Ozone
- (c)Nitric Oxide and Unburnt Hydrocarbons
- (d)Peroxy Radicals and Sub-oxides
Answer
Why
Correct — C, (c) Nitric Oxide and Unburnt Hydrocarbons. These are the two primary pollutants that sunlight acts on; everything else people associate with photochemical smog is produced later, in the air itself.
Where the two come from. Inside an engine cylinder the temperature is high enough for nitrogen and oxygen from the air to combine, giving NITRIC OXIDE, NO. At the same time no engine burns its fuel completely, so a fraction of the petrol or diesel leaves the exhaust as UNBURNT HYDROCARBONS, along with vapour from fuel handling and from solvents. Both are emitted directly, which is what makes them PRIMARY pollutants.
What sunlight then does. Nitric oxide oxidises to nitrogen dioxide, and ultraviolet light splits nitrogen dioxide into nitric oxide and a free oxygen atom. That atom joins molecular oxygen to make OZONE. Meanwhile the hydrocarbons are attacked by radicals and yield aldehydes, peroxy radicals and, in combination with nitrogen dioxide, peroxyacetyl nitrate. Ozone, PAN, aldehydes and the rest are SECONDARY pollutants: nobody emits them, and they are the substances that make photochemical smog sting the eyes, crack rubber and damage crops.
So the question turns on one distinction — primary against secondary. Ask of each substance in the options whether it comes out of a tailpipe or is manufactured in the atmosphere afterwards, and only option (c) contains two substances that are emitted.
One contrast worth carrying alongside. Photochemical smog is an OXIDISING smog and needs warm, dry, sunny, still conditions with heavy traffic — it is often called Los Angeles smog. Classical or London smog is the opposite: a REDUCING mixture of sulphur dioxide, smoke and fog produced by burning coal in cold, damp weather.
Why the others are wrong
- (a)Sulphur Hexafluoride and Soot — Neither substance belongs to this process. Sulphur hexafluoride is an inert gas used as an electrical insulator in switchgear and is notable as an extremely potent greenhouse gas, but it is not photochemically reactive in this way. Soot is particulate carbon from incomplete combustion and is associated with the smoke-and-fog smog of coal-burning cities, which is the other kind of smog entirely.
- (b)Carbon Monoxide and Ozone — The most attractive wrong answer, because ozone is the substance photochemical smog is measured by. That is precisely why it cannot be a reactant here: ozone is the best-known SECONDARY pollutant of the process, made in the air by the action of sunlight on nitrogen dioxide, not emitted by any vehicle. Carbon monoxide is genuinely a primary pollutant from incomplete combustion and does take part in the radical chemistry, but it is not one of the two precursors on which the smog depends.
- (d)Peroxy Radicals and Sub-oxides — Peroxy radicals are real and central to the chemistry, but they are INTERMEDIATES formed during the reaction sequence from hydrocarbons — they are made in the smog, not emitted into it, so they cannot be its reactants. 'Sub-oxides' is not a class of pollutant that figures in this process at all. An option pairing a genuine intermediate with a vague term is designed for a candidate who recognises the first name and stops.
Concept
Photochemical smog is a chain of reactions driven by sunlight in polluted urban air. The precursors are oxides of nitrogen and volatile organic compounds, chiefly unburnt hydrocarbons, both dominated by road traffic. Ultraviolet light photolyses nitrogen dioxide to nitric oxide and an oxygen atom; the oxygen atom makes ozone; hydrocarbons are oxidised through radical chains to aldehydes such as formaldehyde and acrolein and to peroxyacetyl nitrate. The result is a brown haze with high ozone levels, worst on warm sunny afternoons and worst where a temperature inversion traps the air over a city. The health effects are respiratory irritation and eye irritation; the crop and material damage is largely ozone's. Control works on the precursors — catalytic converters on vehicles, which reduce nitrogen oxides and oxidise carbon monoxide and hydrocarbons, cleaner fuels, and control of solvent evaporation.
Environment questions in these papers reliably test the primary-against-secondary distinction, because it is the point at which a candidate either understands a process or has memorised a list of pollutants. The same distinction underlies questions on acid rain and on ground-level ozone. Keeping the two smogs apart — oxidising and sunny against reducing and cold — answers most of what is asked in this area.
Environment questions on this paper reward a causal chain rather than a list. Photochemical smog has a three-link chain — what is emitted, what sunlight does to it, what results — and the examiner can ask about any link while offering substances from the others as distractors. That is exactly what happens here: ozone belongs to the third link and is offered as though it belonged to the first. Before answering any pollution item, place each substance in the option set at its point in the chain; the question then answers itself, and the same discipline handles acid rain, ozone depletion and eutrophication.
Key facts
- The primary precursors of photochemical smog are oxides of nitrogen, chiefly nitric oxide, and unburnt hydrocarbons or volatile organic compounds.
- Both come mainly from vehicle exhaust; nitric oxide forms from atmospheric nitrogen and oxygen at high combustion temperatures.
- Sunlight photolyses nitrogen dioxide into nitric oxide and an oxygen atom, which combines with molecular oxygen to form ozone.
- Ozone, peroxyacetyl nitrate and aldehydes are secondary pollutants, formed in the atmosphere rather than emitted.
- Photochemical smog is oxidising and needs warm, dry, sunny and still conditions — the Los Angeles type.
- Classical or London smog is reducing, and consists of sulphur dioxide, smoke and fog from coal burning in cold, damp weather.
- A temperature inversion traps polluted air near the ground and makes either type of smog worse.
- Catalytic converters and cleaner fuels control the precursors rather than the smog itself.
Study next
Common traps
- Naming ozone as a cause of photochemical smog when it is the best-known product.
- Mixing up the two smogs; sulphur dioxide and smoke belong to the coal-burning type.
- Forgetting that nitric oxide is made in the engine from ordinary air, not from a component of the fuel.
- Choosing an option because it contains one familiar substance without testing the second.
Environmental chemistry items in EPFO papers cluster around smog, acid rain, ozone depletion, the greenhouse effect and water pollution. Each is best prepared as a short causal chain — what is emitted, what happens to it, what the effect is — because the examiner asks for one link in that chain at a time.
Related PYQs
EPFO_APFC_2016_Q63Acid rain is due to
- (a) Sulphur dioxide pollution
- (b) Carbon monoxide pollution
- (c) Pesticide pollution
- (d) Dust particles in the atmosphere
Answer(a) Sulphur dioxide pollution
The companion atmospheric-chemistry item — what causes acid rain, which is the same primary-to-secondary chain worked through sulphur and nitrogen oxides instead.
Practice
- practice — not a real PYQ
Which one among the following is a secondary pollutant in photochemical smog ?
- (a)Nitric oxide
- (b)Unburnt hydrocarbons
- (c)Peroxyacetyl nitrate
- (d)Carbon monoxide
Answer(c) Peroxyacetyl nitrate
- practice — not a real PYQ
Classical or London smog differs from photochemical smog in that it is which one among the following ?
- (a)Oxidising in character and forms in hot sunny weather
- (b)Reducing in character and forms in cold damp weather
- (c)Caused mainly by vehicular nitrogen oxides
- (d)Composed chiefly of ground-level ozone
Answer(b) Reducing in character and forms in cold damp weather