Primary and Secondary Pollutants, and Reading a Smog Curve

You’ll be able to

Distinguish primary from secondary pollutants and classify examples
Explain photochemical smog formation and the daily concentration curve
Compare control technologies and the pollutants each addresses

Primary versus secondary

A primary pollutant is emitted directly from a source: carbon monoxide, sulfur dioxide, nitrogen oxides, particulate matter, volatile organic compounds, lead. A secondary pollutant forms in the atmosphere from reactions among primary pollutants: tropospheric ozone, sulfuric and nitric acids, and the peroxyacyl nitrates in smog. The distinction has a policy consequence that the exam tests directly — you cannot control a secondary pollutant at a smokestack, because it does not exist yet when the exhaust leaves. Ozone is reduced by cutting the precursors, nitrogen oxides and volatile organic compounds, which is why ozone regulations target vehicles and solvents rather than ozone itself.

Photochemical smog, hour by hour

Photochemical smog needs three ingredients: nitrogen oxides, volatile organic compounds, and sunlight. The daily curve is a favorite stimulus and it has a fixed shape. Morning rush hour puts NO and VOCs into the air, so NO peaks first. Sunlight then drives NO to NO₂, which peaks next and gives the brown haze. NO₂ photolysis releases oxygen atoms that combine with O₂ to make ozone, which peaks in the afternoon — several hours after the emissions that caused it. The lag is the point: the worst ozone occurs when traffic has died down, and it occurs downwind of the city rather than in it, which is why suburbs and rural areas downwind often record higher ozone than the urban core. Smog is worst on hot, sunny, still days, and a thermal inversion traps it by placing warm air above cool air so nothing can rise and disperse.

Controls, matched to pollutants

Each technology addresses specific pollutants, and naming the match is what earns credit. Catalytic converters on vehicles convert CO to CO₂, unburned hydrocarbons to CO₂ and water, and NOₓ to N₂ — they do not address CO₂ itself. Scrubbers on coal plants spray a wet alkaline slurry, usually limestone, to remove sulfur dioxide. Electrostatic precipitators charge particles and collect them on plates, removing particulate matter but not gases. Baghouse filters do the same job by physical filtration. Low-NOₓ burners reduce combustion temperature, since NOₓ forms from atmospheric nitrogen at high temperature. Vapor recovery at fuel pumps captures VOCs. Two exam traps: a scrubber does nothing for particulates and a precipitator does nothing for SO₂; and none of these removes carbon dioxide, which is why they reduce smog and acid rain without touching climate.

Photochemical smog sequence

NO (morning peak) → NO₂ (midday peak) → O₃ (afternoon peak), driven by sunlight acting on NOₓ + VOCs

Ozone lags emissions by hours and peaks downwind, not at the source.

Worked example

A monitoring station downwind of a city records peak ozone at 3 p.m. on a hot, cloudless day, hours after morning traffic ended. Explain the timing and the location.

1.Identify the precursors: morning rush hour emits NO and VOCs, so those peak first.
2.Explain the chemistry: sunlight converts NO to NO₂, whose photolysis releases oxygen atoms that combine with O₂ to form ozone. Each step takes time.
3.Explain the timing: because ozone is a secondary pollutant requiring several sunlight-driven steps, its peak lags the emissions by hours, arriving in mid-afternoon when solar intensity has been high for a while.
4.Explain the location: during those hours the air mass carrying the precursors has moved downwind, so the ozone forms over areas away from the source.

Answer: Ozone is a secondary pollutant: morning NO and VOC emissions require several hours of sunlight-driven reaction to produce it, so its peak lags rush hour and arrives mid-afternoon. Over those hours the air mass has drifted downwind, so the highest concentrations occur away from the city that emitted the precursors — which is why downwind suburbs often exceed urban ozone levels.

On the exam

Never propose controlling ozone at the tailpipe or smokestack. It is a secondary pollutant and does not exist at the source. The only available lever is reducing its precursors — nitrogen oxides and volatile organic compounds — and saying so is the point the question is testing.

Checkpoint

Which is a secondary pollutant?

Checkpoint

A wet scrubber installed on a coal-fired power plant primarily removes

Answer the 2 checkpoints as you read.