Primary and Secondary Pollutants & Smog Guide

Air pollutants come in two kinds. Primary pollutants are harmful chemicals released directly from a source — carbon monoxide from a car exhaust, or sulphur dioxide from a factory chimney. Secondary pollutants are never released directly; they form in the air itself when primary pollutants react, often under sunlight.

This split matters because you cannot trap a secondary pollutant at a chimney — it does not exist until the gases mix in the sky. So to clean the air, you must cut the primary pollutants that create them. This is the key to understanding smog, the dirty haze over cities, which comes in two classic types: London smog and Los Angeles smog.

Key Concepts

• Primary pollutants: emitted directly. The main ones are carbon monoxide (CO), sulphur dioxide (SO2), nitrogen oxides (NOx, mainly NO and NO2), particulate matter (soot, dust, smoke) and unburnt hydrocarbons.
• Secondary pollutants: formed by reactions in the atmosphere. Examples: ground-level ozone (O3), peroxyacetyl nitrate (PAN), nitric acid (HNO3) and sulphuric acid droplets (H2SO4).
• London (classical) smog: reducing, sooty smog from coal burning — smoke + fog + SO2, worst in cold, damp weather.
• Los Angeles (photochemical) smog: oxidising smog formed under strong sunlight from NOx and hydrocarbons, giving ozone, NO2 and PAN. It needs no fog at all.

Formulas

• Ozone formation: NO2 + hv → NO + O, then O + O2 → O3. Sunlight splits NO2; the free oxygen atom joins O2 to make ozone. Needs strong sunlight; no sun means no ozone build-up.
• Acid formation: SO2 + 1/2 O2 + H2O → H2SO4 (London smog) and NO2 + OH → HNO3 (photochemical smog). Moist air lets these acids form and fall as acid rain.
• Ozone destruction: NO + O3 → NO2 + O2. Nitric oxide eats ozone back, so the two gases must be tracked together.
• Units: pollutant levels are measured in ppm (parts per million) or μg/m3 (micrograms per cubic metre).

Worked Example

A monitor reports NO2 at 0.20 ppm on a sunny afternoon. One NO2 gives one O atom, which gives one O3 (1:1 ratio). If every NO2 molecule reacted, how much ozone forms?

• Step 1: Starting NO2 = 0.20 ppm.
• Step 2: Ratio NO2 : O3 is 1:1, so maximum O3 = 0.20 ppm.
• Step 3: Real ozone is lower, because NO + O3 → NO2 + O2 destroys it again.

More NO2 in sunlight means more ozone, which is why cities track both gases together on hot afternoons.

Common Misconceptions

• "Smog and fog are the same." Wrong. Fog is pure water droplets. Smog always carries pollutants — London smog added soot and SO2, while Los Angeles smog is a dry mix of ozone, NO2 and PAN.
• "Ozone is always good." Wrong. Stratospheric ozone blocks UV rays, but ground-level ozone is a secondary pollutant that stings eyes and harms lungs.
• "Filters can remove smog." Wrong for photochemical smog. Ozone and PAN form across the whole city in sunlight, so they cannot be caught at one chimney — you must cut the NOx and hydrocarbons that feed them.

Real-World Applications

• Catalytic converters convert CO, NOx and unburnt hydrocarbons in car exhaust into less harmful gases before they can seed photochemical smog.
• Power-plant scrubbers remove SO2, the primary pollutant behind London smog and acid rain.
• Cities use low-emission zones and vehicle rules to cut primary NOx and hydrocarbons, directly lowering secondary ozone.
• Air-quality indices report ground-level ozone daily, warning asthmatics when secondary pollutants peak.

Primary pollutants like CO and SO2 are released directly; secondary ones like ozone and PAN form from reactions in the air. London smog was sooty and reducing, Los Angeles smog is driven by sunlight making ozone from NO2. Cut the primary pollutants, and the secondary ones fall too.