8.3 Urban air pollution

Urban air pollution includes NOx, SO₂, CO and particles; PM2.5 and PM10 are size fractions, so a monitor label tells you which particles it counts.

PM2.5 is no larger than 2.5 µm and PM10 no larger than 10 µm. The finer fraction can penetrate more deeply, but the syllabus labels describe aerodynamic size, not a complete toxicity ranking.

If PM10 is 40 µg/m³ and PM2.5 is 18 µg/m³, the finer fraction is nested inside the PM10 reading; the remainder is larger than 2.5 µm but no larger than 10 µm.

No. PM2.5 is nested within PM10, but each fraction can change by a different amount.

PM10 does not mean ‘ten times worse’; it is a particle-size threshold, not a gas or toxicity score.

A primary pollutant is directly active at emission; its source may be natural or anthropogenic.

Dust lifted by a storm is natural; dust from road building is anthropogenic. Both can be primary PM because they enter the atmosphere directly.

Natural does not mean harmless, and anthropogenic does not mean every later pollutant is emitted directly.

Combustion can emit PM2.5, PM10, CO and SO₂ directly, while tropospheric ozone is secondary and forms in the atmosphere from precursors.

Label a pollutant by where it appears in the pathway. NOx from a tailpipe can react in sunlight later; ozone is therefore not simply an exhaust gas even when traffic supplies its precursor.

A car emits NOx directly at 8 a.m.; sunlight can help form ozone downwind later in the morning.

No. It may show secondary ozone formed from traffic precursors and sunlight.

‘From fossil fuels’ describes the source pathway; it does not make every pollutant primary.

Air-pollution management can reduce the activity, control emissions at release or reduce exposure; choose the level that interrupts the causal pathway.

A bus and cycle network can reduce vehicle kilometres while converters reduce emissions from vehicles that remain; a tree screen alone does not remove the source.

Set a pollutant target, identify the causal level, combine complementary measures and monitor emissions and exposure.

A visible green feature is not automatically source control or sufficient management.

NOx and SO₂ react with oxygen and water in the atmosphere to form nitric and sulfuric acids that return by wet or dry deposition.

Use the chain precursor → atmospheric reaction → acid → deposition. Natural rain is already slightly acidic, so acid rain means additional acidification rather than ‘dirty water’ alone.

SO₂ from fuel burning dissolves in cloud water and is oxidized, adding sulfuric acid to precipitation.

Water and oxygen; naming only the emission source skips the formation mechanism.

Acid rain is not simply rain with visible dirt; explain the gas reactions that create acids.

Acid deposition harms different receptors through different mechanisms: nutrient leaching and aluminium toxicity in ecosystems, corrosion in materials, and particle-related lung inflammation.

Choose receptor → mechanism → effect. Acidified soil can lose calcium; mobilized aluminium can damage fish gills; acids corrode carbonate stone; associated particles can enter lungs.

Acidified soil releases aluminium into a stream, where fish gills are damaged and survival falls.

Nutrient leaching and root/foliage damage reduce uptake; name that pathway instead of saying ‘acid kills trees’.

Acid rain does not usually burn skin directly; respiratory harm follows particle exposure pathways.

Acid deposition can be managed by changing the activity, controlling release, or restoring damage; those levels prevent different parts of the causal chain.

Reduce fossil-fuel use at source, use scrubbers or converters at release, and use healthcare or lake liming for existing damage. Prevention limits new loading; restoration cannot remove upstream emissions.

A scrubber cuts sulfur emissions from a plant, while liming an acidified lake neutralizes stored acidity after deposition.

Restoration such as liming; pair it with source reduction to prevent recurrence.

Restoration is not source control.

Photochemical smog forms when sunlight drives reactions between primary NOx and VOCs, producing secondary PANs and tropospheric ozone.

Name both the emitted precursors and the atmospheric condition. Smog is a chemical transformation, not simply a visible mixture of whatever came from a tailpipe.

Vehicle NOx plus solvent VOCs on a sunny afternoon can produce ozone and PANs downwind of the source.

Primary NOx and VOCs; PANs and ozone belong on the secondary-product side.

Ozone in photochemical smog is secondary, not emitted in the same way as NOx or VOCs.

Sunlight speeds smog chemistry, while weak wind, temperature inversion, mountains or high buildings keep precursors and products concentrated near the ground.

Separate creation from accumulation: weather and topography do not create emissions, but they change reaction time and dispersion. A warm layer above cool polluted air is an inversion.

In a basin with weak wind, NOx and VOCs remain near the surface long enough for sunlight to form more ozone.

No. Check precursor emissions and mixing/dispersion as well as sunlight.

A trapping condition intensifies smog; it does not replace the need for precursor gases.

Direct tropospheric-ozone effects include biological damage to plants and airways and physical deterioration of fabrics and rubber exposed to the gas.

Classify the exposed receptor first. Ozone can damage plant membranes, irritate eyes and inflame airways; it can also oxidize outdoor fibres and rubber. These direct effects are distinct from later healthcare or productivity costs.

A high-ozone episode can reduce plant photosynthesis and make an outdoor rubber seal crack faster.

Indirect: the direct airway injury leads to healthcare costs; classify the effect in the chain.

‘Ozone’ is not automatically good or bad; stratospheric protection and tropospheric exposure are different contexts.

Indirect ozone impacts appear after direct harm: illness raises healthcare demand and missed work, while crop or material damage creates economic costs that may fall unevenly across communities.

Trace direct effect → household, workplace or public cost, then check exposure and capacity to respond. A city average can hide higher burdens near roads or for outdoor workers with limited healthcare access.

If outdoor workers live near a busy road, an ozone episode can combine lost work hours with higher treatment costs.

Housing, occupation, healthcare access and income change both exposure and ability to recover.

Do not treat an average concentration as an equal burden for every group.