AP Syllabus focus:
‘Smog formation depends on environmental factors such as sunlight intensity, temperature, wind, and atmospheric stability.’
Photochemical smog episodes are not determined by emissions alone. Weather and atmospheric structure control how quickly smog-forming reactions proceed and whether pollutants disperse or accumulate near the ground where people breathe.
Core idea: conditions control chemistry and dilution
Smog severity reflects a balance between:
Production of reactive pollutants (faster under strong sunlight and warmth)
Accumulation (greater when winds are light and vertical mixing is weak)
Removal (dispersion by wind/mixing; scavenging by precipitation)
Sunlight intensity
Sunlight provides the energy that drives many photochemical reactions involved in smog.
High solar intensity (clear skies, midday sun, lower cloud cover) increases reaction rates, allowing pollutants to transform more rapidly into irritating oxidants.
Cloudy conditions reduce the light available for photochemistry, typically suppressing smog intensity.
Short winter days and low sun angle generally limit photochemical activity compared with summer.
Temperature
Warmer air tends to increase smog formation by speeding temperature-dependent reactions and changing pollutant behaviour.
Higher temperatures often correlate with stronger smog episodes because chemical reaction rates increase.
Warm conditions can also increase evaporation of fuel-related vapours, raising the pool of reactive gases available for smog chemistry.
Heat waves commonly coincide with stagnant air, compounding the effect of temperature with poor dispersion.
Wind speed and wind patterns
Wind influences both transport and dilution of pollutants.
Low wind speeds promote smog because pollutants remain concentrated near their sources.
Moderate to strong winds can reduce smog locally by dispersing pollutants and importing cleaner air.
Consistent wind direction can shift smog downwind, affecting suburbs or rural areas beyond the emission source region.
Local wind systems (e.g., valley breezes or sea breezes) can recirculate air, allowing pollutants to “age” and build up over multiple hours.
Atmospheric stability (vertical mixing)
Atmospheric stability describes how readily air parcels rise and mix vertically; stable conditions suppress vertical motion and trap pollutants near the surface.
This infographic explains how a temperature inversion forms and why it traps pollutants near the ground. Warmer air aloft acts as a stable cap that limits vertical mixing, so emissions accumulate in the shallow near-surface layer where people breathe. The figure connects atmospheric stability directly to higher ground-level pollution during stagnant conditions. Source
Atmospheric stability: the atmosphere’s resistance to vertical air movement; stable air limits mixing, while unstable air enhances vertical mixing and dilution.
Key ways stability affects smog:
Stable air creates a shallow near-surface layer where emissions accumulate, raising ground-level pollutant concentrations.
Unstable air (strong surface heating, rising thermals) increases vertical mixing, diluting pollutants and lowering peak concentrations.
Stable nights and mornings can allow pollutants to build up; if stability persists into the day, smog episodes can intensify.
Mixing height (practical implication)
A useful way to think about stability is the mixing height (the depth of air near the ground that pollutants can mix into).
This diagram compares a typical sunny day versus a clear night in the planetary boundary layer. Daytime surface heating drives convective mixing and a deeper boundary layer (higher effective mixing height), while nighttime radiational cooling strengthens a near-surface inversion and produces weak mixing in a shallow layer. The result is much poorer dilution at night, increasing the likelihood of pollutant buildup near the ground. Source
Low mixing height → less air volume for dilution → higher concentrations
High mixing height → more dilution → lower concentrations
How these factors combine in real smog events
High-smog days commonly share a “recipe” of conditions:
Strong sunlight
High temperatures
Light winds (stagnation)
Stable atmosphere (weak vertical mixing)
Conversely, smog is often limited when:
Skies are cloudy (reduced sunlight)
Winds are gusty (enhanced dispersion)
The atmosphere is unstable (good vertical mixing)
Weather systems bring ventilation and pollutant export from the area
FAQ
A daytime sea breeze can push polluted air inland, where it may heat and react further.
At night, land breezes can reverse flow, sometimes recirculating pollutants back over the city the next day.
Surrounding high terrain can limit ventilation and promote stagnant air.
Air can pool in low-lying areas, reducing dilution volume and allowing pollutants to accumulate near the surface.
Yes. Higher humidity can promote formation and growth of secondary particles (haze), affecting visibility and irritation.
Very dry air may reduce some particle growth, even when photochemistry is active.
Cities often remain warmer than surrounding areas, especially at night.
This can alter local breezes and delay morning mixing changes, sometimes extending periods of poor dispersion.
Persistent high-pressure conditions are a common signal.
Clues include clear skies, light winds, limited cloud cover, and little storm activity—conditions that maintain warmth, sunlight, and weak atmospheric mixing.
Practice Questions
State two environmental conditions that increase photochemical smog formation. (2 marks)
Any two from: strong sunlight / high temperature / low wind speed / stable atmosphere (1 mark each).
Explain how wind and atmospheric stability together influence ground-level smog severity in an urban area. (6 marks)
Low wind speeds reduce horizontal dispersion, increasing local pollutant concentrations (1).
Higher wind speeds dilute and transport pollutants away, reducing local peaks (1).
Stable air suppresses vertical mixing so pollutants remain near the surface (1).
Unstable air enhances vertical mixing, increasing dilution through a deeper mixing layer (1).
Combined effect: low wind + stable air produces the highest ground-level concentrations (1).
Combined effect: higher wind and/or unstable conditions lower concentrations by dispersion and dilution (1).
