Photochemical Smog

Photochemical smog is a mixture of air pollutants and particulates, including ground-level ozone, that is formed when oxides of nitrogen and volatile organic compounds (VOCs) react in the presence of sunlight.

Any two of:

• Eye and respiratory irritation in people (e.g. stinging eyes, breathing difficulty).
• Decreased crop yields.
• Deterioration of plastics and rubber.

Oxides of nitrogen (NO_x) are produced in the high temperatures of vehicle engines and released in vehicle exhausts. Volatile organic compounds (VOCs) evaporate into the air from fuels, solvents and unburned petrol. In the presence of sunlight, which supplies the energy, these oxides of nitrogen and VOCs react together to form ground-level ozone and other pollutants. This mixture of pollutants and particulates is photochemical smog.

People: photochemical smog causes eye irritation (stinging, watering eyes) and respiratory irritation, making breathing harder and worsening conditions such as asthma, because the smog and ground-level ozone irritate the airways and lungs.

Crops: it causes decreased crop yields — ground-level ozone damages plant tissues and reduces photosynthesis, so plants grow less and harvests are smaller, which can threaten food supply.

Materials: it causes deterioration of plastics and rubber — ozone reacts with and breaks down these materials, so plastics crack and rubber (such as tyres and seals) perishes and weakens.

Formation. Oxides of nitrogen (NO_x) are produced in the high temperatures of vehicle engines and released in vehicle exhausts, while volatile organic compounds (VOCs) evaporate from fuels, solvents and unburned petrol. In the presence of sunlight — which supplies the energy — these react to form ground-level ozone and other pollutants. Together with particulates, this mixture is photochemical smog.

Why some cities are worst. The smog is worst in sunny cities with heavy traffic: lots of vehicles supply plenty of NO_x and VOCs, and strong sunlight drives more reaction. On hot, still days there is no wind to disperse the pollutants, and a temperature inversion (warm air above cooler air) can act like a lid, trapping them near the ground so concentrations build up.

Impacts.

• People: eye and respiratory irritation, worsening conditions such as asthma.
• Crops: decreased crop yields, as ground-level ozone damages plant tissue and reduces photosynthesis.
• Materials: deterioration of plastics and rubber, which crack and perish as ozone breaks them down.

Introduction. Photochemical smog is a mixture of air pollutants and particulates, including ground-level ozone, formed when oxides of nitrogen (NO_x) and volatile organic compounds (VOCs) react in the presence of sunlight. Because much of the NO_x comes from vehicle exhausts, reducing vehicle use is an obvious strategy. This essay weighs reducing vehicle use against other methods, judging by which most effectively cuts the reactant emissions that drive the smog.

The case that reducing vehicle use is effective.

• Vehicles are the main source of NO_x in cities and also release VOCs through fuel evaporation, so fewer vehicles means fewer reactants and less ground-level ozone.
• Measures such as public transport, cycling, congestion charges and car-free zones cut traffic directly, especially in the dense city centres where the smog concentrates.
• Reducing vehicle use tackles the problem at source, before the pollutants are released, rather than treating its effects.

The case that it is not the only effective way.

• Catalytic converters fitted to vehicles cut the NO_x (and other pollutants) leaving the exhaust without anyone reducing their journeys, so emissions fall even if vehicle use stays the same.
• Controlling VOCs — tighter rules on fuels, solvents and paints, and reducing fuel evaporation — attacks the other reactant, which reducing vehicle use alone does not fully address.
• Switching to electric vehicles and cleaner energy reduces tailpipe NO_x at source, and these measures (covered in 7.3) can be introduced alongside, or instead of, cutting journeys.
• Reducing vehicle use is often resisted by the public and can be hard to enforce, so relying on it alone may be impractical.

Judgement. The decisive factor is which approach most reduces the NO_x and VOC emissions that, with sunlight, create the smog. Reducing vehicle use is clearly very effective because vehicles are the main NO_x source, but it is not the only effective method: catalytic converters, VOC controls and cleaner vehicles also cut emissions, and the sunlight that drives the reaction cannot be controlled at all. The statement is therefore an overstatement — the most successful approach combines reduced vehicle use with technological controls (catalytic converters, cleaner fuels and vehicles) and VOC regulation, so that all the reactant sources are reduced together.