Practice a College Board-style free response question on Atmospheric Pollution. Write your response, then reveal the model answer to see exactly what earns each point.
A valley city surrounded by mountains experiences frequent air quality alerts in the summer. Researchers measured ground-level ozone concentration and recorded daily weather conditions over a one-week period.
| Day | Weather conditions | Ground-level ozone (ppb) |
|---|---|---|
| Mon | Overcast, windy | 22 |
| Tue | Sunny, calm, thermal inversion present | 118 |
| Wed | Sunny, calm, thermal inversion present | 125 |
| Thu | Sunny, windy, no inversion | 64 |
| Fri | Rainy, windy | 15 |
The highest ozone concentrations (118–125 ppb) occurred on sunny, calm days with a thermal inversion present (Tuesday and Wednesday). Ground-level ozone is a secondary pollutant formed when sunlight drives a chemical reaction between nitrogen oxides (NOx) and volatile organic compounds (VOCs) — both largely from vehicle emissions — producing ozone (O3) as a byproduct.
A thermal inversion occurs when a layer of warm air sits above a layer of cooler air near the surface, which prevents the normal upward mixing and dispersal of pollutants — trapping the ozone and its precursors close to the ground where concentrations build up. The surrounding mountains physically block horizontal wind flow that would otherwise help disperse the trapped air, making the valley more prone to inversions and pollutant accumulation than open, flat terrain where wind can clear pollutants more easily.
The city could require mandatory catalytic converters and stricter vehicle emission standards, or implement temporary driving restrictions during high-risk weather conditions (sunny, calm days with inversion forecasts). This directly addresses the root cause identified in Part A by reducing the NOx and VOC emissions from vehicles that react with sunlight to form ozone — fewer precursor emissions mean less ozone can form, even under unfavorable weather conditions.