Answer: leaves
Explanation:
Joey should carefully observe the leaves of the plants during his experiment. Photosynthesis primarily occurs in the leaves, where chlorophyll, the pigment responsible for capturing sunlight, is located. The process of photosynthesis converts sunlight, carbon dioxide, and water into glucose and oxygen. Polluted air can negatively impact photosynthesis by reducing the availability of carbon dioxide or by damaging the chlorophyll and other components involved in the process.