During hot summer months a temperature inversion layer may form in the atmosphere which traps pollutants near the ground. Of all atmospheric, ozone is the most important with respect to plant injury. Other important pollutants include sulfur dioxide, nitrogen dioxide, fluorides, methane, ammonia, PAN and ethylene. However, plants take up carbon dioxide and give off oxygen which is opposite to ourselves. This production of oxygen is partially responsible for maintaining the oxygen atmosphere on the planet.
Plants take up and release gases through small pores on the leaf surface called stomata.
The uptake of carbon dioxide and conversion to carbohydrates and energy is called photosynthesis. During this process of uptake of carbon dioxide plants can also take up pollutants. Once the pollutants enter the leaf, the pollutants may injure leaf tissue and upset the normal metabolic functioning of the plant. Depending on the pollutant, the upper or lower leaf surface of the plant will exhibit symptoms. For these pollutants, reliable air monitoring is necessary to associate plant injury with a pollutant episode.