How are acid rain pollution and ground-level ozone pollution connected?

Acid rain and ground-level ozone pollution are closely connected because they are largely derived from the same human activities, primarily the burning of fossil fuels such as coal, oil, and natural gas.12

The relationship between these two forms of air pollution is defined by the following shared factors:

• Common Precursors: The combustion of fossil fuels by power plants, industrial facilities, and vehicles releases nitrogen oxides (NOx) into the atmosphere.12
• Acid Rain Formation: NOx, along with sulfur dioxide, reacts with water in the atmosphere to form nitric acid and sulfuric acid, which fall to the Earth as acid deposition.12
• Ozone Formation: NOx emissions are also key ingredients in the formation of ground-level ozone, often referred to as photochemical [smog]. When NOx combines with hydrocarbon emissions in the presence of sunlight, it creates ozone, which is a widespread and harmful air pollutant.1

Because these pollutants originate from the same industrial and transportation sources, efforts to regulate emissions—such as the U.S. Clean Air Act—often address both issues simultaneously.3