Final answer:
Industrial emissions, particularly the combustion of fossil fuels, are the single major source of photochemical reactants in the United States, contributing to both air pollution and acid rain. The significant increase in greenhouse gases from human activities is the main cause of recent global climate change.
Step-by-step explanation:
The single major source of photochemical reactants in the United States is industrial emissions. These emissions include a variety of pollutants, but in the context of photochemical reactions and air quality, the most notable are sulfur dioxide (SO2) and nitrogen oxides (NOx). These compounds are primarily released during the combustion of fossil fuels in power plants, factories, and motor vehicles. These activities lead to the formation of photochemical smog, which is a complex mixture of pollutants including ozone (O3), particulate matter, and other chemical compounds.
Concerning acid rain, the anthropogenic causes are notably due to the emissions of sulfur dioxide and nitrogen oxides from the combustion of fossil fuels such as coal and oil. The scientific consensus on global climate change strongly indicates that the recent changes are primarily caused by human activities, particularly the increase in greenhouse gases such as carbon dioxide (CO2) from burning fossil fuels, which trap heat in the atmosphere.