What causes acid rain to form in the atmosphere?

Acid rain forms primarily due to the presence of sulfur dioxide (SO2) and nitrogen oxides (NOx) in the atmosphere. These compounds are typically produced from industrial processes, vehicle emissions, and the burning of fossil fuels. When they are released into the air, these pollutants can react with water vapor, oxygen, and other chemicals to form sulfuric and nitric acids. When these acids mix with rainwater, they lower the pH of the rain, resulting in acid rain.

This phenomenon is closely linked to the presence of ash and smoke in the air, as they often contain the very pollutants (sulfur dioxide and nitrogen oxides) that contribute to the formation of acid rain. Hence, the correct answer highlights the importance of these pollutants and their contribution to this environmental issue. Other choices, such as carbon dioxide, sunlight exposure, and high humidity, do not play a direct role in the specific chemical reactions that lead to acid rain, making them less relevant in understanding its formation.