The correct answer is option 2: volcanic eruptions. Volcanic eruptions release large amounts of gases, ash, and particulate matter into the air, which can contribute to air pollution. The gases released during volcanic eruptions include sulfur dioxide, carbon dioxide, and hydrogen sulfide, which can react with other compounds in the atmosphere and form pollutants such as sulfuric acid, acid rain, and smog. Volcanic ash and particulate matter can also affect air quality by causing respiratory problems and reducing visibility. It is important to note that while forest fires, dust storms, and smoke from burning dry leaves can also contribute to air pollution, they are not classified as natural sources of air pollution in this question.