प्रदूषण युक्त वायुमंडल को प्राकृतिक रूप से निम्न में से किसके द्वारा स्वच्छ किया जाता है?

Which of the following is not traditionally classified as a primary direct air pollutant at normal atmospheric concentrations, despite being a greenhouse gas?

Which of the following gases is primarily responsible for the formation of photochemical smog?