How Do scientist study about smog and air pollution.

Yes, they look in the dirt, if you are talking about scientists back in the stone age. These days, I think they analyze the particles found in the air. Any difference in the natural proportions would be considered pollution. Anyway, I'm sure it's more complicated than that.

Air pollution is a chemical, physical (e.g. particulate matter), or biological agent that modifies the natural characteristics of the atmosphere. The atmosphere is a complex, dynamic natural gaseous system that is essential to support life on planet earth. Stratospheric ozone depletion due to air pollution has long been recognized as a threat to human health as well as to the earth's ecosystems