Why sulpher dioxide available on lower atmosphere instead of being heavy than air?

Answer 1

sulpher dioxide IS heavier than air.
Approximate air as the ideal gas, that one mole of molecules occupies the same volume (at same T and P). Hence the gas' molecular weight (or averaged molecular weight) is proportional to gas' density. Now, air is 80% N2 (molecular weight 28) and 19% of O2 (molecular weight 32), and 1% of other gases such as Ar, H2O vapour, CO2, etc. The averaged molecular weight would not exceed 32. The molecular weight of SO2 is 64. It is therefore heavier than air.