Why lichens do not occur in Delhi whereas they commonly grow in Manali or Darjeeling?

Lichens are the bio indicators of pollution, found in non polluted areas and are highly susceptible towards sulphur dioxide. They react with sulphur dioxide to form a poisonous chemical that kills the plant itself. Lichens grow best in Manali and Darjeeling where amount of sulphur dioxide is very less. Delhi is highly populated hence lichens cannot grow and survive there.