Why do not lichens occur in Delhi whereas they commonly grow in Manali or Darjeeling?
Lichens are bio-indicators of air pollution. They are sensitive to Sulphur dioxide (SO2) which occurs in sufficient quantity in the atmosphere of Delhi due to the large number of vehicles, factories using fossil fuels etc. Moreover, Delhi occurs in semi-arid areas where atmospheric moisture is low. In Manali and Darjeeling, the atmosphere is humid and Sulphur dioxide pollution is comparatively low.