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

Lichens act as indicators of air pollution. They are sensitive to sulphur dioxide which occurs in sufficient quantity in the atmosphere of Delhi due to large number of vehicles, homes and factories using fossil fuels., Also Delhi exists in semiarid area where atmospheric moisture is low. In Manali and Darjeeling, the atmosphere is humid and has comparatively low contents of sulphur dioxide.