38. NCERT Exemplar XX

Pollution is indicated by lichens. They are used as bioindicatorsweather indicatorchemicalindicator of pollution in the air. They are sulphur trioxide sulphur dioxide nitrous oxidecarbon monoxide sensitive and will not grow in areas where the air contains a high amount of $NO_2$$SO_2$$CO_2$.

Due to vehicular emission, Delhi has maximum air pollution and a high amount of $SO_2$$CO_2$$NO_2$ in the air. Manali and Darjeeling, on the other hand, are not polluted and are green cities; therefore, the $SO_2$$NO_2$$CO_2$ concentration is comparatively low in these areas. Due to this, the growth of lichens does not occur in Delhi, which is air polluted and has a high $NO_2$$SO_2$$CO_2$ concentration.