The study area and trees were identified. The circumference of each tree selected was measured and recorded. If
the circumference is less than 80 cm, the direction of study is reduced from 4 to 2, either north and south or east and
west. For each tree, a quadrant of 20x50 cm2 is placed vertically on the bark at a height of 1 m from the ground and
facing the north direction. The lichens are then identified according to the three groups, i.e. tolerant, moderate and
sensitive lichen groups. Magnifying len was used to observe the morphology of the lichens and the lichens were
identified with the help of a lichen expert. The number of the lichens were counted and recorded to determine the
frequency of the lichens. The frequency score was calculated and recorded. The previous steps were repeated for
the same tree facing south, east and west direction. The same method was applied for each palm tree in the study
area. The air quality score for each study area was calculated according to equation 1 above. For each quadrant,
about 4 g of the bark of the palm tree was scraped and collected.