( It ( ( is ( ( a ( primary source ) ) ( of ( air pollution ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (NP (NP (DT a) (JJ primary) (NN source)) (PP (IN of) (NP (NN air) (NN pollution))))) (. .)))
It is a primary source of air pollution.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))