( 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 (NP (NNP Ash)) (SBAR (WHNP (WDT that)) (S (VP (VBZ enters) (NP (DT the) (NN air)) (PP (ADVP (RB naturally)) (IN as) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (DT a) (NN volcano) (NN eruption))))))))) (VP (VBZ is) (VP (VBN classified) (PP (IN as) (NP (DT a) (JJ primary) (NN pollutant))))) (. .)))