( ( Primary ( Pollutant ( Air pollutants ) ) ) ( ( that ( ( enter ( the atmosphere ) ) directly ) ) . ) )
(ROOT (S (NP (JJ Primary) (NNP Pollutant) (NNP Air) (NNS pollutants)) (VP (ADVP (IN that)) (VBP enter) (NP (DT the) (NN atmosphere)) (ADVP (RB directly))) (. .)))
Primary Pollutant Air  pollutants that enter the atmosphere directly.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))