( ( Primary ( air pollutant ) ) ( ( Substances ( that ( ( ( become pollutants ) immediately ) ( upon ( entering ( the atmosphere ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Primary) (NN air) (NN pollutant)) (VP (VBZ Substances) (S (NP (DT that)) (VP (VB become) (NP (NNS pollutants)) (ADVP (RB immediately)) (PP (IN upon) (S (VP (VBG entering) (NP (DT the) (NN atmosphere)))))))) (. .)))
Primary air pollutant Substances that become pollutants immediately upon entering the atmosphere.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))