( ( Primary ( air pollutants ) ) ( ( are ( those ( which ( are ( ( emitted directly ) ( into ( the ( ambient atmosphere ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Primary) (NN air) (NNS pollutants)) (VP (VBP are) (NP (NP (DT those)) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (VP (VBN emitted) (ADVP (RB directly)) (PP (IN into) (NP (DT the) (JJ ambient) (NN atmosphere))))))))) (. .)))
Primary air pollutants are those which are emitted directly into the ambient atmosphere.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))