( ( Primary pollutants ) ( ( are ( pollutants ( that ( are ( ( ( put directly ) ( into ( the air ) ) ) ( by ( ( humans or ) ( natural sources ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP are) (NP (NP (NNS pollutants)) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (VP (VBN put) (ADVP (RB directly)) (PP (IN into) (NP (DT the) (NN air))) (PP (IN by) (NP (NP (NNS humans)) (CC or) (NP (JJ natural) (NNS sources)))))))))) (. .)))
Primary pollutants are pollutants that are put directly into the air by humans or natural sources.
(ROOT (S (NP (JJ Primary) (NNS pollutants)) (VP (VBP enter) (NP (DT the) (NN air)) (ADVP (RB directly))) (. .)))