( This ( then ( ( reacts ( with ( ( the oxygen ) ( in ( the ( sir ( to ( form ( nitrogen ( oxide pollutants ) ) ) ) ) ) ) ) ) ) . ) ) )
(ROOT (S (NP (DT This)) (ADVP (RB then)) (VP (VBZ reacts) (PP (IN with) (NP (NP (DT the) (NN oxygen)) (PP (IN in) (NP (DT the) (NN sir) (S (VP (TO to) (VP (VB form) (NP (NN nitrogen) (NN oxide) (NNS pollutants)))))))))) (. .)))
This then reacts with the oxygen in the sir to form nitrogen oxide pollutants.
(ROOT (S (NP (NNP Nitrogen)) (VP (VBZ oxide) (NP (NP (NN form)) (PRN (-LRB- -LRB-) (PRP s) (-RRB- -RRB-)) (SBAR (WHADVP (WRB when)) (S (NP (NN nitrogen) (CC and) (NN oxygen)) (VP (VBP combine) (PP (IN at) (NP (JJ high) (NNS temperatures)))))))) (. .)))