( ( ( These ( compounds interact ) ) ( with nitrous ) ) ( ( oxides ( to ( form ( ( ozone , ) ( ( a ( primary cause ) ) ( of smog ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT These) (JJ compounds) (NN interact)) (PP (IN with) (NP (NNS nitrous)))) (VP (VBZ oxides) (S (VP (TO to) (VP (VB form) (NP (NP (NN ozone)) (, ,) (NP (NP (DT a) (JJ primary) (NN cause)) (PP (IN of) (NP (NN smog))))))))) (. .)))
These compounds interact with nitrous oxides to form ozone, a primary cause of smog.
(ROOT (S (NP (NN Ozone)) (VP (VBZ is) (NP (NP (DT a) (NN compound)) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (EX there)) (VP (VBZ is) (VP (VBG sparking) (NP (JJ electrical) (NN equipment))))))))) (. .)))