( ( Ammonium ( nitrate particles ) ) ( ( ( form only ) ( when ( ( oxides ( of ( ( ( ( nitrogen ( oxide emissions ) ) ( from ( vehicle exhaust ) ) ) and ) ( ( other ( high-temperature ( combustion sources ) ) ) ( -LRB- ( ( such ( as ( power plants ) ) ) -RRB- ) ) ) ) ) ) ( ( ( convert ( to ( nitric acid ) ) ) and ) ( combine ( with ammonia ) ) ) ) ) ) . ) )
(ROOT (S (NP (NN Ammonium) (NN nitrate) (NNS particles)) (VP (NN form) (ADVP (RB only)) (SBAR (WHADVP (WRB when)) (S (NP (NP (NNS oxides)) (PP (IN of) (NP (NP (NP (NN nitrogen) (NN oxide) (NNS emissions)) (PP (IN from) (NP (NN vehicle) (NNS exhaust)))) (CC and) (NP (NP (JJ other) (JJ high-temperature) (NN combustion) (NNS sources)) (PRN (-LRB- -LRB-) (PP (JJ such) (IN as) (NP (NN power) (NNS plants))) (-RRB- -RRB-)))))) (VP (VP (VB convert) (PP (TO to) (NP (JJ nitric) (NN acid)))) (CC and) (VP (VB combine) (PP (IN with) (NP (NN ammonia)))))))) (. .)))
Ammonium nitrate particles form only when oxides of nitrogen oxide emissions from vehicle exhaust and other high-temperature combustion sources (such as power plants) convert to nitric acid and combine with ammonia.
(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)))))))) (. .)))