( It ( ( is ( formed ( when ( sunlight ( interacts ( with ( ( ( hydrocarbons ( and ( nitrogen oxides ) ) ) , ) ( ( most ( of which ) ) ( are ( produced ( by ( coal-burning ( power ( plants ( and ( automobile emissions ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (VP (VBN formed) (SBAR (WHADVP (WRB when)) (S (NP (NN sunlight)) (VP (VBZ interacts) (PP (IN with) (NP (NP (NNS hydrocarbons) (CC and) (NN nitrogen) (NNS oxides)) (, ,) (SBAR (WHNP (JJS most) (WHPP (IN of) (WHNP (WDT which)))) (S (VP (VBP are) (VP (VBN produced) (PP (IN by) (NP (JJ coal-burning) (NN power) (NNS plants) (CC and) (NN automobile) (NNS emissions)))))))))))))) (. .)))
It is formed when sunlight interacts with hydrocarbons and nitrogen oxides, most of which are produced by coal-burning power plants and automobile emissions.
(ROOT (S (NP (PRP We)) (VP (VBP call) (NP (NP (DT the) (JJ solid) (NN form)) (PP (IN of) (NP (JJ hydrocarbons) (NN coal))))) (. .)))