( ( ( ( ( ( Other problems ) ( caused ( by ( air pollution ) ) ) ) , ) ( such ( as ( ( ( acid rain ) and ) ( ( the formation ) ( of ( ground-level ozone ) ) ) ) ) ) ) , ) ( ( ( are mainly ) ( caused ( by ( pollution ( emitted ( in ( ( other parts ) ( of Europe ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (NP (JJ Other) (NNS problems)) (VP (VBN caused) (PP (IN by) (NP (NN air) (NN pollution))))) (, ,) (PP (JJ such) (IN as) (NP (NP (JJ acid) (NN rain)) (CC and) (NP (NP (DT the) (NN formation)) (PP (IN of) (NP (JJ ground-level) (NN ozone)))))) (, ,)) (VP (VBP are) (ADVP (RB mainly)) (VP (VBN caused) (PP (IN by) (NP (NP (NN pollution)) (VP (VBN emitted) (PP (IN in) (NP (NP (JJ other) (NNS parts)) (PP (IN of) (NP (NNP Europe)))))))))) (. .)))
Other problems caused by air pollution, such as acid rain and the formation of ground-level ozone, are mainly caused by pollution emitted in other parts of Europe.
(ROOT (S (NP (JJ Acid) (NN rain)) (VP (MD may) (VP (VB be) (VP (VBN caused) (PP (IN by) (NP (NN air) (NN pollution)))))) (. .)))