( However ( , ( ( ( elevated levels ) ( of acidity ) ) ( ( result ( when ( ( ( ( ( the ( primary ( air pollutants ) ) ) , ) ( sulphur ( dioxide ( and ( nitrous oxide ) ) ) ) ) , ) ( ( ( ( ( enter ( the atmosphere ) ) ( at ( ( ( disproportionately high ) rates ) ( -LRB- ( ( due ( primarily ( to ( ( the burning ) ( of ( fossil fuels ) ) ) ) ) ) -RRB- ) ) ) ) ) , ) and ) ( react ( to ( form ( secondary ( air pollutants ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (ADVP (RB However)) (, ,) (NP (NP (JJ elevated) (NNS levels)) (PP (IN of) (NP (NN acidity)))) (VP (VBP result) (SBAR (WHADVP (WRB when)) (S (NP (NP (DT the) (JJ primary) (NN air) (NNS pollutants)) (, ,) (NP (NN sulphur) (NN dioxide) (CC and) (NNS nitrous) (NN oxide)) (, ,)) (VP (VP (VBP enter) (NP (DT the) (NN atmosphere)) (PP (IN at) (NP (NP (ADJP (ADVP (RB disproportionately)) (JJ high)) (NNS rates)) (PRN (-LRB- -LRB-) (ADJP (JJ due) (PP (RB primarily) (TO to) (NP (NP (DT the) (NN burning)) (PP (IN of) (NP (JJ fossil) (NNS fuels)))))) (-RRB- -RRB-))))) (, ,) (CC and) (VP (VBP react) (S (VP (TO to) (VP (VB form) (NP (JJ secondary) (NN air) (NNS pollutants)))))))))) (. .)))
However, elevated levels of acidity result when the primary air pollutants, sulphur dioxide and nitrous oxide, enter the atmosphere at disproportionately high rates (due primarily to the burning of fossil fuels), and react to form secondary air pollutants.
(ROOT (S (NP (NP (NNP Ash)) (SBAR (WHNP (WDT that)) (S (VP (VBZ enters) (NP (DT the) (NN air)) (PP (ADVP (RB naturally)) (IN as) (NP (NP (DT a) (NN result)) (PP (IN of) (NP (DT a) (NN volcano) (NN eruption))))))))) (VP (VBZ is) (VP (VBN classified) (PP (IN as) (NP (DT a) (JJ primary) (NN pollutant))))) (. .)))