This step of the analysis employs complex computer models that simulate the transport and transformation of emitted pollutants in the atmosphere.
( ( ( This step ) ( of ( the analysis ) ) ) ( ( employs ( ( complex ( computer models ) ) ( that ( simulate ( ( the ( ( transport and ) transformation ) ) ( of ( ( emitted pollutants ) ( in ( the atmosphere ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT This) (NN step)) (PP (IN of) (NP (DT the) (NN analysis)))) (VP (VBZ employs) (NP (NP (JJ complex) (NN computer) (NNS models)) (SBAR (WHNP (WDT that)) (S (VP (VBP simulate) (NP (NP (DT the) (NX (NX (NN transport)) (CC and) (NX (NN transformation)))) (PP (IN of) (NP (NP (JJ emitted) (NNS pollutants)) (PP (IN in) (NP (DT the) (NN atmosphere))))))))))) (. .)))
The analysis can't be done by humans 
( ( The analysis ) ( ( ca n't ) ( be ( done ( by humans ) ) ) ) )
(ROOT (S (NP (DT The) (NN analysis)) (VP (MD ca) (RB n't) (VP (VB be) (VP (VBN done) (PP (IN by) (NP (NNS humans))))))))