(S (S (BASENP (DT The) (NN situation)) (VP (VBD was) (VP (VP (VBN complicated) (PP (IN by) (BASENP (JJ dense) (NN smoke)))) (COMMA ,) (S (VP (VBG necessitating) (NP (BASENP (DT the) (NN use)) (PP (IN of) (BASENP (NN gas) (NNS masks))))))))) (PERIOD .))