Evidence from firn air for recent decreases in non-methane hydrocarbons and a 20th century increase in nitrogen oxides in the northern hemisphere.

Worton, D.R.; Sturges, W.T.; Reeves, C.E.; Newland, M.J.; Penkett, S.A.; Atlas, E.; Stroud, V.; Johnson, K.; Schmidbauer, N.; Solberg, S.; Schwander, J.; Barnola, J.-M.