“…More recent model calculations by Kohlmann and Poppe [1999] adopted a re-measured value of 60% for the branching ratio [Meunier et al, 1996], which -together with a number of other revisions -gave an estimated N 2 O source of only 0.2 Tg N a À1 . Other studies report even lower branching ratios [Lindholm and Hershberger, 1997;Park and Lin, 1996] [Michalski et al, 2003], the contribution of the NH 2 + NO 2 source is likely to be at the higher end of the estimate here, because D 17 O(NO 2 ) is expected to be between 0.9 and 1.5 times D 17 O(HNO 3 ), depending on the formation pathway (hydrocarbon oxidation by NO 3 , heterogeneous hydrolysis of N 2 O 5 or gas-phase reaction of NO 2 + OH).…”