“…However, this isotope effect does not provide an adequate explanation for the observed seasonal changes in d 15 N of NO 3 À at Summit , Ny Å lesund [Heaton et al, 2004], or Bermuda [Hastings et al, 2003]. In fact, recent calculations using this isotope effect and the assumptions made in the Freyer et al [27] Several studies suggest that the d 15 N of NO 3 À most likely reflects the N isotopic composition of NO x sources [Freyer, 1978[Freyer, , 1991Garten, 1996;Russell et al, 1998;Xiao and Liu, 2002;Hastings et al, 2003;Heaton et al, 2004]. The important pre-industrial sources of NO x include biomass burning, biogenic soil emissions, lightning, and stratospheric injection [e.g., Wolff, 1995;Legrand and Kirchner, 1990].…”