“…This contrasts with most other atmospheric O-bearing molecules including molecular oxygen (O 2 ), water (H 2 O), and peroxy radicals (RO 2 or HO 2 ) that have Δ 17 O values near 0‰ (Barkan & Luz, 2005). These differences provide quantitative measures to evaluate NO x oxidation chemistry involving various contributions from O 3 , HO x , RO x , and XO (where X = bromine [Br] or chlorine [Cl]) oxidation pathways (Michalski et al, 2003) and to compare modeled formation pathways (Alexander et al, 2009) with direct observations (Morin et al, 2008;Savarino et al, 2013 (Morin et al, 2011), representing a NO + O 3 branching ratio of 0.72 to 1.0. Post-NO 2 reaction pathways have been assumed to reflect O isotopic mass balance with the associated oxidants, which has been derived in previous works (Alexander et al, 2009;Ishino et al, 2017;Michalski et al, 2003;Morin et al, 2009) Patris et al, 2002;Sanusi et al, 2006), 21±0.1‰ for sea salt SO 4 2− (Rees et al, 1978), 3±3‰ for continental SO 4 2− (Jenkins & Bao, 2006;Li & Barrie, 1993;Norman et al, 1999), and 2.6±0.3‰ for stratospheric SO 4 2− (Castleman et al, 1974), respectively.…”