i<sub>N</sub>RACM: incorporating <sup>15</sup>N into the Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry plays in controlling the isotopic composition of NO<sub><i>x</i></sub>, NO<sub><i>y</i></sub>, and atmospheric nitrate

Abstract. Nitrogen oxides, classified as NOx (nitric oxide (NO) + nitrogen dioxide (NO2)) and NOy (NOx+ NO3, N2O5 HNO3, HNO4+ HONO Peroxyacetyl nitrate (PAN) organic nitrates any oxidized N compound), are important trace gases in the troposphere, which play an role formation of ozone, particulate matter (PM), secondary aerosols (SOA). There remain many uncertainties origin fate atmospheric compounds including understanding cycling, emission budgets, unresolved issues within heterogeneous uptake coefficients N2O5, urban forests, to name a few. A potential tool resolve some these using natural abundance isotopes compounds. Here we have developed photochemical mechanism used simulate tropospheric photochemistry include 15N reactions means δ15N values The 16 96 involving Regional Atmospheric Chemistry Mechanism (RACM) were replicated new called iNRACM. 192 iNRACM tested see if isotope effects relevant with respect significantly changing (±1 ‰) NOx, HONO, and/or HNO3. fractionation factors (α) for assigned based on recent experimental or calculated values. Each reaction was individually concert order assess controlling reactions. their diurnal importance discussed. comparison between predictions observed NO3- from Tucson, Arizona, suggests model, incorporated into it, accurately capturing occurring during NOy. implication is that measurements may be way tracing situ chemistry assessing budgets.