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. Nitrogen oxides, classified as NOx (nitric oxide (NO) + nitrogendioxide (NO2)) and NOy (NOx+ NO3, N2O5HNO3, + HNO4+ HONO + Peroxyacetyl nitrate (PAN) + organicnitrates + any oxidized N compound), are important trace gases in thetroposphere, which play an important role in the formation of ozone,particulate matter (PM), and secondary organic aerosols (SOA). There remainmany uncertainties in the origin and fate of atmospheric N compounds includingthe understanding of NOy cycling, NOx emission budgets,unresolved issues within the heterogeneous uptake coefficients ofN2O5, and the formation of organic nitrates in urban forests, toname a few. A potential tool to resolve some of these uncertainties areusing natural abundance N isotopes in NOy compounds. Here we havedeveloped a photochemical mechanism used to simulate troposphericphotochemistry to include 15N compounds and reactions as a means tosimulate δ15N values in NOy compounds. The 16 N compoundsand 96 reactions involving N used in the Regional Atmospheric ChemistryMechanism (RACM) were replicated using 15N in a new mechanism callediNRACM. The 192 N reactions in iNRACM were tested to see if isotopeeffects were relevant with respect to significantly changing the δ15N values (±1 ‰) of NOx, HONO, and/orHNO3. The isotope fractionation factors (α) for relevantreactions were assigned based on recent experimental or calculated values.Each relevant reaction in the iNRACM mechanism was tested individuallyand in concert in order to assess the controlling reactions. The controllingreactions and their diurnal importance are discussed. A comparison betweeniNRACM predictions and observed δ15N NO3- inparticulate matter from Tucson, Arizona, suggests the model, and isotopefractionation factors incorporated into it, are accurately capturing theisotope effects occurring during the photochemistry of NOy. Theimplication is that measurements of δ15N in NOy compoundsmay be a new way of tracing in situ N chemistry and a means of assessing NOxemission budgets.

geoscientific model development

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