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<p>Nitrogen oxides, classified as NO<span class="inline-formula">_<i>x</i></span> (nitric oxide (NO) <span class="inline-formula">+</span> nitrogen dioxide (NO<span class="inline-formula">₂</span>)) and NO<span class="inline-formula">_<i>y</i></span> (NO<span class="inline-formula">_<i>x</i>+</span> NO<span class="inline-formula">₃</span>, N<span class="inline-formula">₂</span>O<span class="inline-formula">₅</span> HNO<span class="inline-formula">₃</span>, <span class="inline-formula">+</span> HNO<span class="inline-formula">₄+</span> HONO <span class="inline-formula">+</span> Peroxyacetyl nitrate (PAN) <span class="inline-formula">+</span> organic nitrates <span class="inline-formula">+</span> any oxidized N compound), are important trace gases in the troposphere, which play an important role in the formation of ozone, particulate matter (PM), and secondary organic aerosols (SOA). There remain many uncertainties in the origin and fate of atmospheric N compounds including the understanding of NO<span class="inline-formula">_<i>y</i></span> cycling, NO<span class="inline-formula">_<i>x</i></span> emission budgets, unresolved issues within the heterogeneous uptake coefficients of N<span class="inline-formula">₂</span>O<span class="inline-formula">₅</span>, and the formation of organic nitrates in urban forests, to name a few. A potential tool to resolve some of these uncertainties are using natural abundance N isotopes in NO<span class="inline-formula">_<i>y</i></span> compounds. Here we have developed a photochemical mechanism used to simulate tropospheric photochemistry to include <span class="inline-formula">¹⁵</span>N compounds and reactions as a means to simulate <span class="inline-formula"><i>δ</i>¹⁵</span>N values in NO<span class="inline-formula">_<i>y</i></span> compounds. The 16 N compounds and 96 reactions involving N used in the Regional Atmospheric Chemistry Mechanism (RACM) were replicated using <span class="inline-formula">¹⁵</span>N in a new mechanism called i<span class="inline-formula">_N</span>RACM. The 192 N reactions in i<span class="inline-formula">_N</span>RACM were tested to see if isotope effects were relevant with respect to significantly changing the <span class="inline-formula"><i>δ</i>¹⁵</span>N values (<span class="inline-formula">±1</span> ‰) of NO<span class="inline-formula">_<i>x</i></span>, HONO, and/or HNO<span class="inline-formula">₃</span>. The isotope fractionation factors (<span class="inline-formula"><i>α</i></span>) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the i<span class="inline-formula">_N</span>RACM mechanism was tested individually and in concert in order to assess the controlling reactions. The controlling reactions and their diurnal importance are discussed. A comparison between i<span class="inline-formula">_N</span>RACM predictions and observed <span class="inline-formula"><i>δ</i>¹⁵</span>N NO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="9pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="d4ab4f4bbbf3853ed4e712bcab2aae0c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gmd-14-5001-2021-ie00001.svg" width="9pt" height="16pt" src="gmd-14-5001-2021-ie00001.png"/></svg:svg></span></span> in particulate matter from Tucson, Arizona, suggests the model, and isotope fractionation factors incorporated into it, are accurately capturing the isotope effects occurring during the photochemistry of NO<span class="inline-formula">_<i>y</i></span>. The implication is that measurements of <span class="inline-formula"><i>δ</i>¹⁵</span>N in NO<span class="inline-formula">_<i>y</i></span> compounds may be a new way of tracing in situ N chemistry and a means of assessing NO<span class="inline-formula">_<i>x</i></span> emission budgets.</p>