iNRACM: incorporating 15N into the Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry plays in controlling the isotopic composition of NOx, NOy, and atmospheric nitrate

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 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 NOy cycling, NOx emission budgets, unresolved issues within the heterogeneous uptake coefficients of N2O5, 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 NOy compounds. Here we have developed a photochemical mechanism used to simulate tropospheric photochemistry to include N-15 compounds and reactions as a means to simulate delta N-15 values in NOy compounds. The 16 N compounds and 96 reactions involving N used in the Regional Atmospheric Chemistry Mechanism (RACM) were replicated using N-15 in a new mechanism called i(N)RACM. The 192 N reactions in i(N)RACM were tested to see if isotope effects were relevant with respect to significantly changing the delta N-15 values (+/- 1 %) of NOx, HONO, and/or HNO3. The isotope fractionation factors (alpha) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the i(N)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(N)RACM predictions and observed delta N-15 NO3- 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 NOy. The implication is that measurements of delta N-15 in NOy compounds may be a new way of tracing in situ N chemistry and a means of assessing NOx emission budgets.

Automated summary

This study investigates the importance of nitrogen oxides in the troposphere and introduces a new method of using N isotopes to trace compound formations. A photochemical mechanism was developed to simulate N isotopic effects, showing promising results in replicating atmospheric chemical reactions. The incorporation of isotope fractionation factors accurately captures isotope effects during NOy photochemistry, suggesting a potential new approach for tracing in situ N chemistry and assessing NOx emission budgets.