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 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 15N compounds and reactions as a means to
simulate δ15N values in NOy compounds. The 16 N compounds
and 96 reactions involving N used in the Regional Atmospheric Chemistry
Mechanism (RACM) were replicated using 15N in a new mechanism called
iNRACM. The 192 N reactions in iNRACM were tested to see if isotope
effects were relevant with respect to significantly changing the δ15N values (±1 ‰) of NOx, HONO, and/or
HNO3. The isotope fractionation factors (α) for relevant
reactions were assigned based on recent experimental or calculated values.
Each relevant reaction in the iNRACM 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
iNRACM predictions and observed δ15N 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 δ15N in NOy compounds
may be a new way of tracing in situ N chemistry and a means of assessing NOx
emission budgets.
Simulating δ<sup>15</sup>N of atmospheric NO<sub><i>x</i></sub> in CMAQ version 5.2.1, based on <sup>15</sup>N incorporated SMOKE version 4.6 and WRF version 4.0 for assessing the role atmospheric processes plays in controlling the isotopic composition of NO<sub><i>x</i></sub>, NO<sub><i>y</i></sub>, and atmospheric nitrate