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

Fang, Huan; Walters, Wendell W.; Mase, David; Michalski, Greg

Nitrogen oxides, classified as NOinline-formulax (nitric oxide (NO) inline-formula+ nitrogen dioxide (NOinline-formula2)) and NOinline-formulay (NOinline-formulax+ NOinline-formula3, Ninline-formula2Oinline-formula5 HNOinline-formula3, inline-formula+ HNOinline-formula4+ HONO inline-formula+ Peroxyacetyl nitrate (PAN) inline-formula+ organic nitrates inline-formula+ 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 NOinline-formulay cycling, NOinline-formulax emission budgets, unresolved issues within the heterogeneous uptake coefficients of Ninline-formula2Oinline-formula5, 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 NOinline-formulay compounds. Here we have developed a photochemical mechanism used to simulate tropospheric photochemistry to include inline-formula15N compounds and reactions as a means to simulate inline-formulaδ15N values in NOinline-formulay compounds. The 16 N compounds and 96 reactions involving N used in the Regional Atmospheric Chemistry Mechanism (RACM) were replicated using inline-formula15N in a new mechanism called iinline-formulaNRACM. The 192 N reactions in iinline-formulaNRACM were tested to see if isotope effects were relevant with respect to significantly changing the inline-formulaδ15N values (inline-formula±1 ‰) of NOinline-formulax, HONO, and/or HNOinline-formula3. The isotope fractionation factors (inline-formulaα) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the iinline-formulaNRACM 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 iinline-formulaNRACM predictions and observed inline-formulaδ15N NOinline-formula M39inlinescrollmathml normal 3 - 9pt16ptsvg-formulamathimgd4ab4f4bbbf3853ed4e712bcab2aae0c gmd-14-5001-2021-ie00001.svg9pt16ptgmd-14-5001-2021-ie00001.png 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 NOinline-formulay. The implication is that measurements of inline-formulaδ15N in NOinline-formulay compounds may be a new way of tracing in situ N chemistry and a means of assessing NOinline-formulax emission budgets.

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Fang, Huan / Walters, Wendell W. / Mase, David / et al: 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. 2021. Copernicus Publications.

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