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INCORPORATING NITROGEN STABLE ISOTOPES INTO ATMOSPHERIC CHEMISTRY MODELS
thesisposted on 11.03.2021, 06:45 by Huan Fang
The nitrogen stable isotope composition (δ15N) of nitrogen oxides (NOx) may be an effective tool to evaluate the accuracy of the NOx emission inventories, which are based on different assumptions. In order to approach this goal, the understanding of (1) the δ15N(NOx) values of the major emission sources, (2) how atmospheric processes, such as mixing, transport, and deposition, alters the composition of atmospheric NOx, (3) gas-phase isotope effects that occur during the oxidation of NOx into NOy (NOx + NO3, N2O5, HNO3, + HNO4 + HONO + Peroxyacetyl nitrate (PAN) + organic nitrates + any oxidized N compound), and (4) the corresponding δ15N measurement of atmospheric NOx under different scenarios are necessary. Therefore, I developed 15N incorporated CMAQ (The Community Multiscale Air Quality Modeling System), to explore the changes in δ15N driven by atmospheric processes and tropospheric photochemistry after different sources of NOx being emitted to the atmosphere. The 15N was first incorporated into the emission dataset, based on the amount of NOx emission from each source retrieved from the emission inventories, and the corresponding δ15N(NOx) values characterized from the prior studies. Then the 15N incorporated emission dataset was used as input to run CMAQ, to trace how atmospheric processes alter the composition of NOx, by using WRF (Weather Research and Forecasting) model to prepare meteorology conditions. Finally, 15N was incorporated into the chemical mechanisms of CMAQ to explore the gas phase isotope effects associated with NOx oxidation, by adding the 15N of N compounds and replicate the chemical reactions involving N compounds, with the corresponding fractionation factors () based on prior experimental and theoretical studies. The simulated δ15N(NOx) was compared with corresponding measurements at NADP sides within Indiana, Illinois, Ohio, and Kentucky. Overall, this research explores changes in δ15N values along the “journey” of atmospheric NOx, for better understanding the spatial distributions of the NOx emission budgets, as well as the impacts from the associated atmospheric processes and NOx oxidation chemistry.