Assessing the roles emission sources and atmospheric processes play in simulating <i>δ</i><sup>15</sup>N of atmospheric NO<sub><i>x</i></sub> and NO<sub>3</sub><sup>−</sup> using CMAQ (version 5.2.1) and SMOKE (version 4.6)

Abstract. Nitrogen oxides (NOx= nitric oxide (NO) + nitrogen dioxide (NO2)) are important trace gases that affect atmospheric chemistry, air quality, and climate. Contemporary development of NOx emissions inventories is limited by the understanding of the roles of vegetation (net NOx source or net sink), vehicle emissions from gasoline- and diesel-powered vehicles, the application of NOx emission control technologies, and accurate verification techniques. The nitrogen stable isotope composition (δ15N) of NOx is an effective tool to evaluate the accuracy of the NOx emission inventories, which are based on different assumptions. In this study, we traced the changes in δ15N values of NOx along the “journey” of atmospheric NOx, driven by atmospheric processes after different sources emit NOx into the atmosphere. The 15N was incorporated into the emission input dataset, generated from the US EPA trace gas emission model SMOKE (Sparse Matrix Operator Kernel Emissions). Then the 15N-incorporated emission input dataset was used to run the CMAQ (Community Multiscale Air Quality) modeling system. By enhancing NOx deposition, we simulated the expected δ15N of NO3-, assuming no isotope fractionation during chemical conversion or deposition. The simulated spatiotemporal patterns in NOx isotopic composition for both SMOKE outputs (simulations under the “emission only” scenario) and CMAQ outputs (simulations under the “emission + transport + enhanced NOx loss” scenario) were compared with corresponding measurements in West Lafayette, Indiana, USA. The simulations under the emission + transport + enhanced NOx loss scenario were also compared to δ15N of NO3- at NADP (National Atmospheric Deposition Program) sites. The results indicate the potential underestimation of emissions from soil, livestock waste, off-road vehicles, and natural-gas power plants and the potential overestimation of emissions from on-road vehicles and coal-fired power plants, if only considering the difference in NOx isotopic composition for different emission sources. After considering the mixing, dispersion, transport, and deposition of NOx emission from different sources, the estimation of atmospheric δ15N(NOx) shows better agreement (by ∼ 3 ‰) with observations.