Variable effects of spatial resolution on modeling of nitrogen oxides

Abstract. The lifetime and concentration of nitrogen oxides (NO_x) are susceptible to non-linear production and loss, and con- sequently to the resolution of a chemical transport model (CTM). Here we use the GEOS-Chem CTM in its high performance implementation (GCHP) to investigate NO_x simulations over the eastern United States across a wide range of resolutions (13–181 km). Following increasing grid size, daytime surface NO_x concentrations over July 2015 generally decrease over the Great Lakes (GL) region and increase over the Southern States (SS), yielding regional biases (181 km vs. 13 km) of −18 % to 9 %; meanwhile hydrogen oxide radicals (HO_x) increase over both regions, consistent with their different chemical regimes. Night- time titration of ozone by surface nitric oxide (NO) was found to be more efficient at coarser resolutions, leading to longer NO_x lifetimes and higher surface concentrations of nitrogen dioxide (NO₂) over the GL in January 2015. The tropospheric NO₂ column density at typical afternoon satellite overpass time has spatially more coherent negative biases (e.g., −10 % over the GL) at coarser resolutions in July, which reversed the positive biases of surface NO_x over the SS. The reduced NO₂ aloft (> 1 km altitude) at coarser resolutions was attributable to the enhanced HO_x that intrudes into the upper troposphere. Application of coarse resolution simulations for interpreting satellite NO₂ columns will generally underestimate surface NO₂ over the GL and overestimate surface NO₂ over the SS in summer, while uniformly overestimating NO_x emissions over both regions. This study significantly broadens understanding of factors contributing to NO_x resolution effects, and the role of fine resolution to accurately simulate and interpret NO_x and its relevance to air quality.