Variable effects of spatial resolution on modeling of nitrogen oxides

Abstract. The lifetime and concentration of nitrogen oxides (NOx) 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 NOx simulations over the eastern United States across a wide range of resolutions (13–181 km). Following increasing grid size, daytime surface NOx 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 (HOx) 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 NOx 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 NOx over the SS. The reduced NO2 aloft (> 1 km altitude) at coarser resolutions was attributable to the enhanced HOx 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 NOx emissions over both regions. This study significantly broadens understanding of factors contributing to NOx resolution effects, and the role of fine resolution to accurately simulate and interpret NOx and its relevance to air quality.