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<p>Nitrogen oxides (<span class="inline-formula">NO_<i>x</i></span> <span class="inline-formula">=</span> <span class="inline-formula">NO</span> <span class="inline-formula">+</span> <span class="inline-formula">NO₂</span>) play a crucial role in the formation of ozone and secondary inorganic and organic aerosols, thus affecting human health, global radiation budget, and climate. The diurnal and spatial variations in <span class="inline-formula">NO₂</span> are functions of emissions, advection, deposition, vertical mixing, and chemistry. Their observations, therefore, provide useful constraints in our understanding of these factors. We employ a Regional chEmical and trAnsport model (REAM) to analyze the observed temporal (diurnal cycles) and spatial distributions of <span class="inline-formula">NO₂</span> concentrations and tropospheric vertical column densities (TVCDs) using aircraft in situ measurements and surface EPA Air Quality System (AQS) observations as well as the measurements of TVCDs by satellite instruments (OMI: the Ozone Monitoring Instrument; GOME-2A: Global Ozone Monitoring Experiment – 2A), ground-based Pandora, and the Airborne Compact Atmospheric Mapper (ACAM) instrument in July 2011 during the DISCOVER-AQ campaign over the Baltimore–Washington region. The model simulations at 36 and 4 <span class="inline-formula">km</span> resolutions are in reasonably good agreement with the regional mean temporospatial <span class="inline-formula">NO₂</span> observations in the daytime. However, we find significant overestimations (underestimations) of model-simulated <span class="inline-formula">NO₂</span> (<span class="inline-formula">O₃</span>) surface concentrations during nighttime, which can be mitigated by enhancing nocturnal vertical mixing in the model. Another discrepancy is that Pandora-measured <span class="inline-formula">NO₂</span> TVCDs show much less variation in the late afternoon than simulated in the model. The higher-resolution 4 <span class="inline-formula">km</span> simulations tend to show larger<span id="page11134"/> biases compared to the observations due largely to the larger spatial variations in <span class="inline-formula">NO_<i>x</i></span> emissions in the model when the model spatial resolution is increased from 36 to 4 <span class="inline-formula">km</span>. OMI, GOME-2A, and the high-resolution aircraft ACAM observations show a more dispersed distribution of <span class="inline-formula">NO₂</span> vertical column densities (VCDs) and lower VCDs in urban regions than corresponding 36 and 4 <span class="inline-formula">km</span> model simulations, likely reflecting the spatial distribution bias of <span class="inline-formula">NO_<i>x</i></span> emissions in the National Emissions Inventory (NEI) 2011.</p>