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<p>Sub-grid variability (SGV) in atmospheric trace gases within satellite pixels is a key issue in satellite design and interpretation and validation of retrieval products. However, characterizing this variability is challenging due to the lack of independent high-resolution measurements. Here we use tropospheric <span class="inline-formula">NO₂</span> vertical column (VC) measurements from the Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) airborne instrument with a spatial resolution of about <span class="inline-formula">250 m×250 m</span> to quantify the normalized SGV (i.e., the standard deviation of the sub-grid GeoTASO values within the sampled satellite pixel divided by the mean of the sub-grid GeoTASO values within the same satellite pixel) for different hypothetical satellite pixel sizes over urban regions. We use the GeoTASO measurements over the Seoul Metropolitan Area (SMA) and Busan region of South Korea during the 2016 KORUS-AQ field campaign and over the Los Angeles Basin, USA, during the 2017 Student Airborne Research Program (SARP) field campaign. We find that the normalized SGV of <span class="inline-formula">NO₂</span> VC increases with increasing satellite pixel sizes (from <span class="inline-formula">∼10</span> % for <span class="inline-formula">0.5 km×0.5 km</span> pixel size to <span class="inline-formula">∼35</span> % for <span class="inline-formula">25 km×25 km</span> pixel size), and this relationship holds for the three study regions, which are also within the domains of upcoming geostationary satellite air quality missions. We also quantify the temporal variability in the retrieved <span class="inline-formula">NO₂</span> VC within the same hypothetical satellite pixels (represented by the difference of retrieved values at two or more different times in a day). For a given satellite pixel size, the temporal variability within the same satellite pixels increases with the sampling time difference over the SMA. For a given small (e.g., <span class="inline-formula">≤4</span> h) sampling time difference within the same satellite pixels, the temporal variability in the retrieved <span class="inline-formula">NO₂</span> VC increases with the increasing spatial resolution over the SMA, Busan region, and the Los Angeles Basin.</p> <p>The results of this study have implications for future satellite design and retrieval interpretation and validation when comparing pixel data with local observations. In addition, the analyses presented in this study are equally applicable in model evaluation when comparing model grid values to local observations. Results from the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) model indicate that the normalized satellite SGV of tropospheric <span class="inline-formula">NO₂</span> VC calculated in this study could serve as an upper bound to the satellite SGV of other species (e.g., <span class="inline-formula">CO</span> and <span class="inline-formula">SO₂</span>) that share common source(s) with <span class="inline-formula">NO₂</span> but have relatively longer lifetime.</p>