The first step in this project is to qualitatively compare the GCAS column NO2 measurements with coincident measurements from TROPOMI and model output from CAMx. Spatial plots of column NO2 in the early afternoon at the TROPOMI overpass time are shown in Figure for a 10-day average and a single day. By observing the 10-day average (top row), we can see that GCAS measurements are generally larger than both the TROPOMI measurements and CAMx model output. TROPOMI captures the broad spatial patterns observed by GCAS with a notable low bias. The TROPOMI low bias is partially driven by the 3.5