Interpreting summertime hourly variation of NO₂ columns with implications for geostationary satellite applications

Accurate representation of the hourly variation in the NO₂-column-to-surface relationship is essential for interpreting geostationary observations of NO₂ columns. Previous research indicated inconsistencies in this hourly variation. This study employs the high-performance configuration of the GEOS-Chem model (GCHP) to analyze daytime hourly NO₂ total columns and surface concentrations during summer. We use measurements from globally distributed Pandora sun photometers and aircraft observations over the United States. We correct Pandora total NO₂ vertical columns for (1) hourly variations in effective temperature driven by vertically resolved contributions to the total column and (2) changes in local solar time along the Pandora line of sight. These corrections increase the total NO₂ columns by 5-6_1014 molec. cm2 at 09:00 and 18:00 across all sites. Finescale simulations from GHCP (_ 12 km) reduce the normalized bias (NB) against Pandora total NO₂ columns from 19% to 10% and against aircraft measurements from 25% to 13% in Maryland, Texas, and Colorado. Similar reductions are observed in NO₂ columns over the eastern US (17% to 9 %), the western US (22% to 14 %), Europe (24% to 15 %), and Asia (29% to 21 %) when compared to 55 km simulations. Our analysis attributes the weaker hourly variability in the total NO₂ column to (1) hourly variations in column effective temperature, (2) local solar time changes along the Pandora line of sight, and (3) differences in hourly NO₂ variability from different atmospheric layers, with the lowest 500m exhibiting greater variability, while the dominant residual column above 500m exhibits weaker variability..