Summertime Diurnal Variability of Formaldehyde Over the Contiguous United States: Constraints From Pandonia Global Network

The diurnal cycle of Formaldehyde (HCHO) provide critical insights into tropospheric photochemistry. Here we use tropospheric HCHO retrievals from the Pandonia Global Network (PGN), combined with NASA's GEOS Composition Forecast (GEOS‐CF) model to understand the diurnal variation of summertime HCHO across the contiguous US. While PGN HCHO tropospheric column (HCHO trop ) shows a weak diurnal cycle in most regions, a distinct midday peak is found at the urban sites of Southern US (mainly in Houston), likely driven by highly reactive VOC emissions. For the vertical profile of HCHO mixing ratio within the Planetary Boundary Layer (PBL), PGN shows a significant decrease from 0.5 to 2 km while GEOS‐CF exhibits an excessively well‐mixed vertical shape. This discrepancy in HCHO profile leads to an overestimated HCHO trop in GEOS‐CF in Northeast Coastal US and Southeast US. These findings offer valuable insights for interpreting geostationary satellite observations and understanding model biases in surface ozone (O 3 ).