Contrasting the TEMPO and TROPOMI Views of Tropospheric NO ₂ : Implications for Diurnal Variability and Model Evaluation

Abstract Nitrogen dioxide (NO 2 ) is a key indicator of unhealthy air, yet its sub‐daily spatiotemporal variability has not been widely characterized across large spatial scales in North America. Tropospheric Emissions: Monitoring of Pollution (TEMPO), a geostationary hyperspectral UV‐Vis instrument, offers up to 12 hourly observations per location across North America, providing opportunities to better understand NO 2 sub‐daily variability at wide scales. This study evaluates TEMPO tropospheric NO 2 retrievals relative to TROPOspheric Monitoring Instrument (TROPOMI) and highlights the additional value of TEMPO's hourly measurements. When observations overlap, TEMPO and TROPOMI show strong spatial agreement, with some systematic differences: TEMPO NO 2 is lower in unpolluted regions and marginally higher in polluted regions, primarily driven by differing air mass factor (AMF) assumptions, including surface albedo and a priori NO 2 profiles. Applying a geometric AMF to both datasets substantially reduces inter‐sensor differences. TEMPO reveals diurnal NO 2 column patterns that broadly follow surface NO 2 observations but with weaker amplitude and slower morning declines. In rural regions, TEMPO is able to monitor the NO 2 diurnal patterns of wildfires, soils, and long‐range transport. Comparison with a Comprehensive Air Quality Model with extensions simulation shows TEMPO reproduces much of the modeled spatiotemporal variability but highlights some systematic differences. TEMPO NO 2 columns show flatter late afternoon behavior that contrasts with modeled column NO 2 behavior. The simulation underpredicts urban morning NO 2 and overpredicts late‐day NO 2 , indicating potential NO x emission temporalization biases or boundary layer evolution mischaracterizations. Overall, TEMPO's diurnal data enhance our understanding of NO 2 variability and provide robust constraints for air quality models and emissions assessments.