Evaluating TEMPO Formaldehyde Retrievals With Co-Located Ground-Based FTIR and Pandora Observations

Formaldehyde (HCHO) is an important proxy for volatile organic compounds and photochemical activity, with significant implications for air quality and atmospheric chemistry. The recently launched geostationary satellite instrument Tropospheric Emissions: Monitoring of Pollution (TEMPO) provides the first-ever hourly observations of HCHO and other key pollutants over North America. To evaluate the accuracy of the TEMPO Level 2 Version 3 retrieval, we compared HCHO total columns (TC) with co-located ground-based measurements from Fourier Transform Infrared (FTIR) and Pandora spectrometers at three sites: Boulder, Colorado, USA; Mexico City, Mexico; and Toronto, Ontario, Canada. The consistency between FTIR and Pandora HCHO TC measurements was first assessed to characterize the performance of the Pandora instruments relative to established FTIR retrievals, which is important given the rapid global expansion of the Pandora network and limited overlap with reference instruments. Our results show good correlation and consistent hourly and seasonal patterns between FTIR and Pandora, while highlighting the importance of routine field calibrations and performance assessments to ensure long-term data reliability. By establishing agreement and understanding differences between the two independent ground-based data sets, the TEMPO evaluation was strengthened. TEMPO HCHO TC retrievals exhibit a consistent low bias of approximately 30% relative to ground-based observations at all three sites, with the largest discrepancies under elevated HCHO conditions. Nonetheless, TEMPO accurately captures both diurnal and seasonal variability, demonstrating its utility for characterizing short-lived atmospheric species. These findings highlight the value of coordinated satellite and ground-based measurements for validating and improving satellite products for air quality research and monitoring.