Nitrogen dioxide (NO₂) is a trace gas and a “criteria” air pollutant that plays an important role in atmospheric chemistry and serves as one of three inputs to Canada's Air Quality Health Index. Its atmospheric abundance can be measured by ground-based instruments, against which space-based data products are validated for long-term assessment. Polar-orbiting satellite instruments typically provide one measurement per day at a fixed overpass time. The recent geostationary (GEO) constellation, i.e. Geostationary Environment Monitoring Spectrometer (GEMS) over Asia, Sentinel-4 over Europe, and Tropospheric Emissions: Monitoring of Pollution (TEMPO) over North America, now provide continuous daytime observations. Air quality standards and health indices, however, are defined at the surface, while these satellites retrieve a vertical column; addressing that gap requires ground-based reference measurements whose uncertainties are characterized and validated.

The Pandora spectrometer provides both a vertical column comparable to nadir-viewing satellite measurements and a near-surface concentration comparable to in-situ monitors, making it a potential transfer standard for this validation. This work aims to: (i) assess Pandora's NO₂ products for use in air quality applications, and (ii) evaluate TEMPO's relation to ground-based measurements, working toward a satellite-derived Air Quality Health Index. TEMPO tropospheric NO₂ columns are compared with the St. George and CN Tower Pandora measurements in Toronto and Pandora Ultraviolet and Visible surface NO₂ retrievals are evaluated against US Environmental Protection Agency Air Quality System monitors across the Pandonia Global Network. Results will be presented on the consistency of TEMPO with ground-based columns at varying instrument elevations, and on the relative performance of the two Pandora products against surface monitors.