Spectral Calibration for TEMPO (Tropospheric Emissions: Monitoring of Pollution): Algorithm Description and Trending of Spectral Performance

Abstract The Tropospheric Emissions: Monitoring of Pollution (TEMPO) instrument provides continuous, high‐resolution observations of atmospheric pollutants over North America from geostationary orbit. This study introduces an on‐orbit spectral calibration algorithm implemented in the TEMPO Version 3 Level 0–1 processor, covering both operational irradiance and radiance wavelength calibrations and offline slit function retrievals. Irradiance wavelength calibration accuracy was evaluated, with the TSIS‐1 hybrid solar reference spectrum chosen due to its low fitting residuals. Accordingly, first‐ and second‐order Chebyshev‐polynomial fittings are applied to UV and VIS, respectively, to derive the wavelength grid. Earth‐view radiance wavelength calibration updates the wavelength grid based on the latest solar irradiance calibration result by fitting a wavelength shift. To optimize efficiency and accuracy, a narrow spectral window of 100 channels (320–340 nm for UV and 630–650 nm for VIS) was selected, with wavelength shift uncertainties of 0.002 nm (UV) and 0.006 nm (VIS). Radiance calibration results shows that the wavelength shifts of inhomogeneous pixels vary relatively significantly. We perform a 22‐month trend analysis of the TEMPO solar irradiance spectral performance. Compared to first light, the wavelength shift gradually increases, reaching 0.08–0.09 nm in July 2024, and then remains stable. The offline slit function parameters, retrieved from several narrow spectral windows using a super‐Gaussian function, show minor variations during the 22‐month on‐obit operation and did not deviate significantly from prelaunch. This study supports the long‐term L1b data processing for TEMPO and provides an instrument spectral calibration framework applicable for future geostationary orbit spectrometers.