The Impact of 3D Cloud Radiative Effect on Trace Gas Retrievals: Bridging the Gap from Low Earth Orbit to Geostationary Missions

The three-dimensional (3D) cloud radiative effect, specifically radiance contributions via horizontal photon transport from neighboring clouds that operational algorithms cannot capture, is a significant source of structural uncertainty in trace gas retrievals from Low Earth Orbit (LEO) sensors like TROPOspheric Monitoring Instrument (TROPOMI) and Orbiting Carbon Observatory-2 (OCO-2). Our previous studies have shown that these biases are dependent on Solar Zenith Angle (SZA) due to elongated optical paths at high angles. This dependence presents a critical challenge for the new generation of geostationary (GEO) satellites, specifically the Geostationary Environment Monitoring Spectrometer (GEMS) and Tropospheric Emissions: Monitoring of Pollution (TEMPO). While LEO instruments typically favor low SZA overpasses to maximize signal-to-noise ratios, GEO sensors observe the full diurnal evolution of trace gases. This necessitates measurements at high SZA (low sun elevation), where the 3D cloud effect becomes particularly pronounced.

Furthermore, GEMS and TEMPO deviate from the heritage O₂ A-band (760 nm) pressure and cloud properties retrievals used by TROPOMI and OCO-2, instead relying on O₂-O₂ dimer absorption at 477 nm. This shift introduces distinct radiative transfer challenges, as O₂-O₂ absorption scales with the square of pressure due to its collisional nature and exhibits different sensitivities to aerosol layering. This study analyzes the 3D cloud radiative effect specific to GEO viewing geometry and gas retrieval products utilizing O₂-O₂ bands. Specifically, we evaluate the potential for artificial diurnal bias in retrieved NO₂ caused by the interplay of changing solar geometry and the 3D cloud effect, and we assess the effectiveness of current Air Mass Factor (AMF) correction strategies for the O₂-O₂ based retrieval algorithm in the vicinity of clouds.