Near-Simultaneous Observations of XCO2 and NO2 Over Megacities from OCO-3 and GEMS

Synergistic space-based observations of carbon dioxide (CO₂) and air quality (AQ) tracers like NO₂ have immense potential for improving our understanding of polluted urban environments and regions of fossil-fuel based energy production, as well as for optimizing design of future greenhouse gas (GHG) observation strategies. While by the mid-2020s there will be a range of both low-earth orbit and geostationary observation platforms with dual GHG-AQ capabilities, current operational space-based sensors are largely dedicated to either one or the other: no platform provides simultaneous observations of CO₂ and NO₂, and comparisons of these gases are limited by the temporal separation of the measurements from different satellites and confined to fixed times-of-day.

The Orbiting Carbon Observatory 3 (OCO-3), launched to the International Space Station (ISS) in 05/2019, has been making routine measurements of column average CO₂ dry air mole fraction (XCO₂) and solar-induced chlorophyll fluorescence (SIF). OCO-3 observes globally between sunrise and sunset within a latitude band of ±52°, a limit imposed by the inclination of the ISS orbit. Originally built as a spare copy of the OCO-2 sensor, the OCO-3 spectrometer is nearly identical to that of OCO-2 but includes external modifications to facilitate operation on the ISS, most notable a Pointing Mirror Assembly (PMA) to flexibly position the instrument optics at specific locations on the Earth’s surface. Owing to the PMA, OCO-3 capabilities include a Snapshot Area Map observation mode, primarily dedicated to mapping polluted urban environments and other localized emission hotspots, like volcanoes, power plant facilities, etc. They typical size of an OCO-3 Snapshot Area observation is 80×80 km², taken within 120 seconds. The Geostationary Environmental Monitoring Spectrometer (GEMS), launched 02/2020, observes air quality constituents like ozone and NO₂ from geostationary orbit centered over the Korean peninsula, hourly covering a field of regard of about 85°-145°E, 5°S-45°N, depending on season and time of day. For several megacities within the GEMS field of regard, near-coincident observations of GEMS NO₂ and OCO-3 XCO₂ within 30 min or less can be identified (see image of OCO-3 XCO₂ and GEMS NO₂ over Seoul on 8 April 2022).

We report on first results from near-simultaneous OCO-3 XCO₂ and GEMS NO₂ correlations over Asian megacities using the recently released OCO-3 v10.4 and GEMS v2.0 data products that constitute significant improvements over earlier data product versions. This includes the identification of observation pairs during the period of joint operation (01/2021 to present), temporal matching, spatial resampling onto a common grid, and XCO₂–NO₂ correlations in polluted environments. Selected OCO-2 XCO₂ target mode observations will be included. Nearly 1,000 near-simultaneous XCO₂-NO₂ observations with sufficient spatial coverage by both instruments have been identified for the common temporal observation period (12/2020 through 12/2022), and this record is being continuously updated as new observations become available. With the recent extension of the OCO-3 mission until 2030 (end of life of the ISS) and the upcoming launch of several geostationary sensors like NASA’s TEMPO (2023) and ESA’s Sentinel-4/UVN (2024), this record will continue and will be expanded into a long-term data set of XCO₂-NO₂ correlations