The GeoCarb greenhouse gas retrieval algorithm: Simulations and sensitivity to sources of uncertainty

Abstract

Abstract. The Geostationary Carbon Cycle Observatory (GeoCarb) was selected as NASA's second Earth Venture Mission (EVM-2). The scientific objectives of GeoCarb are to advance our knowledge of the carbon cycle, in particular land-atmosphere fluxes of the greenhouse gases carbon dioxide (CO2) and methane (CH4), and the effects on these fluxes on the Earth's radiation budget. GeoCarb will retrieve column integrated amounts of CO2 (XCO2), CH4 (XCH4) and CO (XCO; important for understanding tropospheric chemistry), in addition to Solar-Induced Fluorescence (SIF), which is proportional to the photosynthetic activity of vegetation, from hyperspectral resolution measurements in the O2 A-band at 0.76 um, the weak CO2 band at 1.6 um, the strong CO2 band at 2.06 um, and a CH4/CO band at 2.32 um. Unlike it's polar orbiting predecessors (OCO-2/3, GOSAT-1/2, TROPOMI), GeoCarb will be in a Geostationary orbit with a sub-satellite point centered over the Americas. This orbital configuration combined with its high spatial resolution imaging capabilities will provide an unprecedented view of these quantities on spatial and temporal scales accurate enough to resolve sources and sinks to improve land-atmosphere CO2 and CH4 flux calculations and reduce the uncertainty of these fluxes. This paper will present a description of the GeoCarb instrument and the level-2 retrieval algorithms which will be followed by simulation experiments to determine a relatively comprehensive error budget for each target gas. Several sources of uncertainty will be explored including that from the instrument calibration parameters for radiometric gain, the instrument line shape (ILS), the polarization, and the geolocation pointing, in addition to, forward model parameters including that from meteorology and spectroscopy. The results indicate that the errors (1σ) are less than the instrument's multi-sounding precision requirements of 1.2 ppm, 10 ppb, and 12 ppb (10 %), for XCO2, XCH4, and XCO, respectively. In particular, when considering the sources of uncertainty separately and in combination (all sources included), we find overall RMS errors of 1.06 ppm for XCO2, 8.2 ppb for XCH4, and 2.5 ppb for XCO, respectively. Additionally, we find that, as expected, errors in XCO2 and XCH4 are dominated by forward model and other systematic errors, while errors in XCO, like SIF, are dominated by measurement noise.

Intelligent pointing increases the fraction of cloud-free CO2 and CH4 observations from space

“…Its design is still preliminary and could change prior to eventual launch but the current baseline design has 4 near-infrared -shortwave infrared (NIR-SWIR) bands (Table 2), each with a dedicated focal plane array (FPA). A dispersive spectrometer would use the bands planned for GeoCarb, which they optimized with noise considerations for that instrument type (McGarragh et al, 2023).…”