XCO 2 observations using satellite measurements with moderate spectral resolution: investigation using GOSAT and OCO-2 measurements
Abstract. In light of the proposed space segment of Europe's future CO2 monitoring system, we investigate the spectral resolution of the CO2 
spectrometer, which measures earthshine radiance in the three relevant spectral bands at 0.76, 1.61, and 2.06  µm . The Orbiting Carbon
Observatory-2 (OCO-2) mission covers these bands with fine spectral resolution but limited spatial coverage, which hampers the monitoring of
localized anthropogenic CO2 emission. The future European CO2 monitoring constellation, currently undergoing feasibility studies at
the European Space Agency (ESA), is targeting a moderate spectral resolution of 0.1, 0.3, and 0.3–0.55  nm in the three spectral bands with a
high signal-to-noise ratio (SNR) as well as a spatial resolution of 4  km2 and an across-track swath width >250 km . This spectral
and radiometric sizing is deemed to be favorable for large-swath imaging of point sources of CO2 emission. To assess this choice, we use
real and synthetic OCO-2 satellite observations, which we spectrally degrade to the envisaged lower spectral resolution. We evaluate the
corresponding CO2 retrieval accuracy by taking the Total Carbon Column Observing Network (TCCON) observations as reference. Here, a lower
spectral resolution enhances the scatter error of the retrieved CO2 column mixing ratio ( XCO2 ) but has little effect on the
station-to-station variation in the biases. We show that the scatter error gradually increases with decreasing spectral resolution. Part of the
scatter error increase can be attributed to the retrieval noise error which can be compensated for by a future instrument with improved SNR. Moreover,
we consider the effect of the reduced spectral resolution on the capability to capture regional XCO2 variations and XCO2 plumes from
selected OCO-2 orbits. The investigation using measurements from the Greenhouse gases Observing SATellite (GOSAT) and synthetic measurements
confirms our finding and indicates that one major source of uncertainties regarding CO2 retrieval is the insufficient information on aerosol
properties that can be inferred from the observations. We hence recommend the implementation of simultaneous, co-located measurements that have
a larger information content on aerosols with an auxiliary instrument in the future European observing system.