Long-term portable FTS measurements with GOSAT and OCO-2 preparatory for GOSAT-2

The column-average dry air mole fractions of carbon dioxide (XCO₂) and methane (XCH₄) were retrieved from the surface measurements using direct sunlight at near-IR wavelengths. We have recorded near-IR spectra using a portable Fourier transform spectrometer (FTS), EM27/SUN, in western US field experiments at Caltech in Pasadena, a northern Los Angeles suburb, and Railroad Valley (RRV), a desert playa in Nevada every early summer since 2014. These measurements were conducted during the GOSAT and OCO-2 joint calibration-validation campaigns. We added the second channel for carbon monoxide (XCO) measurement for GOSAT-2 preparation and enhanced the modulation efficiency by optical realignment in 2017. We also demonstrated the measurements at possible emission areas such as a dairy farming region in the east of Los Angeles, air pollution by biomass burning over southern California's Imperial Valley and large cities located near power plant in Japan.

The JAXA EM27 measurements were taken in conjunction with GOSAT and OCO-2 target observations and are being used to assess the value of the EM27 as a portable transfer standard for validating space-based measurements of greenhouse gases. Before and after experiments, the JAXA EM27 was compared with the NIES EM27 and the Tsukuba TCCON station to quantify the measurement bias. After shipment to US, the JAXA EM27 was compared with the Caltech EM27 and the Caltech or JPL TCCON station. We will present the EM27 measurement and performance for 5 years with GOSAT and OCO-2 coincidences.