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Abstract:
This report describes a validation study of Greenhouse gases Observing Satellite (GOSAT) data processing
using ground-based measurements of the Total Carbon Column Observing Network (TCCON) as reference
data for column-averaged dry air mole fractions of atmospheric carbon dioxide (X CO2). We applied the
photon path length probability density function method to validate X CO2 retrievals from GOSAT data
obtained during 22months starting from June 2009. This method permitted direct evaluation of optical path
modifications due to atmospheric light scattering that would have a negligible impact on ground-based
TCCON measurements but could significantly affect gas retrievals when observing reflected sunlight from
space. Our results reveal effects of optical path lengthening over Northern Hemispheric stations, essentially
from May-September of each year, and of optical path shortening for sun-glint observations in tropical
regions. These effects are supported by seasonal trends in aerosol optical depth derived from an offline threedimensional
aerosol transport model and by cirrus optical depth derived from space-based measurements of
the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument. Removal of observations that
were highly contaminated by aerosol and cloud from the GOSAT data set resulted in acceptable agreement in
the seasonal variability of X CO2 over each station as compared with TCCON measurements. Statistical
comparisons between GOSAT and TCCON coincident measurements of CO 2 column abundance show a
correlation coefficient of 0.85, standard deviation of 1.80ppm, and a sub-ppm negative bias of -0.43ppm for all
TCCON stations. Global distributions of monthly mean retrieved X CO2 with a spatial resolution of 2.5°
latitude×2.5° longitude show agreement within∼2.5°ppm with those predicted by the atmospheric tracer transport model.
