Using CALIOP Retrieved Aerosol Parameters to Improve OCO-2 Version 8 Retrievals

The NASA Orbiting Carbon Observatory-2 (OCO-2) was designed to measure the column-averaged dry air mole fraction of carbon dioxide (XCO2) at a sufficient accuracy and precision to improve constraints of regional-scale CO2 fluxes. Achieving this level of data quality is particularly challenging when significant aerosol amounts are within the field of view. Errors in the aerosol properties, in particular the vertical distribution and the scattering properties, can increase both bias and variance in the XCO2 retrieval. The current operational OCO-2 retrieval (Version 8) uses an aerosol climatology derived from the NASA Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis from 2009-2010. The climatology is used to specify the aerosol type and scattering properties as prior information in the physical retrieval algorithm. The algorithm includes the aerosol optical thickness and aerosol pressure level as part of the retrieval state vector (OCO-2 L2 Algorithm Theoretical Basis Document).

This research explores the use of co-located aerosol data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the CALIPSO spacecraft as prior information in the OCO-2 retrieval. The close formation flying between OCO-2 and CALIPSO since August 2015 has yielded a long term dataset with CALIOP and OCO-2 ground footprints overlapping when OCO-2 collects data in Nadir geometry. The CALIOP retrieved aerosol altitude is used as a tight constraint for the aerosol pressure height, while the CALIOP retrieved optical depth is used as a loose constraint. Analysis has been done on observations of two primary locations: Lamont, OK, to facilitate comparison to Total Carbon Column Observing Network (TCCON); and the Saharan Desert, where common dust aerosol layers and high reflectance surfaces create challenging scenes for retrieval of XCO2. Retrieval performance is evaluated through a variety of methods, including comparisons to TCCON and surface reflectance retrieved from the Aqua Moderate resolution imaging spectrometer (MODIS) instrument.