The ACOS (Atmospheric CO2 Observations from Space) team is preparing to release data from their new algorithm, build version 2.9. Numerous changes are anticipated in the Level 1B and Level 2 products. These changes will be numerated and listed in a Data User’s Guide document we will post on our GES DISC/ACOS Web site at the time of public release. Users will have to carefully observe the version id, “2.9” , in the GES DISC domain: the ACOS on-line archive, and the Mirador search engine.
Even though the GES DISC is not publicly distributing Level 1B ACOS products, it should be known that changes are affecting both Level 1B and Level 2 data. An important enhancement in Level1B will address the degradation in the number of quality-passed soundings in v2.8 (Figure 1).
Elimination of many systematic biases, and better agreement with TCCON (Total Carbon Column Observing Network), is expected in Level 2 retrievals. The key changes to the L2 algorithm include scaling the O2-A band spectroscopy (reducing XCO2 bias by 4 or 5 ppm); using interpolation with the instrument lineshape [ ILS ] (reducing XCO2 bias by 1.5 ppm); and fitting a zero level offset to the A-band.
Users will have to also carefully familiarize themselves with the disclaimer in the new upcoming documentation. An important element to note are the updates on data screening. Although a Master Quality Flag is provided in the data product, further analysis of a larger set of data has allowed the science team to provide an updated set of screening criteria. These are listed in the data user’s guide, and are recommended instead of the Master Quality Flag.
Lastly, users should continue to carefully observe and weigh information from three important flags:
- sounding_qual_flag – quality of input data provided to the retrieval processing
- outcome_flag – retrieval quality based upon certain internal thresholds (not thoroughly evaluated)
- master_quality_flag - four possible values: “Good”, “Caution” and “Bad”, and “Failed”, as determined from other flags in the L2 product
Read more ACOS updates at the ACOS/OCO web site.
Recipes for simple reading and screening of ACOS data have been added at our ACOS web portal .
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Figure 1. Two-month aggregates of the current Version 2.8, Level 2 xCO2 retrieval, for Fall 2009, Spring 2010, and Fall 2010.
Only “Good” and “Caution” qualities are used in the aggregation process. The degradation in the number of soundings that pass the quality control in Version 2.8 is evident in the “September-October” plots. This issue is addressed and removed from the upcoming version 2.9 build.
The “March-April” plot manifests the seasonal accumulations of xCO2 in the Northern Hemisphere appearing by the end of the winter.