Scripps Institution of Oceanography

This report documents changes to the CO2 calibration scale of the Scripps O2 program to bring it in line with the SIO X12 scale. The update involves new assignments on six primary CO2 reference gases used by the O2 program, which were used to assign cubic coefficients to the instrument response function for the Siemens CO2 analyzer used by the O2 program.  The new assignments correct for drift in the CO2 concentration of these cylinders which is now well resolved. The update also entails changing the functional form for the instrument response function for the Licor CO2 analyzer, also used by the Scripps O2 program, from a cubic to a double inverse hyperbole. This update impacts all data from the Scripps O2 program from program inception onwards.  We designate the updated CO2 scale as the VH344-2020 scale. The previous scale, which had no designation, is now called the VH344-2015 scale.  The changes from the VH344-2015 to VH433-2020 scales involve changes at the level of a few tenths of a ppm, typically towards higher CO2 .  This report also compares the VH344-2020 scale to the NOAA X2007 scale based on tanks provided by Britt Stephens of the National Center of Atmospheric Research.