Scripps Institution of Oceanography

We report here on several details that apply to updating the primary in situ Mauna Loa CO2 record from the Scripps CO2 program to the X12 calibration scale from the previous X08A scale.  The update incorporates revised determinations of the primary reference gases on the constant-volume mercury-column manometer (CMM) based on improved assessments of manometer volumes and other parameters (Keeling et al., 2016).  The changes, which impact data only after 1974, are typically of order 0.1 to 0.2 ppm or less at ambient concentrations. In this update, we also incorporate two changes that are unrelated to the revised manometric determinations and are relevant only for the in situ Mauna Loa record but not relevant for flask data from Mauna Loa or other stations. The first involves a reassignment of a secondary calibration cylinder used for the in situ Mauna Loa measurements starting April 2015.  The second involves eliminating a +0.12 ppm adjustment that had been applied only to the in situ Mauna Loa record to correct for offset between flasks and the continuous analyzer.