This change is primarily due to the changes in the adjustment for drifting local measurement time. (realclimate.org)
These monitors provided detailed local measurements but due to their limited range, oceans and low - population areas were poorly represented. (scientificamerican.com)
They, IMO, are mostly Beck and ZJ when making global conclusions from local measurements from primary authors whose first purpose were not always CO2 levels, even local (see for instance Lockhart and Court 1942 available in docs.lib.noaa.gov/rescue/mwr/070/mwr-070-05-0093.pdf, about... oxygen depletion in Antarctica). (rabett.blogspot.ru)