If you can point me to a source that demonstrates clearly that no Co2 outgassing from the Mauna Loa volcano reaches the instrumentation, I’ll gladly revise my post.
Well, there is no such source, because of course the CO2 reaches the instrumentation when the wind is coming from a particular direction. However, scientists are of course aware of that, and take that into account. There is even an FAQ at the Scripps Institution of Oceanography:
Isn’t the Mauna Loa record influenced by CO2 emitted by the volcano?
If one looks at the minute-by-minute data from Mauna Loa, one finds rare occasions when the CO2 is elevated from emissions from fumaroles upwind on the mountain. The fumaroles are emitting constantly, so the timing of the events depends on wind direction and not changes in volcanic activity. These events impact only a tiny faction of the data and are easily distinguished from rest of the record. The reported version of the Mauna Loa record has been “filtered” to remove these events, as well as other certain other local effects, as described in the early publications (see Keeling 1960 Tellus paper).
Keeling himself writes (1960):
At Mauno Loa Observatory, Hawaii, a less prominent variability has been found in approximately half of the records. This is attributed to release of carbon dioxide by nearby volcanic vents; combustion on the island associated with agricultural, industrial and domestic activities; and lower concentration of carbon dioxide transported to the station by upslope winds. The values reported here are averages of downslope winds or strong lateral winds when the concentration remained nearly constant for several hours or more.
In that paper Keeling also shows CO2 measurements from other locations, all in good agreements, and points to problems these measurements have, and how he accounted for them.
Global CO2 data are available here.