Believe it or not the standard is to report the CO2 level measured at the top of an active volcano (Mauna Loa). This is for historical reasons, because it's whe

Believe it or not the standard is to report the CO2 level measured at the top of an active volcano (Mauna Loa). This is for historical reasons, because it's where the first measurements were taken, but also they claim it's more accurate because the measurement site is surrounded by miles of lava and thus there's no trees or vegetation which could absorb CO2 locally nor any human settlements that could emit CO2. There is of course the small problem of it being a volcano. Other measurements come from a collection of sensors on TV or telecom towers. Some of them come from planes. You can see a map of sites here: https://gml.noaa.gov/dv/iadv/index.php?code=sgi https://gml.noaa.gov/dv/iadv/index.php?code=sgi The trend is the same in every site, although absolute readings vary somewhat. There are cleaner sites than Mauna Loa, like in Antarctica, but those show about 10ppm lower. Others give the same reading as at Mauna Loa but with more seasonal variance. It's unclear whether these differences really matter and they tend to be ignored.