Mauna Loa in Hawaii has a long data record of CO2 concentrations in the atmosphere.
The actual concentrations found in different areas varies draatically depending on the season and local factors such as forests or industrial emissions.
To give comparable data, the results collected at a single location are used (Mauna Loa). At the start of 2016, CO2 levels broke 400ppm for the first time. They were about 320 ppm 50 years ago. The levels now are 406ppm. A rise of over 1% in one year is a cause of significant scientific interest/concern.