Global Monthly Methane Observations

The Global Monitoring Division of NOAA’s Earth System Research Laboratory has measured methane since 1983 at a globally distributed network of air sampling sites. A global average is constructed by first smoothing the data for each site as a function of time, and then smoothed values for each site are plotted as a function of latitude for 48 equal time steps per year. Global means are calculated from the latitude plot at each time step.

Methane is reported as a “dry air mole fraction”, defined as the number of molecules of methane divided by the total number of molecules in the sample, after water vapor has been removed. The mole fraction is expressed as nmol mol-1, abbreviated “ppb” (for parts per billion; 1 ppb indicates that one out of every billion molecules in an air sample is CH4).
https://gml.noaa.gov/ccgg/trends_ch4/