The concentrations of carbon dioxide in the atmosphere are expressed as parts per million by volume (abbreviated as ppmv or just ppm). To convert from the usual ppmv units to ppm mass, multiply by the ratio of the molar weight of CO to that of air, i.e. times 1.52 (44.01 divided by 28.96).
The first reproducibly accurate measurements of atmospheric CO were from flask sample measurements made by Dave Keeling at Caltech in the 1950s. Measurements at Mauna Loa have been ongoing since 1958. Additionally, measurements are also made at many other sites around the world. Many measurement sites are part of larger global networks. Global network data are often made publicly available.