"Mole fraction" is a term used to describe the amount or concentration of a particular substance in a mixture. It represents the ratio of the number of moles of that substance to the total number of moles in the mixture. Full definition
The enhancement of the CO2 mole fraction in the atmosphere over pre-industrial is expressed both as ppm and as a percentage change since the year 1800. (esrl.noaa.gov)
Both sensors enable the retrieval of near - surface sensitive column - averaged dry air mole fractions of CO2 and CH4, i.e., XCO2 (in ppm) and XCH4 (ppb). (esa-ghg-cci.org)
One study using evidence from stomata of fossilized leaves suggests greater variability, with carbon dioxide mole fractions above 300 ppm during the period seven to ten thousand years ago, [70] though others have argued that these findings more likely reflect calibration or contamination problems rather than actual CO2 variability. (en.wikipedia.org)