"Mass fraction" refers to the portion or amount of a specific substance within a mixture or solution, expressed as the ratio of its mass to the total mass of the mixture. Full definition
We find that TRAPPIST - 1 c and e likely have largely rocky interiors, while planets b, d, f, g, and h require envelopes of volatiles in the form of thick atmospheres, oceans, or ice, in most cases with water mass fractions less than 5 %.» (planetaria.ca)
It then assumes a constant initial mass fraction to estimate the RC age. (climateaudit.org)
The Greenhouse gas doesn't not involve the heat capacity of CO2, and so the mass fraction of C02 in the atmosphere has nothing to do with this. (climateaudit.org)