Gas mixtures are usually specified in terms of volumetric gas fraction: by percentage, parts per thousand or parts per million, which converts trivially to partial pressure ratio, following Dalton's law of partial pressures.
Gas mixtures are usually specified in terms of molar gas fraction (which is closely approximated by volumetric gas fraction for many permanent gases): by percentage, parts per thousand or parts per million. Volumetric gas fraction converts trivially to partial pressure ratio, following Dalton's law of partial pressures.