Mass mixing ratio ($q={\rho }_a/\rho$)

The mass mixing ratio used in a dilute radiative transfer model is absorber mass density divided by total atmospheric mass density. A hydrostatic pressure interval contributes absorber mass path $qdp/g$. Some meteorological conventions divide by dry-air mass instead; the convention must be stated. A volume or mole mixing ratio instead contributes absorber moles $xdp/(gM)$, with background molar mass $M$.