The fixed proportions in which two or more substances may become combined, such as the amount of nitrogen in the air compared to the rest of the air. Atmospheric scientists routinely discuss the gas phase concentrations of trace components in mixing ratios expressed in ratios of VOLUMES, as in "the present tropospheric mixing ratio for methane is approximately 1.7 ppmv."
[Science; v261: pages 1130-1134; 1993.] [Nature; v354: pages 382-284; 1991.]
Source: Atmospheric Chemistry Glossary