These terms give scientists a way to describe how much of a substance is contained in a sample: parts of analyte per million parts of sample, for instance. In atmospheric chemistry these become volume parts of analyte per volume parts of atmosphere: ppmv, ppbv, etc. At low analyte gas phase concentrations the analyte is assumed to act as an ideal gas. For instance, a 1 ppmv concentration of formaldehyde would represent 1 liter of formaldehyde per every 1,000,000 liters of air; also equivalent to 1 microliter of H2CO per 1 L air. In gas phase concentrations these units are also called gas phase mixing ratios. The reason is because they are just that, ratios of analyte volume to sample volume; the volume of the sample doesn't matter. [Ecology A to Z; P. Fleisher; p.150-161; 1994; Dillon Press; New York] [Occupational Environment; S. DiNardi; p.1320; 1997; AIHA Press; Fair Fax, Virginia]

This definition is in context to Atmospheric Chemistry. See more contextual defintions for Parts Per Million (ppm) And Parts Per Billion (ppb).