Ppbv/ppmv/pptv

The high atmospheric abundances of CO2, SO2, OCS, HCl, and HF on Venus are due to the high temperatures at Venus surface (Fegley and Treiman, 1992). All these gases are present at much lower abundances in the Earth s atmosphere. For example, average mixing ratios in the terrestrial troposphere of CO2, SO2, OCS, HCl, and HF are 360 ppmv, 20-90 pptv, 500 pptv, —1 ppbv, and —25 pptv, respectively (cf. Table 3). Also the major sources and sinks for these gases on Earth are different from their probable sources and sinks on Venus. 

Mole fraction Number density of substance divided by number density of air (dimensionless variable). The mole fraction is generally expressed in cmol/mol, mmol/mol, pmol/mol, nmol/mol, and pmol/mol, which corresponds to mole fractions of 10-2, 10-3, 10-6, 10-9, and 10-12, respectively. The terms parts per million by volume (ppmv), parts per billion per volume (ppbv), or parts per trillion per volume (pptv) are often being used to express mixing ratios by volume. Volume mixing ratio is equivalent to mole fraction only for an ideal gas. 

Before discussing the compositions of planetary atmospheres, it is helpful to define the units in which they are described. The most common is the volumemixing ratio (or simply mixing ratio), which is a dimensionless quantity given by the ratio of the gas partial pressure (Pi) to the total pressure (Pt). It is also commonly referred to as the mole (or volume) fraction of the gas. Mixing ratios are given in terms of percent (%), parts per million (ppmv), parts per billion (ppbv), and parts per trillion (pptv) by volume. 

Ppmv parts per million volume, ppbv parts per billion volume, pptv parts per trillion volume... 

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