Some journals emphasize this by writing these units as ppm (v:v), etc.įor example, a background concentration of 03 may be 0.04 ppm this is 4 pphm, 40 ppb, or 40,000 ppt. Although we shall use the term concentration frequently, if the units are ppm, ppb, or ppt, it should be understood that this is really a mixing ratio. If water vapor is included, the mixing ratio would vary with humidity, which could induce a variation of a few percent. Although not expressly stated in most cases, mixing ratios are usually referenced to dry air. Since the total air pressure and hence total concentration of air decreases with altitude, a constant mixing ratio does not imply a constant concentration (i.e., as expressed in moles or molecules per unit volume). The use of mixing ratios is widespread for expressing the relative amounts of a species at various altitudes throughout the atmosphere. This ratio of moles, molecules, or volumes of the species to the number of moles, molecules, or volumes of dry air is more commonly known as the mixing ratio. The latter unit is often used in isotope studies and is denoted parts per mille, or %c.Īlternatively, because numbers of molecules (or moles) are proportional to their volumes according to the ideal gas law (PV = nRT), these units may be thought of as the number of volumes of the pollutant found in 106, 10x, 109, or 1012 volumes of air, respectively. Care must also be taken to ensure that ppt means parts per trillion and not parts per thousand. As a result, in some journals when these units are used, they are accompanied by a definition, e.g., ppb (parts in 109). It should be noted that, although these are commonly used, confusion may arise in that in some European countries a billion means 1012 and a trillion means 1018. These units express the number of molecules of pollutant found in a million (106), a hundred million (10x), a billion (an American billion (109)), or a trillion (1012) molecules of air, respectively. Parts per Million, Parts per Hundred Million, Parts per Billion, and Parts per Trillionįor gas-phase species, the most commonly used units are parts per million (ppm), parts per hundred million (pphm), parts per billion (ppb), and parts per trillion (ppt). For a review of IUPAC recommendations, see Schwartz and Warneck (1995).Ī. A number of different units are used in expressing the concentrations of various species in the atmosphere we shall review these here.
0 kommentar(er)
