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H20 Saturation Mixing Ratio
Temperature, K
FIGURE 1.3 U.S. Standard Atmosphere temperature and saturation water vapor mixing ratio at 45°N and at the equator.
Conversion from Mixing Ratio to ng m 3 Atmospheric species concentrations are sometimes expressed in terms of mass per volume, most frequently as |ig m 3, Given a concentration mi, in |ig m~3, the molar concentration of species i, in mol m~\ is
_ 10~6m, Ci ~ M, where M, is the molecular weight of species i.
Noting that the total molar concentration of air at pressure p and temperature T is c = p/RT, then
Mixing ratio of t in ppm =-x concentration of i in jig m~i pM;
If 7" is in K and p in Pa (see Table A.6 for the value of the molar gas constant, R)
Mixing ratio of i in ppm
As an example, let us determine the concentration in ng m-3 for 03 corresponding to a mixing ratio of 120ppb at p = latm and T = 298 K. The 120ppb corresponds to 0.12 ppm. Then
(1.013 x 105)(48) ~ 8.314(298) = 235.6 (igra"3
Now let us convert 365 jig m 3 of S02 to ppm at the same temperature and pressure:
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