Mixing ratios, concentrations and different units:
Amounts of gases are often given in different units:
concentrations: molecules/cm3 or µg/m3
or mixing ratios: ppt (pmol/mol), ppb (nmol/mol), ppm (µmol/mol), % (10 mmol/mol)
Mixing ratios are often more helpful for scientist, because if air rises and expands the volume grows and therefore the concentration change, but the mixing ratio (relation of the gases) remains the same.
The conversion from one unit into the other depends on the pressure (= the altitude) and molecular weight. If we do the calculation for the Earth surface for normal pressure of about 1 bar we can express the total molecules per air volume in the following way:
1 mol = 22.4 L = 6x1023 molecules =>
1 cm3 = 2.7x1019 molecules
1 dm3 = 1 L = 2.7x1022 molecules
1 m3 = 2.7x1025 molecules
Example for a rough estimate:
2 µg/m3 = 2x10-6 g/m3 NO2 is a typical value for nitrogen dioxide in a non-urban area.
molecular weight M(NO2) = 46 g/mol
This means: 2x10-6 g/m3 = 4.3x10-8 mol/m3 = 2.6x1016 molec/m3
So the mixing ratio is about 2.7x1016 / 2.7x1025 = 10-9 = 1 ppb
Since ozone has a similar molecular weight, M(O3) = 48 g/mol, we can also roughly say
2 µg/m3 of ozone = 1 ppb
This calculation is valid only for the Earth surface, where we live. So, for ozone smog events in urban areas we can now calculate:
120 µg/m3 = 60 ppb -> high level
240 µg/m3 = 120 ppb -> very high level, no sports, risk for health
360 µg/m3 = 180 ppb -> extremely high level, very unhealthy for the lung, stay at home!
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