EddyPro Units – Distinguishing Between Molar Density, Mole Fraction, and Mixing Ratio

EddyPro Units – Distinguishing Between Molar Density, Mole Fraction, and Mixing Ratio

Confused by the units provided by your gas analyzer? Which units should you use when processing eddy covariance data sets in EddyPro? This chart summarizes the definition of units, different ways to express measurements, and which instruments record data in which units.

Measurement Type Intended Meaning Common Units Typical Instruments
Molar Density • A measurement representing the number of moles of a molecule present in a unit volume of air.
• It can also be the mass per unit volume.
• No distinction is made regarding the presence of water vapor in the unit volume.
• mmol/m3;
• µmol/m3;
• g/m3;
• mg/m3;
• µg/m3;
• Open path and enclosed path instruments (e.g., LI-7500A, LI-7200, LI-7700)
Mole Fraction • A measurement representing the amount of a molecule present per unit amount of wet (moist) air.
• It is assumed that the air has not been dried or water vapor has not been subtracted from air.
• mmol/mmol (parts per thousand);
• µmol/mol (ppm);
• nmol/mol (ppb);
• Closed path instruments that have fast (10Hz or more) temperature and pressure measurements and calculate mole fraction at 10Hz (e.g., LI-7200).
• OR Closed path instruments that have optical paths where temperature and pressure are changing slowly and are measured to calculate mole fraction at 10Hz (e.g., LI-7000, LI-6262).
Mixing Ratio • A measurement representing the amount of a molecule present per unit amount of dry air.
• This is essentially mole fraction per mole of dry air, or dry mole fraction.
• A distinction is made in that water vapor has either been removed using a dryer or the instrument measures water vapor and subtracts it before reporting the values.
• mmol/mmol (parts per thousand);
• µmol/mol (ppm);
• nmol/mol (ppb);
• Closed path instruments that have fast (10Hz or more) temperature and pressure measurements to calculate mole fraction of both the gas and water vapor and subtract water vapor from total, all at 10Hz or faster (e.g., LI-7200).
• OR Closed path instruments that have optical paths where temperature and pressure are changing slowly and are measured to calculate mole fraction of the gas and water vapor and subtract water vapor from the total, all at 10Hz or faster (e.g., optionally applied in the LI-6262).
Comments are closed.