When they say the $\ce{CO2}$ concentration is $\pu{350 ppm}$ what does this actually mean? Does this mean that if we took a sample of air say $\pu{1 m3}$ then $\pu{350 mg}$ of that cubic meter would be $\ce{CO2}$? Is it simply $\pu{mg//m3}$?
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The unit parts per million refers to the unit molecules. Therefore, 350 ppm CO2 means that for every million molecules of a fluid, there are 350 molecules of CO2.
Concentrations of chemicals in air are typically measured in units of the mass of chemical (milligrams, micrograms, nanograms, or picograms) per volume of air (cubic meter or cubic feet).
However, concentrations may also be expressed as parts per million (ppm) or parts per billion (ppb) by using a conversion factor. The conversion factor is based on the molecular weight of the chemical and is different for each chemical. Also, atmospheric temperature and pressure affect the calculation.
Typically, conversions for chemicals in air are made assuming a pressure of 1 atmosphere and a temperature of 25 degrees Celsius.
For these conditions, the equation to convert from concentration in parts per million to concentration in milligrams per cubic meter (mg/m3 ) is as follows:
Concentration (mg/m3 ) = 0.0409 x concentration (ppm) x molecular weight
To convert from mg/m3 to ppm, the equation is as follows: Concentration (ppm) = 24.45 x concentration (mg/m3 ) ÷ molecular weight
Source: https://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/display.files/fileid/14285
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$\begingroup$ Ok great, Why do they sometimes express it as mg/m^3? $\endgroup$ – user66634 Dec 7 '18 at 23:04
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$\begingroup$ See updated answer (feel free to upvote and accept the answer by clicking the check mark) $\endgroup$ – H. Khan Dec 7 '18 at 23:11
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$\begingroup$ Upvoted. Thank you! So if we say there are 300ppm of C02 that is somewhat arbitrary or dimensionless ? I guess thats what is confusing me, the units as in 330ppm what are the units? $\endgroup$ – user66634 Dec 8 '18 at 0:23
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$\begingroup$ @user66634 When ppm is based on molar concentration (as it usually is for gas concentration) the result is a unitless ratio. We are comparing molecule counts to molecule counts. This works well for gases as it the same ratio applies to partial pressures and volumes at constant pressure. IF other contaminants like particulates are the topic, though, this doesn't work and a mass/volume ratio might be used. This will have units of weight/volume. $\endgroup$ – matt_black Dec 8 '18 at 15:45
\pu{ ... }. $\endgroup$ – Martin - マーチン♦ Dec 8 '18 at 12:26