micro atm pCO2 is partial pressure of CO2. For gaseous species, the mole fraction is represented in units of ppm (for e.g. moles of CO2 per million parts of air). Now assuming ideal gas equation number of moles will be directly proportional to partial pressure of an idea gas.  So 950 micro atm (950E-6 atm) CO2 in 1 atm air will be equivalent to 950 ppm of CO2.

Dear Gnanamoorthy and Vinod,

that is in principle true, but the mixing ratio (in ppm) is defined as 'mixing ratio in dry air', i.e. in going from partial pressure you have first to substract out the water vapor pressure. The conversion is

pCO2 = xCO2 * ( p(air) - p(H2O) )

where pCO2 is the partial pressure of CO2, xCO2 is the mixing ratio, p(air) is the air pressure and p(H2O) is the water vapor partial pressure. Because of that, the number for pCO2 is somewhat lower than that for xCO2. How much depends in temperature and relative humidity, but for an xCO2 of 360 ppm you get typically numbers of pCO2 around 350 microatm.

Cheers, Christoph

good answer Dr. Christoph

regards

 good day, i agree with dr.Christoph Voelker

Might i add a similar question?

Of this equation:

pCO2 = xCO2 * ( p(air) - p(H2O) )

My project is an underway measurement of CO2 using an equilibrator. I had come along this equation by referring to Dickson's Guide to Ocean measurement; the p(air) and p(H2O) were substituted into p(equilibrator) and vapor pressure of H2O.

So my question here is that

(1) if the system is a closed system (i.e. equilibrator system), it is logical to subtitute the original equation to fit the system?

(2) because its a closed system, is it that we have to use vapor pressure of H2O instead partial pressure of H2O?

Thanks