We are considering buying an air cooler to cool and dry our sample air stream. We are interested in the CO2 concentration of the air stream, and we are wondering how much CO2 in the air stream will dissolve in the condensed water when cooled. Which alkalinity and pH can be assumed at the moment of condensation (tropical air far from a city)? How much CO2 will dissolve (as CO2 as well as other DIC products) until an equilibrium has been reached with the outside air?
We have an air stream of 1 L a min with ~400 ppm CO2. We calculated to have ~1 ml condensed water produced per 30 min.
The effect on CO2 in the air will be negligible. Assuming warm air is 40 degrees C it would contain about 0.051 g of water per L of air at saturation (based on water vapor partial pressure of 7.4 kPa, which you can calculate with a saturated steam table calculator--there are several online, and the ideal gas law--close enough). (Note that this is not too much higher than your estimate of 1 g water per 30 L air.) Assuming unlimited CO2 (g) is present at a partial pressure of 0.04 kPa (400E-6 * 101 kPa) and in equilibrium with pure water, the resulting solution will have a pH of 5.55 and a total inorganic carbon (TIC) concentration of 2.95E-5 mol/kg (this is based on PHREEQC, also available online, about 90% of the TIC would be CO2 (aq) and H2CO3, because of the low pH). In this worst-case scenario (maximum transfer of CO2 to water, no kinetic limitations) then CO2 captured by water would be about 1.5E-9 mol per L of air (2.95E-5 mol/kg * 0.051 g/1000 g/kg). In contrast, air at 40 degrees C with 400 ppmv CO2 has about 1.5E-5 mol/L (based on the ideal gas law). So loss in water would be < 0.01% of total CO2 (assuming there are no big mistakes--no promises). Air pollution could affect CO2 partitioning, but a high concentration of a basic pollutant is unlikely (unless you are working on a farm, where NH3 will be present).
Dear Sasha,
Thank you for your elaborate answer, and sorry for the delay in my response. I am not so familiar with the PHREEQC software, but I have found (few) values in literature which confirm your calculation.
I have looked for DIC concentrations in rainwater (which should be of a similar magnitude), and I found values between 10-40 umol p L, and higher values of up to 80 umol p L for areas affected by biomass burning. See below for the references.
As you also calculated, this will only have a minimal effect of less than 0.01%, which will bring our air stream from 400 to 399.96 ppm. This is close to negligible for most scientific research purposes.
However, in our case, we will use the REA (Relaxed Eddy Accumulation) method, wherein the CO2 concentration difference between two air streams is being measured and used. These two different air streams might differ slightly in DIC uptake potential (aerosol content/alkalinity). When the CO2 difference (delta CO2) between the 2 air streams is small (lets say 0.2 ppm), this DIC uptake by the condensate will influence the delta CO2 significantly.
Thank you again for your help. We will now look further into other methods of sample drying.
Lara et al 2001: Chemical composition of rainwater and anthropogenic influences in the Piracicaba River Basin, Southeast Brazil
Das et al 2005: Carbon isotope ratio of dissolved inorganic carbon (DIC)
in rivers draining the Deccan Traps, India: Sources of DIC and their magnitudes
Neu et al 2016: Dissolved Organic and Inorganic Carbon Flow Paths in an Amazonian Transitional Forest
Fred Mackenzie, Carbon in the Geobiosphere, Carbon Dioxide in Natural waters
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