Stable carbon isotope labelled carbon dioxide as tracer gas for air change rate measurement in a ventilated single zone

Carbon dioxide (CO2) has often been used as tracer gas for measurement of the air change rate λ (h−1) in buildings. In such measurements, a correction is required for the presence of indoor CO2, which commonly consists of atmospheric CO2 mixed with human respired CO2. Here, 13C isotope-labelled CO2 was employed as tracer gas, and cavity ring-down spectroscopy (CRDS) was used for simultaneous measurement of the two isotope analogues 12CO2 and 13CO2. This enabled the simultaneous measurement of the 13CO2 tracer gas, with correction for background 13CO2, and the concentration of indoor CO2, allowing for presence of occupants. The background correction procedure assumes that the isotope delta of the background indoor CO2 equals δB = −19‰, based on the prior information that the carbon isotope ratio RB = 13C/12C of all carbon in the bio-geosphere of earth is in the interval 0.010900 < RB < 0.011237. Evidence supported that λ could be accurately measured, using the new 13CO2 tracer method, even when the background 13CO2 concentration varied during the measurement time interval, or when the actual δB value differed from the assumed value. The measurement uncertainty for λ was estimated at 3%. Uncertainty in λ due to uncertainty in RB, uRB(λ), was estimated to increase with a decreasing amount of 13CO2 tracer. This indicated that at least 4 ppm tracer must be used, in order to obtain uRB(λ)/λ < 2%. The temporal resolution of the λ measurement was 1.25/λ h.