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

Publication date: April 2017
Source:Building and Environment, Volume 115
Author(s): K.E. Anders Ohlsson, Bin Yang, Alf Ekblad, Christoffer Boman, Robin Nyström, Thomas Olofsson
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 δ B  = −19‰, based on the prior information that the carbon isotope ratio R B  = ¹³C/¹²C of all carbon in the bio-geosphere of earth is in the interval 0.010900 < R B  < 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 δ B value differed from the assumed value. The measurement uncertainty for λ was estimated at 3%. Uncertainty in λ due to uncertainty in R B , u RB (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 u RB (λ)/λ < 2%. The temporal resolution of the λ measurement was 1.25/λ h.