Air exchange rate measurements in naturally ventilated dairy buildings using the tracer gas decay method with 85Kr, compared to CO2 mass balance and discharge coefficient methods

Emission fluxes from naturally ventilated cattle buildings are usually calculated as the product of gas concentrations and the air exchange rate (AER). One key issue is the measurement of the AER throughout such buildings. The aim of this study was to investigate three different methods for AER estimation in naturally ventilated dairy buildings: (1) the concentration decay of the tracer gas Krypton85 using two different evaluation procedures, (1a) sum85Kr, in which the impulses from all the single 85Kr sensors are combined into one cumulative curve and (1b) ave85Kr, which averages the AERs obtained from single 85Kr sensors; (2) the carbon dioxide mass balance model (CO2M); and (3) the discharge coefficient method (DCM). These three methods were compared with each other using both our own measurements and results from the literature. The comparison was statistically evaluated using analysis of variance (ANOVA). The effects of the external conditions of wind velocity and temperature and the effect of nearby buildings on the differences between the methods were predicted. The concentration decay of 85Kr and the DCM generally produced higher AER values than the CO2M (mean ± standard error (SE), 22.8 ± 7.2 h−1). These differences were determined to be 8.5 ± 3.1 h−1 (P = 0.007) for the sum85Kr method and 12.9 ± 3.1 h−1 (P < 0.0001) for the DCM. The concentration decay of 85Kr can also be used to estimate higher AER ranges when the difference between the CO2 concentrations inside and outside the building is below 200 ppmv. The sum85Kr method is the recommended procedure for tracer decay analysis for AER estimation.