Establishing tracer gas dilution as a potential reference method for in-stack CO2 emission metrology

Facility-level CO 2 emission quantification faces persistent challenges, including significant uncertainties from flow measurement biases in in-stack continuous emission monitoring system (CEMS) or alternatively the dependence of top-down methods on uncontrollable meteorological conditions. However, there remains a lack of an in-situ, traceable measurement approach that can provide reference-level emission estimates under non-ideal stack flow conditions and serve to validate or calibrate existing monitoring systems. In this study, we applied the Tracer Gas Dilution (TGD) method as a potential solution to this gap, enabling direct, in-situ quantification of CO 2 emissions from the main stack of a power plant. Unlike emission rates computed from pitot-tube flow measurement, which are susceptible to errors under non-uniform or swirling flow conditions, and CO 2 concentrations from CEMS, the TGD method determines emission rates from the dilution ratio of a tracer gas, making it inherently insensitive to velocity profile distortions. Our TGD method achieved a standard uncertainty of 1.6% (expanded uncertainty of 3.2%), demonstrating a high measurement precision. Both computational fluid dynamics simulations and field experiments confirmed sufficient mixing of the tracer with flue gas, limiting mixing-related uncertainty to below 1% even under turbulent flow regimes. Emission rates derived from TGD were benchmarked against measurements from a CEMS and a UAV-based top-down mass balance approach. The TGD results were consistent with the UAV-derived estimates, with average deviations less than ±10%. Moreover, validation using TGD reduced the CEMS measurement bias from approximately 40% to 2%. These results demonstrate that TGD achieves reference-level metrological performance and addresses a critical methodological gap by providing an independent in-situ benchmark for calibrating and validating existing stack emission monitoring systems, thereby enhancing the accuracy and credibility of reported CO 2 emissions for environmental management and climate policy implementation. • Tracer gas dilution offers a reliable benchmark for stack CO 2 emission measurement. • Field tests show improved accuracy over conventional monitoring under complex flows. • Supports credible carbon accounting through calibration of existing monitoring systems.