Tracer decay method for estimating CH4 emission from tent covered slurry tanks

Robust estimates of gas emissions from full-scale slurry storage tanks are critically needed but remain challenging under farm-scale conditions. Tent-covered slurry tanks are naturally ventilated systems in which air exchange varies over time. The tracer decay method offers a practical approach to quantify air exchange rates and, when combined with concentration measurements, enables emission estimation. This study evaluated tracer gas selection and concentration, assessed horizontal mixing within the headspace, and compared different fitting approaches for tracer decay under full-scale conditions. Pure nitrous oxide and diluted butene were tested as tracers following pulse injection into a tent-covered digestate tank. Decay curves were analyzed using constant and time-varying approaches to estimate air exchange. The choice of fitting method had little influence on average methane emission estimates but substantially affected short-term emission dynamics. Under variable ventilation conditions, time-varying approaches were required to represent temporal changes in air exchange. The Kalman-based approach provided a stable reconstruction of ventilation dynamics without introducing excessive noise. Tracer concentrations at two sampling positions located approximately 30 m apart were consistent after an initial homogenization period, indicating effective horizontal mixing at the measurement height. However, butene concentrations approached background levels during decay, resulting in unstable air exchange estimates, whereas nitrous oxide remained clearly distinguishable from background throughout the monitoring period. Estimated air exchange rates ranged from approximately 90 to 300 m 3 h -1 for a tent-covered tank containing 4500 m 3 slurry. The tracer decay method provided robust average methane emission estimates when tracer concentration, monitoring duration, and mixing conditions were adequate. • Application of the tracer decay method in tent-covered storage tanks • Homogeneous gas mixing achieved via point injection • Choice of decay fitting method affects dynamics estimates but not average emissions • Kalman filter provides the best estimates of emissions dynamics • Tracer gas concentration must be sufficiently high for reliable estimates