Emission measurements at NV dairy barns: assessing individual barn measurability with the direct air flow rate method.

Measuring emissions in naturally ventilated (NV) dairy barns presents significant challenges. Currently two methods are employed: the indirect method, using tracer gases for estimating ventilation rates, and the direct method, utilizing ultrasonic anemometry.
Evaluating the measurability of a barn is crucial for accurate emission measurements, but is currently typically assessed qualitatively based on expert judgement. To address this, recent studies focused on refining the direct method, utilizing ultrasonic anemometers (UAs) to measure air velocities to determine airflow rates through barn openings. This allows, in theory, for achieving a closed mass balance between incoming and outgoing air flow on the barn level. Therefore, we introduced a ‘ventilation balance check’, a quantitative method for assessing the measurability of individual NV dairy barns for determining emissions using the direct method. This involves establishing a relationship between air in- and outflow rates of the barn, through orthogonal linear regression. These regressions are performed separately for the main air flow directions in the barn, and for the complete dataset, to assess differences under different flow regimes. Based on scaled experiments in a mock-up building, a maximum relative difference of 20 % between incoming and outgoing flow was proposed[1].
In this study, we present the results of a ventilation balance check of six NV dairy barns, with relative differences between in and outgoing flow ranging from 5% up to more than 40%. We discuss the possible factors which influenced these results and give some suggestions to decrease differences by improving the measuring setup.