Spatial variability of mixing ratios of ammonia and tracer gases in a naturally ventilated dairy cow barn
Résumé
The use of the tracer gas ratio method to estimate emissions fromnaturally ventilated (NV)
livestock barns excludes the need of monitoring ventilation rates. However, it requires
accurate measurement of tracer release rate (QT) and a representative estimate of the
mixing ratio between pollutant (P) and tracer (T) gasesð½P =½T Þ. While the quality of QT
simply depends on using an accurate commercial mass flow controller, determination of a
representative mixing ratio ½P =½T is not trivial, since the NV livestock barn airspace presents
complex movements that might be dependent on spatial vertical and cross horizontal
dimensions. The goal was to assess the spatial variability of concentrations of the
artificial tracer gas sulphur hexafluoride (SF6), the metabolic carbon dioxide (CO2) and the
pollutant ammonia (NH3), along with their mixing ratios ([NH3]/[CO2], [NH3]/[SF6], [CO2]/
[SF6]), inside a NV dairy cow barn. The results indicated that the vertical variability of the
calculated mixing ratios became more stable with increase in height, reaching approximately
constant values above the animal occupied zone. Using both the metabolic CO2 and
the artificially injected SF6 as tracer gases led to a homogeneous spread in behaviour of
mixing ratios along V and HC directions. Finally, the possibility of finding a zone within the
barn airspace where mixing ratios are considered to be representative for the whole barn,
and the implications of applying artificial or metabolic tracers are discussed.