Integrating an HF analyzer into a flux detector

A commercially available instrument, the LasIR®, which uses near-IR tunable diode laser technology for in situ remote sensing has been available to the industrial sector for quite some time. This instrument is in widespread use industrially, for example in the aluminium industry, where it is used as an HF analyzer to monitor levels in exhaust ducts and potrooms in smelters worldwide.

Unisearch, in collaboration with the Alberta Research Council and the University of Alberta, undertook to develop a LasIR® based system for determining fluxes (flows) of environmentally sensitive gases in an agricultural setting, for example fluxes of methane or ammonia emanating from a fertilised field. The system used a LasIR instrument (HF analyser), four sets of remote optics, and a weather station. The LasIR and remote optics were used to obtain gas concentrations averaged across an open air path of up to 50 m, while the weather station was used to record local micrometeorological information such as windspeed, air and soil temperature, humidity, soil heat flux, and total radiation.

It was necessary to select an appropriate model for turning these experimentally measurable quantities into a flux value. Among the models in use are mass balance, the eddy correlation (EC) technique, and the flux gradient (FG) method. Application of both the eddy correlation and flux gradient methods requires accurate concentration measurements, as well as wind speed and other micrometeorological data. The flux gradient method further depends on being able to accurately measure a concentration gradient. This experimental methodology provides detailed information on concentration gradients above a semi-infinite surface by averaging across four long paths. Gas fluxes are then calculated by applying an integrated flux gradient (FG) approach based
on Monin-Obukhov similarity theory. This system integrating LasIR technology as an HF analyzer delivered better results and higher accuracy rates.

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