An evaluation of the wind-tunnel technique for estimating ammonia volatilization from land: Part 1. Analysis and improvement of accuracy

Experiments were carried out to check the validity of the method used to ca lculate gas fluxes in the wind-tunnel technique. This method assumes that t he airflow is completely mixed downwind of the emission plot. This hypothes is was tested using a homogeneously distributed CO2 source installed on the ground under the tunnel cover. Carbon dioxide was used instead of ammonia to allow measurement of its concentration fluctuation. The measured vertical profiles of wind speed and concentration were non-uni form in the measurement section, showing that the flow was far from being c ompletely mixed, and leading to a recovery rate ranging from 77 to 87%. Usi ng the measured concentration and wind profiles, the efficiency of several sampling systems was numerically simulated. Using this approach, the conventional sampling system led to an underestima tion of the actual flux of up to 11%, while the non-uniformity of the wind profiles was responsible for an overestimation of 3%. The turbulent compone nt of the horizontal flux was found to be less than 0.5% of the mean compon ent. This study suggests that an optimal sampling system could decrease the error due to sampling to a small percentage, while a flow modifier could h alve the error due to the wind profile. (C) 1999 Silsoe Research Institute.