A review of ammonia emission measured using wind tunnels compared with micrometeorological techniques

Ammonia (NH3) emission from livestock manure constitutes a loss of crop-available nitrogen (N) and poses a threat to the environment. Therefore, low NH3 emission slurry application technologies have been developed, the reduction efficiency of which has typically been estimated through measurements using wind tunnels or integrated horizontal flux (IHF) micrometeorological techniques. A recovery of 100% of released NH3 can be obtained if wind tunnels are designed to avoid pulses of wind into the tunnel through the canopy opening and leaks from the tunnels. The NH3 emission measured with wind tunnels adjusted to an air flow of 1 m/s deviated significantly (P < 0.05) from the emission measured using IHF methods, which are generally considered to give reliable emission estimates for ambient conditions. If wind tunnel air flow was adjusted to the ambient wind speed at height 0.25-0.30 m, then measured emissions were not significantly different from those measured using IHF methods. Wind tunnels influence the air flow pattern and cause turbulent convection in the air layers above the emitting surface, so the similarity in measured emissions for the two measuring techniques is perhaps surprising. This may be because the soil surface resistance to NH3 transport is often the most important rate-regulating variable, so that the absence of a laminar boundary air layer resistance as caused by wind tunnels is of minor importance. It is concluded that the wind tunnels are well suited to test the emission reduction efficiency of new technologies