Factors influencing local dry deposition of gases with special reference to ammonia

A model is developed that is used to compute the dry deposition of gases as a function of the downwind distance from a source. The model is applied to examine the fraction of the ammonia emission (Fr) from a point source that is deposited within different distances from the source in relation to factors affecting dispersion and deposition. The results show that Fr at 2000 m distance from the source may be as large as 60% for a 3 m high source when ammonia deposits to mature forest at rates limited only by atmospheric transfer. It is shown how the dry deposition of ammonia depends on source height, wind speed, atmospheric stability, surface resistance, surface roughness length and compensation points. The model’s application to area sources is illustrated with urine patches. Differences between emissions measured in the field and in the laboratory are attributed to dry deposition in between the urine patches.