Intensive field measurements of NH3 and CO2 exchange were made over a wet heathland in the vicinity (10%). Corrections were applied using a local-scale dispersion-exchange model. The analysis highlights how advection modifies the classical one-dimensional inferential resistance approach. It is concluded that ecosystems in the vicinity of agricultural land receive more dry deposition than would be estimated using NH3 concentration monitoring and standard inferential models. In the present study, this effect represented an overall increase in total NH3 deposition of 32%. ¿ 2001 American Geophysical Union