The air-surface exchange of atmospheric ammonia (NH3) and measurements of the canopy and stomatal compensation points (chi(cp) and chi(st), respectively) and the stomatal and soil emission potentials (Gamma(st) and Gamma(g), respectively) are reviewed. A database of these values has been developed, to be used for the development of input parameters and for model evaluation. The compensation points are dependent on canopy type, nitrogen (N) status, temperature, growth stage, and meteorological conditions. Canopies that receive high atmospheric nitrogen input generally have high chi(cp) values. chi(cp) values also tend to be higher over intensively managed vegetated surfaces than semi-natural vegetation, due to the higher nitrogen content in these surfaces. Increased nitrogen concentrations from fertilization and cutting practices have been observed to increase the compensation points and therefore the emission from these canopies. The decomposition of litter leaves has been found to play a dominant role and significantly increase the values of chi(cp) over agricultural vegetation and fertilized grasslands. By modifying an existing big-leaf dry deposition model to allow NH3 emission from leaf stomata and soil surfaces, a bi-directional air-surface exchange model has been developed for applications in regional-scale air-quality models. The model predicts chi(cp) values that vary with canopy type, nitrogen content, and meteorological conditions. chi(cp) values predicted by the model are around 1-2 mu g m(-3) over forest canopies and 4-10 mu g m(-3) over grasslands and agricultural canopies during a typical summer daytime; ccp values are similar to 10 and similar to 3 times lower over forests and agricultural lands, respectively, in nighttime and/or winter conditions. These results are similar to the range of measured values. The new bi-directional air-surface exchange model will reduce the dry deposition fluxes by 20-100 ng m(-2) s(-1) compared to the original dry deposition model over low-N forests and agricultural lands during typical summer daytime conditions, which can be the difference between whether deposition or emission occurs. For example, this new model produces emissions fluxes of 0-90 ng m(-2) s(-1) over croplands when the atmospheric NH3 concentrations are below 10 mu g m(-3).
