Bi-directional air-surface exchange of atmospheric ammonia - Review of measurements and development of a big-leaf model for general applications

The air-surface exchange of atmospheric ammonia (NH3) and measurements of the canopy and stomatal compensation points (χc and χst, respectively) are reviewed. The exchange of NH3 between the atmosphere and the surface is bi-directional; with emission dominating during the daytime and in warmer, drier climates and deposition dominating nighttimes and cooler climates. Measured χc values tend to be higher over intensively-managed vegetated surfaces, such as agricultural crops, than semi-natural vegetation, such as forests. Higher nitrogen concentrations from fertilization and cutting practices have been observed to increase these compensation points. The conditions of the ground cover have also been found to have an impact on χc and the emission and deposition of NH3. Litter leaves have been observed to significantly increase the values of χc over agricultural vegetation. Recent measurements of emissions from soil surfaces have been important in developing our understanding of the exchange processes of NH3. In this study, a bi-directional air-surface exchange model has been developed for NH3 and compared to existing measurements. This two-layer big-leaf model, which simulates both emission and deposition of NH3 over 26 land-use categories during different seasons, can be incorporated into large-scale air-quality models. In our simulations, χc varied with vegetation type, with daytime values ranging from 4.6 μg/m3 over a forest to 18.4 μg/m3 over non-fertilized agricultural grassland, high in nitrogen, at a temperature of 25C. These results are similar to the range of measured values. Variations in temperature have also been shown to affect the compensation points. A rise in atmospheric temperature from 10 to 33C caused the χst value of an agricultural crop to increase from 0.8 to 12.6 μg/m3. The resulting χc values ranged from 8.9 to 16.1 μg/m3 for these temperatures, which are comparable to measured values for similar vegetation.