Dynamic ammonia exchange within a mixed deciduous forest canopy in the Southern Appalachians

Area of Science:

• Atmospheric Chemistry
• Forest Ecology
• Biogeochemical Cycles

Background:

• Ammonia (NH3) plays a crucial role in forest ecosystems, influencing air quality and nutrient cycling.
• Understanding NH3 exchange between the atmosphere and forests is vital for ecological and regional air quality assessments.
• Existing models often simplify NH3 bi-directional exchange, potentially missing key in-canopy processes.

Purpose of the Study:

• To analyze ammonia (NH3) concentration and flux measurements in a mixed deciduous forest.
• To assess a multi-layer, one-dimensional column model's ability to represent in-canopy NH3 profiles and exchange processes.
• To investigate the influence of canopy conditions, particularly dew, on NH3 dynamics.

Main Methods:

• Utilized a multi-layer, one-dimensional column model with detailed canopy physics.
• Simulated NH3 concentrations and fluxes for dry canopy periods (April) and a period with dew (July).
• Compared model outputs with field measurements of in-canopy NH3 profiles and above-canopy fluxes.

Main Results:

• The model accurately reproduced in-canopy NH3 profiles under dry canopy conditions.
• Model performance degraded during rapid drying periods, underestimating NH3 concentrations, likely due to unaccounted dew release.
• Simulated NH3 fluxes above the canopy were generally small, with seasonal variations in deposition and emission.

Conclusions:

• Dew significantly influences in-canopy ammonia (NH3) concentrations, a process not captured by current models.
• Multi-layer models are necessary to accurately represent NH3 bi-directional exchange and in-canopy variations in forest ecosystems.
• Further research is needed to understand NH3 exchange with dew and its implications for regional air quality modeling.