Citation:

Wu, Z., JohnT Walker, X. Chen, C. Oishi, T. Duman, AND D. Schwede. Estimating Sources, Sinks, and Fluxes of Reactive Nitrogen and Sulfur within a Mixed Forest Canopy using Eulerian and Lagrangian Inverse Models. National Atmospheric Deposition Program Fall Science Symposium, Boulder, Colorado, November 04 - 08, 2019.

Impact/Purpose:

Canopy-scale flux measurements and inferential models are useful for developing estimates of net emission or deposition of trace gases and aerosols above forests. However, more detailed measurements and models are needed to relate net fluxes to biological, physical, and chemical processes occurring within the air-canopy-soil system, which occur over multiple time scales. The objective of this work is to develop modeling tools that can be used to link atmosphere-biosphere fluxes of reactive compounds to specific ecosystem compartments (e.g., canopy, understory, ground).

Description:

Estimating the source/sink distribution and vertical fluxes of air pollutants within and above forested canopies is critical for understanding the biological, physical, and chemical processes influencing the soil-vegetation-atmosphere exchange. The vertical source-sink profiles of reactive nitrogen and sulfur were examined using multiple inverse modeling methods in a mixed hardwood forest in the southern Appalachian Mountains, which is a region sensitive to deposition of nutrients and acidity. Measurements of the vertical concentration profiles of ammonia (NH3), nitric acid (HNO3), sulfur dioxide (SO2), and ammonium (NH4+), nitrate (NO3-), and sulfate (SO42-) in PM2.5 were measured at the Coweeta Hydrologic Laboratoryduring five intensives between May 2015 and August 2016. The mean concentration of NH3 decreased with height in the upper canopy and increased below the understory toward the forest floor. All other species exhibited patterns of monotonically decreasing concentration from above the canopy to the forest floor. Using the measured concentration profiles and within-canopy flow fields, we estimated the vertical source-sink flux profiles using three inverse approaches: a Eulerian high-order closure model (EUL), a Lagrangian localized near-field (LNF) model, and a new full Lagrangian stochastic model (LSM). Models predicted positive (upward) NH3 fluxes near the forest floor, indicating emissions from the litter or soil. The modeled above-canopy flux of NH3 tended to be negative (downward), indicating that the forest was a net sink of NH3. The modeled flux profiles of HNO3 and SO2 presented a monotonically decreasing trend with most uptake occurring in the upper canopy. Significant differences in the estimated flux profiles can be found between different models and for different timescale inputs. The vertical distributions of fluxes from the inverse models were compared with net canopy-scale, stomatal, cuticular, and soil fluxes estimated from resistance-based big-leaf models. This study provides new insight into atmosphere-biosphere exchange of reactive compounds in forest ecosystems and advances the development of soil-vegetation-atmosphere models capable of partitioning canopy-scale deposition of nitrogen and sulfur to specific ecosystem compartments.

Record Details: