CMAQv5.0 Mesophyll resistance

Mesophyll Resistance September 27th 2011

Jesse Bash

Mesophyll resistances have only been experimentally identified for a few atmospheric pollutants but can represent a significant barrier to deposition for insoluble atmospheric gases. The dual pathway resistance model of Wesely (1989) was implemented to scale the uptake rates of pollutants in extracellular solution covering the leaf mesophyll using Henry's Law constants and the reactivity of the pollutant with biological substances.

Effects of Changes:

Peak NO₂ deposition velocities were increased by approximately 50% and NO dry deposition velocities were reduced by approximately 50% in areas covered in vegetation. This resulted in lower NO₂ concentrations due to a larger dry deposition sink. The reduction in the ambient NO₂ resulted in reduced ambient NO and HNO₃ concentrations due to a reduction in the photochemical production of NO and HNO₃. These changes resulted in improved model evaluations against SEARCH NO₂ concentrations, CASTNET HNO₃ and O₃ concentrations, and aerosol nitrate observations. On a CONUS domain wide basis these model improvements are on the order of 1%-3%, however there are larger regional model improvements in the Eastern US (5%-10% for nitrogen species and ~1 ppb for O₃).

References

Wesely, M.L., 1989, Parametrization of surface resistances to gaseous dry deposition in regional-scale numerical models, Atmospheric Environment, 23, 1293-1304