Abstract
Abstract. Dry deposition has an impact on nitrogen status in forest environments. However, the mechanism for the high dry-deposition rates of fine nitrate particles (NO3-) observed in forests remains unknown and is thus a potential source of error in chemical transport models (CTMs). Here, we modified and applied a multilayer land surface model coupled with dry-deposition and aerosol dynamic processes for a temperate mixed forest in Japan. This represents the first application of such a model to ammonium nitrate (NH4NO3) gas–particle conversion (gpc) and the aerosol water uptake of reactive nitrogen compounds. Thermodynamics, kinetics, and dry deposition for mixed inorganic particles are modeled by a triple-moment modal method. Data for inorganic mass and size-resolved total number concentrations measured by a filter pack and electrical low-pressure impactor in autumn were used for model inputs and subsequent numerical analysis. The model successfully reproduces turbulent fluxes observed above the canopy and vertical micrometeorological profiles noted in our previous studies. The sensitivity tests with and without gpc demonstrated clear changes in the inorganic mass and size-resolved total number concentrations within the canopy. The results also revealed that within-canopy evaporation of NH4NO3 under dry conditions significantly enhances the deposition flux of fine-NO3- and fine-NH4+ particles, while reducing the deposition flux of nitric acid gas (HNO3). As a result of the evaporation of particulate NH4NO3, the calculated daytime mass flux of fine NO3- over the canopy was 15 times higher in the scenario of “gpc” than in the scenario of “no gpc”. This increase caused high contributions from particle deposition flux (NO3- and NH4+) to total nitrogen flux over the forest ecosystem (∼39 %), although the contribution of NH3 was still considerable. A dry-deposition scheme coupled with aerosol dynamics may be required to improve the predictive accuracy of chemical transport models for the surface concentration of inorganic reactive nitrogen.
Highlights
The dry deposition of inorganic reactive nitrogen gas (e.g., HNO3 and NH3) and particles (e.g., NO−3 and NH+4 ) is one of the major pathways of nitrogen input into forest ecosystems
Nakahara et al (2019) observed a higher concentration gradient of fine NO−3 than of fine SO24− in cool–temperate forests using a thermodynamic equilibrium model to explain this difference via the evaporation of NH4NO3 particles in the NH4NO3–NH3– HNO3 triad within the canopy
A new multilayer land surface model fully coupled with dry deposition and aerosol dynamics was developed to evaluate the impact of NH4NO3–NH3–HNO3 conversion in temperate forests
Summary
The dry deposition of inorganic reactive nitrogen gas (e.g., HNO3 and NH3) and particles (e.g., NO−3 and NH+4 ) is one of the major pathways of nitrogen input into forest ecosystems. Recent observational studies at forests revealed that the drydeposition flux of inorganic reactive nitrogen in the form of fine NO−3 was markedly higher than that expected from theory Despite many uncertain factors (e.g., emission inventory, grid resolution, chemical and physical dynamics, and deposition modules), Shimadera et al (2014) demonstrated that the surface concentration of total nitrate could be reproduced by increasing the dry-deposition velocity of HNO3 by a factor of 20 with respect to previous studies. The deposition velocity of NO−3 in fine particles and/or HNO3 are among the major uncertainties in the CTMs
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