Aerosol dynamics and gas-particle conversion in dry deposition of inorganic reactive nitrogen in a temperate forest

Abstract

Although dry deposition has an impact on nitrogen status in the forest environments, the mechanism for high dry deposition rates of fine nitrate aerosols (NO − 3 ) observed in forests remains unknown and is a potential source of error in chemical transport models. Here we developed a new multi-layer land surface model coupled with dry deposition and aerosol dynamics processes for a temperate mixed forest in Japan. The processes of thermodynamics, kinetics, and dry deposition for mixed inorganic aerosols are modeled by a triple-moment modal method. The new model overall reproduces observed turbulent 5 fluxes above the canopy and vertical micrometeorological profiles, as well as inorganic mass and size-resolved total number concentrations within the canopy. Sensitivity tests revealed that the within-canopy evaporation of ammonium nitrate (NH 4 NO 3 ) under dry conditions significantly enhances deposition flux for fine NO − 3 and NH + 4 aerosols, while reducing deposition flux for nitric acid gas (HNO 3 ). 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. 10 15 Sakamoto et al., 2018; Nakahara et al., 2019). As long as physical deposition processes are dominant, the deposition velocities of SO 2− 4 and NO − 3 aerosols are expected to be similar because both species exist in the same sub-micron size range (e.g., Wolff et al., 2011). However, Sakamoto et al. (2018) showed observed deposition velocity of NO − 3 as high as those of HNO 3 at a temperate mixed forest, using the relaxed eddy accumulation method (Matsuda et al., 2015). Nakahara et al. (2019) also observed a higher concentration gradient of fine NO − 3 than of fine SO 2− 4 at a cool-temperate forest, using a thermodynamic 20 equilibrium model to explain this difference by evaporation of NH 4 NO 3 aerosols in the NH 4 NO 3 -NH 3 -HNO 3 triad within1 https://doi.the canopy. Numerical studies of chemical transport models for the East Asian region have demonstrated that the models have overestimated total (gas + aerosol) nitrate concentration at many locations (Kajino et al., 2013; Shimadera et al., 2018; Morino et al., 2015; Sakurai et al., 2015). Despite the many uncertain factors (e.g., the emission inventory; grid resolution; chemistry, physics, dynamics, and deposition modules), Shimadera et al. (2014) demonstrated that the surface concentration 25of total nitrate could be reproduced by increasing dry deposition velocity of HNO 3 by a factor of twenty, in reference to past numerical studies. Hence, the deposition velocity of NO − 3 of fine aerosols and/or HNO 3 is among the major uncertainties in the chemical transport modeling.Modeling studies for dry deposition have demonstrated the importance of NH 4 NO 3 volatilization during dry deposition of gaseous and particulate nitrates (Brost et al., 1988;van Oss et al., 1998;Kramm and Dlugi, 1994). All these studies used 30 the "big-leaf" concept for the canopy represen...