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New Model for Dry Deposition of Aerosols.
Citation:
Pleim, Jon, R. Saylor, F. Binkowski, J. Willison, AND L. Ran. New Model for Dry Deposition of Aerosols. Air-Surface Exchange, Atmospheric Deposition and Ecosystem Impact; 25th Conference on Atmospheric Chemistry; 103rd American Meteorological Society Annual Meeting, Denver, CO, January 08 - 12, 2023.
Impact/Purpose:
The purpose of this research is to improve representation of aerosol dry deposition in air quality and climate models thereby reducing the large uncertainty associated with this process. The result is improved accuracy in simulating aerosol deposition and concentrations in the atmosphere for models such as CMAQ and any other AQ or climate model that uses this new scheme.
Description:
Dry deposition is an important process for aerosols that has significant effects on aerosol concentrations in the atmospheric boundary layer. The dominant removal processes (gravitational settling, Brownian diffusion, inertial impaction, and interception) are all dependant on particle size. However, the parameterizations of some of these processes, particularly impaction and interception, are very uncertain, especially for complex vegetative canopies. Most aerosol dry deposition models used in air quality and climate models are based on the seminal work by W. G. N. Slinn published in several papers in the 1970s and 1980s. In recent years, advanced measurement techniques, including size-resolved eddy correlation fluxes, have enabled accurate measurements of aerosol dry deposition in a variety of landscapes. Several recent studies have aggregated dry deposition velocity data by particle size from many published field experiments and compared to models used in air quality models. These studies show very large discrepancies between models and measurements, especially in forested areas, that potentially have significant effects on aerosol concentration and thus, air quality and climate forcing. In this study we re-derive the aerosol dry deposition calculations for the Community Multiscale Air Quality (CMAQ) model using formulations for the most important processes based on Slinn but adapted for better agreement with the consensus of measured data. The key innovations are dependence on leaf area index (LAI) for the vegetated part of the grid cell and two terms for inertial impaction for both macroscale obstacles (e.g., leaves and needles) and microscale obstacles (e.g., leaf hairs and microscale ridges). The resulting model compares better to size-resolved observations than other models, especially in forests. When the modally integrated form is applied in CMAQ, the accumulation mode deposition velocities increase by more than an order of magnitude in highly forested areas resulting in lower concentrations of PM2.5, which reduces bias compared to the base model. The new aerosol dry deposition model has been released in the latest version of CMAQ (version 5.4).