You are here:
New Model for Dry Deposition of Aerosols
Citation:
Pleim, Jon, R. Saylor, F. Binkowski, AND L. Ran. New Model for Dry Deposition of Aerosols. International Technical Meeting On Air Pollution Modelling And Its Application, Barcelona, SPAIN, October 18 - 22, 2021.
Impact/Purpose:
To improve the CMAQ model for use in research and air quality policy
Description:
Dry deposition is an important process for aerosols that has large 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 George 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 in 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 of quasi-laminar boundary layer resistance 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). When the modally integrated form is applied in CMAQ, the accumulation mode deposition velocities increase by more than an order of magnitude in highly vegetated areas resulting in lower concentrations of PM2.5, which reduces bias compared to the base model.