A Re-Evaluation of Particle Dry Deposition over Forested Landscapes

Dry deposition is a significant loss process for aerosol particles in three-dimensional air quality (3-D AQ) models.  Process analysis results indicate that dry deposition is the single largest process controlling particle concentrations near the surface on days not impacted by wet deposition. Particle deposition parameterizations currently used in most 3-D AQ models have their roots in measurements and theoretical analyses made more than 30 years ago and have not changed substantially since, even though evidence has accumulated over time that the model parameterizations do not adequately mirror reality as reflected in particle flux measurements.  In particular, model predictions and measurements have greatest divergence in the accumulation size range (diameter of 0.1-1.0 mm) for densely vegetated canopies, such as forests. In this work, several dry deposition parameterizations are implemented in a sectional version of the CAMx modeling system (Comprehensive Air quality Model with eXtensions). Impacts of the dry deposition parameterizations on surface PM_2.5 concentrations and total particle deposition are examined and compared for a domain over the heavily forested Southeast U. S.  Data from the 2013 Southeast Atmosphere Study and existing network measurements over the region are used to evaluate model results. If particle deposition velocity measurements over forests are correct, model results suggest that particle dry deposition is much larger over forested landscapes than has been previously recognized.