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Comparison of Aerodynamic Resistance Parameterizations and Implications for Dry Deposition Modeling
Citation:
Walker, Johnt, D. Schwede, J. Bash, AND C. Oishi. Comparison of Aerodynamic Resistance Parameterizations and Implications for Dry Deposition Modeling. National Atmospheric Deposition Program, Indianapolis, IN, October 22 - 24, 2014.
Impact/Purpose:
Multiple atmospheric models are being used by Federal research groups and Academia to develop total nitrogen deposition budgets for North America. In some cases, the models yield differences in deposition of large enough magnitude to preclude the development and testing of effective atmospheric regulations for ecosystem health. This work examines the reasons for such disagreement among models and ultimately will lead to reduced uncertainty in atmospheric deposition budgets used to support the secondary NAAQS and critical loads policies.
Description:
Nitrogen deposition data used to support the secondary National Ambient Air Quality Standards and critical loads research derives from both measurements and modeling. Data sets with spatial coverage sufficient for regional scale deposition assessments are currently generated from several distinct modeling platforms, which yield substantially different estimates under certain conditions. For the ecological and atmospheric science communities to provide the best science for policy development, differences in these data sets must be reconciled. One source of bias in deposition estimates across data sets is the choice of model formulation for dry deposition. While most dry deposition models employ a similar conceptual framework, the well-known resistance analogy, the details of the models differ. The resistance framework describes the process of dry deposition as consisting of three components in sequence: turbulent transfer from the atmosphere to the receptor surface, diffusion across the laminar boundary layer of air at the receptor surface and uptake by the surface. The resistance to transfer by these processes, referred to as the aerodynamic (Ra), boundary layer (Rb), and canopy (Rc) resistances, respectively, controls the rate at which the gas or particle deposits (i.e., deposition velocity). For some compounds such as nitric acid (HNO3), an important contributor to the dry N flux, the deposition process is not influenced by the chemical (e.g., acidity), physical (e.g., morphology), or biological (e.g., stomatal behavior) characteristics of the surface. For such compounds, Ra is the limiting resistance. In many cases, differences in the parameterization of Ra cause large differences in model estimates of dry nitrogen deposition. This study investigates the impact of differences in Ra parameterizations on dry deposition estimates, particularly nitrogen compounds. Parameterizations include those used in the CASTNet multi-layer model (MLM), versions of the Community Multi-scale Air Quality Model (CMAQ) with input from the Weather Research Forecast model (CMAQ-WRF) and 5th generation Mesoscale Model (CMAQ-MM5), and the Big Leaf Model (BLM) of Zhang et al. (2003) used within the Canadian Meteorological Service’s ‘‘A Unified Regional Air quality Modelling System” (AURAMS), the Canadian Air and Precipitation Monitoring Network (CAPMoN) and the Comprehensive Air Quality Model with Extensions (CAMx). This suite of models represents the most commonly used methods for developing nitrogen deposition budgets across North America.Parameterizations are compared using a common set of micrometeorological data collected over a grass field (Duke Forest, NC), a mixed hardwood forest (Coweeta, NC), and a coniferous forest (Howland Forest, MA). Differences in parameterizations are analyzed with respect to atmospheric stability and the impact of differences in Ra on dry deposition calculations is illustrated by comparing cumulative seasonal and annual HNO3 deposition for the three case study sites.