A Case Study Analyzing Updates to the AERSURFACE Tool

摘要

When conducting an air dispersion modeling study using the United States Environmental Protection Agency's (U.S. EPA) AERMOD modeling system, representative meteorological data is typically processed using the AERMET meteorological processing program. A required input to AERMET are values of three surface characteristics: surface roughness length, noontime albedo, and daytime Bow en ratio. These parameters are used in the determination of planetary boundary layer (PBL) parameters for convective conditions driven by the surface sensible heat flux. The U.S. EPA developed the AERSURFACE tool back in 2008 to "aid users in obtaining realistic and reproducible surface characteristic values". The tool reads digital land cover data from the United States Geological Survey (USGS) and look-up tables of surface characteristic values which vary by land cover type and season. While AERSURFACE is not a required tool for regulatory use, it provides an objectively determined, reproducible set of values following required methodology for estimating surface characteristics. The most recent regulatory release of AERSURFACE, dated 13016, is limited to reading USGS National Land Cover Database (NLCD) data files from the original 1992 archives covering only the continental United States (CONUS). Since many studies utilize meteorological data from the most recent 5-year period preceding the modeling (e.g., 2014-2018), this can create a division between the representativeness of data records taken recently compared to surface characteristics estimates based on data 20-25 years old. Some jurisdictions address this discrepancy by recommending alternative methodology for surface characteristic definition. On February 21, 2019, the U.S. EPA released a draft version of the AERSURFACE tool, dated 19039DRFT. While not yet available for use in regulatory applications, the new tool includes the ability to process the most recent data from the NLCD including 2001, 2006, & 2011 land cover which can be supplemented with percent impervious and percent tree canopy data. Expansion of NLCD also allows use of AERSURFACE in regions beyond CONUS which were previously unsupported including Alaska, Hawaii, and Puerto Rico. This paper will review the updates to the AERSURFACE tool and perform a comparison of surface characteristic values generated using the new 19039_DRFT release against both the regulatory version (13016) and alternative methodology. Also included will be a brief analysis of AERMET-calculated PBL parameters and impacts on ground-level concentrations modeled using AERMOD.