When applying the AERMET meteorological processor (EPA, 2004a) to process meteorological data for the AERMOD model (EPA, 2004b), the user must determine appropriate values for three surface characteristics: surface roughness length (zo), albedo (r), and Bowen ratio (Bo). The surface roughness length is related to the height of obstacles to the wind flow and is, in principle, the height at which the mean horizontal wind speed is zero based on a logarithmic profile. The surface roughness length influences the surface shear stress and is an important factor in determining the magnitude of mechanical turbulence and the stability of the boundary layer. The albedo is the fraction of total incident solar radiation reflected by the surface back to space without absorption. The daytime Bowen ratio, an indicator of surface moisture, is the ratio of sensible heat flux to latent heat flux and, together with albedo and other meteorological observations, is used for determining planetary boundary layer parameters for convective conditions driven by the surface sensible heat flux. Further details regarding the AERMOD model formulations and their dependence on surface characteristics are provided in Cimorelli, et al. (2004). The AERSURFACE tool has been developed to aid users in obtaining realistic and reproducible surface characteristic values, including albedo, Bowen ratio, and surface roughness length, for input to AERMET. The tool uses publicly available national land cover datasets and look-up tables of surface characteristics that vary by land cover type and season. This users guide provides a technical description of the AERSURFACE tool, including information on the data used by AERSURFACE to provide these surface characteristics for AERMET. Detailed user instructions for application of AERSURFACE are also provided.