Modeling the Upwind Pollutant Source Footprint Along Backward Trajectories Using the MM5/CALMET/CALPUFF Modeling System

Cities and their associated industrial areas are dynamic entities that consume and metabolize atmospheric trace gases in daily and seasonal cycles. The metabolites of urbanization directly affect public health and the environment on local, regional, and global scales. The Urban Metabolism and Trace Gas Respiration project funded by the National Aeronautical and Space Administration (NASA) is studying urban and industrialized areas as entities that consume, metabolize and respire a wide range of trace gases. As part of this project, an urban footprint model system is being developed to predict upwind pollutant source areas from downwind concentration measurements. The focus is on creating a modeling system that is both easy to use and applicable to regulatory agencies for determining possible locations of fugitive sources and accidental releases.

Plume diffusion modeling along a forward trajectory maps out the distribution of pollutant concentrations due to an upwind source. Application of plume diffusion theory along a back-trajectory yields the upwind source distribution affecting a receptor at the trajectory initial point. Thus, our approach is to use available modeling systems to derive a detailed wind field, and then apply the CALPUFF model in reverse along backward trajectories. The results yield the upwind source distribution or footprint for sources affecting a downwind receptor.

Three components comprise this urban source distribution modeling strategy: 1) Mesoscale modeling of the regional wind field, 2) Application of the CALMET meteorological model for a finer scale resolution, then inversion of the resulting wind field, 3) Application of the CALPUFF plume dispersion model to the inverted wind field. The MM5 mesoscale meteorological model developed by Pennsylvania State University and the National Center for Atmospheric Research (NCAR) is a three-dimensional non-hydrostatic model. MM5 provides an initial guess wind field to the CALMET diagnostic wind field model utilized primarily to interpolate the winds to a finer grid resolution for input into CALPUFF. Finally, the winds are inverted and the CALPUFF model is applied. CALPUFF is a multi-layer non-steady-state puff dispersion model simulating the effects of time and space varying meteorological conditions on pollutant transport, transformation, and removal. By applying CALPUFF to the inverted wind field, the resulting plume trajectory and puff dispersion indicates the upwind pollutant source area affecting a downwind receptor.

Results from a field campaign conducted May 21-29, 1999 in Boston, Massachusetts will be correlated with the model results. The field campaign consisted of real-time and integrated sampling of a suite of urban pollutants and SF6 tracer studies.