United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA-600/S3-84-057 May 1984 &EPA Project Summary Development of the MESOPUFF II Dispersion Model J. S. Scire, F. W. Lurmann, A. Bass, and S. R. Hanna The development of the MESOPUFF II regional-scale air quality simulation model is described. MESOPUFF II is a Lagrangian variable-trajectory puff- superposition model that has been designed to treat transport, transformation, diffusion, and removal processes of pollutants emitted from multiple point and/or area sources at transport distances beyond the range of conventional straight-line Gaussian model (i.e., beyond ~ 1O-50 km). The major features of this model and enhancements over its predecessor, MESOscale PUFF (MESOPUFF), include the use of hourly surface meteorological data, twice-daily rawinsonde data, and hourly precipitation data; separate wind fields to represent flows within and above the boundary layer; parameterization of vertical dispersion in terms of micro- meteorological turbulence variables; transformation of sulfur dioxide (SO2) to sulfate (SO<) and nitrogen oxides (NOx) to nitrate (NO3~): a resistance model for dry deposition; time- and space-varying wet removal; and a computationally efficient puff- sampling function. The scientific and operational bases of the methods used in the model are discussed. The resultsfrom several model algorithms also are compared against experimental data. This Project Summary was developed by EPA's Environmental Sciences Research Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully documented in a separate report of the same title (see Project Report ordering information at back). Introduction The regional and long-range transport and transormation of sulfur oxides and nitrogen oxides emitted from major point sources have been of considerable concern.There is a need for easily usable, cost-efficient air-quality models that can realistically treat the various physical processes important on these scales The MESOscale PUFF (MESOPUFF) model has been extensively modified to revise and more realistically treat the transport, vertical dispersion, chemical transformation, and dry and wet removal processes. The new model, designated MESOPUFF II, is one element of an integrated modeling package that includes components for preprocessing of meteorological data (READ56, MESO- PUFF II) and for postprocessing of pre- dicted concentrations (MESOFILE II). Major Model Features MESOPUFF II uses a puff-superposi- tion approach to represent continuous plumes. The pollutant material in each puff is transported independently of that in other puffs and is also subjected to dispersion, chemical transformation, and removal processes. Some of the general features of the MESOPUFF II modeling system are as follows: (1) Hourly surface meteorological data, twice-daily rawinsonde data, and hourly precipitation data are read from magnetic tapes. (2) Wind fields to represent the mean flow in the boundary layer and above the boundary layer are constructed, although several options are given. ------- (3) Boundary-layer structure is treated in terms of micrometeorological parameters that includethesurface friction velocity, mixing height, convective velocity scale, and Monin-Obhukov length. (4) Space- and time-varying chemical transformation can be performed simultaneously for up to five pollu- tant species, including sulfur diox- ide (SO;,), sulfate (S0,:), nitrogen oxides (NOx=NCH NO?), nitric acid (HNO3), and nitrate (N03 ). (5) Dry deposition is prescribed by a resistance model in the surface- depletion mode, and the source- depletion method is optional. (6) Space- and time-varying wet removal is parameterized according to precipitation rate and scavenging coefficients. The meteorological data inputs required by the preprocessors consist of the routine twice-daily upper air sound- ings, hourly surface meteorological observations, and hourly precipitation measurements reported by the National Weather Service. The preprocessor pro- grams have been designed to read the standard-formatted meteorological data tapes available from the National Climatic Center in Asheville, North Carolina. Wind fields for MESOPUFF II are con- structed from the hourly surface wind observations, as well as from the twice- daily rawinsonde wind profile data. The surface station network data allow better temporal and spatial resolution than do the upper air sounding data, which involve much larger distances and less frequent measurements. The wind fields are constructed at two user-specified levels--a lower level representing the mean boundary-layer flow and an upper level representing flow above the boundary layer. Boundary-layer structure is parameter- ized in terms of micrometeorological variables computed from the surface station data and information about surface characteristics (land use, or roughness lengths) provided by the user for each grid point. The surface friction velocity, u*. the convective velocity scale, w*, the Monin-Obukhov length, L, and the boundary-layer height, Zi, are computed. Chemical transformation rate express- ions were developed from the results of photochemical model simulations over a wide range of environmental conditions. The rate expressions include gas-phase N0« oxidation, gas- and aqueous-phase components of SOj oxidation, and the chemical equilibrium of the nitric acid, ammonia, and ammonia nitrate system. The parameterized transformation rates depend on solar radiation, background ozone concentration, and atmospheric stability. The SOj oxidation rate is empir- ically increased at high relative humidity to account for aqueous-phase reactions. In the case of NOx, the transformation rate also depends on the NOx concentra- tion. The spatial and temporal variations of dry deposition are treated by a resistance model. The pollutant flux is proportional to the inverse of a sum of resistances to pollutant transfer through the atmosphere to the surface. The resistances depend on the characteristics of the pollutant and the underlying surface and atmospheric conditions. MESOPUFF II contains options for the commonly used source-depletion method or for more realistic surface depletion, where pollutants are removed only from a surface layer in the three-layer mode. Precipitation scavenging can be the dominant pollutant removal mechanism during precipitation periods. MESOPUFF II contains a scavenging ratio formulation for wet removal. The scavenging ratio depends on the type and rate of precipita- tion (derived from hourly precipitation measurements, if available) and the char- acteristics of the particular pollutant. In addition, improvements were made in the method of summing the con- tributions of individual puffs to the total concentration at a receptor location. The model uses an integrated form of the puff-sampling function that eliminates the problem of insufficient puff overlap commonly encountered with puff-super- position models. This development allows continuous plumes to be more accurately simulated with fewer puffs, thereby saving computational time and reducing computer storage requirements. MESOPUFF II Modeling System The MESOPUFF II modeling package is schematically illustrated in Figure 1. The two meteorological preprocessor rou- tines are READ56 and MESOPAC II. READ56 processes the rawinsonde data, and MESOPAC II reads the output file created by READ56 and the standard- formatted hourly surface meteorological data and precipitation data. A single output file is produced that includes all the time- and space-interpolated fields of meteorological variables required by MESOPUFF II. All source, receptor, and program- control information is input by formatted- card images. The control parameter inputs determine which options are used in the computations and what type of output is produced. The model was run for a two-day period, to evaluate the SO; to SOj" transformation mechanism and to qualitatively demonstrate the behavior of some other model algorithms. The modeled period was taken from the August 1 978 Tennessee Plume Study, which was conducted near the Cumberland power plant in northwestern Tennessee. The two-day period (August 22-23) included chemical measurements by aircraft traverses through the plume at distances of 18 km to 160 km and represented 2 to 10 hours of travel. The predicted transformation rates were generally close to the observed rates derived from the pollutant measurements, especially for the drier, sunny period on August 22. During this period, SO3 oxidation was probably dominated by gas-phase reactions, whereas on the 23rd, when plume-cloud interactions were observed, the transformation rates were underpredicted (values were two-thirds the observed rates). The larger observed values are attributed to greater contributions by aqueous-phase reactions. Conclusions The scientific and operational ap- proaches to modification of the model are completely described in the report. The results of the model runs, including a comparison of observed and predicted transformation rates, the qualitative behavior of plume growth, plume fumigation, and dry deposition (surface depletion) are also presented. A companion report entitled "User's Guide to the MESOPUFF II Model and related Processor Programs" provides a brief technical description of the methods and a complete set of user instructions. ------- I READ56 Control I Parameter Inputs 'Twice-Daily\ Ra winsonde Data Files (TDF5600 F or mat I * READ56 Upper Air Preprocessor Program fME SOP AC II I Control Parameter I Inputs Hourly Meteorological Data Files (CD 144 Format! Formatted Twice Daily Rawinsonde V Data Files No f MESOPUFFII I Control Parameter I Inputs (Optional) MESOPAC II Meteorological Preprocessor Program Hourly Ozone Measurements Hourly Meteorological Variables MESOPUFFII Dispersion Model f MESOFILEII I Control Parameter I Inputs Concentration Tables Predicted Concentrations MESOFILEII Postprocessor Program Direct Access Concentration Files lor Plotting Concentration Tables Figure 1. MESOPUFF II modeling package. ------- J. S. Scire, F. W. Lurmann. A. Bass, and S. R. Hanna are with Environmental Research and Technology, Inc., Concord. MA O1742. James M. Godo witch is the EPA Project Officer (see below). The complete report, entitled "Development of the MESOPUFF II Dispersion Model," (Order No. PB 84-183 753; Cost: $11.50. subject to change) will be available only from: National Technical Information Service 5285 Port Hoy a I Road Springfield, VA 22161 Telephone: 7O3-487-4650 The EPA Project Officer can be contacted at: Environmental Sciences Research Laboratory U. S. Environmental Protection Agency Research Triangle Park. NC 27711 U.S. GOVERNMENT PRINTING OFFICE: 1984 — 759-O15/7715 United States Environmental Protection Agency Center (or Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 ------- |