United States Environmental Protection Agency Atmospheric Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA/600/S3-86/024 May 1986 4>EPA Project Summary Preliminary Evaluation Studies With the Regional Acid Deposition Model (RADM) This summary focuses on the find- ings of preliminary evaluation studies of the National Center for Atmospheric Research (NCAR) Regional Acid Deposi- tion Model (RADM). Current efforts in evaluating the gas-phase chemistry submodel have met with significant success, and an understanding of the smog chamber databases has been de- veloped. Due to lack of adequate obser- vation data, evaluation of aqueous- phase chemistry and dry deposition submodels has not been possible. The cloud process submodel is consistent with the available but small data set. Preliminary evaluation of the full RADM system using OSCAR (Oxidation and Scavenging Characteristics of April Rains) meteorology and wet chemical deposition data has achieved initial success, though it is far from conclu- sive. Much larger and extensive data- bases are required to test the system thoroughly. The meteorology driver for the RADM has shown considerable skill in forecasting the OSCAR IV meteorol- ogy. Simulations of sulfate and nitrate wet deposition for the first day of OSCAR IV are quite good. RADM- simulated three-dimensional chemical species distributions are consistent with preliminary data measured by NCAR under a separate program. The first test of RADM's capability for ana- lyzing "what if studies with hypotheti- cal reductions in sulfur emissions is demonstrated. The findings confirm the complexity of directly observing poten- tial benefit from emissions reductions. This Project Summary was devel- oped by EPA's Atmospheric Sciences Research Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering infor- mation at back). Introduction A comprehensive Regional Acid Dep- osition Model (RADM) is being devel- oped at the National Center for Atmos- pheric Research. The RADM is an evolving advanced Eulerian computer model simulating the processes and pathways related to acid deposition in eastern North America. The RADM sys- tem consists of a mesoscale meteoro- logical model which drives a transport/ deposition model containing modular descriptions of the gas-phase chem- istry, cloud processes and aqueous- phase chemistry, and dry deposition. The objectives for the RADM are threefold: (1) to result in a state-of-the- art modeling system suitable for con- ducting source-receptor assessment studies, (2) to be sufficiently flexible to integrate current and developing repre- sentations of the relevant physical and chemical processes, and (3) to describe the spatial and temporal distributions of pollutants resulting from known source emissions. Progress toward these ob- jectives has been very favorable. A first version of the RADM was completed in January 1985 and since that time has undergone numerous sensitivity stud- ies and preliminary evaluation against existing field and laboratory data. A summary of the preliminary evaluation of the RADM and of sensitivity and ap- plication studies is presented here. The evaluation of the RADM submodels are discussed first. Next, 72-h RADM simu- lations of two OSCAR (Oxidation and Scavenging Characteristics of April Rains field study, conducted in the ------- northeastern United States in April 1981) cases are compared to measured precipitation chemistry data. For one of these cases, a preliminary evaluation of the meterological model is presented. Emissions sensitivity studies and emis- sions reduction experiments are also presented for one of the OSCAR cases. Finally, databases available for evalua- tion of the RADM and their limitations are discussed. RADM Submodel Evaluation The RADM version I gas-phase chem- istry mechanism has been compared with both smog chamber data and more complex chemical mechanisms. In gen- eral, these tests show that the RADM mechanism gives a good representa- tion of those aspects of tropospheric chemistry necessary for the modeling of acid deposition. Initial tests were made with data obtained from the Satewide Air Pollution Research Center (SAPRC), and agreement between RADM simulations and experimental values is generally very good for NO2, NO, and 03. The agreement between the predicted and experimental values for all the reactive species is excellent. Since reactions with HO radicals is the dominant loss process for reactive or- ganics during photooxidation, the agreement between the simulation and measured concentrations of reactive or- ganic species shows that the RADM mechanism is correctly predicting HO concentrations within the uncertainty imposed by wall sources. Comparison of the RADM gas-phase chemistry predictions with the Univer- sity of North Carolina smog chamber data is only in the beginning stages. Procedures used to establish the error bounds of these data, along with com- parisons with RADM predictions for a single case, are reported. In general, RADM predictions for O3, NO, and PAM are in good agreement with the chamber data, considering the uncer- tainties of the J-values. The simulation also suggests that for definitive tests of the chemical mechanisms for the pro- duction of H202 in the atmosphere, the photolytic rate constant must be accu- rately known. The RADM gas-phase chemical mech- anism was compared to the explicit mechanism of Leone and Seinfield and the carbon bond mechanism, version CBM-X, for conditions ranging from very clean to highly polluted. Both of these latter mechanisms were modified to include sulfur chemistry. Oxidation rates for S02 and NOX are in reasonable agreement by the three mechanisms, while large differences exist for the total H202 produced. A sensitivity analysis was performed to determine reasons for the differences in H2O2. The RADM cloud processes and aqueous-phase chemistry module was subjectively evaluated against limited field data and more complex models. Comparisons with OSCAR aircraft ob- servations of cloudwater composition suggest that the RADM submodel is ca- pable of inferring cloudwater pH over the range of initial conditions encoun- tered during the observation period. RADM predictions of aqueous oxidation within clouds are shown to agree rea- sonably with the difference in the ratio of S02 to total sulfur at the base and top of typical cumulus clouds. Scavenging ratios predicted by the RADM are shown to lie between the extremes ob- served in field observations. In addition to predicting cloud and rainwater com- position, the RADM cloud processes model also predicts the rate at which pollutants are vertically transported by sub-grid scale cumulus clouds. RADM parameterization for this process was compared to a high resolution cloud model, and both predicted similar sig- nificant depletion of a passive tracer from the boundary layer and "venting" of this material to the cloud layer. The method used by the RADM to model dry deposition assumes a series of three resistances (aerodynamic, sub- layer, and surface) and is based on a highly empirical parameterization, rely- ing heavily on a relatively sparse data- base of dry deposition measurements. Therefore, it was not possible to evalu- ate independently the performance of the dry deposition model since all avail- able measurements of dry deposition have been used to tune the current model parameterizations. Data needs for comprehensive dry deposition model evaluation include heat and mo- mentum fluxes in the surface layer, wind speed, isolation, land type (onto which substances are deposited), sur- face roughness, and a quantification of surface moisture. Preliminary Evaluation of RADM with OSCAR Database The overall performance of the mete- orological model and RADM in predict- ing the meteorological processes and wet concentration/deposition during the OSCAR IV period (April 22-24,1981) was examined. A number of quantita- tive measures of the accuracy of the mesoscale meteorological model are described. These measures will be usec to compare the accuracy of different versions of the mesoscale model and tc provide quantitative estimates of the error in the meteorological data sup plied to the RADM. Interpretation of the verification scores and methods from the OSCAR IV case are provided. Correlations between RADM model results and OSCAR measurements oi precipitation chemistry variables are presented for two OSCAR cases for both hourly and event totals of concentra- tions and deposition. Parameters com pared were sulfate, nitrate, ammonium and hydrogen ions in rainwater. In al cases, the correlations were much higher for the total event comparisor and for concentrations. Correlations were also higher for the OSCAR IV case than OSCAR I. RADM Sensitivity Studies and Applications Ultimately, RADM will be used to evaluate source-receptor relationships and the effects of changes in emissions on downwind receptors. This requires an understanding of the basis of the pat- terns of concentration and deposition forecast by the RADM along with sensi- tivity of the forecasts to various input parameters and physical parameteriza- tions. Toward this end, wet and dry dep- osition patterns for the OSCAR I and IV cases were analyzed. Results are alsc presented for RADM simulations of twc "what if" scenarios of 50% and 90°/< source reduction in the Ohio Valley re gion. The patterns and magnitudes of dn deposition of S02 over the northeasterr United States and southeastern Canada integrated over the three-day episode: were similar for both the OSCAR I anc IV cases. In Canada, peaks showed uf near Sudbury and Noranda. In the United States, the peaks showed UF over south-central Pennsylvania; at th( conjunction of Ohio, Pennsylvania, anc West Virginia; and in New Jersey, f maximum dry deposition of 1.14 kg/hi after three days occurred over south central Illinois during OSCAR IV. Fo HNO3, the accumulated dry depositor for the two three-day episodes wen again quite similar, and mass deposi tion was considerably less than S02. / maximum deposition of 0.24 kg/ha oc curred from northern New Jersey t< ------- southern New Hampshire. The accumu- lated wet deposition differed more be- tween the two experiments than did dry deposition. This reflects primarily the difference in rainfall patterns for the two episodes. The percentage of total sulfur deposition that was wet was about 50% and 60% fo the OSCAR I and IV cases, respectively. The percentage of nitro- gen deposition that was wet was 45% and 60% for OSCAR I and IV, respec- tively. Several sensitivity tests were run in which the SOX emissions were reduced by 50% and 90% in the Ohio Valley (states of Indiana, Ohio, Kentucky, Ten- nessee, and West Virginia). In response to this regional reduction in emissions, the maximum regional reduction for dry and wet deposition occurred in the re- duced emissions area, but only weakly in more distant receptor states. The de- crease in dry deposition was greater than the decrease in wet deposition only in the four states with decreased emissions. The decrease in sulfur emis- sions had very little effect on nitrogen chemistry or nitrogen deposition. Databases for RADM Evalua- tion Studies The evaluation of RADM performance requires the assessment of the data- bases used for model execution and evaluation. Included in the report are a description and evaluation of the avail- able air quality databases for evaluating regional models, a description of qual- ity control procedures employed in preparing the emissions databases (EPRI and NAPAP) for model applica- tions, and the precipitation database used in verification of the precipitation predictions. Also discussed are the limi- ations of smog chamber data in evaluat- ing RADM chemistry. The National Center for Atmospheric Research is located in Boulder, CO 80307. John F. Clarke is the EPA Project Officer (see below). The complete report, entitled "Preliminary Evaluation Studies with the Regional A cid Deposition Model (RA DM)," (Order No. PB 86-175 692/A S; Cost: $22.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Atmospheric Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 ------- United States Center for Environmental Research ,, s* ""--•,, .,..,yj jj i Environmental Protection Information .•-'•... -.«. Agency Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S3-86/024 0000329 PS U S ENVIR PROTECTION AGENCY REGION S LIBRARY 230 S DEARBORN STREET CHICAGO TT IL 60604 ------- |