EPA/600/A-97/083 6.7 THE PLUME-IN-GRID TREATMENT OF MAJOR ELEVATED POINT-SOURCE EMISSIONS IN MODELS-3 Noor V. Gillani1*, Arastoo Biazar1, Yu-Ling Wu' James Godowitch2, Jason Ching2, Robert Imhoff 1 University of Alabama in Huntsville, Huntsville, AL 2 Atmospheric Sciences Modeling Division, Air Resources Laboratory, National Oceanic & Atmospheric Administration, Research Triangle Park, NC 3 Tennessee Valley Authority, Muscle Shoals, AL 1. INTRODUCTION Modets-3/CMAQ (Byun et ai., 1998) is the new comprehensive, multiseale, multi-pollutant Euierian modeling system currently in development under the sponsorship of the U.S. EPA. in regional applications, as with its predecessor (RADM), its coarse grid resolution is likely to be between 20 and 80 km. Unlike RADM, however, CMAQ can achieve finer spatial resolution by the use of nested grids in urban-industrial areas with concentrated sources, and by plume-in-grid treatment (PinG) of major elevated point-source emissions (MEPSEs). Most MEPSEs are related to large fossil-fuel power plants. In 1993, the utility sector was estimated to contribute more than 72% and 33%, respectively, of the US emissions of SOx and NOx (EPA, 1994). Just the largest 100 and 200 of the thousands of "major point sources" in the US contribute about 35% and 50%, respectively, of the total NOx emissions of this source category. Thus, these largest MEPSEs constitute an important class, by themselves, of the sources of ozone and aerosol precursors. Since these MEPSEs do not also emit VOC (the other key precursor of the ozone-aerosol chemistry), the mixing of MEPSE NOx and background VOC is critical for plume chemistry which is, therefore, diffusion-limited. As this chemistry is also very non-linear, proper simulation of such mixing is of critical importance for accurate determination of the impact of MEPSE emissions. In RADM and its contemporary and predecessor regional grid models, such emissions were instantly mixed with the VOC of the entire emission grid cell, thereby completely bypassing the diffusion-limited subgrid- scale (SGS) evolution of the plumes. Current practice is shifting towards use of PinG to simulate the SGS transport and chemistry in grid models. PinG in Models- 3 is significantly different from that in other grid models. It is aimed particularly at realistic treatment of rural MEPSE plumes in a coarse regional grid. This paper provides an overview of PinG in Models-3. 2. OPERATIONAL PinG APPROACHES PinG has been implemented in the PARIS (Seigneur et al., 1983), UAM-V (Morris et al., 1992), SAQM (Myer et * Corresponding author address: NoorV. Giani, Earth System Science Lab., Univ. of Alabama, Huntsville, AL 35899. email: gillani@atmos.uah.edu al., 1996) and URM (Kumar and Russell, 1996) grid models. In all of these, multiple SGS Lagrangian plumes (or puffs) within a domain can be simulated, but they are assumed not to interact directly with each other. Each plume expands as it advects, entraining the back- ground, and the plume-background mixture then reacts. Spatial resolution is achieved through discretization of the plume cross-section: PARIS used a rectangular plume cross-section divided into vertically well-mixed pillars; the other models use an elliptical plume section divided into concentric rings. The grid size was 4 km in the single application of PARIS, and as high as 12 km in UAM-V and SAQM. In all three, the outermost pillars or elliptic rings are sequentially shed off to the grid model as the remaining plume attains the width of the grid cell, and the SGS simulation stops when the core of the plume attains grid size. In URM, the elliptic plume simulation is limited to a fixed one hour, when the whole plume is handed over. Strictly speaking, the elliptic plume model formulation is valid only as long as the plume is fully elevated. In the convective boundary layer (CBL), this condition typically lasts for less than one hour. The common observation in all of the above PinG applications has been that the PinG treatment does not make much difference except very locally. This is not surprising, since the above formulations/applications have been focussed on the detailed simulation of plume-core chemistry only in the early phase of plume evolution. 3. Models-3/PinG : CONCEPTUAL APPROACH Models-3 is scheduled to be released in 1998. It will include a preliminary developmental version of PinG which will be best suited for rural MEPSEs within a coarse regional grid (A > 12 km or so) where the SGS error in the representation of MEPSEs is largest without PinG treatment. Figure 1 provides a graphical perspec- tive of the number and distribution of such MEPSEs in eastern USA. The "rural" cells represent likely locations of rural MEPSEs with total NOx emission rate (QNO*) exceeding approximately 0.3 kg/s or 10,000 tons/y (approximately, the cut-off source strength for the top 200 MEPSEs in the US); "urban" ceils are likely locations of urban areas (with or without MEPSEs). The urban and rural cells together occupy only 3% of the land mass shown, but contribute more than 50% of the ------- total NOx emissions of the region (Gillani and Pleim, 1996). Some of the "rural" cells represent suburban MEPSEs, but many are truly rural. There is a concen- tration of MEPSEs in the Ohio River valley, but many of the other rural MEPSEs are quite isolated. PinG currently assumes linear superposition of SGS MEPSE plumes. Rural MEPSEs have the lowest likelihood of interactions, not only with other MEPSEs, but also with the thousands of non-MEPSE "major point sources". Figure 2 illustrates the typical chemical evolution of a large isolated rural power plant plume, consisting of three stages (Gillani and Wilson, 1980). In stage 1, the relatively fresh plume is dominated by primary emissions and a deficit of ozone due to its titration by NO. The chemistry is mostly inorganic and VOC-limited, In stage 3, at the other end, the plume is substantially diluted and has a high VOCrNQx ratio, and is chemically mature, with little NOx remaining and an excess of ozone; the chemistry here is mostly NOx-Iimited, much as in the background. Stage 2 marks the transition of the chemistry from VOC-limited (Stage 1) to NOx-Iimited (Stage 3). It is marked by growing ozone wings at plume edges, resulting from vigorous chemistry there fed by a good supply of both VOC (background entrainment) and NOx (diffusion from plume core). This is the diffusion- limited chemical stage with a complex SGS structure. Its proper simulation is the key to accurate characteri- zation of SGS plume chemistry. Indeed, such charac- terization requires that a chemical criterion of success- ful completion of stage 2 chemistry be satisfied in the PinG solution before plume handover to the grid solution (Gillani and Pleim, 1996), This chemical criterion is not observed in the PinG treatments of UAM-V, SAQM and URM, because they are necessarily limited to near-field and plume core chemistry (as in stage 1) as a result of Figure 1. Distribution of 20 km x 20 km "rural" (D) and "urban" ( •) cells with NO, emission flux, qNO, , greater than 101J molecules cm"J s"1, "Urban" cells are defined by qvoG/QNo.3" 1 (C:N, by volume), and "rural" cells by quoc/qNOl[< 1. (Figure from Gillani and Pleim, 1996). their elevated elliptic ring formulation. As shown in Figure 2 (stage 2), there is much fine structure in the plume even when it is about 20 km wide. The plumes of very large MEPSEs in the eastern USA (QNOx - 3 kg NOj/s or more) generally do not attain full chemical maturation until their width is about 30 km; smaller ones mature at smaller widths. In rural Tennessee, such maturation typically requires about 100 km and 6 h of plume transport in the summer daytime CBL for large SO, Stage I 100 ppb Stage2 ••.A /'-.Ozone Sulfate 20 ppb Bscat Stage 3 _ Bscat .*' Sulfate 10x10 1131 1138 1224 1231 1557 TIME 1606 Figure 2. Aircraft data of SO2 (surrogate for NOx), ozone, sulfate and Bscat (surrogate for fine aerosol mass) during crosswind traverses on 23 Aug 1978 through the plume of a large MEPSE (TVA's Cumberland power plant in northwest Tennessee, QNOx - 3 kg/s), illustrating the three stages of plume chemical development. The three sets of profiles shown represent quasi-Lagrangian information at three downwind sections of the - 0700 plume release. Plume (age.width) at the distances shown are approx. (4 h, 10 km), (5.5 h, 20 km) and (8 h, 32 km). The plume was in elevated stable layers until almost 1030, and had just completed fumigation at 80 km, where it was still more or less in stage 1. (Figure taken from Gillani et al., 1981). ------- MEPSEs, and less for smaller ones (Giilani et al, 1997), During much of this transport, the plume is fully mixed over the CBL and in contact with the ground. The elevated elliptical plume formulation of UAM-V, SAQM and URM is not the appropriate framework for such plume transport-chemistry simulation. The rectangular plume geometry of PARIS is much more appropriate. Unfortunately, the single application of PARIS (Seigneur et al., 1983) was within a 4 km grid and the detailed plume simulation was confined within the inner 4-km core, dumping the critical plume edges with much SGS structure prematurely to the coarse grid solution. Based on such observational evidence, our goal in PinG of Models-3 is to perform detailed plume physical and chemical simulations until plume chemical maturation. Figure 3 illustrates our approach in concept, in keeping with the available hourly meteorological information, our detailed SGS plume simulation in PinG is designed, in essence, to transform each steady hourly emission rate (™v) of primary species "p" from the target MEPSE source to a steady hourly delivered rate (™j , excess over background) of all modeled species *f, at the appropriate plume handover time (two) and location, to the grid model. Such transformation is achieved by means of a detailed Lagrangian simulation of plume dynamics and kinetics in which plume transport after touchdown (within the CBL) is expected to last typically about 3 to 7 hours in the summer in eastern USA (depending mainly on source strength, plume spread rate, background chemical composition, and jre_3. Conceptual model of a subgrid scale plume simulation in PinG of Models-3, sunlight). Such simulation is facilitated by a formulation based on a rectangular plume cross-section. A major limitation of this Lagrangian plume approach is that conditions of high plume shear (particularly speed shear) cannot be modeled accurately. Such conditions are most likely during the nocturnal transport of a vertically well-mixed daytime plume of 1-2 km depth. Most daytime releases, except those of the late afternoon, are likely to be chemically mature before the collapse of the CBL in the evening. Night-time releases of MEPSEs in the elevated stable layers typically result in vertically confined plumes which are spared much shear because of their small vertical extent. Our practice is to hand the plume over to the grid model whenever conditions prevail, e.g., high shear, which the plume model cannot simulate with acceptable accuracy. 4. Mode!s-3/PinG : THE BUILDING BLOCKS For each MEPSE plume treated in PinG, its SGS plume dynamics and chemistry must be simulated for each release. We have developed a new plume dynamics model (PDM). For chemistry, we have adapted the U. of AL Huntsville Lagrangian Reactive Plume Model; UAH-LRPM (Giilani, 1986), as the building block. The Lagrangian entity tracked is the expanding plume section representing emissions into a thin semi- infinite vertical slab as it passes the target MEPSE source. The new PDM simulates plume rise, plume transport and plume crosswind spread (horizontal and vertical). For releases into the nocturnal stable layer above the shallow mixing layer, it also simulates fumiga- tion into the rising mixing layer in the morning. PDM uses state-of-the-art parameterizations of plume rise, and of oy and o, representing horizontal and vertical plume spread, or is assumed to consist of two components representing turbulent spread (o,,) and shear spread (a^), as follows: af = oy/ + oyihz. oyjh is due to directional shear in the mean wind field, it is small in coherent nocturnal plumes, but within the CBL, its contribution can be significant after the plume has attained substantial vertical depth. The implementation of PDM, of course, requires appropriate meteorological inputs, including dynamic 3-D temperature and wind fields, as well as 2-D information about mixing height, u* and w* (friction and convective velocities). PDM outputs (typically every 10 minutes) include, most importantly, the plume cross-sectional geometry, i.e., the co-ordi- nates of the centroid and of the four plume edges, left/right and top/bottom, The chemistry model (UAH-LRPM; Giilani, 1986) simulates the complete mass balance of reactive species within such an expanding plume section. Figure 4 illustrates the processes simulated in the LRPM. The upper plot shows a time-height view of night-time and daytime plume releases. The lower plot shows the plume cross-section, discretized into vertically well- mixed pillars, and its expansion from t to t+At, where At ------- (a) ?H?H??&; roc nhoca H^I!H'.!*i!!(!l!Hiiii!i iiiiiiiiii'i vaas-pnase iirssissisii^s'iisrssrmrt Surface emissions & depositions (b) N ~£ •>. s~ ^s-~ -v^ Di ^ fusi ^ DP — > -jTi ^N Plume cross-section ~^~ at (t+At) Entrainment from sides D i L u T 1 0 N jre4. An illustration of the formulation of the UAH-Lagrangian Reactive Plume Model showing the processes simulated : (a) the time-height view ; (b) the cross-sectional view. denotes the transport (advection) time step. In the figure, the pillar widths are shown as equal, but the model can simulate any arbitrary non-uniform discretization also (e.g., "Gaussian" framework, as in PARIS). At each plume expansion, horizontally and vertically, the plume mass not only dilutes, but there is also entrainment from the sides (the background) and from aloft, as well as entrainment and detrainment between adjacent pillars. Also, there is (turbulent and shear) diffusion of mass across pillar interfaces. In addition, the model permits uniform surface emissions and dry deposition to/from each pillar. Finally, LRPM performs detailed gas-phase chemistry in each pillar, with the same options of chemical mechanisms as available in the Chemistry-Transport Model (CTM) of Models-3. LRPM also uses the same chemistry solvers as does CTM. It derives background and aloft information from the appropriate grid cell solution. It receives dynamic plume geometry from PDM simulations. 5. Models-3/PinG : IMPLEMENTATION Figure 5 shows the relationship of PDM and PinG to Models-3/CMAQ. The main Chemistry-Transport Model (CTM) of CMAQ has options to use three levels of nested gridding, any one of several state-of-the-art gas- phase chemical mechanisms, aqueous chemistry and an aerosol module. It has two main pre-processors: MEPPS for emissions, and MM5 for meteorology. MEPPS also performs the selection of MEPSEs from the "major point sources", based on the following two criteria: 1. QNO, > Qc, where Qc is a critical cut-off value (default Qc = 0.3 kg/s = 10 kT/y as NO2); 2. h(stack) > hc, where hc is a critical cut-off value of stack height (default hc = 100 m). Interface processors ECIP and MCIP process the outputs of MEPPS and MM5, respectively, to generate appropriate inputs for CTM, PDM and PinG. PDM runs as a pre-processor of PinG, as shown. It performs the complete plume dynamics for each hourly release of each MEPSE selected for PinG treatment, generating Emissions Chemistry- Transport Plume Dynamics Meteorology Figure 5. Placement of PDM and PinG in Models- 3/CMAQ ------- output files containing dynamic plume geometry as well as other related information. The actual plume transport- chemistry simulation (as in LRPM) occurs in PinG, which is embedded in CTM and operates interactively with it at the level of each CTM advection time step (= 10 min). For a typical 5-day run of CTM including 100 MEPSEs, PDM and PinG must process a total of 12000 hourly plume releases. At each interaction with CTM, PinG initiates new plumes (if any) and expands every "live" plume (typically 500 - 1500 for a 100 MEPSE run, depending on time of day) according to the PDM- supplied plume dynamics. Consequently, each plume entrains mass from the grid solution, and also receives surface emissions (after plume touch-down). Also, gas- phase chemistry (same as in CTM except for night-time plume releases, for which a much simpler chemistry is implemented), dry deposition and other process operations are implemented. PinG then checks each live plume for the handover decision, and if necessary, performs plume handover to the appropriate grid cells, updates the grid solution and returns control to CTM. The following handover criteria are used. 1. Plume reaches the boundary of the grid domain; 2. Plume is exposed to precipitation > critical level; 3. Plume enters an "urban" cell, as defined previously, after it has touched down. Night-time releases of MEPSEs in such cells may continue to remain alive as elevated plumes transporting above the city. 4. Major point source emissions of NOx in the cells on either side of the plume exceed a cutoff level. This criterion is forced because of the uni-layer plume limitation of the current LRPM. 5. Plume chemistry attains maturity based on the criterion that plume average value of the chemical age parameter, NOz/NOy, exceeds a prescribed fraction (= 0.9) of the corresponding average value in plume environment. 6. Plume width exceeds grid size. In addition, PinG also performs partial plume handover. If along a plume trajectory in the CBL, the mixing height decreases by more than a prescribed fraction, then the portion of the plume above the diminished mixing layer is handed over, since LRPM currently has only one vertical plume layer. According to this criterion, a well- mixed daytime plume will be handed over piecemeal as the mixing height collapses at the end of the solar day. In this manner, the possibility of high shear of such a plume during nocturnal transport is also avoided. 6. CONCLUSION Before the release of Models-3/CMAQ in 1998, the initial operational versions (lOVs) of PDM and PinG will be tested with multi-plume data of the SOS/Nashville Ozone Field Study of 1995. Major areas of anticipated future improvements of PinG include: 1. Higher vertical resolution of the plume to at least match that of MM5 and CTM, both of which include about seven layers from ground up to 800 mb (~ 2 km). Such an improvement will greatly improve the vertical distribution of surface emissions as well as pave the way for better treatment of non-MEPSE elevated major point-source emissions; 2. Inclusion of aqueous-phase chemistry (same as in CTM); 3. Inclusion of an aerosol module (same as in CTM); 4. Improved treatment of SGS plume-plume interactions. Acknowledgements/Disclaimer: This work is supported jointly by TVA (under Contract TV-88817V to UAH) and by EPA (under Contract DW64937190 to TVA). This paper has been reviewed in accordance with EPA's peer and administrative review policies and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. 7. REFERENCES Byun, D., et al., 1998: Description of Models-3 Community Multiscale Air Quality (CMAQ) modeling system. 10th Joint AMS-AWMA Conf. on Air Poll. Met., Phoenix AZ, 11-16 Jan. 1998. EPA, 1994: National Air Quality and Emissions Trends Report for 1993, EPA 454/R-94-026, OAQPS, U.S. EPA, Res. Triangle Park, NC . Gillani, N.V. and W.E. Wilson, 1980: Formation and transport of ozone and aerosols in power plant plumes. Annals N. Y. Acad. Sci. 338, 276-296. Gillani, N.V., S. Kohli and W.E. Wilson, 1981: Gas-to- particle conversion of sulfur in power plant plumes: I. Parameterization of the gas-phase conversion rate in moderately polluted ambient conditions. Atmos. Environ. 15, 2293-2313. Gillani, N.V., 1986: Ozone formation in pollutant plumes: Development and application of a reactive plume model with arbitrary crosswind resolution. EPA- 600/S3-86-051, U.S. EPA, Res. Triangle Park, NC. Gillani, N.V. and J.E. Pleim, 1996: Subgrid-scale features of anthropogenic emissions of VOC and NOx in the context of regional Eulerian models. Atmos. Environ. 30(12), 2043-2059. Gillani, N.V., et al., 1997: Relative production of ozone and nitrates in urban and rural power plant plumes: I. Composite results of ten days of field measure- ments. Submitted, J. Geophys. Res. Kumar, N. and A.G. Russell, 1996: Development of a computationally efficient reactive subgrid-scale plume model. J. Geophys. Res. 101, 16737-16744. Morris, R.E., et al., 1992: Overview of the variable-gid UAM-V. 85th Annual Meeting, AWMA, Kansas City. Myer, T., et al., 1996: The implementation of a plume-in- grid module in SAQM. Report SYSAPP-96/06, Systems Applications Int., San Rafael, CA. Seigneur, C., et al., 1983: On the treatment of point- source emissions in urban air quality modeling. Atmos. Environ. 17(9), 1655-1676. ------- TECHNICAL REPORT DATA 1. REPORT NO. PA/600/A-97/083 2. 4. TITLE AND SUBTITLE The Plume-in-Grid Treatment of Major Elevated Point- Source Emissions in Models-3 S.REPORT DATE 6.PERFORMING ORGANIZATION CODE 7. AUTHOR(S) Noor V. Gillani1, Arastoo Biazar1, Yu-Ling Wu1, James Godowitch2, Jason Ching2, and Robert Imhoff3 8.PERFORMING ORGANIZATION REPORT NO. 9. PERFORMING ORGANIZATION NAME AND ADDRESS *Univ. Of Alabatna-Huntsville, Huntsville, KL ^ame as Block 12 3Tennessee Valley Authority, Muscle Shoals, AL 10.PROGRAM ELEMENT NO. 11. CONTRACT/GRANT NO. 12. SPONSORING AGENCY NAME AND ADDRESS U.S. Environmental Protection Agency Office of Research and Development National Exposure Research Laboratory Research Triangle Park, NC 27711 13.TYPE OF REPORT AND PERIOD COVERED Preprint, FY-98 14. SPONSORING AGENCY CODE EPA/600/9 15. SUPPLEMENTARY NOTES 16. ABSTRACT The plume-in-grid modeling approach for treating the subgrid scale dispersion, transport, and chemistry of pollutants contained in plumes released from major point source stacks is described. A plume dynamics model provides the plume rise height, plume position, and the horizontal and vertical plume dispersions in the subgrid scale phase when the pollutants are to be simulated by the Lagrangian reactive plume module. The Lagrangian plume algorithm is embedded in the Eulerian chemistry- transport grid model and operates simultaneously. A coupling exists between the models as the grid concentrations provide boundary conditions for entrainment to the subgrid plume sections, and a feedback ocurrs when the plume size and chemical criteria indicate a plume's pollutants are to be transferred to the grid system for further simulation. The methods for simulating the plume processes are also presented and the input/output data involved with these two components of the Models3 plume-in-grid approach are discussed. 17. KEY WORDS AND DOCUMENT ANALYSIS DESCRIPTORS b,IDENTIFIERS/ OPEN ENDED TERMS C.COSATI 18. DISTRIBUTION STATEMENT RELEASE TO PUBLIC 19. SECURITY CLASS (This Report) UNCLASSIFIED 21.NO. OF PAGES 20. SECURITY CLASS (This Page; UNCLASSIFIED 22. PRICE ------- |