Chapter 9 PLUME-IN-GRID TREATMENT OF MAJOR POINT SOURCE EMISSIONS

Transcription

1 Chapter 9 PLUME-IN-GRID TREATMENT OF MAJOR POINT SOURCE EMISSIONS N. V. Gllan Earth System Scence Laboratory Unversty of Alabama n Huntsvlle Huntsvlle, Alabama James M. Godowtch * Atmospherc Modelng Dvson Natonal Exposure Research Laboratory U.S. Envronmental Protecton Agency Research Trangle Park, North Carolna ABSTRACT A plume-n-grd (PnG) technque has been developed to more realstcally treat the dynamc and chemcal processes mpactng selected, major pont source pollutant plumes n the Communty Multscale Ar Qualty (CMAQ) modelng system. The prncpal scence algorthms nclude a Plume Dynamcs Model (PDM) and a Lagrangan reactve plume code. The PDM processor smulates plume rse, horzontal and vertcal plume growth, and transport of each plume secton durng the subgrd scale phase. It generates a data fle of ths nformaton for use by the PnG module. In contrast to the tradtonal Euleran grd modelng method of nstantly mxng the emssons from each pont source nto an entre grd cell volume, the PnG module smulates the relevant physcal and chemcal processes durng a subgrd scale phase whch allows each plume secton to expand n a realstc manner and to evolve chemcally. The PnG module s fully ntegrated nto the CMAQ Chemcal Transport Model (CCTM) n order to utlze the grd concentratons as boundary condtons and t provdes a feedback of the plume pollutants to the grd model concentraton feld at the proper tme and grd locaton. The techncal approaches and the model formulaton of the relevant processes treated by ths plume-n-grd approach are descrbed. The capabltes and lmtatons of the ntal verson of PnG are also dscussed and selected results from a test applcaton for a sngle pont source are brefly descrbed. * On assgnment from the Natonal Oceanc and Atmospherc Admnstraton, U.S. Department of Commerce. Correspondng author address: James M. Godowtch, MD-80, Research Trangle Park, NC E-mal: jug@hpcc.epa.gov

2 9.0 PLUME-IN-GRID TREATMENT OF MAJOR POINT SOURCE EMISSIONS 9.1 Introducton Sgnfcant emssons of anthropogenc ntrogen oxdes (NO x ) and sulfur oxdes (SO x ) are released from ndvdual elevated pont sources nto the atmosphere at varous levels. These major pont source emssons are dstrbuted throughout the U.S. The physcal dmensons of ther plumes are relatvely small ntally and expand at fnte growth rates. Ths dffuson-lmted nature of plumes s n sharp contrast to the tradtonal method appled n Euleran photochemcal grd modelng, whch has been to unformly and nstantly mx pont source emssons nto the entre volume of a model grd cell. Dependng on the meteorologcal condtons, however, pollutant plumes may requre up to several hours to grow to the typcal sze of a regonal model grd cell. Snce the horzontal grd resoluton of regonal scale domans for Euleran models has generally been km or greater, a real-world pollutant plume may reman a subgrd scale feature at current regonal model grd resolutons for a substantal tme perod and consderable dstance after release. Thus, the wdely-used grd modelng approach nadequately treats subgrd scale plume transport and dffuson. There are also mportant mplcatons on the chemcal processes n models due to the nablty of large grd cells to adequately resolve a major pont source plume. As a real-world plume gradually grows, t concurrently evolves chemcally as t entrans surroundng ambent ar often rcher n volatle organc compounds (VOCs). However, these dynamc and chemcal processes of plumes are neglected when large pont source emssons are nstantly dluted nto large grd cells wth other anthropogenc area emssons, whch may actually be separated spatally from a plume. The effect s that the smultaneous avalablty of NO x and VOC's n a large grd cell prematurely ntates rapd photochemstry leadng to dstorton n the spatal and temporal features of secondary speces concentratons. Ths overdluton of pont source emssons and the subsequent dstorton of chemcal processes contrbute to model uncertanty. Consequently, t has been recognzed that a realstc, subgrd scale modelng approach s needed whch smulates the relevant physcal and chemcal processes mpactng ths notable class of large pont source emssons, and n partcular, that has the capablty of properly resolvng the spatal scale of plumes and ther growth. There are approaches whch have been appled n attempts to resolve fne scale emssons n regonal grd models. These have ncluded unform, nested grd modelng (Odman and Russell, 1991; Chang et al., 1993), non-unform grd modelng (Mathur et al., 1992), and the plume-ngrd modelng (PnG) technque. The nested grd (telescopng) modelng approach employs the same Euleran model and smulatons are performed wth progressvely fner-mesh grd szes wthn a doman to better resolve small scale emsson features. Whle an advantage of ths approach s the use of a sngle model algorthm, a dsadvantage s the successvely hgher computatonal burden as the grd cell sze s reduced n order to resolve major emsson sources. Efforts to mplement and apply the plume-n-grd technque, n partcular, have been performed by Segneur et al. (1983) n a verson of the Urban Arshed Model (UAM-PARIS), by Morrs et al. (1992) n the UAM-V model, by Kumar and Russell (1996) n the Urban-Regonal Model 9-1

3 (URM), and by Myer et al. (1996) n the SAQM (SARMAP Ar Qualty Model) model. Brefly, each of these PnG efforts smulate multple plumes or puffs n a doman wth no nteracton between ndvdual plumes. Segneur et al. (1983) employed a rectangular plume cross-secton dvded nto vertcally well-mxed plume cells exhbtng varyng wdths to mantan equal pollutant mass among the cells. It was appled to an urban doman wth a 4 km grd cell sze. The other efforts employ an ellptcal plume secton dvded nto concentrc rngs, except for the Kumar and Russell (1996) PnG. Plume materal n an outermost ellptc rng or shell s sequentally transferred to the grd model as t attans the grd sze. In the treatment by Kumar and Russell, the plume was returned to the grd model after one hour. The general result of applcatons from these PnG mplementatons was that notceable dfferences were found prmarly wthn or near the grd cells contanng the major pont sources and neglgble mpact was found far downwnd. Ths fndng s to be expected snce these formulatons seemed to focus on the plume-core chemstry only n an early phase of plume chemcal evoluton wth the feedback of plume pollutants occurrng rather quckly to the grd model. A cooperatve research and development effort was undertaken to desgn and mplement a plume-n-grd (PnG) capablty n the Models-3 CMAQ Chemcal Transport Model (CCTM) n order to address the need for an mproved approach to treat major pont source emssons. The current PnG approach has been desgned to be sutable for the largest, solated pont source emssons wth grd resolutons of regonal modelng domans, where the subgrd scale error n the representaton of these sources s expected to be greatest wthout a PnG treatment. An mportant consderaton s to be able to spatally resolve a pollutant plume and to adequately smulate physcal expanson so that the chemcal evoluton can occur gradually. Therefore, the objectves wth ths PnG technque are to provde an mproved characterzaton of the near-source pollutant feld from the subgrd scale plume concentratons and to also generate better regonal pollutant concentratons downwnd due to the feedback of the PnG concentratons to the grdded doman. Consequently, when the PnG approach s appled to the largest pont source emssons n a regonal, coarse-grd doman, mproved ntal and boundary condtons are antcpated for use wth subsequent smulatons for a subdoman of the same grd resoluton or for fne-mesh nested domans that also need the regonal grdded concentraton felds. In ths chapter, the conceptual desgn of the PnG approach s dscussed. The key modelng components are a Plume Dynamcs Model (PDM) and a Lagrangan reactve plume model desgnated as the PnG module snce t s fully ntegrated and coupled wth the CCTM Euleran grd model. The PDM has been developed to serve as a processor program to generate plume dmensons, plume poston nformaton and related parameters needed by the PnG module. The descrptons of the mathematcal formulatons and numercal technques contaned n these modelng components are also presented. Ther solutons form the bass of the physcal-chemcal smulatons of subgrd scale plumes as mplemented n the prncpal algorthms of the PnG modelng components. 9-2

4 9.2 Overvew of the Conceptual Framework of the Plume-n-Grd Treatment The PnG technque s ntended for the largest pont sources, desgnated as major elevated pont source emssons (MEPSEs), whch are solated from notable area emsson sources. A complete descrpton of the varous crtera avalable to classfy a pont source as a MEPSE s provded n the emsson processng system n chapter 4. In the context of photochemcal modelng, the NO x emsson rate s a useful crteron to classfy a major pont source as a MEPSE from the thousands of ndvdual major pont sources n an nventory. Based on a NO x emsson rate crteron of 50 tons/day as a lower lmt, Fgure 9-1 depcts the group of fewer than 100 MEPSEs n a 36 km grdded doman coverng the eastern US wth emsson rates greater than ths crteron. The MEPSE sources are stuated away from major metropoltan emsson source areas. It reveals that numerous MEPSEs are dstrbuted throughout the Oho Rver and Tennessee Rver valley regons. Snce the current PnG technque also reles on lnear superposton of MEPSE plumes for determnng the mpact on grd concentraton, the relatvely solated MEPSEs located n these remote rural envronments also provde for the lowest lkelhood of plume-plume nteractons. An mportant feature of a PnG treatment s that t must be able to reproduce the varous stages n the chemcal evoluton found n a large, rural pont source plume. Based on daytme expermental feld study plume data, Gllan and Wlson (1980) documented three dstnct stages, as dsplayed n Fgure 9-2, n the chemcal evoluton of ozone (O 3 ) n a pollutant plume from a hgh NO x source. The O 3 data measured across a plume at dfferent downwnd dstances n Fgure 9-2 llustrate ozone evoluton n a large NO x pont source plume. Although NO x was not obtaned, t would dsplay a smlar varaton to SO 2 shown n Fgure 9-2. Durng the frst stage, the relatvely fresh plume s domnated by prmary NO x emssons and an O 3 defct exsts n the plume due to ttraton by very hgh NO concentratons. The chemstry n stage 1 s mostly norganc and VOC-lmted snce MEPSE plumes generally exhbt low VOC emssons. The second stage represents a transton n the chemstry wth rapd producton of O 3 along the plume edges and some O 3 recovery n the plume core. Plume growth from dsperson processes allows for entranment of background ar whch generally contans a rcher supply of ambent VOC concentratons. Consequently, ths promotes rapd photochemcal O 3 producton whch leads to the characterstcally hgher O 3 concentratons observed at the edges of a plume durng ths stage. Fgure 9-2 reveals that stage 2 certanly exhbts consderable subgrd scale plume structure n speces concentratons. Therefore, the proper smulaton of plume chemstry throughout stage 2 s a key phase for a realstc, overall characterzaton of plume chemcal evoluton. In fact, full completon of stage 2 s an mportant requrement for a chemcal crteron durng the PnG smulaton before the plume pollutants are transferred to the grd system. In stage 3, the plume s chemcally mature and substantally dluted. In ths mature stage, the broad plume would also exhbt a hgh VOC/NO x rato, low NO x concentratons, and O 3 concentratons n excess of background levels. In the modelng system, background (boundary) concentratons are provded from a CCTM grd cell contanng the subgrd scale plume secton. The chemcal evoluton, as descrbed above, for a large MEPSE plume n the eastern US generally may take up to several hours to reach full maturty even durng the daytme perod. The photochemcal cycle s strongly nfluenced by the plume growth rate, partcularly the rate of 9-3

5 horzontal spread, wth stage 3 acheved when a typcal MEPSE plume attans a wdth of about 30 km durng the daytme perod. In addton, the tme perod to reach chemcal maturty s also a functon of the emsson rate. There s a range of emsson rates even wthn a MEPSE group. For example, partcular MEPSE plume wth lower NO x emssons s expected to exhbt a faster chemcal evoluton than a MEPSE wth a hgher emsson rate, assumng other factors beng the same. Nevertheless, durng the subgrd scale smulaton perod, the plume may travel a consderable dstance downwnd (.e. several model grd cells) of ts source locaton n the daytme planetary boundary layer (PBL) durng the summer before reachng a wdth comparable to the grd cell sze. The conceptual desgn of the Models-3 PnG s llustrated n Fgure 9-3. PnG components smulate the relevant processes mpactng the hourly emsson rate of all pollutant speces durng the subgrd scale phase and t provdes a feedback, or "handover", of the modeled plume speces concentratons at an approprate handover tme (t H/O ) to the CCTM grd framework. The PnG module operates n a Lagrangan modelng framework to smulate the dynamcs and knetcs n a movng plume secton durng the subgrd scale phase whose duraton depends prmarly upon source strength, plume growth rate, chemcal composton of the background CCTM grd concentratons, and sunlght. The PnG module smulates the plume processes concurrently durng the smulaton of the CCTM grd model. The PnG module contans a seres of rectangular plume cross-sectons, each of whch are composed of a contguous, crosswnd array of plume cells wth the depth of each cell extendng vertcally from plume bottom to a top heght. As dsplayed n Fgure 9-3a, the vertcal depth of the sngle-layer plume model may be elevated ntally, however, t eventually extends from the surface to the PBL heght (z ). Ths one-layer plume structure n the ntal PnG module also presents a lmtaton under hgh wnd speed shear condtons snce ths stuaton cannot be modeled adequately. Whle strong speed shears are more frequent durng the nocturnal perod, the vertcal extent of a plume s also generally confned, whch lessens the possble mpact of speed shear. Nevertheless, ths condton s dentfed and approprately handled by the PnG components. 9.3 Formulaton of the Plume-n-Grd Modelng Components Descrpton of the Plume Dynamcs Model The PDM was developed to serve as a processor program n order to provde data needed for the CCTM/PnG smulaton. Therefore, the PDM and the PnG module are closely lnked through the PDM data fle. The key processes smulated by the PDM nclude plume rse, plume transport, and plume dsperson of each MEPSE plume cross-secton. The PDM also specfes when a partcular plume secton s to be transferred to the grd model based on an ndcator flag whch communcates to the PnG module when ths process s to be performed. The PDM s capable of smultaneously treatng up to 100 MEPSEs n a sngle smulaton wth the mpact on computatonal tme a functon of the number of MEPSEs smulated. In addton, the maxmum 9-4

6 smulaton perod s currently lmted to 24 hours. Therefore, a sngle smulaton of the CCTM wth PnG s also lmted to a 24-hour tme perod Aspects of Plume Rse of MEPSE Sources The plume rse treatment mplemented n PDM contans the same algorthms appled n the Emssons-Chemstry Interface Processor (ECIP). A detaled dscusson of the plume rse equatons and treatment has been provded n secton of chapter 4 on the MEPPS system and therefore s not repeated heren. The plume rse algorthms handle the entre durnal cycle snce plume sectons are released hourly throughout the 24 hour cycle. The temporal varaton of the fnal effectve plume heght for hourly MEPSE plume releases s llustrated n Fgure 9-4. In ths case, a durnal varaton n the fnal plume heght s evdent wth lower plume rse occurrng at nght beleved to be due n part to stronger wnd speeds. The plume heght ncreases durng the mornng perod as the PBL heght grows and the relatvely hgh values found n the afternoon dsplay lttle varaton untl the end of the daytme perod. A notable feature of MEPSE plumes also revealed n Fgure 9-4 s the rather sgnfcant heght often attaned by ths class of pont sources. Statstcs of the stack parameters from MEPSE and non-mepse groups of major pont sources were determned. A comparson ndcates notable dfferences n certan stack parameters between these source groups. The average values from a MEPSE group contanng 84 stacks compared to the other major pont source class contanng 6539 stacks show a stack heght of 207 m versus 45 m, a stack velocty of 20 m/s versus 12.5 m/s, a stack dameter of 7.5 m versus 2.3 m, a stack ext temperature of 410 o K versus 424 o K, and a stack ext flow rate of 908 m 3 /s versus 105 m 3 /s, respectvely. Clearly, these results ndcate why MEPSE plume heghts attan hgher levels than most pont sources. In general, MEPSE stacks are consderably hgher and ther plumes exhbt greater buoyancy flux due prmarly to greater physcal dmensons whch contrbute to hgher plume rse heghts Plume Dsperson Methods A key aspect of modelng a subgrd scale reactve plume, as noted earler, s the mportance of realstcally specfyng plume dmensons durng travel tme downwnd. Practcal methods have been appled n the ntal verson of PDM to determne the horzontal and vertcal dmensons of each Lagrangan plume secton throughout ts lfe cycle. Each rectangular plume cross-secton has a wdth (W p ) and an overall heght from plume bottom to top denoted by H p. The tradtonal dsperson parameters n the horzontal () y ) and vertcal () z ) have been employed n order to derve these plume dmensons. In the current verson, the plume wdth and plume heght values are determned accordng to W p a) y (9-1a) 9-5

7 H p h T h b (9-1b) where a has been set to whch s obtaned from 2(%) ½, an adjustment parameter from an ellptcal shape to the rectangular plume. The top and bottom heghts of each plume crosssecton are denoted by h T and h b, respectvely. The methods used to determne values for the parameters n Equatons 9-1a and 9-1b are dscussed next. Durng the plume rse phase, a plume experences ntal growth due to buoyancy-nduced turbulence. To account for ths process, practcal methods have been used to determne ntal values for the dsperson parameters. The ntal value of ) y s computed accordng an expresson suggested n Irwn (1979), as advanced by Pasqull (1976), gven by ) yo h/3.5 (9-2) In Equaton 9-2, the ntal value s a functon of the plume rse (h) above the stack top. Two methods have been nstalled to prescrbe an ntal vertcal plume thckness. The wdely used approach advanced by Brggs (1975) s to defne H p to be equvalent to the amount of plume rse (h). Ths method can result n a rather thck plume when there s consderable plume rse, partcularly, at nght when expermental evdence suggests nocturnal plumes may be relatvely thn. Consequently, an optonal approach provded as an expermental alternatve has been ncluded. It conssts of an emprcal form developed by Gllan (1996) whch s based on analyses of observed plume data taken durng a feld study (Gllan et al., 1981). Hs regresson analyses produced an expresson gven by ) zo Ae ( BdT/dz) (9-3) where dt/dz s the vertcal ambent temperature gradent at the plume centerlne heght and the best-ft values for A and B are 15 and 117, respectvely. A mnmum ) zo value s set to 3 m. Then, the ntal H p s determned from SZ0FA tmes ) zo. SZ0FA s a user-specfed nput parameter for a PDM smulaton and t s currently set to Atmospherc turbulence and wnd drecton shear contrbute to lateral plume growth durng travel downwnd. Durng the daytme convectve perod, the turbulence component s often domnant, although drectonal shear across the PBL also has an mportant role n contrbutng to plume spread wth travel dstance. In contrast, turbulence s generally weak at nght, and horzontal 9-6

8 plume expanson may be domnated by wnd drecton shear over a plume's depth. Thus, the composte horzontal (lateral) dsperson parameter () y ) can be defned by ) 2 y )2 yt ) 2 ys )2 yo (9-4) where the turbulence () yt ) and drecton shear () s ) terms may both contrbute to horzontal plume growth after the ntal plume spread. A general form for ) yt can be expressed by ) yt ) v Tf(T/- L ) (9-5) where ) v s the standard devaton of the lateral wnd (v) component, T s travel tme, and - L s the Lagrangan tme scale. Although varous expressons for the functon, f(t/- L ), have emerged (e.g., Draxler, 1976; Irwn, 1983), the followng form from Wel (1985;1988) and appled by others (eg.,venkatram, 1988) has been adopted. f 1 (1 T/2- L ) ½ (9-6) Ths form s advantageous snce t fts dsperson at both the short (t < - L ) and also long travel tmes (t>>- L ). Snce the Lagrangan tme scale s small relatve to the plume secton travel tmes whch are often several hours, the expressons above reveal the famlar relaton that ) yt ncreases wth the square root of travel tme. Therefore, the followng form accordng to Wel (1988) at long travel tmes, whch has been examned by Clarke et al. (1983), s gven by ) 2 yt 2) 2 v - L T (9-7) The above equaton also dsplays the square root of tme ncrease for the lateral dsperson parameter. Usng modeled meteorologcal parameters from data fles generated by MCIP, methods have been ncorporated to determne the key parameters needed to solve the above expressons for ) yt. Values for ) v and - L are computed wth a set of formulas presented n Hanna et al. (1982) and Hanna (1984), respectvely, for varous stablty condtons based on Monn- Obhukov length (L) lmts. For unstable condtons wth L < 0 9-7

9 ) v u (12 0.5z /L) 1/3 (9-8) - L 0.15z /) v (9-9) where z s the PBL heght and u * s the surface frcton velocty. Under stable condtons defned by L > 0, ) v 1.3u (1 z/z ) (9-10) - L 0.07z /) v (z/z ) 0.5 (9-11) and z s the heght of the plume centerlne. For neutral condtons (L = 0), values are determned by ) v 1.3e ( 2fz/u ) (9-12) - L 0.5z/) v (1 15fz/u ) (9-13) where f s the Corols parameter. Usng ) v and - L values, the contrbuton from the turbulence component s computed ncrementally from the dervatve of Equaton 9-7 and solved at each tme step. The turbulence term s accumulated wth tme. Other sem-emprcal methods avalable n PDM provde an alternatve set of parameterzatons for ) v as provded by Wel (1988), Neuwstadt(1984), and Arya(1984) for the dfferent stablty regmes, whch may be selected from an nput opton when exercsng PDM. Wnd drecton shear due to turnng of the wnd over the vertcal extent of a plume can also provde an mportant contrbuton to horzontal plume growth. Whle turbulence often domnates n the daytme PBL, drecton shear durng the nocturnal perod s the prncpal mechansm for lateral plume expanson snce turbulence s generally weak. An expresson by Pasqull (1979) has been appled for all condtons to derve the drecton shear term and s gven by ) 2 ys 2 X 2 (9-14) 9-8

10 where s 0.03, X s dstance traveled by a plume secton over a partcular tme nterval, and s the wnd drecton dfference (radans) over the vertcal extent of the plume. The shear and turbulence terms are combned to solve for the total ) y 2 n Equaton 9-4. After the ntal plume thckness has been determned, the treatment for ) z dffers based on whether the plume heght s above or below the PBL heght. When the plume centerlne heght s above z, then the followng expresson from Gllan (1996) s appled to determne ) z. ) 2 z )2 zo (1 bt 0.5 ) (1 bt 0.5 o ) (9-15) where b s 2.3 and T s travel tme. Equaton 9-15 s appled for T < 4 hours. At longer travel tmes, ) z s set to zero for plumes exstng above the PBL n the free atmosphere. When a plume secton s nsde the PBL, the parameterzaton for the vertcal plume dsperson parameter can be expressed by the same general form as ) yt. ) z ) w T f(t/- Lz ) (9-16) As before, T s the travel tme, and f has the same functonal form as used for ) yt although t now contans the Lagrangan tme scale n the vertcal (- Lz ). The standard devaton of the vertcal wnd component () w ) s computed from the followng expressons as gven by Wel (1988) for unstable condtons and from Venkatram et al (1984) for neutral/stable condtons, respectvely. ) w 0.6w (convectve case) (9-17) ) w 1.3u (1 z/z ) 3/4 neutral/stable (9-18) The MCIP data fles provde values for the fracton velocty (u * ), the convectve velocty scale (w * ) and z. The plume centerlne heght s represented by z. The value of - Lz s computed from the followng expressons by Hanna et al. (1982) for unstable and by Venkatram et al. (1984) for neutral/stable condtons. - Lz 0.15 z ) w [1 e 5z/z ] (unstable) (9-19) 9-9

11 In the latter expresson, the length scale (l) s derved accordng to Venkatram et al. (1984). An example applcaton of these methods from a PDM smulaton for a plume released at nght from a MEPSE tall stack s shown n Fgure 9-5. It shows the plume remans rather narrow and t - Lz l ) w (neutral/stable) (9-20) stays elevated durng the nght. However, once the PBL grows beyond the plume heght durng the mornng perod, the plume thckness expands to fll the entre PBL wth the plume top heght matchng z untl t reaches a maxmum heght. The temporal varaton of the plume wdth for the same example case s dsplayed n Fgure 9-6. It shows that snce the plume has a lmted vertcal extent at nght, the plume wdth ncreases at a relatvely slow rate as suffcent drecton shear exsted over the plume depth to cause lateral spreadng. Durng the daytme, however, horzontal plume expanson occurs at a faster rate due to both greater turbulence and shear contrbutons. For a regonal grd cell sze of 36 km, the plume wdth for ths release reached the physcal grd sze durng the md-mornng hour. These results and other smulaton results obtaned from a daytme release versus observed values (Godowtch et al., 1995) provde prelmnary evdence that these methods gve a realstc depcton of the temporal behavor of the magntudes of plume wdth and depth. Addtonal cases have also been exercsed, although results are not shown heren, n order to assess the robustness and capablty of the algorthms for dfferent days. An extensve evaluaton of these dsperson methods n PDM s planned aganst feld study data obtaned n the vcnty of major pont sources n the Nashvlle, Tennessee area durng the Southern Oxdant Study expermental perod n the summer of Plume Transport Wth the updated plume top and bottom heghts, the mean wnd components are determned by averagng the wnds over the layers spanned by the plume from h b to h T. Ths approach s currently applcable because of the sngle vertcal layer structure of the current PnG module. The mean plume transport speed s used to determne an updated plume centerlne poston over the tme nterval. In addton, the postons of the plume edges at the bottom and top are also found n order to derve grd ndces for later use n applyng the proper grd cell concentratons for boundary condtons of each plume secton Formulaton of the Plume-n-Grd Module Overvew of the Plume Conceptual Framework The Lagrangan plume model descrbed orgnally by Gllan (1986) provded the conceptual bass for the Models-3 PnG module. However, the computer algorthms n the CCTM/ PnG 9-10

12 have been rewrtten to contan updated methods for the key processes to be descrbed n a later secton. In a modelng framework, a PnG plume cross-secton can be descrbed as a sem-nfnte vertcal slab movng along a Lagrangan trajectory wth a mean wnd flow. A plume cross-secton s consdered rectangular wth a vertcal heght (H p ) and a wdth (W p ). Temporally, plume spread n the vertcal and horzontal s specfed by growth rates dh p /dt and dw p /dt, respectvely. A plume cross-secton s dscretzed laterally nto an array of attached plume cells or pllars wth the same H p at tme t, as depcted n Fgure 9-7. Each plume secton conssts of N plume cells (currently, N = 10) wth the wdth of each cell beng equal. Wth respect to the plume centerlne, there are N L cells on the left sde and N R cells to the rght sde, such that N L N R W p M 1 w L M 1 w R (9-21) N N L N R Currently, N L = N R as L and R refer to the cells on the left and rght sdes, respectvely. Wth respect to the plume centerlne, the rght sde of the plume secton expands to the rght and the left sde expands to the left. The wdth (w ) of each plume cell s gven by (w ) L/R (y 1 y ) L/R (9-22) where y values represent dstances from the plume centerlne poston. Normalzaton of the plume cell wdths s performed such that as the plume expands laterally, each cell wdth ncreases n the same proporton as the overall plume wdth. Thus, snce the ndvdual cell wdth s normalzed wth respect to the total plume wdth, the result s a transformed movng crosswnd grdded array whch s nvarant wth respect to tme. The transformed crosswnd coordnate () of the model s gven by y W p (9-23) and d /dt s zero. Therefore, although W p and y are tme dependent, remans constant durng a smulaton. Addtonally, although the fractonal wdth of the cells across the plume secton n the PnG has been prescrbed to be equal n the current setup of the module, the algorthms have been generalzed to allow for unequal plume cell wdths and a dfferent number of cells on the left and rght sdes of the plume centerlne as descrbed n Gllan (1986) Formulaton of the Plume Mass Balance Equaton 9-11

13 The relevant processes, as noted n Fgure 9-7, have been ncorporated nto the plume equaton for the mass balance of ndvdual speces. They nclude dluton and entranment due to vertcal plume expanson, dluton and entranment/detranment due to horzontal plume expanson, lateral dffuson, gas-phase chemstry, surface removal, and surface emsson. The subscrpt denotes plume cell (left or rght of the plume centerlne), subscrpt j denotes speces j, and superscrpt t denotes a partcular tme. Thus, C t j s the concentraton of speces j n cell at tme t, and m t j s mass of speces j n cell at tme t. In the followng dervaton of the terms of the mass balance equaton for any speces j n cell, the subscrpt j s omtted for convenence; also, the equatons and terms do not show L or R representng a left or rght plume cell, snce the equaton and terms apply smlarly to both sdes. Consequently, consder the mass balance of (left or rght) cell (for speces j) correspondng to the ts expanson durng a small tme nterval dt. 0m (0m ) Dsp (0m ) Ems (0m ) Chem (0m ) Dep (9-24) Now, the followng expresson s also applcable for the plume cell mass. 0m m tdt m t C tdt lh tdt p W tdt p C t lh t p W t p (9-25) Heren the alongwnd dmenson n the downwnd drecton (l = Udt) s prescrbed, whch s determned from the ntal mean transport speed (U) over the tme nterval dt. Mathematcal expressons are presented n subsequent sectons for the updated concentraton of speces j n cell due to the acton of the ndvdual processes durng tme nterval dt. In the current verson of PnG, only surface emssons after plume touchdown are ncluded n the emsson term snce (non-mepse) pont-source emssons nto grd cells (above the lowest cell layer) neghborng the MEPSE plume are released unformly nto the entre emsson grd cell and mpact the MEPSE plume only through the boundary condtons. Surface removal s due to dry deposton at the ground. Gas-phase chemstry s ncluded wth the full chemcal mechansms of the CCTM n the chemstry term. In the future, the PnG wll be adapted to employ the same aerosol module used by the CCTM. No wet processes are treated n the ntal PnG verson. The current approach to address lmtatons n the ntal verson of PnG s to transfer the plume over to the grd soluton whenever condtons exst (e.g., precptaton) whch the PnG treatment cannot adequately accommodate. Ths ssue wll be addressed n more detal n the secton on plume handover Treatment of Plume Expanson and Dffuson Processes 9-12

14 Dsperson affects the mass balance of a plume cell as a result of dluton and entranment/detranment processes related to the lateral and vertcal expanson of a plume crosssecton durng a tme nterval dt, as well as mass dffuson whch mpacts cell concentratons as a result of concentraton gradents between adjacent cells. The dsperson processes can be expressed by (0m ) Dsp (0m ) Dl/E/D (0m ) Df (9-26) where the frst term on the rght sde of Equaton 9-26 represents the dluton / entranment / detranment processes and the last term s the eddy dffuson process. The relatonshps developed heren apply to cells on ether sde of the plume centerlne, snce the only dfference occurs n ther applcaton when dfferent boundary condtons occur at each edge. (0m ) Dl/E/D (m tdt m t ) Dl/E/D (9-27) refers to the change of mass n plume cell (for any transported speces j) durng dt as a result of dluton and entranment/detranment. The changes due to just the horzontal and vertcal exchange processes can be expressed by m tdt m t (0m ) Latdl/E/D (0m ) Vertdl/E/D (9-28) where Latdl refers to horzontal (lateral) dluton. In practce, the lateral dluton/ entranment/detranment term n Equaton 9-28 s determned and then the last term s solved. The lateral dluton term n Equaton 9-28 s gven by (0m ) Latdl/E/D lh t p (C t 10y 1 C t 0y ) (9-29) However, the followng relatonshp also exsts. 0y 0W p (9-30) 9-13

15 snce s nvarant. Therefore, Equaton 9-29 can be revsed to the followng form. (0m ) Latdl/E/D lh t p ( 1 C t 1 C t )0W p (9-31) At ths tme, (.e., after lateral dluton/entranment/detranment) the wdth of a plume cell s w t+dt. Wth the cell wdths remanng unchanged, plume vertcal expanson occurs (.e., n the up and down drecton for an elevated plume, and up only after plume touchdown). Let C a t+dt and C b t+dt denote the concentratons of speces j above and below plume cell, respectvely, for the elevated plume case. C a and C b represent the CCTM grd concentratons at t+dt. Once the plume becomes surface-based, only C a s relevant. The equaton for the vertcal dluton term s gven by (0m ) Vertdl/E/D lw tdt (C tdt a 0H a C tdt b 0H b ) (9-32) where the changes n H a and H b denote plume vertcal expanson of the upper and lower plume boundares, respectvely. An assumpton s made that 0H a 0H b 0.50H p (9-33) for an elevated plume cross-secton, and after a plume reaches the surface 0H a 0H p 0H b 0 (9-34) are applcable. Equaton 9-32 can be rewrtten accordngly as (0m ) Vertdl/E/D lw tdt C tdt 0H p (9-35) where C tdt (C a C b ) (elevated plume) C tdt C tdt a (surface based plume) (9-36) 9-14

16 for an elevated plume and a plume bottom at the surface, respectvely. The combnaton of Equatons 9-31 and 9-35 produces the followng form. (0m ) Dl/E/D lhp0w t p ( 1 C t 1 C t ) l0h p w tdt C tdt (9-37) However, by substtutng the next expresson, (0m ) Dl/E/D C tdt lh tdt p w tdt C t lh t p w t (9-38) and rearrangng, the concentraton after the total dluton/entranment/detranment step s C tdt C H t p w t H tdt p w tdt H t p H tdt p 0W p w tdt ( 1 C t 1 C t ) 0H p H tdt p C tdt (9-39) The frst term on the rght-sde s related to the dluton effect and the next two terms are the lateral and vertcal entranment/detranment effects, respectvely. The subsequent expressons are also applcable. w tdt w t y t 1 y t W p ( 1 ) W tdt p ( 1 ) ( 1 )(W p 0W p ) (9-40) p 0H p H tdt p H t By substtutng the above expressons nto Equaton 9-39, and rearrangng gves C tdt C t (H p W p ) t (H p W p ) tdt 0H p H tddt p C tdt H t p H tdt p 0W p ( C t 1 1 C t ( 1 ) W p ) (9-41) However, Equaton 9-41 can be smplfed and re-wrtten as 9-15

17 C tdt 1 1 H ( H C tdt 1 [(C t 1 W w 1 ( 1 C t 1 C t )] (9-42) where H 0H p H p, W 0W p W p. (9-43) When applyng Equaton 9-42, certan boundary condtons must be specfed. In partcular, f 1 ; 1 0 N L, N R ; C 1 C tdt bg (9-44) where C bg t+dt and C * t+dt are the partcular grd cell concentratons from the CCTM soluton for the current tme step. In the executon of the CCTM/PnG, both the grd-cell and plume-cell concentratons are updated by numercal ntegraton of the correspondng mass balance equatons from tme t to t+dt, where dt s the CCTM advecton tme step (MSTEP). The sequence s that CCTM performs ths ntegraton and updates the concentraton feld frst, generatng C t+dt n each grd cell, before PnG s called to do the same. PnG performs the ntegraton n a fractonal step approach; frst for plume dsperson (step 1) then for the surface emsson and removal processes (step 2), and fnally for plume chemstry (step 3). The chemstry ntegraton s actually performed n smaller chemstry tme steps, nternally determned for the plume cells by the chemcal solver, n sequence, untl the ntegraton s completed for the tme nterval dt. The emsson/removal step s mplemented n a sngle ntegraton tme step. The dsperson step s performed sequentally n subtme steps untl dt s reached. The PnG dsperson tme step s constraned by a restrcton when applyng Equaton 9-42 arsng from the fact that, n a gven tme step, the locaton after expanson of the nner nterface of a gven plume cell (.e. y t+dt ) cannot pass beyond the locaton of the outer nterface of that cell at the start of the ex expanson. Therefore, the followng constrant apples. y tdt y t 1, 2 N L, R (9-45) 9-16

18 Ths crteron requres that the followng lmt for the PnG tme step must be satsfed. 0t dsp ( 1 ) 1 W (9-46) W 1 W p dw p dt (9-47) It should also be noted that W * s related to w accordng to w tdt t wdt (9-48) Durng the model smulaton, W p (t) and dw p /dt are provded to the PnG module from the PDM data fle. Thus, W * s determned drectly, and W can also be computed usng the plume wdths from tme t and t+dt. The dsperson step s not complete wthout ncluson of the dffuson term. The dffuson equaton of a speces concentraton for any plume cell may be expressed by 0C 0t 0 0y (K y 0C 0y ) (9-49) where K y s the horzontal eddy dffuson coeffcent. It can be expressed by K y 1 2 d dt ()2 y ) (9-50) By applyng the relatonshp between ) y and W p n Equaton 9-1, the followng form for K y can be derved. K y W 2 p a 2 w (9-51) 9-17

19 Numercal ntegraton of a form lke Equaton 9-49 s solved wth the Crank-Ncholson method n whch centered dfferencng s used for both tme and space dervatves to solve the dffuson equaton. Ths technque provded stable numercal solutons compared to an explct method. The resultng fnte-dfference equaton s then solved by matrx decomposton of the coeffcent-matrx nto upper and lower (LU) trangular matrces. For convenence n settng up the matrx form of the fnte dfference dffuson equaton for all plume cross-secton cells, a set of smultaneous equatons s developed whch are wrtten n a vector-matrx form for solvng by LU decomposton. The dervaton of the set of equatons s provded. By averagng Equaton 9-49 n tme to obtan 0C/0t at t+0t, and usng center-dfferencng to solve for 0 2 C/0y 2, smplfcaton leads to C k1 1 [ C k1 1 [ K y 0t (y 1 y )(y 1 y 1 ) ] C k1 K y 0t [1 (y 1 y )(y 2 y )] K y 0t (y 1 y )(y 1 y 1 ) ] K y 0t (y 1 y )(y 2 y ) ] C k K y 0t 1 (y 1 y )(y 1 y 1 ) ] C k [1 K y 0t (y 1 y )(y 2 y ) K y 0t y 1 y )(y 1 y 1 ) ] C k 1 [ K y 0t (y 1 y )(y 2 y ) ] (9-52) where small k s used as a tme ndex for tme t, and k+1 represents t+0t, s the ndex to the left edge of plume cell, and y s the dstance from the new reference poston at the left background locaton of the plume cross-secton. By usng the relatonshp that y = W p, the followng relatons can be defned. K y 0t W 2 p 1 ( 1 )( 1 1 ) (9-53) K0t W 2 p 1 ( 1 )( 2 ) (9-54) 9-18

20 (9-55) By substtutng the above expressons nto Equaton 9-52, the followng formula s obtaned. C k1 1 (1 )C k1 C k1 1 C k 1 ( )C k C k 1 (9-56) Equaton 9-56 represents a system of smultaneous equatons n the form, Ax b (9-57) where, x = (C k C N+2 k+1 ) t and N s the total number of plume cells. Addtonally, the expresson, A = I - G, s defned where A s a square matrx of sze (N+1)x(N+2), and b = (I+G)C k. In these expressons, I s the dentty matrx, and G s a trdagonal matrx of the form, G = trdag (,, ), and s of sze (N+2)x(N+2). Subsequently, the smultaneous system of equatons gven by Equaton 9-57 s solved usng LU decomposton. The boundary condtons are also ncluded n the above system of equatons snce N+2 s used. Approprate boundary condtons are provded by the CCTM grd concentratons for the rght and left edges of each plume cross-secton. At left edge boundary, 1 = 1 = 1 and b 1 = C 1 = C bgl. At the rght edge boundary, N+2, N+2, and N+2 are zero whch leads to b n+2 = C N+2 = C bgr Surface Area Emssons and Dry Deposton Durng the PnG smulaton, surface emssons are njected nto ndvdual surface-based plume cells. The surface emssons, contaned n layer 1 values of the 3-D emssons fle whch s also employed by the CCTM, are used n all plume cells of a plume cross-secton. For each grd cell, there s an emsson rate for certan speces (q j ). In partcular, a MEPSE plume cross-secton passes over such grdded surface emssons. Consequently, the change n concentraton of plume cell durng a tme step from surface emssons s 0C j q j 0t k j (9-58) xy H p where k j heren s an approprate speces-specfc factor for the converson of concentraton from a mass unt to the approprate volume unt. 9-19

21 Snce the PnG formulaton s composed of a sngle vertcal layer, t was not desgned to have the capablty to properly ngest other elevated pont-source emssons nto the MEPSE plume crosssecton. As long as such an along-path, pont-source emsson reman at a relatvely low value, the mpact on the MEPSE concentraton feld s assumed to be felt through background entranment of the expandng MEPSE plume. If the other pont source emsson was ngested, of course, such entraned mass s currently dspersed nstantaneously throughout the vertcal extent of the plume. For ths reason, when pont-source emssons larger than a crtcal value are encountered by the MEPSE plume, the current logc s to handover the MEPSE plume to the grd soluton. Dry deposton s a snk term and occurs at the bottom of each plume cell based on the deposton velocty concept. 0C j 0t V j d H p C j (9-59) where C j denotes the concentraton of speces j and the speces-specfc deposton velocty (V dj ) values are avalable from a grdded data fle. The same deposton velocty s appled to all cells of a plume secton usng the V d values from the grd cell n whch the plume centerlne s located Gas-phase Chemstry of Plumes A gas-phase chemstry mechansm mplemented n the CCTM s also nvoked by the PnG module. The current mechansms nclude the RADM2 and carbon bond (CB-4) chemcal mechansms. Detals about these chemcal mechansms are provded n secton 8. In addton, there s a separate PnG module verson for each chemcal solver (.e., SMVGEAR and QSSA). Mnor revsons were needed n the PnG versons of the solver codes to customze them to deal wth the plume concentraton array whose dmensonalty dffers from the CCTM grdded concentraton array. Nevertheless, the PnG gas photochemstry treatment s dentcal to that of the CCTM. Snce only gas-phase plume chemstry has currently been mplemented, when condtons conducve to extensve aqueous chemstry are encountered, a plume secton s transferred to the CCTM grd system, as dscussed n the handover secton below. Future plans nclude mplementaton of the exstng CCTM aerosol module nto PnG so that t also has the capablty to treat aerosol and partculate speces Plume Intalzaton The ntalzaton of each plume cross-secton s the frst key procedure performed at the start of ts smulaton. The PnG ntalzaton of a new MEPSE release occurs when a plume crosssecton wdth reaches a mnmum wdth. The wdth crteron s a user-specfed varable and t s specfed for the PDM smulaton. A flag ndcator varable n the PDM fle communcates to PnG when a plume cross-secton s ready to be ntalzed. The current mnmum wdth for ntalzaton of a MEPSE plume secton has tentatvely been set at 2 km. At ntalzaton, t s 9-20

22 assumed that the lateral concentraton dstrbuton across the plume secton of the prmary emsson speces exhbts a Gaussan shape. For all other speces, ntal concentratons have been set to a machne mnmum-value (e ). The lateral concentraton dstrbuton of the prmary speces n the plume cells (.e., ntal condton of the plume concentraton feld) s gven by C j (0) k j q j 1 exp[ 0.5(y UH p 2%) /) y )2 ] (9-60) y where, y * s (y - y o ), the dstance of the outer edge of cell from the plume centerlne, and U s the mean transport speed, q j s the MEPSE emsson rate of speces j, and k j s the approprate speces specfc mass-to-volume converson factor. In PnG, t s assumed that the MEPSE emssons are at an hourly resoluton, and that we are performng plume smulatons of hourly releases. The assumpton s made that the emsson rate (q j ) remans constant over the hour. Consequently, at the handover tme, the transformed mass mpacted by the varous plume dynamc and chemcal processes correspondng to one hour of emsson s released to the grd soluton Methodology for Plume Feedback to the Euleran Grd When the subgrd scale phase of a plume secton smulaton has been completed, the plume materal s ready for transfer to the CCTM grd. The total concentraton of each speces n any plume cell s composed of a component equal to the background concentraton, and an addtonal component consstng of the plume concentraton, whch dffers from the background grd level. In performng the handover, the conceptual bass s that only the plume component s related to the correspondng MEPSE emssons. Thus, the feedback s restrcted to the plume contrbuton. Addtonally, snce hourly plume releases are smulated from each MEPSE source n PnG, and snce the assumpton has been made that each such hourly emsson remans constant for the full hour, the current practce s to handover the contents of a plume secton correspondng to a full hour release. Thus, for each plume speces, m P V p (C p C bg ) (9-61) where average values for the plume and background grd concentratons are employed. The average plume concentraton (C p ) of each speces s determned from all plume cells. The background grd value (C bg ) represents the average of the CCTM grd-cell concentratons on each sde of the plume for all approprate layers over the vertcal extent of the plume. The handover plume mass s dstrbuted nto the column of cells up to H p n whch the plume 9-21

23 centerlne s located at the handover tme. The feedback of plume mass represents an adjustment to the CCTM grd cell concentratons from an average concentraton (C adj ). From a mass balance perspectve, C adj V G (C p C bg )V p (9-62) where V G s the CCTM grd cell volume. The plume volume (V p ) s determned from V p W p H p (Ut) (9-63) where U s the mean horzontal wnd speed, t s a 1-hour nterval and Ut corresponds to the alongwnd dmenson (x p ) spanned by the plume secton. In a smlar fashon, the grd volume mpacted by the plume s gven by V G x G y G (Z t G Z b G ) (9-64) where x G and y G are the horzontal grd cell szes, and Z G t and Z G b are the heghts of the top and bottom of the model layers spanned by the plume. Substtutng the above expresson nto Equaton 9-62 yelds: C adj (C p C bg ) H p W p x p V G (9-65) In the ntal release verson of PnG when plume handover s trggered, an entre hour s worth of plume release n ts transformed state s transferred to the approprate column of grd cells durng a CCTM advecton tme step. In the future, the ntenton s to ncrementally transfer the same plume excess to the mpacted grd cells over a one hour perod of model tme steps. In ths manner, the contnuous emssons after beng treated n PnG would be transferred durng a one hour perod followng the handover tme. PnG smulatons are prmarly ntended for the coarse-grd, regonal modelng grd szes wth the horzontal resoluton () of nomnally km. Daytme MEPSE plumes reach or are close to chemcal maturty when the plume wdth attans these grd szes. However, PnG smulatons at fner grd szes are possble, however, test runs have not been performed but are needed n order to assess the benefts of ths approach at urban grd cell szes. As the prmary purpose of PnG s 9-22

24 to mprove the subgrd-scale treatment of large pont-source plumes, the key handover sze crteron based on plume wdth relatve to grd cell sze has been prescrbed accordng to W p cr (9-66) The default value for cr s 1.0. Therefore, when a plume secton wdth attans the grd cell sze, a flag ndcator varable generated by PDM trggers PnG to perform the handover process. Currently, the CCTM system s operated n a 1-way nestng mode, as multple nested grds are not performed n a sngle smulaton. Consequently, PnG s not equpped to handle the movement of a plume from a larger to a fner grdded doman wthn the same smulaton. Ideally, plume handover also occurs when the plume has also reached chemcal maturaton when concentraton dfferences wthn the plume become rather small. A chemcal crteron also ncorporated nto PnG nvolves a default condton recognzng chemcal maturaton. It s based on an average plume concentraton rato of O 3 /O x,where O x = O 3 +NO 2 denotes the prncpal oxdant speces usng concentratons from all cells n a plume secton. Ths rato s employed as a surrogate ndcator of plume chemcal age. The specfc chemcal maturaton crteron s: O 3 O x r chem (9-67) where r chem represents the crtcal value. The default value has been set at Tests have ndcated ths value to be a good ndcator correspondng to a chemcally-mature stage 3 plume. The use of O 3 / O x as a surrogate for plume age s not common compared to NO x / NO y where NO y ncludes all the reactve oxdes of ntrogen. However, NO x /NO y has not been appled as a chemcal crteron because njecton of fresh emssons from large pont sources has been observed to suddenly decrease the chemcal age of the background whch leads to premature actvaton of the chemcal crteron. Experence from testng wth the frst chemcal crteron has proved t to be more satsfactory. In the ntal verson of the Models-3 PnG, addtonal crtera are needed whch can also trgger plume handover to the Euleran grd cells. As the PnG module s upgraded n the future, the these crtera may be avoded. The next two crtera are related to non-mepse emssons n the mmedate vcnty of a MEPSE plume cross-secton. Urban Crteron 9-23

25 PnG currently does not have the faclty to njest dfferent surface emssons nto ndvdual plume cells. Such a condton s lkely to be encountered by a MEPSE plume after touch-down when t passes over an urban-ndustral area wth a very nhomogeneous spatal dstrbuton of emssons. Gllan and Plem (1996) have dentfed such urban areas based on the followng crteron pertanng to the emssons of NO x. f q (NO x ) {10 12 molecules/cm 2 /s (9-68) Ths condton denotes a farly hgh NO x surface emsson flux (f q ). The crtcal value of chosen by Gllan and Plem (1996) was based on an emsson nventory wth a horzontal spatal resoluton of about 20 km. For such a grd sze, ths condton corresponds to an emsson rate of about 0.3 kg/s. Thus, the selected urban emssons handover crteron s: q NOx 0.3 kg/s (9-69) The above condton must be satsfed to trgger plume secton handover, and the plume bottom must also be at the ground for ths crteron to actvate. Pont-source Emssons Crteron The ntal PnG verson does not possess multple vertcal layer resoluton of the plume. Thus, f a plume secton entrans the elevated concentraton of a prmary speces n a background grd cell from a fresh NO x emsson from a non-mepse major pont source, such entraned mass s nstantaneously mxed vertcally throughout the MEPSE plume. If the non-mepse pont source emsson s large enough, the related error of ts treatment n the MEPSE plume relatve to ts treatment n the grdded soluton becomes large. Thus, t s advantageous to handover the MEPSE plume under ths crcumstance. A handover crteron of 3.33 x 10-6 ppm/s has been set whch corresponds to about 2 ppb/10 mnutes. If the new non-mepse pont-source emssons n the mmedate vcnty of the MEPSE plume are large enough to rase the average background concentraton (left/rght and vertcally-averaged over MEPSE plume heght) by more than 2 ppb n 10 mnutes, then handover s also trggered. Precptaton Crteron PnG has no faclty to handle wet removal of the plume. Consequently, f the total (convectve and non-convectve) precptaton rate n the cell contanng the MEPSE centrod exceeds a 9-24