MUSE - a multilayer dispersion model for reactive pollutants P ...

MUSE - a multilayer dispersion model for

reactive pollutants

P. Sahm, N. Moussiopoulos

Laboratory of Heat Transfer and Environmental Engineering,

Aristotle University Thessaloniki, 54006 Thessaloniki, Greece

Abstract

As a new constituent of the EUMAC Zooming Model (EZM) system, themultilayer model MUSE allows efficiently simulating the formation of photo-chemical smog on smaller computers (e.g. workstations). In its first application,MUSE is used to describe the dispersion and chemical transformation of airpollutants in the Athens basin and the Mesogia plain for various meteorologicalsituations. Emphasis is put on discussing how airport emissions affect airquality assuming that the airport is operating either at Hellenikon or at Spata.Comparison of simulation results achieved with MUSE for one selectedscenario with corresponding results of the validated three-dimensionalphotochemical dispersion model MARS reveals that the model MUSE iscapable of reproducing the spatial and diurnal variation of the majorphotochemical air pollutants in the Athens basin and the Mesogia plain.

1 Introduction

The EUMAC Zooming Model (EZM) system is a complete system for simula-tions of wind flow and pollutant transport and transformation in the mesoscale[1]. Main modules of the EZM are the nonhydrostatic mesoscale model MEMO[1,2] and the photochemical dispersion model MARS [1,3]. MEMO is opera-tional both on mainframes and workstations. MARS, however, is not designedfor case studies on small scale computers (e. g. workstations) as it has highmemory demands and requires much CPU time. As a new constituent of theEZM, the multilayer photochemical model MUSE was developed as asimplified version of the fully 3D photochemical dispersion model MARS.

After a brief model introduction, in this paper the application of MUSE toanalyze the impact of the emissions of the Athens airport on the air quality in

Transactions on Ecology and the Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541

360 Air Pollution Theory and Simulation

the Athens basin is described. In these simulations it is alternatively assumedthat the airport operates (i) at Hellenikon (current location: in the interior of theAthens basin) and (ii) in Spata (planned new location: in the Mesogia plain).

2 The multilayer model MUSE

The model equations solved by MUSE are those of the fully 3D version ofMARS. However, the multilayer model MUSE differs from MARS in thefollowing points:• Instead of discretizing the model domain into 10-30 non-equidistantly

distributed vertical layers, three layers are being used in the vertical direc-tion. The depth of the lower layer (which practically corresponds to thesurface layer) is prescribed and kept constant in time. The depth of themiddle layer is permitted to vary following the diurnal variation of themixing height. The latter is described by Deardorff s prognostic equation[4] which was shown to lead to realistic results when the variation of themixing height with time is strongly influenced by surface heating [5]. Forperiods of stability, an algebraic parameterization based on the frictionvelocity and the Monin-Obukhov length is applied [6]. The limit of theupper layer coincides with the fixed domain top. The assumption of a timedependent depth of the middle and upper layer leads to the need of anentrainment term [7].

• The description of vertical transport due to turbulent diffusion at the lowerlimit of the middle layer is based on the turbulent kinetic energy defined atthe interface of the lower and middle layers. To avoid unrealistic verticaldiffusion rates associated with the growth of the mixing height, one-sidedconcentration gradients are calculated at this interface. Turbulent diffusionin the upper limit of the middle layer is neglected.

• Advective transport is described with the scheme of Smolarkiewicz [8].The average wind speed in the middle and upper layers is calculated byintegrating the fluxes of the corresponding layers of the fully 3D wind fieldmodel.

• The differential equation system in MUSE is solved with a backwarddifference (BDF) solution procedure (by applying the Gauss-Seideliteration scheme) [9]. Because of the nature of this semi-implicit algorithm,vertical diffusive transport and chemical transformation of pollutants haveto be treated separately. In view of the feasible error caused by splitting theoperators associated with vertical diffusion and chemistry [10], thejustification for this separate treatment should be controlled by the aid ofselected applications of the full 3D version of MARS.With the above simplifications, the MUSE code becomes 5-7 times faster

than the full 3D version of MARS. At the same time, the memory requirementsare reduced by 90 %.


Air Pollution Theory and Simulation 361

3 Case specification

The transfer of the Athens airport from Hellenikon to Spata will have a majorimpact on the air quality in the Athens basin and the Mesogia plain [11]. Inorder to investigate how the airport operation, either at Hellenikon or at Spata,affects the air quality in the Athens basin and the Mesogia plain, numeroussimulations with the EUMAC Zooming Model (EZM) were performed. A firstseries of simulations aimed at the description of the wind flow and the disper-sion of inert pollutants [12]. Aspects related to the formation of photochemicaloxidants were analyzed with the multilayer model MUSE in conjunction withthe chemical reaction mechanism KOREM (a modified version of the Botten-heim-Strausz mechanism [13]) for five different meteorological situationsfavouring pollutant accumulation in the Athens basin. The wind field and theturbulent characteristics for these situations were calculated with the nonhydro-static mesoscale model MEMO and are discussed in detail elsewhere [14].

Individual pollutant transport and transformation calculations were per-formed for the study area shown in Figure 1 and the emission scenarios: Noairport (a), operation of the airport at Hellenikon in the years 1993 (b), 2002 (c)and 2012 (d), operation of the airport at Spata in the years 2002 (e) and 2012(f). An emissions inventory for Athens was compiled especially for the needsof the present study [15]. Estimates for the airport emissions are discussedelsewhere [16].

Figure 1: Topography of the study area and locations of stations for whichsimulation results are presented. Altitude isopleths are contoured at 100 mintervals. The two frames indicate the areas for which ozone concentrationdifference contour plots are given in Figure 4.



4 Simulation results

The detailed simulation results can be found elsewhere [17]. In the following,sample results are presented for the meteorological situation of July 7, 1994and three emission scenarios. On the basis of the simulation results, first thecharacteristics of photosmog formation without airport operation are brieflydiscussed and then the contribution of the airport operation (either atHellenikon or at Spata) to the ozone levels in Athens in the year 2002 areestimated.

Predictions of the model MUSE for the diurnal variation of the groundlevel concentration of NO, NOi and ozone in July 7, 1994 at four stations in theAthens basin and the Mesogia plain for scenario (a) - no airport operation - areillustrated in Figure 2. The location of these stations is indicated in Figure 1.Furthermore, Figure 3 shows the predicted ground level concentration patternsof ozone in the entire study area predicted with both the model MUSE and thefull 3D version of MARS for scenario (a) - no airport operation - at 10, 14 and18LSTonJuly7, 1994.

NOA AGP

HEL ppb120 -r100 --

80 -60 --40

200

SPA

__*—

NO2NOO3

Figure 2: Diurnal variations of NO], NO and ozone concentrations at NationalObservatory of Athens (NOA), Aghia Paraskevi (AGP), Hellenikon (HEL) andSpata (SPA) on July 7, 1994 predicted with the multilayer model MUSE forscenario (a) - no airport operation.

At night, the ozone levels are negligible in the city area and along thecoastline of the Saronic gulf. It is evident, that this is associated with the



presence of sufficient NO amounts in these areas during the night. In the morn-ing hours, the formation of ozone appears to be very intensive over the Saronicand the Petalic gulfs, to a large extent because of polluted air masses whichwere advected onto the sea during the night. Until noon, the phenomena in theinterior of the Athens basin remain independent of the situation in Mesogia. Asea breeze cell gradually develops, which advects air mass with a high photo-chemical oxidant content from the Saronic gulf into the Athens basin. This cellcannot penetrate to the east (i. e. to the Mesogia plain), as it is blocked by asecond sea breeze cell formed in the east part of the Attica peninsula. The twosystems meet around Aghia Paraskevi, to the west of Mt. Hymettus, thusleading to relatively high ozone concentrations there during the day. In theevening, NO (and VOC) emissions serve as fast-acting sinks for ozone.Consequently, the ozone levels gradually decrease; the nocturnal ozone levelsare close to zero in the city of Athens.

In the vicinity of the Hellenikon airport higher ozone concentrations arepredicted during the day compared to the city. The local emissions and theadvection of polluted air masses from the interior of the Athens basin, however,lead to a complete destruction of ozone in the evening. Similar phenomena canbe observed in the north of the Athens basin: At Aghia Paraskevi an early-evening concentration peak of primary pollutants (which is also related to theflow configuration associated with the sea breeze) leads to a completedestruction of ozone.

The situation in the Mesogia plain appears to be very similar: Pollutantsemitted in the plain are transported onto the sea during the night and are re-advected by the already mentioned second sea breeze cell to the Mesogia plain.This second sea breeze cell appears to be more intense compared to the cell inthe Athens basin; therefore pollutants from the Mesogia plain may enter theAthens basin, mainly affecting the suburbs of Athens at the northern andwestern slopes of Mt. Hymettus.

Figure 3 reveals that the multilayer model MUSE yields ozone concentra-tion patterns which are in general similar to those obtained with the full 3Dversion of MARS. As a main exception to this, the multilayer model MUSE isfound to underestimate the ozone concentration in the northwestern part of thestudy area. This deviation makes the limitations of a multilayer model overnon-homogeneous terrain apparent: While after sunrise the mixed layer overland begins to grow, the conditions over the sea remain stable. Pollutantsemitted by industry move offshore and reach the upper layer above the Saronicgulf. So, they are inevitably diluted up to a height of 2000 m, i.e. well abovethe mixing height over land. At the same time, the ozone formation in thelower layer is less intense. With the ongoing sea breeze polluted air masses arere-advected only partially into the well mixed middle layer. Therefore, thepredicted ozone formation over the Thriasion plain, the industrialized westernpart of the Greater Athens area, is in the case of MUSE less intense then inMARS.



Figure 3: Predicted near ground concentration patterns of ozone in ppb at 10,14 and 18 LSI on July 7, 1994, for scenario (a) - no airport operation.Left column: results of the multilayer model MUSE,Right column: results of the full 3D model MARS.



10 LSI

14LST

18LST 18 LSI

Figure 4: Predictions of the multilayer model MUSE for the near ground ozoneconcentration differences in ppb between the scenarios with airport operation(left column: Hellenikon, right column: Spata) and that with no airportoperation. Results are valid for the situation in 2002 and are plotted for relevantsubdomains (cf. Figure 1) at 10, 14 and 18 LST on a day with meteorologicalconditions as on July 7, 1994. Negative values (thin lines) correspond to lowerozone concentrations due to airport operation.



Figures 4 and 5 show differences in MUSE predictions for the concentra-tion of photochemical oxidants between scenarios (c) or (e) and (a), i.e. theimpact of the airport operation, either at Hellenikon or at Spata, on thephotosmog levels in the year 2002 for meteorological conditions as on July 7,1994. Specifically, Figure 4 refers to the difference patterns of the ground levelozone concentrations in two subdomains of the study area, whereas Figure 5shows the differences of the diurnal variation of the NO], ozone and O% (= NO]+ Og) concentrations at Hellenikon and Spata. Figure 5 contains alsocorresponding results of the full 3D model MARS.

Figure 5: Predictions of the models MUSE and MARS for the impact of theairport operation at Hellenikon (left) and Spata (right) on the diurnal variationof the NO] (upper thick lines) O] (lower thick lines) and O% (= NO] + Og, thinlines) concentrations in the surroundings of the individual airport for thesituation in 2002 and meteorological conditions as on July 7, 1994. Ox may beconsidered as an indicator for the oxidation potential.

The assumed operation of the airport at Spata does not affect air quality inthe interior of the Athens basin until noon. Small decreases in the ozone con-centration in the Athens basin (up to 6 ppb) are related to primary pollutantsemitted at Spata which penetrate to the Athens basin by advection around Mt.Hymettus. The emissions due to the operation of the airport at Hellenikonaffect mainly the local air quality, resulting in a strong decrease of the ozoneconcentration around the airport. Under the influence of the sea breeze, airportemissions are transported to the north, dampening the ozone formation in thecity of Athens. Apparently, reductions of the ozone concentration in the cases



of both Spata and Hellenikon do not correspond to an air quality improvement:As shown in Figure 5, ozone reductions are accompanied by larger increases inNOi, i.e. net increases in the oxidation potential.

From Figure 5 it can also be inferred that the predictions of the multilayermodel MUSE for the differences in NOi and ozone concentrations are verysimilar to those obtained with the model MARS. In view of the strikingefficiency of MUSE, this model may prove very useful for practicalapplications.

5 Conclusion

The multilayer model MUSE has been applied to analyze the impact of theemissions of the Athens airport on the air quality in the Athens basin and theMesogia plain for various meteorological conditions which favour theoccurrence of air pollution episodes. Individual simulations were performedassuming that the airport operates (i) at Hellenikon (current location: in theinterior of the Athens basin) and (ii) at Spata (planned new location: in theMesogia plain). The comparison of the simulation results with correspondingresults of the full 3D photochemical dispersion model MARS shows that themodel MUSE is capable of reproducing most features of the spatial and diurnalvariation of the major photochemical air pollutants in the Greater Athens area.Hence, the model MUSE is a reasonable extension of the EZM system, as itmay be used in conjunction with the prognostic mesoscale model MEMO forair quality studies on small scale computers (e. g. workstations).

Acknowledgments

This study was part of a major project coordinated by the University of Athensand financially supported by the Organization of the Airport of Athens.

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