Coupling GEOS-CHEM with a regional air pollution model for ...acmg.seas.harvard.edu/publications/2009/Tombrou_et_al_2009.pdf · describe the ozone-NOx-hydrocarbon–sulphur chemistry

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Atmospheric Environment 43 (2009) 4793–4804

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Atmospheric Environment

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Coupling GEOS-CHEM with a regional air pollution model for Greece

M. Tombrou a,*, E. Bossioli a, A.P. Protonotariou a, H. Flocas a, C. Giannakopoulos b, A. Dandou a

a National and Kapodistrian University of Athens, Department of Physics, Division of Environmental Physics and Meteorology, Build. Phys. V, University Campus,Zografou, 157 84, Athens, Greeceb National Observatory of Athens, Institute for Environmental Research and Sustainable Development, V.Pavlou and Metaxa Str., P.Pendeli GR-15236 Athens, Greece

a r t i c l e i n f o

Article history:Received 21 April 2008Received in revised form27 March 2009Accepted 1 April 2009

Keywords:Global to mesoscale model-chainBoundary conditionsSynoptic categoriesGlobal GEOS-CHEM modelCOO3

NOy

* Corresponding author. Tel.: þ30 210 7276935; faxE-mail address: [email protected] (M. Tomb

1352-2310/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.atmosenv.2009.04.003

a b s t r a c t

The sensitivity of regional air quality modeling simulations to boundary conditions over Greece isinvestigated, for various synoptic conditions. For this purpose, a global to mesoscale model-chain isdeveloped and applied, coupling the individual models’ simulations. The global chemical transportmodel GEOS-CHEM, applied in a one-way nested procedure, is used to drive the regional UAM-Vchemical dispersion model with time-varying lateral and top boundary conditions. The results of thecoupling procedure are compared with the MINOS campaign measurements at Finokalia (SouthernGreece) during the period from 1 to 16 August 2001 which is mainly characterized by an interchange oftwo synoptic types, High-Low and Long Wave trough.The comparison between the simulation results and the measurements reveals that the couplingprocedure captures satisfactorily the range of observed CO concentrations at the southern part of Greece.The most severe deviations are observed under strongly variable atmospheric circulation, when nodistinct synoptic circulation is allowed to be established in the area. Regarding O3, the highest, thoughunderestimated, surface concentrations are simulated under Long Wave trough conditions due to theinfluence of the ozone inflow predicted by GEOS-CHEM at the western boundary of the innermostdomain and/or under enhanced NOy emissions arriving at Finokalia from urban and ships plumes.

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1. Introduction influence on the regional model predictions. In particular, we mainly

Until recently, air quality simulations had to rely on the use ofclimatological and often fixed boundary conditions (BCs), repre-sentative of remote and non-polluted areas (Chang and Cardelino,2000). Therefore, for most species, time and space invariant initialand boundary conditions are usually provided with the regionalmodel codes. As global chemical transport models simulations arebecoming available, the necessary BCs for use in regional air qualitymodels can be derived from global model runs. The most commonmethodology that employs global models is their coupling toregional atmosphere-chemistry models (Jonson et al., 2001; Bauerand Langmann, 2002; Langmann et al., 2003; Byun and Schere,2006; Tang et al., 2007; Appel et al., 2007; In et al., 2007).

This paper presents a tropospheric chemistry study over Greeceunder different types of synoptic scale atmospheric circulation, bycoupling the global GEOS-CHEM to the regional UAM-V photo-chemical model. We first examine the boundaries’ variability overGreece, as these are calculated by the nested-grid configuration ofthe global model (Protonotariou et al., in preparation) and then their

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focus on primary CO and photochemical O3, as both of them aresubject to long-range transport, CO mainly inside the boundary layerand O3 in the free troposphere. For a model-observation comparison,we further focus on measurements of O3, NO, NOy and CO collectedat Finokalia during the Mediterranean INtensive Oxidants Study(MINOS) in August 2001 (Lelieveld et al., 2002; Salisbury et al., 2003).The station of Finokalia is located at the northern part of Crete, on topof a well defined hill (130 m above sea level height and slope 0.334).This site is located near the southern boundary of the inner simu-lation domain, therefore, it provides an ideal site to further examinethe effectiveness of the dispersion mechanisms inside this domain.Nevertheless, this particular location does not profit to themaximum from the anticipated impact of the coupling, as the gain isexpected to be higher at the upwind borders of the regional domain.

2. Methodology

2.1. Modeling setup

The global to mesoscale model-chain, developed in this study,consists of two Eulerian models: the global chemical transportmodel GEOS-CHEM (Bey et al., 2001), applied in the one-waynested-grid configuration in Europe (Protonotariou et al., in

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M. Tombrou et al. / Atmospheric Environment 43 (2009) 4793–48044794

preparation) and the combination of the regional atmospherePSU/NCAR Mesoscale model (MM5) (Grell et al., 1994) with thechemistry Urban Airshed Model, UAM-V (SAI, 1999).

The global GEOS-CHEM chemical transport model (v7-01-02) isdriven by assimilated meteorological data from the Goddard EarthObserving System (GEOS-3) of the NASA Global Modeling andAssimilation Office (http://gmao.gsfc.nasa.gov). This modelincludes 41 tracers, over 80 chemical species and 300 reactions todescribe the ozone-NOx-hydrocarbon–sulphur chemistry mecha-nism coupled to aerosol chemistry, also referred to as full-chem-istry mechanism (Park et al., 2003, 2004). For the purposes of thepresent study, a 2-year (January 2000 to December 2001) full-chemistry simulation of the global model is initially performedwith 4� latitude � 5� longitude horizontal grid-resolution. Hourlyboundary conditions are calculated for the second year (allowing1-year model spin up) around the window domain presented inthe upper-right plot of Fig. 1 (20�W to 45�E and 22� N to 74� N),including Europe and parts of neighbouring countries in Africa,Middle East and Asia, in order to be implemented in the nested-grid model simulation. Then, the nested-grid model configurationis applied over the nested-window for 2001, with 1� latitude � 1�

longitude grid-resolution, while, at the same time, results aresaved around the innermost domain, covering the greatest area ofGreece (bottom-left plot in Fig. 1).

The MM5 meteorological model is driven by ECMWF (EuropeanCentre for Medium range Weather Forecast) 0.5� � 0.5� analysis

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Fig. 1. Modeling domains considered in the present study for the nested-window of GEOS-Cdots inside the rectangle indicate the measuring locations of Aliartos (north) and Finokaliadepicted for GEOS-CHEM (top-right plot) and UAM-V (bottom-left plot).

fields, in combination with Sea Surface Temperature (SST) 1� � 1�

data, provided every 6 h. The 25-category USGS land-use classifi-cation scheme was adopted in order to provide land-cover data forthe model domains. The MM5 numerical simulations were per-formed by applying a two-way nesting, with the first simulateddomain covering the extended area of Eastern Mediterranean(149 � 149 grid points with a spatial resolution of 18 km � 18 km)and the second domain spanning the extended area of Greece(147 � 147 grid points with a spatial of resolution 6 km � 6 km).Regarding the model’s physical parameterizations, the followingschemes were applied: the Grell cumulus scheme (Grell et al.,1994), the MRF PBL scheme (Hong and Pan, 1996), the simple iceexplicit moisture scheme (Dudhia, 1989), the cloud radiationscheme (Dudhia, 1989), and finally the five-layer soil model (Dud-hia, 1996). In the vertical axis, 23 sigma layers were used, extendingfrom surface up to 100 hPa. The UAM-V model was applied to thesecond domain (over Greece), following the same grid system asMM5 (Fig. 1, bottom-left plot). The hourly meteorological fields,required by the photochemical model, include horizontal windspeed, temperature, vertical diffusion coefficients, pressure andwater vapor. The chemical mechanism applied by the model is CB-IV-TOX (Ligocki and Whitten, 1992), an extension of the widelyused CB-IV mechanism. CB-IV-TOX contains more than 100 reac-tions and over 30 chemical species.

The regional chemical model was applied for short periods of2001, representing different types of synoptic scale atmospheric

[molecules/cm2/s]

HEM in Europe and the UAM-V regional model over Greece (embedded rectangle). The(south). Mean daily surface anthropogenic emissions of CO (molecules/cm2/s) are also

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M. Tombrou et al. / Atmospheric Environment 43 (2009) 4793–4804 4795

circulations that appear over Greece. In this paper, results arepresented and discussed for the selected MINOS period from 1 to 16August 2001, characterized by a sequence of two different cate-gories of synoptic scale atmospheric circulation: High-Low (HL)and Long Wave trough (LW).

2.2. Coupling procedure/necessary adjustment

Because of differences between the chemical mechanisms of theglobal and the regional chemical models, a matching was neces-sary, especially with respect to the organic part. This matching ispartly based on the study of Moon and Byun (2004) and is furtherextended in order to capture the more detailed study of aldehydesincorporated in the CB-IV-TOX mechanism. The correspondencebetween the species of the two mechanisms is presented in Table 1.GEOS-CHEM organic species are linked to PAR (parafins), OLE(olefins), ISOP (isoprene), ISPD (products of isoprene reactions),HCHO (formaldehyde), PACET (acetaldehyde) and ALDx (higheraldehydes). For the species of the CB-IV-TOX mechanism, with norelevant information from GEOS-CHEM (e.g. aromatics), zero BCshave been used.

2.3. Emissions

Comprehensive emission inventories in the global modeldescribe the natural sources from biosphere, oceans, volcanism andlightning (Price and Rind, 1992), anthropogenic activities (Benko-vitz et al., 1996; Piccot et al., 1992; Wang et al., 1998), biofuels(Yevich and Logan, 2003) and biomass burning, updated for 2001(Duncan et al., 2003), in order to include significant fire activityobserved in the Sea of Azov (Salisbury et al., 2003). Stratosphere –troposphere exchange of ozone is simulated with the Synoz(synthetic ozone) scheme of McLinden et al. (2000), using a globalcross-tropopause O3 flux of 475 Tg y�1.

For the regional simulations, the emission inventories for theterritory of Greece cover transportation, industrial and centralheating sectors, as well as emissions from agricultural activities andforests (Greek Ministry of the Environment). The inclusion ofanthropogenic emissions at the urban areas of the neighbouringcountries, at the northern and eastern boundaries of Greece, wasbased on the land use information. In particular, based on thepercentage coverage of the urban index of each grid cell, theanthropogenic emissions from the EMEP inventory (0.5� � 0.5�

Table 1The correspondence between the species (ppbv) of the CB-IV-TOX and the O3–NOx-hydrocarbon chemical mechanisms.

CB-IV-TOX O3–NOx-hydrocarbon chemistry

[NO2] [NOx][O3] [Ox]–[NOx][N2O5] [N2O5][HNO3] [HNO3][PNA] [HNO4][H2O2] [H2O2][CO] [CO][SO2] [SO2][PAN] [PAN]þ[PPN][PANX] [MPAN][NTR] 1/2*[R4N2][ISOP] (1/5)*[ISOP][ISPD] [MVK]þ[MACR][PHCHO] [CH2O][PACET] (1/2)*[ALD2][ALDX] [RCHO][OLE] 1/3*[PRPE][PAR] [ALK4]þ[C3H8]þ(1/5)*[C2H6]þ[ACET]þ[MEK]þ(1/3)*[PRPE]þ[RCHO]

horizontal grid-resolution) were added to areas characterized asurban. The hourly emissions of NOX, SO2, NMVOC and CO wereprovided for typical summer and winter weekdays. The speciationof the anthropogenic NMVOC reflects the specific characteristics ofthe NMVOC emissions, as measured in Athens urban plumes(Bossioli et al., 2002). The organic compounds emitted from forests(isoprene, monoterpenes and unidentified hydrocarbons) havebeen divided into existing organic classes of the mechanism, apartfrom isoprene which is treated explicitly. The unidentified biogenicspecies have been assumed as slower reacting alkanes, with part ofthe mass as alkenes and as non-reactive (Roselle et al., 1991). InFig. 1, the spatial distribution of the mean daily anthropogenic COemissions is depicted for both the nested-grid window of GEOS-CHEM in Europe (upper-right plot) and the regional modelingdomain in Greece (bottom-left plot).

3. Results

In this section, we first analyze the background large-scaleozone and CO concentrations, as derived from the GEOS-CHEMnesting simulations, at the boundaries of the inner domain,centered over Greece (see Fig. 1, bottom-left plot), for the wholeyear of 2001. This analysis is based on 8 different types of synopticscale atmospheric circulation at 850 hPa isobaric level, that areformed over Greece, following the classification scheme of Kasso-menos et al. (1998): South Westerly flow (SW), North Westerly flow(NW), Long Wave trough (LW), Closed Low (CL), Zonal flow, OpenAnticyclone (OA), Closed Anticyclone (CA) and High-Low (HL).Then, the global to mesoscale model-chain results and comparisonswith observations are presented and discussed for the period from1 to 16 August 2001 of MINOS campaign.

3.1. Background concentrations, measurements and simulationswith the nested-grid configuration of the global model

In Fig. 2a and b, the simulated O3 concentrations by the nested-grid simulation of GEOS-CHEM, in relation to the synoptic scaleatmospheric circulation, is given at the boundaries of Greece, for2001. This representation allows viewing the strong variation of themean hourly surface ozone concentrations throughout the year, attwo selected grids of the GEOS-CHEM nested domain, located at thenorthern and southern boundaries of Greece. It was found that thebackground O3 concentrations, as determined by the nested-gridsimulation of the global model with high grid-resolution (1� �1�),differ substantially (up to 30 ppbv for some hours) from South toNorth (Fig. 2a and b). Significant differences are also evidentbetween the surface and the higher levels (not shown). Further-more, a strong variation of O3 background levels is apparent, amongthe different synoptic types, at lower levels. The lower maximumvalues are calculated for the CL and LW synoptic types, while the HLand OA types are both related to higher O3 values, especially duringsummer.

A similar behavior is also reflected in the surface measurementsat the rural sites of Aliartos and Finokalia, presented in Fig. 2c and d.For the measurements in Aliartos, the variation of the meanconcentrations between the synoptic types reaches up to 10 ppbv,with the lowest mean value corresponding to Zonal flow (9 ppbv)and the largest one (19 ppbv) to LW. Furthermore, at this stationthere is no apparent variation of the minimum concentrationbetween the synoptic types, as these values are close to zero for allcases. Nevertheless, the variation among the correspondingmaximum concentrations reaches up to 30 ppbv, with the smallestvalue (33 ppbv) corresponding to CL and the largest one (63 ppbv)to OA, LW and HL categories, during the summer. Correspondingly,at the Finokalia station, the variation between the mean

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Fig. 2. Mean hourly O3 concentrations during 2001 (a), (b) predicted by GEOS-CHEM and (c), (d) measured, over Northern and Southern Greece, respectively, in relation to thesynoptic scale atmospheric circulation.


concentrations is 18 ppbv, with most of the categories havinga mean value of 25–30 ppbv apart from OA, CA and HL that rangebetween 36 and 41 ppbv. The corresponding variation between themaximum values is 16 ppbv, with the largest one (74 ppbv) beingobserved for OA, CA, LW and HL and the lowest one (58 ppbv)measured under CL atmospheric conditions. These strong spatialand temporal variations justify the need for this study. Besides, it isinteresting to observe if the application of spatially and temporallyaveraged BCs profiles can lead to satisfactory simulation results,when there is an apparent variation under the same synopticcategory.

3.2. Simulations with the regional model

To address the points raised above, simulations were performedfor the Greek domain with UAM-V model using (a) spatially andtemporally varying BCs from GEOS-CHEM nesting simulation and (b)fixed BCs representative of remote and non-polluted areas (referencecase). We have not applied the scenario with mean temporal and

spatial profiles along each boundary, as was done in the studies ofTang et al. (2007) and Appel et al. (2007), since there is no reason fornot making full use of the global model predictions available.

In particular, in the first case (a), hourly fields of the chemicalspecies, reported in Table 1, were applied to the top and the lateralboundaries of UAM-V for selected periods during 2001, but wefocused mainly on the period from 1 to 16 August 2001 of MINOScampaign.

For the reference case (b), we have performed simulations usingconstant BCs, representative of remote areas. The values of 35 ppbvfor winter- and 50 ppbv for summer-days were considered asconstant boundary values of O3. It should be mentioned that thesevalues have been concluded from an extensive review based on thereported works on regional background O3 measurements,collected at rural areas in continental Greece (e.g. Glavas, 1999;Kourtidis et al., 2002). In particular, these measurements showozone values of 50 ppbv or even higher, during summer daytime.Also, at the rural site of Finokalia, O3 measurements range between35 ppbv during winter and 59 � 5 ppbv during the dry period


(Kouvarakis et al., 2000). The CO concentration was set equal to150 ppbv, a representative value in the lower troposphere at theEastern Mediterranean region, especially for air masses originatingfrom Eastern Europe, as measured during the MINOS experimentalcampaign (Traub et al., 2003; Good et al., 2003; Salisbury et al.,2003). For the SO2 and the organic compounds, zero values havebeen used as boundary conditions.

3.3. Top and lateral boundary conditions

The prevailing synoptic conditions, during the selected MINOSperiod correspond mainly to an interchange of the synoptic typesHL and LW (from 5041 to 5473 Julian hours, Fig. 2), being associatedwith strong northeasterlies surface winds over the Aegean Sea forthe first type and northerlies/northwesterlies for the second type.However, for both types, the prevailing wind direction overnortheastern Crete (including Finokalia) is northwesterly, as can beseen in Fig. 8b. More specifically, on 30 July, the HL type prevailsover Greece, characterized by the combination of the Pakistan lowin the east and an anticyclone being centered over central Europe.Then, the circulation changes to LW for the following three days,when the Pakistan low extends further west over the Aegean Sea.From 3 to 5 of August, the HL type is formed again, being followedby another three days of LW type. During the last eight daysof the examined period, HL interchanges with LW, depending onthe depth of the westward extension of the Pakistan low into theAegean Sea, while there is one day (12 August) that is assigned toNW type, being characterized by synoptic northwesterly flow. Thischange of the atmospheric type, on a daily basis, from one type tothe other during the last days does not allow the prompt adjust-ment of the simulated meteorological fields and therefore couldaffect the related dispersion patterns.

Salisbury et al. (2003), based on both the trajectory results andchemical tracers at Finokalia (in particular CO, black carbon andacetonitrile), concluded that for the first MINOS period (29 July to 7August) the influence from the biomass burning was at its minimum.On the contrary, in the period 8–12 August, the peak in the abovespecies was attributed to the large number of fires around the Sea ofAzov, apparent from the MODIS satellite pictures available fromNASA (http://veimages.gsfc.nasa.gov/2009/Ukraine.A2001222.0840.1km.jpg). During the remaining period of the campaign (12–21August), the biomass burning influence was still high, and wasattributed to recent biomass burning events (by the above authors).

In order to examine the concentrations levels that we expect atthe boundaries of the regional domain, indicative lateral BCs for CO

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and O3, used by UAM-V, are shown at 14:00 LST, for 8 and 15August, as representative of LW and HL categories, correspondingly(Figs. 3 and 4).

During a typical summer HL day, the combination of the Pakistanlow over Eastern Mediterranean and the subtropical high overcentral Europe (as in the beginning and at the end of MINOS period)establishes strong North Easterlies (the so called ‘‘Etesians’’) overthe Aegean Sea. This synoptic meteorological condition favors thetransport along the northeastern boundary, most likely fromTurkey, with CO values comparable with those in the winter period(not shown), although the summer anthropogenic emissions aresubstantially lower. During summer, the atmospheric conditionsfavor the intense mixing, enabling CO from high polluted areas todisperse to longer distances. However, along its transport overregions with limited human activities, CO is strongly controlled bythe balance between photochemical production and destructiondue to oxidation by OH. Therefore, CO concentrations, at theboundary periphery of UAM-V domain, have in general similarvalues with those below 2 km, during the winter (not shown). Thesubstantially higher CO concentrations presented on 15 August, atthe northern boundary (Fig. 3, upper plot), are attributed to the largenumber of fires around the Sea of Azov, as mentioned previously.There is also a strong spatial variation below 2 km, exceeding50 ppbv, with the lower values (140 ppbv) provided by the GEOS-CHEM at the southwest corner and the highest ones (>200 ppbv)shifted towards the north and northwest of the boundary periphery.

Regarding O3, the HL synoptic condition favors its accumulationall over Europe (not shown), with the higher surface values overthe central Mediterranean (60 ppbv, Fig. 4) and the Middle Eastand Northern Africa at higher levels (not shown). As for CO, thetransport along the northeastern boundary also exists for O3;nevertheless, the surface concentrations simulated by GEOS-CHEMare rather low (45 ppbv) in comparison to the higher levels values.The most intense spatial variation inside the mixing layer occursduring the nighttime, with the larger values being calculated at thewestern and southwestern boundaries (55 ppbv) and the lowerones along eastern and northern boundaries (35 ppbv) (notshown). During the day, there is an increase of the lower valuesthat results in reducing the largest spatial difference to 15 ppbv(Fig. 4). Between nighttime and daytime period, the largestdifference is observed at the regions with the lower values (eastand north). As expected, there is also a strong O3 concentrationgradient with height, but substantially weaker in comparison tothe winter period, as the tropopause in summer is shifted at higheraltitudes.

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When the Pakistan low extends westwards into the Aegean Sea(LW category), the wind turns to northwestern sector over centraland Southern Aegean, including Crete, retaining its high intensity.Under these synoptic conditions, CO continues transporting fromthe Black Sea, initially southwestwards and then southeastwardsalong the Turkish coast. For this reason, even in cases withremarkable accumulation of CO over the Black Sea, the transportedmasses towards the southern boundary are expected to be muchlower, compared to the HL summer case. On the other hand, there issignificant transport along the western boundary from Italy, sup-ported by moderate northwesterlies, which, however, drive to thesouthwestern edge of the examined area. Furthermore, COconcentrations are confined near the surface, due to the formationof lower boundary layer height under cyclonic conditions in thelower troposphere.

Regarding O3, the spatial variability is higher compared to theHL day (Fig. 4). During the daytime hours the larger values arecalculated at the western and southern boundaries (65 ppbv) andthe lower ones along eastern and northern boundaries (40 ppbv).Thus the spatial variability of the surface ozone reaches up to25 ppbv during daytime while it is increased up to 35 ppbv duringthe night (not shown). The O3 concentrations present high values inthe upper troposphere between 10 and 12 km along the south-eastern border (up to 105 ppbv) that are typical of a tropopausedescending in the presence of an upper air trough, contrasting theprevious HL case. The above mentioned flow patterns for COaccount for O3 along the northern and western border.

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3.4. Influence of temporal and spatial variations of boundaryconditions

In the present section, CO, NO, reactive nitrogen species (NOy)and O3 hourly concentrations predicted by the UAM-V modelconsidering (a) spatially and temporally varying BCs from GEOS-CHEM or (b) fixed BCs, are compared with the available surfaceobservations from 1 to 16 August (Giannakopoulos et al., 2002;Salisbury et al., 2003). In order to evaluate the coupling procedure,statistical measures have been calculated for all the modelsinvolved in the nesting chain at Finokalia station (Tables 2–5).Moreover, the spatial distribution of CO concentrations accompa-nied by the spatial wind pattern, are presented in Figs. 6a and 7a, at14:00 LST, for the HL and LW summer days (15 and 8 Augustrespectively). In the same figures, the corresponding differences inCO concentrations between the simulations with BCs calculated byGEOS-CHEM and those with constant BCs (150 ppbv) are alsopresented (Figs. 6b and 7b). The measured wind speed and direc-tion (Krol et al., 2005) and model predictions, as well as theboundary layer heights, calculated during the whole period, areshown in Fig. 8, so as to facilitate the discussion.

Representative trajectories during MINOS have shown that the airmasses throughout all three periods arrived at Finokalia from thenorth, having passed over the Aegean Sea (Salisbury et al., 2003). Forthis reason, Krol et al. (2005) claimed that the local emissions fromCrete can generally be ignored under such weather conditions at thestation of Finokalia. Also, in the present study, the calculated wind

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inokalia station (Southern Greece) during the period 1–16 August 2001.

Table 2Statistical parametersa for the evaluation of CO predictions (UAM-V, GEOS-CHEM) atthe station of Finokalia.

l–16Aug 1–7Aug 8–11Aug 12–16Aug

Mean (ppbv)Observed 160.7 143.7 199.7 157.9UAM-V with fixed BCs 131.7 130.4 133.3 132.5UAM-V with variable BCs 128.1 126.0 126.8 133.5GEOS-CHEM 169.2 168.1 176.4 164.3

Standard deviation (ppbv)Observed 35.8 25.2 40.6 14.1UAM-V with fixed BCs 3.9 3.9 3.4 3.8UAM-V with variable BCs 16.6 19.3 12.7 11.9GEOS-CHEM 17.4 17.5 18.8 13.4

Mean Bias (ppbv)UAM-V with fixed BCs �29.1 �13.3 �66.4 �25.4UAM-V with variable BCs �32.7 �17.8 �72.9 �24.4GEOS-CHEM 8.2 24.1 �23.3 6.4

Mean Error (ppbv)UAM-V with fixed BCs 35.0 23.8 68.6 25.4UAM-V with variable BCs 34.1 20.4 73.1 24.6GEOS-CHEM 23.7 25.7 32.1 11.9

R2

UAM-V with fixed BCs 0.002 0.000 0.794 0.059UAM-V with variable BCs 0.316 0.652 0.520 0.939GEOS-CHEM 0.416 0.480 0.548 0.854

a The definition of the statistical parameters is given in the Appendix.

Table 4As in Table 2 but for NOy.




Mean Bias (ppbv)UAM-V with fixed BCs �0.17 �0.37 0.01 �0.05UAM-V with variable BCs �0.14 �0.33 0.02 0.01GEOS-CHEM 1.29 1.11 1.79 1.14


R2



direction, systematically deviates (a few degrees) towards north fromthe measured wind direction at this station where the prevailingdirection is less than 270 degrees, while it rarely exceeds 300 degrees(Fig. 8b). In general, there is a good agreement during midday, whilethe largest differences during nighttime, could be explained by thefact that under weak stability the air masses tend to go around the hilland to be finally channeled by the existing gully, resulting to a changeof the local wind direction. Moreover, considering that a coastalmeasurement site is compared with a grid cell of 6 km � 6 km andthat a large portion of the cell is covered by sea, it is reasonable thatthe diurnal variation, that is obvious in the measurements, issmoothening out in the simulations. Also, the boundary layer heightwas always calculated higher than 600 m during that period, while

Table 3As in Table 2 but for NO.




Mean Bias (ppbv)UAM-V with fixed BCs �0.001 0.002 0.000 �0.007UAM-V with variable BCs �0.002 0.002 �0.001 �0.008GEOS-CHEM 0.032 0.037 0.054 0.008


R2


most of the days it even exceeded the height of 1200 m, indicatingthat the air masses might have been influenced by the terrain beforereaching the Finokalia station.

The period from the beginning of the MINOS campaign and until 7August is characterized by Salisburyet al. (2003) as ‘minimum’ biomassburning. During that period, the calculated CO concentrations by theUAM-V with variable BCs (Fig. 5), are obviously influenced by thevariation of GEOS-CHEM CO concentrations (not shown) and followsatisfactorily the measured values (MB¼�17.8 ppbv, ME¼ 20.4 ppbv).The slightly lower underestimation when fixed BCs are used(MB¼�13.3 ppbv) is associated with the choice of the constant valuethat was extracted from the MINOS campaign measurements. Thesubstantially higher variation of the measured values, however, not ata constant frequency during the daytime, is rather related to the localemission patterns (mainly by the nearby sea transport and other

Table 5As in Table 2 but for ozone.




Mean Bias (ppbv)UAM-V with fixed BCs �16.8 �14.2 �23.8 �14.9UAM-V with variable BCs �18.0 �16.8 �24.0 �14.9GEOS-CHEM �8.3 �8.6 �13.7 �3.6


R2


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020406080100120140160180200

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Fig. 6. (a) Wind fields (MM5) and mean hourly surface CO concentrations (ppbv), calculated with UAM-V by considering spatially/temporally varying BCs from GEOS-CHEM, and (b)differences in CO concentrations (ppbv) between the two UAM-V simulations: with spatially/temporally varying and constant BCs, over Greek modeling territory at 14:00 LST for anHL summer day (15 August).


activities that are substantially enhanced during the summer period).Also, the coupling procedure gives better statistical measurescompared to the global simulations (MB¼ 24.1 ppbv, ME¼ 25.7 ppbv),for the same period. The calculated surface CO concentrations, withvariable BCs, are also satisfactorily compared with the observationsduring the latest period (12–16 August), although slightly under-estimated (MB¼�24.4 ppbv). This difference, during the latest period,is well explained by the recent biomass burning events as mentionedby Salisbury et al. (2003). In fact, when we added characteristic valuesfor biomass burning emissions at the northern part of the innerdomain, at the area being indicated by the satellite images, the simu-lated concentrations almost matched the measured ones. Therefore,we believe that inclusion of biomass burning emissions in the regionalmodel’s inventory could partly offset the discrepancy between modelresults and observations.

A strong deviation between the calculated and measured valuesoccurs from 8 to 11 August, when the atmospheric circulation isstrongly variable, not allowing a distinct synoptic circulation to beestablished in the area. In particular, during that period, days with LW,HL and NW characteristics, interchange almost on a daily basis. Theimportance of this interchange to the simulated surface CO concen-trations is also apparent from the spatial pattern of the simulatedsurface CO concentrations, shown in Figs. 6 and 7, respectively. Forboth synoptic types, the simulated surface CO concentrations have thesame range of values during the daytime (14:00 LST), but they aredistributed in a substantially different pattern. Therefore, althoughboth the calculated and the observed wind fields show a similarpicture at Finokalia station, the daily variability of the atmospheric

LW summer

5 m/s

020406080100120140160180200

a b

Fig. 7. As in Fig. 6 but for a LW

circulation might effect the meteorological simulations (especiallyover the northeastern inner domain) and consequently the concen-tration patterns over the inner domain. This becomes more apparent,when there is a remarkable accumulation of CO towards the upwindboarder. In addition, the large boundary layer heights calculated byMM5 (Fig. 8c) dilute the CO concentrations effectively over the AegeanSea and do not permit the high peaks to be observed atthe far downward station of Finokalia. The comparison betweenobservations and UAM-V simulations reveals a larger bias duringthat period equal to �72.9 ppbv. The lower bias of the GEOS-CHEMmodel (MB¼�23.3 ppbv) is probably attributed to the predeterminedboundary layer heights which are relatively low (from 100 to 400 m).

As it was expected, the long-range transport does not play a keyrole in the formation of the NO concentrations at Finokalia station.This assumption is supported by the almost identical mean hourlyNO concentrations calculated either with fixed or variable BCs(Table 3). Besides, the larger biases observed in GEOS-CHEM simu-lations are attributed to the coarser emission inventory not incor-porating local NOX emissions (e.g. sea transport, industry). Thenegative bias in UAM-V predictions (Table 3) during the wholeperiod (1–16 August) is mainly attributed to the zero NO calculatedvalues during the early morning and night hours due to the fastreaction of NO with ozone. The typical daily emission profile used inthe UAM-V model leads to an almost identical daily profile of NOconcentrations and in a steady increase around 8:00–9:00 LST(Fig. 9). This increase sharpens only during the 12th of August.During that day an NW synoptic flow is established over the Aegean,shown both in measured and calculated wind direction, while the

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Fig. 8. Mean hourly values of (a), (b) measured and simulated by MM5 wind speed, and wind direction, and (c) simulated mixing height, at Finokalia station (Southern Greece)during the period 30 July to 16 August 2001.


site is in the influence of a western wind during the morning hours(Fig. 8b). As a result the station receives not only the pollution fromthe close vicinity but also the transported Athens plume. As it isshown in Fig. 9, the model successfully reproduces the measure-ments on the 12th of August. The small underestimation of thecalculated NO concentrations is due to the wind speed over-estimation (Fig. 8a). For the rest days it is shown that the calculatedmorning NO peaks are in better agreement with the measured onesunder HL days, while they are overestimated under LW daysbecause of the transport of the Athens and/or the ships plumes.

Similar are the results for NOy (Fig.10). According to the precedingdiscussion, the diurnal variation of the measured values remainsalmost the same during the whole period. Higher concentrations areobserved during the second (8–11 August) and the third (12–16August) period while the highest ones are observed on the 12th ofAugust. Despite the enhanced NOy concentrations observed duringthese days, their low correlation versus CO measurements

(R2<0.0125) implies that the NOy concentrations arriving at Finokaliastation are not tighten to biomass burning (Kondo et al., 2004).Regarding the predicted NOy concentrations, their daily variation isalmost identical during the first period and relatively underestimated(MB ¼ �0.33 ppbv) compared to measurements (Table 4). Theincrease of the calculated NOy concentrations during the second andthe third period agrees very well with the measurements (reflected inMB and means estimations) and seems to be related with thereception of aged plumes (Athens/ships, not shown). This is alsoreflected in the partitioning of the calculated NOy concentrations (notshown). In particular, from 7 to 13 August and in contrast to the restdays, HNO3 consists the major component of NOy (60–80% versus10%) while the contribution of NOX does not exceed 50% (versus 80%).The time-lag between measured and calculated concentrations maybe attributed to inaccuracies of the emission inventory as well as inlimited photochemical activity simulated by the regional model.Comparing the two scenarios (fixed and variable BCs), the results of

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Fig. 9. Measured and simulated (UAM-V with GEOS-CHEM BCs) surface NO concentrations at Finokalia station during the period 1–16 August 2001.


the regional model get improved for the latter one (Table 4),explained perhaps by the provision of secondary pollutants (e.g. PAN,HNO3) concentrations at the borders of the inner domain that mayimpact the spatial distribution of the oxidized nitrogen species.

Regarding O3, the concentration values were measuredsubstantially higher (ranging from 45 to 60 ppbv) from the calcu-lated ones (that ranged from 35 to 50 ppbv), while during the periodfrom 7 to 11 August they even exceeded the value of 70 ppbv(Fig. 11). The highest O3 concentrations are predicted under LWconditions due to the increased O3 inflow, at the western edge of thedomain (e.g. 8 August, Fig. 4) and/or under enhanced NOy emissionsarriving at Finokalia (e.g. 7, 12 and 13 of August, Fig. 10) from urbanand ships plumes (Fig. 7a). Under these conditions, the slope of theO3/NOy correlation results in the highest ozone production effi-ciency, 7.5 molecules of O3 per molecule of oxidized NOX, a valuewhich is in accordance to the O3 production efficiency calculated forFinokalia by other researchers (Heland et al., 2003). Depending onthe examined period, the MB ranges between �14.2 and �24 ppbv(Table 5). The underestimation of the UAM-V surface O3 predictionsis attributed both to internal and external issues. For example,limitations related to mixing processes in the regional model pre-venting the downward mixing from the rich in O3 layers aloft(Bossioli et al., 2009) as well as the relatively low O3 productionefficiency of the CB-IV mechanism (Appel et al., 2007) inhibit surfaceO3 accumulation. Moreover, when the synoptic circulation favorsthe transport of the Athens plume towards the island of Crete, thisremains narrow and dense due to the limited dispersion over thesea. Consequently, the local topography and related flows are

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Fig. 10. As in Fig. 9

decisive in determining the final receptor area over the island. Theunderestimated surface ozone concentrations predicted by GEOS-CHEM under LW days (Fig. 2) worsens further the regional model’spredictions. The fact that the scenario with fixed BCs providessimilar or sometimes slightly better results (Table 5) is probablyassociated with the ‘correct’ literature-based ozone inflow. Never-theless, the net impact of high temporal resolution BCs is notexpected to be of major importance for surface ozone peaks whichare mainly due to local photochemistry. However, it is anticipatedthat the gain will be higher close to the borders of the regionaldomain and at higher altitudes (Szopa et al., 2009; Tang et al., 2007).

4. Conclusions

In the present study, a global to mesoscale model-chain focusingon Eastern Mediterranean including Greece has been applied toinvestigate the effects of temporal and spatial variations of theboundary conditions (BCs) on the regional model’s air pollutionpredictions during different synoptic atmospheric conditions.

The analysis of background surface CO and O3 concentrations, asdetermined by the global model for different synoptic meteorolog-ical circulations, shows a strong variation. For example, regarding O3,

the larger concentrations are frequently calculated under an OpenAnticyclone (OA) and High-Low (HL) conditions, while the lowerones under Long Wave trough (LW) and Closed Low (CL) conditions.The variability of surface O3 concentrations in relation to thedifferent synoptic types is confirmed by the measured values at therural Sites of Aliartos and Finokalia (central and Southern Greece),

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Fig. 11. As in Fig. 9 but for O3.


though the highest concentrations are observed during OA, HL, andLW conditions and the lower ones during CL conditions. Apart fromthe surface, the global model predicts a strong spatial variation withheight along the boundary periphery for both CO and O3.

The global to mesoscale model-chain was applied for a selectedMINOS period characterized by an almost continuous interchangebetween the HL and LW synoptic types. From the comparison withmeasurements, it is shown that under conditions of minimumbiomass burning intensity, the coupling procedure captures satis-factorily both the hourly variation and the range of the observedsurface CO concentrations at the Finokalia site (southern region).During the days of maximum biomass burning activity, the surfaceCO concentrations are seriously underestimated. Apart from theomission of the biomass burning emissions in the regional model’sinventory, this result may be also associated with the extremelyvariable atmospheric conditions (continuous interchange of HL andLW synoptic types) which might prevent the prompt adjustment ofthe simulated meteorological fields. Moreover, the large boundarylayer heights, predicted by the meteorological model, dilute the COconcentrations effectively over the Aegean Sea and do not permitthe high peaks to be observed at the far downward station ofFinokalia. The calculated differences between the two scenarios,with varying and fixed BCs, reach up to 40 ppbv, showing that thebackground CO, as determined by the nested-grid simulation of theglobal model, affects significantly the regional model’s CO simula-tions results.

Regarding O3, the concentration values are strongly under-predicted from the measured ones. The highest, though under-estimated, surface concentrations at Finokalia are simulated underLW conditions because of the influence of the O3 inflow, at thewestern boundary of the innermost domain (Greece) and/or underenhanced NOy emissions arriving at Finokalia from urban and shipsplumes. Under LW conditions, the contribution of the varying, inrelation to the fixed, BCs reaches up to 10 ppbv, mainly along thenorthwestern coast, while the influence increases with height andcontributes up to 15 ppbv all over the modeling domain. Never-theless, limitations related to mixing processes in the regionalmodel prevent its downward mixing.

Acknowledgements

We wish to thank the Greek General Secretariat for Researchand Technology for providing funding for this work through itsbilateral collaboration project.

Appendix

N ¼ the number of simulation hours, Co ¼ observed value,Ce ¼ estimated value, sC ¼ standard deviation of value

Mean Observation : MO ¼1N

XN

i¼1

Coi

Mean Estimation of the predicted values : MES ¼1N

XN

i¼1

Cei

Standard deviation of observations : Sdobs

¼

0@1

N

XN

i¼1

jCoi �Moj21A

1=2

Standard deviation of estimations : Sde ¼

0@1

N

XN

i¼1

jCei�Mej21A

1=2

Mean Bias : MB ¼ 1N

XN

i¼1

ðCe � CoÞ;

Mean Error ðMean Absolute Gross ErrorÞ :

ME ¼ MAGE ¼ 1N

XN

i¼1

jðCe � CoÞj

Correlation coefficient : R2 ¼

26664PN

i¼1�Coi � Co

��Cei � Ce

�sCo

sCe

37775

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Coupling GEOS-CHEM with a regional air pollution model for ...acmg.seas.harvard.edu/publications/2009/Tombrou_et_al_2009.pdf · describe the ozone-NOx-hydrocarbon–sulphur chemistry