Atmospheric Environment 41 (2007) 6445–6464 On the influence of meteorological input on photochemical modelling of a severe episode over a coastal area G. Pirovano a, , I. Coll c , M. Bedogni b , S. Alessandrini a , M.P. Costa a , V. Gabusi d , F. Lasry c , L. Menut e , R. Vautard f a CESI Ricerca S.p.A., via Rubattino 54, 20134 Milano, Italy b Mobility and Environment Agency, via Beccaria 19, 20122 Milano, Italy c Laboratoire Interuniversitaire des Syste`mes Atmosphe´riques, CMC, Universite´Paris 12, 61 av. du Ge´ne´ral de Gaulle, 94010 Cre´teil Cedex, France d DEA, University of Brescia, via Branze 38, 25123 Brescia, Italy e Laboratoire de Me´te´orologie Dynamique, Institut Pierre Simon Laplace, 91128Palaiseau, France f LSCE/IPSL Laboratoire CEA/CNRS/UVSQ, F-91198 Gif sur Yvette Cedex, France Received 7 September 2006; received in revised form 30 March 2007; accepted 11 April 2007 Abstract The modelling reconstruction of the processes determining the transport and mixing of ozone and its precursors in complex terrain areas is a challenging task, particularly when local-scale circulations, such as sea breeze, take place. Within this frame, the ESCOMPTE European campaign took place in the vicinity of Marseille (south-east of France) in summer 2001. The main objectives of the field campaign were to document several photochemical episodes, as well as to constitute a detailed database for chemistry transport models intercomparison. CAMx model has been applied on the largest intense observation periods (IOP) (June 21–26, 2001) in order to evaluate the impacts of two state-of-the-art meteorological models, RAMS and MM5, on chemical model outputs. The meteorological models have been used as best as possible in analysis mode, thus allowing to identify the spread arising in pollutant concentrations as an indication of the intrinsic uncertainty associated to the meteorological input. Simulations have been deeply investigated and compared with a considerable subset of observations both at ground level and along vertical profiles. The analysis has shown that both models were able to reproduce the main circulation features of the IOP. The strongest discrepancies are confined to the Planetary Boundary Layer, consisting of a clear tendency to underestimate or overestimate wind speed over the whole domain. The photochemical simulations showed that variability in circulation intensity was crucial mainly for the representation of the ozone peaks and of the shape of ozone plumes at the ground that have been affected in the same way over the whole domain and all along the simulated period. As a consequence, such differences can be thought of as a possible indicator for the uncertainty related to the definition of meteorological fields in a complex terrain area. r 2007 Elsevier Ltd. All rights reserved. Keywords: Chemistry-transport model; Meteorological model; Ozone; ESCOMPTE; Sensitivity analysis ARTICLE IN PRESS www.elsevier.com/locate/atmosenv 1352-2310/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2007.04.011 Corresponding author. Tel.: +39 02 3992 4625; fax: +39 02 3992 4608. E-mail addresses: [email protected] (G. Pirovano), [email protected] (I. Coll), [email protected](M. Bedogni), [email protected] (V. Gabusi).
Transcript
ARTICLE IN PRESS
1352-2310/$ - se
doi:10.1016/j.at
�CorrespondE-mail addr
(M. Bedogni), g
Atmospheric Environment 41 (2007) 6445–6464
www.elsevier.com/locate/atmosenv
On the influence of meteorological input on photochemicalmodelling of a severe episode over a coastal area
G. Pirovanoa,�, I. Collc, M. Bedognib, S. Alessandrinia, M.P. Costaa, V. Gabusid,F. Lasryc, L. Menute, R. Vautardf
aCESI Ricerca S.p.A., via Rubattino 54, 20134 Milano, ItalybMobility and Environment Agency, via Beccaria 19, 20122 Milano, Italy
cLaboratoire Interuniversitaire des Systemes Atmospheriques, CMC, Universite Paris 12, 61 av. du General de Gaulle,
94010 Creteil Cedex, FrancedDEA, University of Brescia, via Branze 38, 25123 Brescia, Italy
eLaboratoire de Meteorologie Dynamique, Institut Pierre Simon Laplace, 91128 Palaiseau, FrancefLSCE/IPSL Laboratoire CEA/CNRS/UVSQ, F-91198 Gif sur Yvette Cedex, France
Received 7 September 2006; received in revised form 30 March 2007; accepted 11 April 2007
Abstract
The modelling reconstruction of the processes determining the transport and mixing of ozone and its precursors in
complex terrain areas is a challenging task, particularly when local-scale circulations, such as sea breeze, take place. Within
this frame, the ESCOMPTE European campaign took place in the vicinity of Marseille (south-east of France) in summer
2001. The main objectives of the field campaign were to document several photochemical episodes, as well as to constitute
a detailed database for chemistry transport models intercomparison.
CAMx model has been applied on the largest intense observation periods (IOP) (June 21–26, 2001) in order to evaluate
the impacts of two state-of-the-art meteorological models, RAMS and MM5, on chemical model outputs. The
meteorological models have been used as best as possible in analysis mode, thus allowing to identify the spread arising in
pollutant concentrations as an indication of the intrinsic uncertainty associated to the meteorological input.
Simulations have been deeply investigated and compared with a considerable subset of observations both at ground level
and along vertical profiles. The analysis has shown that both models were able to reproduce the main circulation features
of the IOP. The strongest discrepancies are confined to the Planetary Boundary Layer, consisting of a clear tendency to
underestimate or overestimate wind speed over the whole domain.
The photochemical simulations showed that variability in circulation intensity was crucial mainly for the representation
of the ozone peaks and of the shape of ozone plumes at the ground that have been affected in the same way over the whole
domain and all along the simulated period. As a consequence, such differences can be thought of as a possible indicator for
the uncertainty related to the definition of meteorological fields in a complex terrain area.
ARTICLE IN PRESSG. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–64646446
1. Introduction
One of the major difficulties in forecasting airquality over complex terrains is to correctly describethe processes determining the transport and mixingof ozone and its precursors. In particular, therestitution by models of local terrain-inducedcirculations, such as sea breezes, that take place incoastal zones constitutes a real challenge in theassessment of ozone behavior and thus in air qualitymanagement over those regions (Millan et al., 1996;Hastie et al., 1999; Rosenthal et al., 2003; Liu et al.,2002; Tov et al., 1997). Measurement campaignsprovided experimental evidences that sea breezecirculations can transport polluted air masses up to100 km inland (Bastin et al., 2004) and, in combina-tion with mesoscale and synoptic winds, govern theredistribution of anthropogenic pollutants over thesurrounding areas.
The Mediterranean basin is a critical zone,submitted to intense photochemical activity and astrong occurrence of well-established sea breezecirculation. Simulation studies show that the mainfeatures of the sea breezes can be well understoodand reproduced by models, when working at a fineresolution required by the complexity of the terrain(Svensson, 1998; Liu et al., 2002). Anyway theauthors also notice that the calculation of localflows is very sensitive to the synoptic situation, theirexact direction and intensity having strong implica-tions on ozone levels in the basins (Cheng, 2002).Such dependence is a critical point in modellingsummer coastal recirculations, and has to bequantified when using the model outputs for airquality issues.
How much meteo-dependant are model outputs?Clearly, the choice of any input data strongly affectsthe simulated concentration field features. Largeeffects of meteorological calculations (Sistla et al.,1996; Kumar and Russell, 1996) and physicalparameters such as photolysis rates (Dimitroulo-poulou and ApSimon, 1999; Seefeld and Stockwell,1999) on modelled ozone concentrations field havebeen reported in the literature. Nevertheless, manyof those studies have been conducted for simplifiedsituations, and need to be investigated for realsituations, in particular over complex terrain. Butoverall, in air quality forecasting and management,the main question about model input data currentlyarises in the following terms: which intrinsicuncertainty should be associated to the restitutionby a model of a given situation of pollution?
The study that is proposed here aims to describeand quantify the impacts of meteorological calcula-tions on the chemical model outputs. In coastalenvironments, this module indeed may induce thelargest uncertainties on chemical calculations. Thestudy is based on a sensitivity analysis of therestitution of ozone concentration fields to meteoro-logical input, using the three-dimensional (3D)chemistry-transport model (CTM) CAMx (ENVIR-ON, 2003) over a specific pollution episode on theMediterranean French coast. The chosen episode isextracted from the ESCOMPTE campaign and theCTM was driven by two different meteorologicalmodels, RAMS (Pielke et al., 1992) and MM5(Dudhia, 1993), run with the same temporal andvertical resolution, and used as best as possible inanalysis mode. Model outputs have been analyzedin order to quantify the spread in pollutantconcentration and with the objective to draw someindication about the uncertainty to be associatedwith the model representation of a given photo-chemical situation.
2. Site of the study
The investigated domain (Fig. 1) is located in theFrench Mediterranean area and centered on thecities of Marseille, Toulon and Aix-en-Provence. Itis characterized by the presence of a large amount ofhuman activities mainly grouped into two differentpoles: the Marseille-Aix agglomeration including1.3 million inhabitants, and the Fos-Berre industrialarea.
On the contrary, the inland part of the domainpresents rural and forested areas that generate highemissions of biogenic VOCs. It is also characterizedby a complex topography, with increasing oro-graphic altimetry toward the Alps (north-east (NE)of the domain), constraining the air masses trans-port in preferential directions.
This site frequently experiments severe ozoneepisodes: as an example, the information threshold(90 ppbv) is exceeded during 15–40 days per year inthe Bouches-du-Rhone area, making this depart-ment the most affected by photochemical pollutionin France. Typically, during the day, the sea breezesettles in a perpendicular way to the coastlinegenerating a south-west (SW) wind componentover Marseille and a south component overToulon and over the industrial area. Thus, thecoast curve induces a breeze convergence toward
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Fig. 1. (a) The number indicates the position and the meteorological zone of each ground-level meteorological station; black dots show the
radiosounding trajectory (Aix-Les-Milles); (b) the number indicates the position and the chemical zone of each station; the black cross
shows the ozone LIDAR position (LI-ALTO); (c) trajectories of the selected flights; refer to the text for an explanation of the letters.
Orography is shown in gray scale with a contour increment of 100m. Stations that are explicitly displayed in subsequent figures are circled
and labelled.
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–6464 6447
NE bringing polluted air originating from the twourban areas.
Because of the high occurrence of photochemicalevents, this area has been subject to a researchprogram, called ESCOMPTE, aiming to constitutea 3D documentation of several pollution episodes,for chemical and dynamical parameters (Cros et al.,2004; Drobinski et al., 2007). This database hasbeen especially designed for CTM validation andfor an intercomparison exercise at the regionalscale. In this frame, an important field campaigntook place in this area in summer 2001. In additionto the operational observations, the supplementarymeasurements had to give information on the breezecells formation, on the boundary layer develop-
ment, as well as on the chemical composition of theair masses along their transport from the emitter toreceptor areas, and their extension in altitude andover the sea. To this aim, several institutesperformed measurements of meteorological andchemical parameters at ground level, along verticalprofiles (LIDAR, balloons) and in altitude duringseveral flights. Specific ground chemical stationshave also been implemented along three axes,representing the preferential plume directions, inorder to document many VOCs and radical speciesconcentrations. At last, the ESCOMPTE campaignallowed documenting four various and representa-tive photochemical episodes (14 days), so-calledintensive periods of observation.
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3. The modelling system
3.1. Model configurations
Photochemical simulations have been carried outusing the CAMx CTM. In the frame of this studyCAMx has been applied taking into account onlygas phase transformations, described by theSAPRC99 mechanism (Carter, 2000). Meteorologi-cal fields have been supplied by two mesoscaleatmospheric models: MM5 and RAMS.
The PSU/NCAR mesoscale model, known asMM5, is a non-hydrostatic, terrain-following sig-ma-coordinate model designed to simulate orpredict mesoscale atmospheric circulation. Moreinformation on MM5 is given at http://www.ncar.ucar.edu. The MM5 model version used in thiswork includes classical parameterizations forboundary layer (the ‘‘MRFPBL’’ MM5 scheme),radiation, microphysics and convection. Slightmodifications of microphysics and boundary layerschemes are considered and described in Chiriacoet al. (2006). Vertical diffusion and boundary layerheights are calculated by the Troen and Mahrt(1986) formulation.
RAMS 4.4 is a prognostic model based on non-hydrostatic, compressible equations of air dynamicsand thermodynamics conservation equations. TheLEAF-2 model is used to describe storage andvertical exchange of water and energy in multiplesoil layers (Walko et al., 2000).
Planetary boundary layer (PBL) height andmicrometeorological parameters have been com-puted by means of a diagnostic approach, based onCarson (1973) and Holtslag and van Ulden (1983).Then, vertical diffusion coefficients have beenestimated, in stable and unstable conditions, follow-ing the similarity theory approach of Holtslag andNieuwstadt (1986).
3.2. Computational grids, input data and simulated
periods
The computational domain covers the wholeESCOMPTE area, centered on the city of Marseille,and including the city of Avignon at the north-west(NW) and that of Toulon at the east, with a hori-zontal resolution of 3 km (48� 50 cells for approxi-mately 150� 150 km2). Both RAMS and MM5calculated dynamical fields on nested grids (RAMS:30, 10 and 3.3 km, MM5: 9 and 3km resolution)centered on the Marseille agglomeration.
The computational domain extended up to14000ma.g.l. and it has been subdivided in 25vertical levels in MM5 and 30 in RAMS. First layerthicknesses were respectively 35m in RAMS and15m in MM5 grid. Then meteorological fields havebeen interpolated in order to feed the same CAMxvertical grid having 15 vertical layers, whose topsrange between 40 and 8000ma.g.l. The first 10CAMx levels lie inside the convective PBL, while theremaining ones describe the free troposphere.Interpolation has been carried out trying to preservethe information provided by the meteorologicalmodels as much as possible, reproducing almost thesame grid structure inside the PBL and aggregatingthe vertical levels lying in free troposphere.
In both cases boundary conditions were obtainedfrom European Center for Medium-range WeatherForecast (ECMWF) reanalysis fields, available witha time step of 6 h. With regards to chemical inputdata, CHIMERE (Schmidt et al., 2001; Menut,2007) chemical fields issued from simulations at thecontinental scale (0.51 of horizontal resolution) havebeen used to define chemical boundary conditions.The emission inventory used here has been derivedfrom the ESCOMPTE inventory, which provided inparticular a detailed speciation of NMVOC at aresolution of 1 km2 (AIRMARAIX, 2002). Pointsources were also available in the inventory.
The largest ESCOMPTE intense observationperiods, the IOP no. 2 (June 21–26, 2001), waschosen for this study. This IOP divides into twodistinct periods that differ for the synoptic meteor-ological situation (Menut et al., 2005). During thefirst period, the so-called IOP2a (June 21–23, 2001),the region is under the influence of low to moderatewesterly and north-westerly winds that lead to theformation of an oxidant plume at the east of thedomain, in Toulon and off its coastline, due to airmasses transport from Fos Berre and Marseille.During the first 2 days, the observed diurnal ozonemaxima at ground level mainly range from 70 to80 ppbv, but locally exceed the 90 ppbv air qualitythreshold in the eastern part of the domain. Thesecond part called IOP2b (June 24–26, 2001), showsa decrease in the wind speed and well-establishedsea breezes coming from the SW. Over the 3 days,ozone measurements frequently exceed the 90 ppbvlimit in several parts of the domain. A maximum of130 ppbv is reached in Aix on June 25.
In order to evaluate the model performances, arepresentative subset of measurement stations hasbeen selected from the whole ESCOMPTE database
ARTICLE IN PRESSG. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–6464 6449
(Fig. 1). Both meteorological and chemical stationshave been grouped into different subsets of similarbehavior. Meteorological stations have been sub-divided into four groups (Fig. 1a): zones 1 and 2 referto the inland part of the domain, while stations ofzones 3 and 4 are placed along the coast. Chemicalstations have been subdivided in six groups (Fig. 1b)according to the classification provided in the frameof the ESCOMPTE project. Zone 1 refers to theBerre pond industrial area and zone 2 coversAvignon, placed inside the Rhone valley. Zone 3covers Aix-en-Provence and its surroundings whilezone 4 refers to the rural areas NE of Marseille,including the Durance valley. Finally, zones 5 and 6refer to Marseille and Toulon agglomerates.
4. Meteorological simulations
The models are skillful in reproducing thedifferent features of the atmospheric circulation of
Fig. 2. MM5 (above) and RAMS (below) wind fields at ground level on
represent the measured values at the meteorological stations. Compute
both IOP2a and IOP2b. During IOP2a, bothmodels reproduce very well the Mistral synopticconditions, characterized by a quite homogeneouswind blowing from the NW of the domain. A morerelevant and interesting result has been achievedduring IOP2b, as the models have been able tocorrectly describe the diurnal evolution of thesea–land breeze, both along the coast and in theinland part of the domain, however with somediscrepancies. Results are expressed in CentralEuropean Time (CET) which corresponds toUTC+1.
4.1. Ground-level wind field analysis
Fig. 2 is a 2D representation of observed andsimulated wind fields at ground level for 2 h on June25. This figure shows that the models correctlyrestitute the diurnal breeze cycle, with a dominatingland breeze at 07:00 CET and a well-established seabreeze at 16:00 CET.
June 25. Left: 07:00 CET and right: 16:00 CET: The white arrows
d wind fields are plotted every three grid steps.
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In the early morning, when land breeze and weakwinds dominate, the models well identify thenorthern flows on the western part of the domainthat turn to NW over Marseille, as well as aneasterly component in the wind direction on thenortheastern mountainous part of the domain.Nevertheless, as can be seen in the measure-ments (white arrows), the predominance of verylow winds makes the measurements poorly repre-sentative of the area and the model does not capturethe quite high local fluctuations in the winddirection and intensity over and around the mainrelief.
During the day on the reverse, in a better-definedsituation, the global features of the wind fields aremore easily captured by the models: the globalincrease in the wind intensity from 7:00 UT to 16:00CET is simulated, and both models reproduce asouthern flow in Port-de-Bouc turning to a south-western flow on the Toulon coast, with a predomi-nant transport of inland air masses to the NE of thedomain along the Durance valley. Furthermore, ascan be seen on the right graphs at 16:00 CET, theconvergence zone between the breeze and thewesterly flow is correctly located, which is animportant parameter for the location of the ozoneplume maximum.
Fig. 3. Observed and computed hourly wind vectors in five selected me
ground level.
Fig. 3 compares the observed and computed windvectors at five ground-level stations. During IOP2a,synoptic conditions are observed in all the stationsduring daytime hours. The only exception is for theToulon station where local-scale effects due tobreeze circulation prevail. As shown, both modelsreproduce such synoptic conditions, also highlight-ing a correct reconstruction of the diurnal cycle ofthe wind speed. They produce quite similar windfields at the different stations, with the exception ofLa Bastide and overall Toulon, center of theobserved ozone plume, where their restitutions ofdominant flows differ.
During IOP2b, the wind at all stations exhibits amarked sea–land breeze cycle. It is worth notingthat both models capture quite well the timing of thesea breeze, starting in Toulon at about 7:00 in themorning, in Istres at around 11:00, and going inlandat around noon in Aix-les-Milles. However, thecorrespondence is not so clear in La Bastide as thelocal effects due to orography partially dominatethe mesoscale features and no marked diurnal cyclecan be observed. In Avignon, the sea breeze ispresent mainly on June 25 and 26 and is wellestablished at 12:00 or 13:00, a couple of hours laterthan Istres. In Avignon, breeze is well reproducedby RAMS, while MM5 shows a weaker and less
teorological stations. Vectors indicate wind speed and direction at
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variable wind. During night-time, the observationsare clearly influenced by local-scale effects thatMM5 and RAMS do not capture completely. Still, asystematic overestimation of the wind speedscalculated with RAMS in anticyclonic situationsappears in those graphics.
4.2. Statistical analysis at the ground
Table 1 summarizes the mean values, averagednormal bias (ANB) and root mean square error(RMSE) of ground-level wind speed for bothmodels in the different meteorological zones. Thecalculations show that during IOP2a, both modelsoverestimate wind speed with a RMSE of 2.2, whilewind speed variability is well reproduced, as bothmodels showed a standard deviation around2.0m s�1 like the observations (not shown). DuringIOP2b the models exhibit some significant differ-ences: MM5 better reproduces wind speed, andparticularly low values as highlighted by thenormalized error (0.06 for MM5 against 0.58 forRAMS). Nevertheless, MM5 provides slower windsthan observed, in all coastal regions (zones 3 and 4).On the contrary, RAMS wind fields appear toostrong over the whole domain, except in the coastalzone 3 at night. For wind directions (not shown),MM5 and RAMS display a more similar behaviorthan for wind speed. For both IOPs, the fraction ofdirections that differ by less than 301 from theobservations ranges between 40% and 50% for bothmodels. Along the coast (zones 3 and 4) the models’
Table 1
Ground-level wind speed (m s�1): evaluation of models performance
Zone Mean ANB RMSE
Obs MM5 RAMS MM5 RAMS MM5
IOP2a All 3.5 4.4 4.6 0.68 0.63 2.2
IOP2b All 2.4 2.0 3.3 0.06 0.58 1.8
1 2.5 2.2 3.4 0.11 0.58 1.8
2 2.2 2.4 3.3 0.36 0.70 1.8
10:00–18:00h 2 2.8 1.8 4.3 �0.41 0.71 1.6
3 2.6 1.8 3.2 �0.13 0.36 1.8
4:00–10:00 h 3 1.5 1.0 1.6 �0.22 0.25 1.5
4 2.3 1.6 3.2 �0.15 0.61 1.6
4:00–10:00 h 4 1.9 1.3 2.4 �0.12 0.52 1.5
ANB is the averaged normalised BIAS and RMSE is the root mean sq
account the hourly values of all the stations. For IOP2b, statistics h
performances have been evaluated also taking into account a smaller s
10:00 for zones 3 and 4).aMetrics have been computed also taking into account only pairs
observations.
performances are reasonable, even though thedevelopment of the breeze during morning hoursof IOP2b is not well captured; in fact between 4:00and 10:00 in the morning, the fraction of directionsthat differs by less than 301 decrease to 28% forMM5 and 23% for RAMS, but it is worth notingthat during those hours the observed mean windspeed is lower than 2m s�1, thus decreasing thesoundness of wind direction measurements. On thecontrary, the models well capture the direction ofthe inland penetration of the sea breeze duringdaytime hours of IOP2b. Between 10:00 and 18:00the fraction of directions differing by less than 301 is47% for RAMS and reaches 55% in MM5 runs.Statistical analyses have been performed also on asubset of data, including only pairs where simulateddirections by both models differ by less than 301from the observations. Such a subset is character-ized by higher wind speed in both IOPs, thusconfirming that model performances are worseduring weak circulation conditions. Model perfor-mances do not highlight relevant discrepancies withrespect to the whole data set, although in some casesRAMS overestimation is slightly emphasized.
Simulated wind and temperature profiles havealso been compared with the radiosounding avail-able in Aix-les-Milles for June 24 at 15:00.Discrepancies between models and observationsare clearer near the ground than at higher levels,where both models are able to correctly capture themesoscale features of wind circulation and tempera-ture profile (Fig. 4). The similarity between MM5
Meana ANBa RMSEa
RAMS Obs MM5 RAMS MM5 RAMS MM5 RAMS
2.2 4.5 5.1 5.5 0.42 0.49 2.1 2.2
1.8 3.0 2.4 4.4 �0.03 0.69 1.8 2.0
1.7 3.0 2.7 4.5 0.07 0.75 1.9 2.0
1.8 2.6 2.8 4.1 0.29 0.80 1.6 2.1
2.1
1.6 3.8 2.2 4.4 �0.37 0.31 2.2 1.7
1.2
1.9 2.7 1.8 4.5 �0.20 0.89 1.5 2.4
1.5
uare error. For IOP2a, statistics have been computed taking into
ave been supplied for each zone as well. For zones 2–4 model
ubset of hours (from 10:00 to 18:00 for zones 2 and from 4:00 to
where simulated wind direction differs less than 301 from the
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Wind speed
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Fig. 4. Comparison between observed and computed vertical profiles at Aix-les-Milles on June 24— 15:00. Black: observations, light gray:
RAMS, deep gray: MM5.
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–64646452
and RAMS profiles in altitude result from theircommon ECMWF analyses forcing fields.
4.3. Consistency of meteorological parameters in
altitude
Wind field computations have also been com-pared along three flights (Fig. 1c) performed duringJune 25 in the morning (ARAT 2501), around noon(ARAT 2502) and in the afternoon (AZTEC 2502).For each point of the flight trajectories, themodelled wind components (u,v) of the nearestvertical levels have been linearly interpolated inorder to match the coordinates of the observation.
Along all the flights (Fig. 5), the models show aquite similar behavior and a reasonable agreementwith observations. During both morning and after-noon the models reproduce the north-westerlysynoptic winds that take place above 1500ma.s.l.(points A1, A2, A3), and that represent the large-scale meteorological situation. Nevertheless, duringthe morning, the models are not able to capture thecomplex wind structure at 800m a.s.l. characterized
by easterly winds in the Rhone Valley (B) andwesterly (C) and south-westerly winds (D) elsewhere(Khaltoff et al., 2005), as they simulate a ratherconstant direction with winds blowing from thenorth. The observation in altitude of this phenom-enon, which was previously observed in Fig. 2 atground level, implies that the lack of easternstructure in the models mainly comes from themeteorological forcing. At noon (between 12:00 and13:00 CET on the figure), in a well-establishedsouthern sea breeze, the models better reproducewind directions both in Marseille (F) and near theDurance valley (E). They underestimate the windstrength at 3000m a.s.l. while in the boundary layer,model values are closer to the measurements: there,RAMS almost always simulates higher values thanMM5. MM5 values lay below or close to themeasured points while the reverse is observed forRAMS which well estimates (zone E) the windintensity or overestimates it.
The afternoon flight (Fig. 1c—AZTEC 2502)takes off and lands in Avignon and covers theDurance valley at four different heights. Inside
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the Durance valley (from 15:45 to 17:30 CET)both models capture very well the vertical variationin wind direction, from SW (G, H1, H2) to theNW synoptic direction (A4). There, MM5 repro-duces the observed wind speed with a fair agree-ment while RAMS overestimates values within thePBL (estimated by Khaltoff et al. at around1500m).
The analysis carried out on meteorologicalsimulations has first shown that both models wereable to reproduce the main evolution in the synopticsituation from IOP2a to IOP2b. Although discre-pancies were noticed at the end of the night, bothmodels propose a similar interpretation of thecombination of synoptic and local flows and theresulting modelled dynamical situations are close.Indeed, the models do not show disorganizeddiscrepancies, but a pronounced tendency over thewhole domain to overestimate (RAMS) or under-estimate (MM5) the intensity of air masses trans-port along some prevailing directions.
Significant uncertainties on the modelled meteor-ological situation could anyway be identified (i) inthe temporal restitution of the breeze diurnal cycle,(ii) in local flow specificity when the wind is low and(iii) in the amplitude of the recirculation cells inrelation to wind speed at the ground.
Finally, PBL height and vertical diffusion coeffi-cients (Kv) have been compared in order to highlightthe main differences between the two differentestimation techniques. Due to a lack of space, theresults are not illustrated here. Anyway, thecomparison highlights the fact that MM5 provideshigher maximum PBL heights, especially in theinland area. On the reverse, calculated Kv meanvalues inside the PBL are quite similar.
5. Photochemical simulations
Simulations have been conducted using the twometeorological inputs presented above. The simula-tion using MM5 inputs will constitute our referencecase (so-called ‘‘CAMx_base’’), while the simulationconducted with RAMS calculations will be labelledas CAMx_rams. As discussed in Section 4, bothmeteorological modelling systems have proved tocorrectly restitute the evolution of the episode.Subsequently, the two CAMx simulations also showa correct understanding of the photochemicalsituations, providing primary and secondary plumesthat are satisfactorily formed and transported insidethe domain, with maximum values of the right order
of magnitude, but significantly dependant on theconfiguration.
In both situations, the model generates locallyhigh—albeit variable—primary pollutant concen-trations in the urban, industrial and harbor areas, aswell as along the shipping routes. The comparisonwith ground-based stations (Fig. 6) shows thatCAMx is able to distinguish the different features ofIOP2a and IOP2b (e.g. in Salon-de-Provence), andcorrectly reproduces the spatial distribution ofnitrogen dioxide concentrations as can be inferredfrom stations of different typology. As it could beexpected, the model performances are better in therural stations than in the urban and suburban ones,owing to the difficulty for Eulerian models to fullyfit the large and complex temporal variability ofNO2 concentrations in source areas. Anyway, theNO2 spatial gradients and its day-to-day variabilityare well captured by CAMx. The statistical analysisof the NO2 daytime means (not presented in thetables) indeed shows a very good agreementbetween both model outputs and measurements.The gradient from urban (mean value around8 ppbv) to rural (mean value between 2 and 4 ppbv)locations is also well reproduced, although CAMxshows a tendency to overestimate the daily NO2
mean values (RMSE of 2–8 for both configurationsaccording to the zones).
5.1. Ozone daily maximum
During IOP2a, CAMx correctly describes theformation of secondary pollution, with an ozoneplume that tends to be swept away along thecoastline downwind the Berre area, and backgroundvalues in the rural inland part of the domain, asobserved in the measurements (Cros et al., 2004;Menut et al., 2005).
Fig. 7a presents a 2D view of ozone concentra-tions observed at 16:00 CET at all ground-basedstations on June 21. Figs. 7b and c correspond tothe CAMx outputs at the same time. On that day, inthe inland part of the domain, the model mainlyreflects large-scale transport, associated with mod-erate and homogeneous ozone concentrations, asprovided by CHIMERE chemical boundary condi-tions: in both configurations, CAMx simulatesozone values ranging from 62 to 65 ppbv, whichare quite consistent with the background ozonevalues measured at the north-western stations of thedomain (such as Avignon). Statistics howeverindicate that the maximum diurnal values of ozone
Fig. 6. Observed and computed hourly concentrations of nitrogen dioxide in three selected ground-level stations. Black dots:
observations, black line: CAMx_base, gray line: CAMx_rams.
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–6464 6455
modelled over the coastal zones are 5–10 ppbv lowerthan the means of the observed maxima that rangefrom 68 to 72 ppbv (see Table 2). This bias probablystems from the non-restitution by the model of theslight ozone production observed between Berreand Marseille in Fig. 7a. Standard deviationanalysis (Table 2) confirms that modelled concen-tration fields inland are globally too homogeneous,probably due to a slight overestimation of the windspeed during IOP2a.
Contrarily to inland patterns, the neat under-estimation by the model of the regional ozone peakobserved in the coastal Toulon area (zone 6) onJune 21 is more likely due to a bias in the simulatedsynoptic wind than to a lack of oxidant productionin the model. Indeed, both model configurations doproduce ozone in a well-defined plume with amaximum value comprised between 85 and 90 ppbvon that day (against measurements of 85–92 ppbvaround Toulon), but the modelled plume formsdownwind Toulon and at out at sea, instead ofbeing partly located inland at Toulon. It is worth
noting that, as it could be expected from the lowdiscrepancies between the modelled meteorologicalsituations of IOP2a, the impact of regional meteor-ological calculations on the restitution of thischemical episode by the two models is low, albeitit is still visible through the differences in theregional maxima intensity and plume locations.
When breeze circulations develop (June 24–26,2001), the model simulates an ozone plume that isbuilt up between 10:00 and 15:00 under the seabreeze influence, downwind the main anthropogenicareas. In these conditions, ozone is locally producedand then accumulated inland mainly toward NE.Figs. 8 and 9 present the observed and modelledozone concentrations on the domain for June 25–26at 16:00 CET and 13:00 CET, respectively.
As mentioned in Section 4, the RAMS modulecalculates faster winds (with a mean ANB of 0.5)than those observed over the whole domain, whichcauses the pollutants to be excessively dispersed anddiluted in the PBL when reaching the rural areas.On the reverse, MM5 shows a pronounced negative
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Table 2
Ozone daily maximum: evaluation of the model performance during IOP2a and IOP2b
Fig. 8. Ground-level ozone fields (ppbv) on June 25 at 16:00. Observations (a); meteo from MM5 (b); RAMS (c).
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–6464 6457
configuration are much lower than those predictedby CAMx_base during the whole IOP2b.
During IOP2b, the analysis of ozone maps atground level reveals local disagreements in theozone plume features restituted by the two config-urations. On June 24 and 25, observations indicatean ozone plume lying inland parallel to the coastand moving toward the NE, between Aix andAvignon, with maximum values exceeding 90 ppbvand reaching 120 ppbv at Aix on the 25th. Modeloutputs show quite similar ozone plume patterns,but systematic differences appear in the intensity ofthe ozone values, CAMx_rams being permanently5–15 ppbv lower than CAMx_base over thewhole of the domain. At 16:00 CET on the 25th(see Figs. 8b and c), the CAMx_base presents, asobserved, an ozone plume on the north of domainbetween Salon-de-Provence and Cadarache and lowvalues south of Aix, although coastal values close toToulon remain much too elevated, probably due to
the specific recirculation of air masses around Pland’Aups (NE of Toulon) underestimated by MM5(as observed in Fig. 2). Although peak values are ingood agreement with observations, the plume issomehow closer to the sources than observed, due tothe low winds inland. On the east of the domain,CAMx_base simulates its highest ozone values, butthis could not be evaluated due to the absence ofmeasurement stations over all this area. CAMx_rams presents the same structure as the base version,with an ozone plume located further at the north,but with clearly underestimated concentrations(maximum 85 ppbv). The observation of modelledozone concentration maps at the ground for June 26(see Figs. 9b and c) reveals that not only theintensity but also the shape of the secondary plumeis strongly affected by the meteorological config-uration choices. On that day, the breeze combinesinland winds with a north-westerly synoptic circula-tion: observations (Fig. 9a) show that the ozone
Fig. 9. Ground-level ozone fields (ppbv) on June 26 at 13:00. Observations (a); meteo from MM5 (b); RAMS (c).
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–64646458
plume extends from west to east inland, oneremaining over the NW of the domain, while theeastern edge is transported toward the east. At 13:00CET, CAMx_base very well captures the plumepatterns (although the eastern part is a little too farat the north) and simulates, as observed, a largearea of elevated ozone up to Avignon. Peak valuesare satisfactory and exceed 100 ppbv. On thecontrary, CAMx_rams shows a thin plume asso-ciated with concentration maxima lower than85 ppbv and located much too far at the north ofthe domain.
5.2. Ozone time series and vertical profiles
Time series of modelled and measured ozoneconcentrations are presented in Fig. 10. Theobservation of those graphs leads to the followingremarks: (i) Ozone temporal evolution is quite wellreproduced by CAMx in both configurations, which
includes the distinction of the two IOP specificities,the fit of the main diurnal ozone evolution, and theoccurrence of peaks, with the exception of La Ciotatand Toulon on June 21 as previously discussed. Themissed events on June 25 in zone 4 are also due toan inexact location of the plume as mentionedabove. The night-time behavior of ozone is knownto be very difficult to reproduce, mainly because ofits very local origin. Thus, as seen in the graphs, theozone nocturnal behavior is only well restituted inVitrolles (on the top of a hill) and at the mobilestation LISA (far from anthropogenic sources in anopen area). (ii) Ozone underestimation in CAMx_rams is visible on each day experiencing ozoneproduction. The wind speed overestimation ob-served in RAMS outputs thus leads to anticipatedozone maxima compared to CAMx_base (Table 3),systematically underestimated peaks, and the sub-sequent incapacity of the model to catch theobserved events in the northern rural areas (e.g.
Fig. 10. Observed and computed ozone hourly concentrations in six selected ground-level stations. Black dots: observations, black line: CAMx_base, gray line: CAMx_rams.
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Table 3
Evaluation of the delta time between modelled and observed
daily peak of ozone during IOP2b for rural and suburban stations
Zone Mean Sigma
Base Rams Base Rams
No_urb 1.22 0.10 2.23 2.43
3 1.89 0.94 2.03 1.86
4 0.26 �1.15 2.16 2.49
No_urb indicates the whole domain with the exception of
Marseille and Toulon areas. Delta time is defined as the
difference (in h) between the modelled and the observed peak
time.
G. Pirovano et al. / Atmospheric Environment 41 (2007) 6445–64646460
June 26 in IMK). Indeed, daytime ozone peak, onaverage, varies from 6 to 8 ppbv from the base to theRAMS configuration, inducing a doubling of therelative bias in CAMx_rams. In the most sensitiveareas (3 and 4) ozone peaks may vary to more than20 ppbv. (iii) The center of the domain (Vitrolles,Salon-de-Provence) is well understood by the modelbase run during IOP2b. In those areas locatedimmediately downwind the main emitters, thedifference in meteorological fields is already dis-criminating as CAMx_rams does not capture theday-to-day evolution of ozone (Fig. 11).
Ozone concentrations have been also comparedwith vertical profiles provided by the ozone LIDARplaced in Aix-les-Milles (Fig. 1). Observationsreveal a very complex vertical layering due to theinteraction of different phenomena. Inside the PBL,varying from 500 to 1500m along the day, thetemporal evolution of the lowest levels of theobserved and computed profiles are well correlatedwith ground-level observations collected in a stationplaced a few kilometers far from the LIDARplatform (Fig. 1). During the morning and eveninghours, dry deposition and NOX titration togetherwith stable conditions induce the development ofstrong vertical gradients. At noon, ozone exhibitsdifferent behaviors: on June 24, well-mixed condi-tions favor a more homogeneous distribution ofozone concentrations, while on June 25 a sharpgradient is observed inside the PBL. This could bedue to polluted air masses recirculating from thesea, laying above the marine BL during night-time,and then superimposing horizontal advectionof ozone to the local chemical production (Cousinet al., 2005; Kalthoff et al., 2005). Above1500–2000m, additional ozone layers are visible,probably due to stratospheric intrusion as well as tosynoptic scale transport (Dufour et al., 2005;
Ancellet and Ravetta, 2005). CAMx reproducesrather well the ozone daily dynamics in the lowest1000m of the troposphere, mainly related to local-scale processes as well as to sea breeze circulation.However, modelled daytime vertical profiles remainmuch flatter than observations, due to the verticalcutting of the model, the nature of the verticalhomogeneities that are driven by local events, andprobably a too strong vertical mixing. It is worthnoting that meteorological models discrepancieshave a strong effect not only on ground-levelconcentrations but also along the whole PBL. At ahigher altitude, CAMx fails in reproducing thecomplex vertical structure observed both near thePBL top and inside the free troposphere. There, itsimulates, in both configurations, a constant verticalprofile forced by CHIMERE boundary conditions,although both meteorological models have beenable to reproduce the complex vertical structure ofwind circulation (Fig. 5).
6. Conclusions
The CAMx model has been applied in order toinvestigate the sensitivity of CTM to meteorologicaldrivers in a complex terrain area. Simulations havebeen carried out in the frame of the ESCOMPTEproject that was aimed to constitute a 3D docu-mentation of several pollution episodes, for chemicaland dynamical parameters, in the Marseille area.The IOP2 (June 21–26, 2001) was chosen for thisstudy, as it is characterized by the presence of well-defined synoptic pattern (Mistral) during the firstpart, then moving toward sea breeze circulationconditions during the second one.
Two meteorological models, RAMS and MM5,have been used as best as possible in analysis mode,in order to identify the resulting spread in thepollutant concentration fields as an indication of theuncertainty to be associated to meteorologicalinput. Firstly, meteorological simulations have beendeeply analyzed and compared with a considerablesubset of observations both at ground level andalong vertical profiles. Comparisons among modelsand observations have highlighted the fact that thestrongest discrepancies are confined to the PBL,where processes are terrain-driven and sensitive toeach model principles and parameterizations, whilesynoptic flows and altitude transport tend to bebetter reproduced and with few differences betweenmodels. It is worth noting that, at the ground,models do not show disorganized discrepancies, but
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station, black line: CAMx_base, gray line: CAMx_rams. Hours refer to local time (CET).
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pronounced tendencies over the whole domain. Inour case, the models tended to overestimate or tounderestimate the intensity of the air mass transportthrough the restitution of the wind speed.
The photochemical simulations showed that thechoice of the meteorological module was crucial forthe restitution of the location as for the calculationof the intensity of the secondary pollution peaks.This was observed especially under anticyclonicconditions, when low wind speeds emphasize theinfluence of transport and accumulation processeson the whole regional pollution patterns, above allin complex terrain areas.
As a consequence, from time series at ground-based stations we could observe that NO2 is notmuch affected here by meteorological calculationsas most of its variability is located very close to theemission points, and as its concentration quicklydrops to very low values when transportedaway from the sources. Its local variability may bemore sensitive to vertical mixing as parameterized inthe model core. Analogous conclusions can beinferred concerning the intra-day variability ofozone. Diurnal evolution, night-time titration in-tensity and deposition may be related to local-scalephenomena, probably of chemical nature butalso dependent on the nature of the site andits description in the model. Differently the intensityof the ozone peaks and, consequently, the shapeof ozone plumes at ground are clearly influencedby the choice of meteorological driver. Differencesin the location and shape of the secondaryplume thus appear to be linked strongly, andin a direct manner, to the calculation of windspeed that seems more influent than the PBLand turbulence fields. For example, MM5 provi-ded higher PBL height than RAMS during day-time, but ozone peaks were higher with theformer meteorological driver. In fact ozone peakstake place during daytime, when the atmosphere iswell mixed, hence differences in turbulent mixingcan be less relevant than differences in horizontaltransport. Globally, high wind speeds make thesecondary plume thinner, less intense and locatedtoo far from the main sources. This result confirmsthat the intensity of the ozone episodes is stronglyrelated to the variation of the horizontal transport,or, in other words, to the effectiveness of thedilution processes at regional scale.
Differently, high altitude concentrations are notinfluenced by the choice of the meteorologicaldriver, because above the PBL, pollutant concen-
trations are mainly driven by synoptic flows that, inthis study, showed very similar patterns due to thecommon ECMWF boundary fields.
As shown, regional ozone maximum valuesduring IOP2b, on average, may vary from 6 to8 ppbv, corresponding to a doubling of the relativebias. But in the most sensitive areas (such as 3 and 4)ozone peak may vary to more than 20 ppbv. Itmeans that, on average, the sensitivity related to themeteorological driver represents about 10% of thedomain-mean ozone peak, while in the mostsensitive area the sensitivity can reach 20% of thedaily peak observed in those zones (Table 2). It isworth noting that such a variability in ozoneconcentrations derives from a discrepancy insimulated wind fields that during IOP2b, onaverage, corresponds to more than 50% of thedomain-mean wind speed, but reaching 90% of theobserved wind speed in the NE part of the domainduring afternoon hours (Table 1).
In conclusion, the study has put in evidence that,near the ground, circulation shape and intensitymay be the main source of uncertainty in photo-chemical studies over complex domain, descendingfrom the meteorological driver. Such an uncertaintycan give rise to significant discrepancies in ozoneconcentrations, although previous observationshave indicated that meteorological differences havean influence that is less than linear on the ozonemaximum. This was expected as the ozone peakdepends not only on horizontal regional transportof locally produced pollution, but also on large-scale background and vertical exchanges with high-er atmospheric layers. It is worth also noting thatthe observed meteorological discrepancies do notshow random variability between the two config-urations but rather systematic biases that affect thechemical calculations in the same way.
Acknowledgments
The authors are grateful to Fabien Brocheton andVincent-Henri Peuch (Meteo-France) for providingthe ESCOMPTE emission and observation dataused in this study.
CESIRICERCA contribution has been financedby the Research Fund for Italian Electrical Systemestablished with Ministry of Industry Decree DM26/1/2000.
The Municipality of Milan has sustained Mobi-lity and Environment Agency contribution.
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