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model and application to a transboundary pollution
episode between northern France and southern England.
Pierre Tulet, V. Crassier, F. Solmon, D. Guédalia, R. Rosset
To cite this version:
Pierre Tulet, V. Crassier, F. Solmon, D. Guédalia, R. Rosset. Description of the Mesoscale nonhydro-static chemistry model and application to a transboundary pollution episode between northern France and southern England.. Journal of Geophysical Research, American Geophysical Union, 2003, 108, (D1), pp.4021. �10.1029/2000JD000301�. �hal-00136468�
Description of the Mesoscale Nonhydrostatic Chemistry model
and application to a transboundary pollution episode between
northern France and southern England
Pierre Tulet,1 Vincent Crassier,2 Fabien Solmon, Daniel Guedalia, and Robert Rosset
Laboratoire d’Ae´rologie UMR Centre National de la Recherche Scientifique/ Universite´ Paul Sabatier 5560, Toulouse, France Received 27 December 2000; revised 25 September 2001; accepted 25 September 2001; published 14 January 2003. [1] The mesoscale air quality Mesoscale Nonhydrostatic Chemistry (Meso-NH-C) model
is applied to a complex pollution episode over Western Europe during the period 11 to 12 August 1997. As observed in satellite pictures and as simulated, the complexity of this episode is related to the presence of anticyclonic clear-sky areas and regions with deep convective activity in the simulation domain. A brief presentation of the model is made that covers in particular the on-line coupling capability for calculating meteorological and chemical concentration fields at each time step. Then, emphasis is put upon the simulation of transboundary pollution fluxes from London to northern France in a zone of large horizontal wind gradients. Comparison with data from the French Agence De
l’Environnement et de la Maitrise de l’Energie (ADEME) pollution network indicates that ozone concentrations and time of arrival of the pollution plume are correctly predicted at surface stations in northern France. A sensitivity analysis relying upon local ozone production and pollution transport has shown that30% of ozone maxima levels could be attributed to regional transboundary fluxes. INDEXTERMS: 0368 Atmospheric Composition and Structure: Troposphere—constituent transport and chemistry; 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; 3362 Meteorology and Atmospheric Dynamics: Stratosphere/troposphere interactions; KEYWORDS: ozone transport, online model, transboundary pollution, mesoscale pollution modeling
Citation: Tulet, P., V. Crassier, F. Solmon, D. Guedalia, and R. Rosset, Description of the Mesoscale Nonhydrostatic Chemistry model and application to a transboundary pollution episode between northern France and southern England, J. Geophys. Res., 108(D1), 4021, doi:10.1029/2000JD000301, 2003.
1. Introduction
[2] Deterministic air quality simulation and prediction is
a challenge that heavily relies upon advances in both atmospheric physics ( pollution diffusion and transport for different stability regimes at various scales) and in atmos-pheric chemistry (chemical schemes and reaction kinetics, heterogeneous chemistry, emission inventories, etc.). Vari-ous domains in pollution could benefit from such advances: pollution peak prediction, local, regional, and transboun-dary pollution fluxes, particulate pollution, to name a few. Present air quality models (e.g., RADM [Chang et al., 1987]; DRAIS [Fiedler, 1987; Baer and Nester, 1992]; UAM-V [Scheffe and Seaman, 1990]; EURAD [Jacobs et al., 1995]) are generally of a Chemisty Transport Model (CTM) type with decoupled meteorological and chemical modules. Moreover, such models are usually run in anti-cyclonic situations, for which it can be assumed that chemistry and dynamics can be treated independently. This is only one particular meteorological regime, since typically
and most frequently, as in the case of August 1997 studied here, local heavy pollution episodes during a few days in midlatitude western Europe are interspersed with precipitat-ing perturbed situations sweepprecipitat-ing parts of this domain. In such contrasted regimes, as will be seen later, western Europe generally displays highly polluted clear-sky anti-cyclonic areas next to stormy ones. In such situations, chemical transformations and dynamical transports display a large range of characteristic time constants [Andreae and Crutzen, 1997]. This results, at least in some areas where rapid evolution takes place, in the need for dynamical and chemical on-line coupling. This has been realized in the Mesoscale Nonhydrostatic Chemistry (Meso-NH-C) model. On-line capability of Meso-NH-C is a distinctive character-istic allowing for the computation of meteorological and chemical fields at each time step, resulting in simultaneous advection and turbulent diffusion transport, respectively for dynamics and chemistry. This means that all implicit and explicit transport formulations are the same for both chem-ical species and thermodynamchem-ical variables (advection, convective fluxes, turbulent diffusion). This on-line cou-pling has been a primary goal in designing the Meso-NH-C model in order to simulate complex spatially nonhomoge-neous and rapidly evolving situations such as occur in summer with both stormy (rapid aqueous phase chemistry)
1Now at Meteo France, DP/ENV, Toulouse, France. 2
Now at ORAMIP, Colomiers, France.
Copyright 2003 by the American Geophysical Union. 0148-0227/03/2000JD000301
and clear-sky conditions. This is hardly be done with CTMs. Also, Meso-NH-C can be run with high vertical resolution, in the present study 64 levels of which 33 being in the boundary layer. Three further consequences ensue from the argumentation above. First, particular care must be taken in the subgrid parameterization of shallow and deep convective transports, both at grid and subgrid scales. Second, chemical initialization must reflect the nonhomo-geneous spatial structure of the various fields. Finally, special effort must be devoted to biogenic emissions with strong diurnal modulation. These three points will be addressed in section 2.
[3] Meso-NH-C is then applied to a complex situation in
August 1997 over western Europe, with strongly polluted clear-sky areas adjacent to deep convective areas. This paper will particularly focus on the evolution of transboun-dary pollution fluxes in a limited area encompassing part of the Channel, southeastern England, and northern France. In a cloud-free zone a detailed study is made for ozone pollution at the surface and in the boundary layer, with comparisons between model results and available data.
2. Mesoscale Nonhydrostatic Chemistry Model
[4] A schematic overview of Meso-NH-C is depicted in
Figure 1. Meso-NH is a meteorological model jointly developed by CNRM (Meteo France) and Laboratoire d’Aerologie (Centre National de la Recherche Scientifique) [Lafore et al., 1998]. The chemical modules displayed in Figure 1 are coupled on-line with Meso-NH, which means that the meteorological and chemical fields [Suhre et al., 1998] are simultaneously computed at each time step and each grid point.
[5] Meso-NH-C allows for small scale (LES type) to
synotic scale simulations (horizontal resolutions ranging from a few meters to several tens of kilometers). Meso-NH-C can be run in a two-way nested mode involving up to eight nesting stages. Another important feature is its easily variable vertical resolution, allowing for detailed description of the
boundary layer as well as of the upper troposphere-lower stratosphere region. A preliminary study describing the above quoted on-line capabilities can be found in [Tulet et al., 1999].
2.1. Gas-Phase Chemical Module
[6] Various gas-phase chemical modules can be
imple-mented using the modular structure (Figure 1) of Meso-NH-C. The Regional Atmospheric Chemistry Mechanism (RACM [Stockwell et al., 1997] has been selected as it is widely used and provides consistency with the European Genemis emission inventory [Wickert et al., 1999]. Through validation tests made with the full RACM, a reduced version of RACM has been developed to reduce the computational costs of intensive three-dimensional (3-D) on-line chemical simulations and sensitivity studies. The reduced scheme ReLACS comprises 37 species (against 77 species in RACM) and 126 reactions. As shown by Crassier et al. [2000], it compares favorably with RACM and has been used in our simulations.
2.2. Dry Deposition
[7] The general resistance parametrization for dry
deposi-tion velocities of Wesely and Hicks [1977] has been intro-duced into Meso-NH-C. The surface resistance incorporates both the physical and biological surface characteristics together with the solubility of deposited species [Baer and Nester, 1992]. For vegetated surfaces [Baldolcchi et al., 1987; Wesely, 1989], one further considers the relative contributions of stomatas, mesophyllic tissues, and cuticle whereas for liquid surfaces, Erisman and Baldocchi’s [1994] parameter-ization is used. These parameterparameter-izations have been included in the submodel Interaction Soil-Biosphere-Atmosphere (ISBA) [Noilhan and Mahfouf, 1995] and coupled with the 255 surface classification types of Meso-NH. ISBA calculates such evolving parameters as aerodynamical, quasi-laminar, stomatal resistances, and drag coefficients for different vegetation types. So, chemical dry deposition velocities evolves at each time step together with surface wind, turbulent conditions and chemical specificities (Henry’s solubility constant, biological reactivity [Wesely, 1989]).
2.3. Anthropogenic Emissions
[8] The Eurotrac Generation and Evaluation of Emission
Data (GENEMIS) anthropogenic emissions inventory is used [Wickert et al., 1999]. Basically, it consists of an European hourly emission inventory at 5-km resolution for SO2, NOx, CO, NH3, and for 30 hydrocarbon classes
previously defined in an American context [Middleton et al., 1990]. This inventory, presently available for the years 1990 and 1994, is based upon the CORINAIR [CITEPA, 1992] database. Major pollutant sources include traffic, industry, solvent use, waste treatment, and agriculture. A preliminary comparative analysis has been made upon sur-face temperatures between the 11 – 12 August 1997 simu-lated period and the summer 1994 to select optimal days in the GENEMIS database.
2.4. Biogenic Emissions
[9] Isoprene and monoterpenes, mainly emitted by
for-ests, are the biogenic species in Meso-NH-C. Parametriza-tions of these volatile organic compounds (VOC) fluxes follow Guenther et al.’s [1991] algorithms. The biogenic Figure 1. Synoptic overview of the Mesoscale
fluxes are expressed as products of emission potentials (normalized to standard meteorological conditions) multi-plied by environmental correction factors that are functions of the surface temperature and the photosynthetically active radiation (PAR). Emission potential maps for isoprene and monoterpenes have been drawn for France, based on the Coordination of Information on the Environment (COR-INE) Land Cover database, statistical data on French forests issued from the French National Forest Inventory (IFN) and bibliographic data on tree species emissions. These different data hava been merged using a Geographic Information System (GIS) to integrate the emission properties from leaf to regional scales [Geron et al., 1995]. Once inserted into Meso-NH-C, these emission potentials (typically at a 2-km resolution) are used to compute the isoprene and mono-terpene emission fluxes following [Guenther et al., 1993]. Conversion from emission potentials to emission fluxes requires surface temperatures and PAR fields issued from Meso-NH-C, after corrections considering fractional cloudi-ness and introduction of a radiative attenuation factor within the vegetation cover [Roujean, 1996]. Such a procedure allows on-line diurnal evolution of the biogenic emission fluxes within Meso-NH-C (F. Solmon et al., Regional inventories of isoprene and monoterpenes emission poten-tials over France for mesoscale chemistry applications, submitted to Journal of Geophysical Research, 2001), for France only. No such computations are attempted outside of France.
2.5. Convective Transport Scheme
[10] The convection parameterization [Bechtold et al.,
2001] is based upon the Kain and Fritsch [1993] mass flux scheme. It has been implemented within Meso-NH-C to calculate the subgrid scale transport of chemical species. The mass flux parameterization represents the vertical transport in convective drafts, i.e., updrafts bringing boun-dary layer air upward and downdrafts that represent down-ward transport of midtroposheric air. Furthermore, the convective drafts horizontally exchange mass with their environment through detrainment of cloudy air and entrain-ment of environentrain-mental air.
2.6. Chemical Initialization
[11] This a crucial problem in all pollution simulations.
Here most reactive species (hydrocarbons, nitrogen oxides, etc.) have been initially set equal to zero. CH4is initialized
to a typical background value of 1700 ppb. Owing to strong vertical stratification and tropopause deformations in perturbed conditions, ozone initialization requires another procedure. In the troposphere, ozone profiles are initialized using the closest in time Measurements of Ozone by Airbus in Service Aircraft (MOZAIC) soundings [Marenco et al., 1998]. In the stratosphere, above the dynamical tropopause defined as a high potential vorticity surface (over 2 PVU), one adopts a linear relationship between ozone and PV, 1 PVU corresponding to 100 ppb of ozone [Ebel et al., 1991]. Now, owing to possible stratosphere-troposphere exchanges, one problem is to determine the location of the stratospheric air. Three criteria have been used. First, stratospheric air is assumed to be found only above 700 hPa; second, its PV must be higher or equal to 2 PVU; and third, its relative humidity must be lower than
70%. Another important point is that PV correspondence to ozone is also used at the model domain lateral boundaries. So, it is possible to account for tropopause evolution during the whole integration period, through lateral forcing by the dynamical analysis. For nonstratospheric air, lateral boundary conditions of zero gradient chemical fluxes are applied.
3. The 11 – 12 August 1997 Pollution Episode
3.1. Synoptic Study and Satellite Pictures
[12] A complex situation with adjacent clear-sky and
stormy regions, from 11 to 12 August 1997 has been selected, associated with ozone peaks over western Europe, particularly in the north of France. The model domain (Figure 2a) is comprised between latitudes 41°N and 54°N and longitudes 10°W and 16°E. In Figure 2 the GM symbol is located at 45°N on the Greenwich meridian.
[13] For 11 August 1997 the geopotential map on the 300
hPa surface in Figure 2a shows a deep cutoff low over Barcelona, associated in the satellite picture in Figure 3a with characteristic cloud wrapping from southwestern France to England [Browning, 1997].
[14] A cloud band is also extending from southwestern
France to northern England, with a band of very dry air behind. Several convective systems are developing. The first one (number 1 in Figure 3a), very intense, is located within an ascending area east of the cutoff low [Hoskins et al., 1985]. The convective zone number 2 (Figure 3a) is embedded within the cloud wrapping area.
[15] East of these cloudy regions, prevailing anticyclonic
conditions maintain clear skies over northeastern France and northern Spain (Figure 2a; Figure 3a). On 12 August 1997 at 1200 UTC, the low has moved eastward filling rapidly (Figure 2b) and France is now influenced by anticyclonic conditions. Another front is seen approaching the British Isles before dissipating near the above ridge. As shown in the satellite picture (Figure 3b) for 12 August 1997 at 0700 UTC, most of France and Europe are under clear skies.
3.2. Ozone Observations From the French ADEME Surface Network
[16] Figures 4a and 4b illustrate the maximum hourly
ozone values over the French Agence De l’Environnement et de la Maitrise de l’Energie (ADEME) network for the period 11 to 12 August 1997, respectively.
[17] On 11 August the Mediterranean low is associated
with convection, high winds, and cloudy conditions that are not favorable for ozone formation. Indeed, southwest of the line Caen-Marseille (Figure 4a), low ozone concentrations are observed, much below the European threshold level 1 of 55 ppb (Figure 4a). This value is exceeded only at a few urban sites like Marseille or Lyon. Northeast of this line, ozone levels are generally above the 55 ppb level. For example, ozone concentrations in Paris reach the threshold level 2 (90 ppb), while concentrations in Alsace (at Colmar and Mulhouse) exceeded 90 ppb. On 12 August, anti-cyclonic conditions have settled over France and are asso-ciated with a general increase in ozone concentrations. Nevertheless, these values are still below 55 ppb in south-western France. Measured ozone concentrations exceed 90 ppb in three regions: northern France (e.g., at Lens and
Bethune near Lille), Alsace, and the Paris area. With the exception of a few local sites near Lyon, ozone levels are between 55 and 90 ppb. Lyon reveals a particular situation with nearby stations displaying both high and low ozone values with an average value in the range of 55 to 90 ppb: this can be tentatively attributed to complex topography and strong spatial heterogeneity of pollutant sources. So, the period 11 – 12 August 1997 can be described as a regional ozone pollution episode over northern France with ozone increasing from 11 to 12 August.
4. Meso-NH-C Simulation for 11 – 12 August 1997
[18] Simulation of the 11 – 12 August 1997 episode thus
appears quite challenging. This was done using 64 levels in the vertical between the surface and 16 km, with a 40 m
resolution near the surface, stretching to 800 m at the model top. This stretching gives 33 levels in the boundary layer up to 2000 m, while preserving sufficient resolution in the free troposphere and at the tropopause. The horizontal resolution is 30 km in a 1920*1620 km2 domain large enough for depiction of transboundary fluxes in western Europe. The simulation starts at 0000 UTC on 11 August using the French operational analysis from the model ARPEGE. It is intended to represent a complete diurnal cycle of the boundary layer while possibly taking into account early morning pollution peaks due to road traffic.
4.1. Ozone in the Upper Troposphere
[19] Figure 5a displays the simulated ozone field at
11,500 m on 11 August 1997, at 1300 UTC. This field can be directly compared to the satellite picture in Figure 3a.
Figure 3. Infrared National Oceanic Atmospheric Admin-istration (NOAA) satellite pictures: (a) 11 August 1997 at 1300 UTC and (b) 12 August 1997 at 0700 UTC.
Figure 2. Arpege wind analyses (arrows with reference scale below) and geopotential (isocontours in mgz) fields on the 300 hPa surface: (a) 11 August 1997 at 1200 UTC and (b) 12 August 1997 at 1200 UTC.
[20] Typically, at this level, ozone can be assumed to be a
dynamic tracer reflecting the dynamics of the tropopause. In connection with low geopotential heights off Barcelona, high ozone values of stratospheric origin (over 200 ppb) are observed there (cf. the isocontour in Figures 5a and 5b). Furthermore, a band of high ozone values (though lower than 200 ppb) is seen behind the upper level cold front, from southwestern France to Scotland. This band is asso-ciated with dry air observed behind the cirrus clouds in the satellite picture in Figure 3a. Elsewhere lower ozone con-centrations (100 ppb) are typical of upper tropospheric air. The wind field wrapping around the cutoff low in Figure 5a
is characteristic of an upper level trough over France. In Figure 5a two zones are identified as A and B. In A, ozone concentrations of500 ppb depict tropopause descent; the same holds in zone B where dry air intrusion and still higher ozone values (700 ppb) are indicative of deep stratospheric descent down into the troposphere [Danielsen, 1964]. Above the Pyrenees, there is descent of the dynamic tropopause (according to the 200 ppb isocontour) down to 7000 m (vertical cross section in Figure 5b). Just ahead of this descent, the convective system 1 observed in the satellite picture (Figure 3a), is characterized in the vertical cross section (Figure 5b) by ascendant wind vectors denoted
Figure 4. The French Agence De l’Environnement et de la Maitrise de l’Energie (ADEME) ozone network. Hourly maxima ozone concentrations displayed according to the European ozone thresholds levels 1 (55 ppb) and 2 (90 ppb). (a) 11 August 1997 and (b) 12 August 1997. See color version of this figure at back of this issue.
Figure 5. For 11 August 1997 at 1300 UTC (after 13 hours of simulation): (a) Ozone concentrations (scale in ppb at the right) and wind vectors (scale at the bottom) at 11,500 m; (b) vertical cross section along the heavy dashed line in Figure 5a (the solid isocontour is for 200 ppb).
as 1. Thus the Meso-NH-C simulation favorably compares to the synoptic and mesoscale observations: this is a pre-requisite before any chemical study.
4.2. Transport and Pollution in the Boundary Layer [21] In the boundary layer, NOx can be considered a
typical anthropogenic primary species. Figure 6a displays the surface NOxconcentrations over western Europe on 12
August at 1500 UTC.
[22] NOxis used hereafter to trace several anthropogenic
pollution plumes in the simulation domain. At this time of the day the mixed layer is thick enough to prevent high NOx
values. Two polluted areas clearly appear in Figure 6a, one at local and regional scales, the other at regional and synoptic scales. In the first area, near the Mediterranean coast, individual NOx pollution plumes are emitted, e.g.,
from Barcelona or Roma. There are a few exceptions: for example, in the Po valley, between Milan and Marseille or in the Rhone valley between Marseille and Lyon where several plumes are merging into extended pollution areas. The second area, densely populated and covering Benelux (Belgium, Netherlands, Luxembourg), parts of Germany and England, appears more continuous. There, even at 1500 UTC, when strong heating and vertical mixing occur, NOx concentrations are everywhere above 1 ppb, as
opposed for example to France and Spain where concen-trations are typically 0.5 ppb. In Figure 6a, a large intense (above 10 ppb) NOx plume extends from Benelux to
England, leading to transboundary pollution fluxes. Partic-ular attention should be given to the horizontal wind shear between Benelux and northern France, which generates, in parallel to pollutant transport from Benelux to England, an opposite westerly flow from London and southern England to northern France. For ozone the result of this latter flow appears in Figure 7 for 12 August 1997 at 1500 UTC. There is an ozone maximum between London and northern France, corresponding in location and intensity to the reported ADEME surface measurements in Figure 4b. The simulated ozone levels agree reasonably well with observa-tions of low pollution (lower than 55 ppb) over central and western France against values over 55 ppb at Marseille. In the Lyon region the coarse horizontal resolution cannot
Figure 6. Reactive odd (a) nitrogen and (b) ozone surface concentrations (in ppb) superimposed upon wind fields on 12 August 1997, at 1500 UTC (after 39 hours of simulation). NOxisocontours for 1 and 10 ppb against 55
ppb (level 1) and 90 ppb (level 2) for ozone. The heavy dashed line in Figure 6a is the surface trace of the vertical cross section in Figure 8. See color version of this figure at back of this issue.
Figure 7. Zoom on northern France of Ozone surface concentrations (in ppb) superimposed upon wind fields on 12 August 1997 at 1500 UTC (after 39 hours of simulation). O3isocontours for 55 ppb (level 1) and 90 ppb (level 2). See
account for the observed large variability (30 to 100 ppb) between stations only a few kilometers apart. There, Meso-NH-C gives threshold level 1 ozone values within 30-km grid cells, in agreement with the average concentrations observed. As for the high concentrations observed in Alsace
(northeastern France), they are underestimated by20 ppb. Apart from possible flaws in the GENEMIS emission inventory, several arguments could explain such systematic ozone underestimation. The horizontal resolution of 30 km is not really consistent with the complex topography of Figure 8. Vertical cross sections on 12 August 1997, along the dashed line in Figure 6b for (a) NOx, (b)
O3at 0600 UTC, (c) NOx, (d) O3at 1200 UTC and (e) NOxand (f ) O3at 1500 UTC (concentration scales
at the right). Wind vectors (scale at bottom left) are superimposed upon the ozone fields in Figure 8b, Figure 8d, and Figure 8f ). The criteria of 10% on the turbulent kinetic energy maximum value appears as a plain line depicting the top of the mixed boundary layer (0.03 m2s 2for Figure 8a, 0.12 m2s 2for Figure 8c, and 0.09 m2s 2for Figure 8e). See color version of this figure at back of this issue.
Alsace and the Rhine valley between the Vosges and Black Forest massifs. Moreover, biogenic emissions have been introduced only in France, not in Germany. On a positive note, Meso-NH-C correctly simulates level 2 ozone con-centrations observed in the flatter Paris and northern France areas. In Figure 7, there is clearly no connection between these two polluted areas. While the Paris area is submitted to a northwesterly flow, this is not the case in the north of France where the cities of Lille, Bethune, and Lens are in the flow coming from London. This strong connection between these two latter areas is now illustrated through a time series of vertical cross sections, the trace of which appears as the heavy dashed line in Figure 6b.
4.3. The London to Northern France Polluted Area [23] For a detailed analysis of this extended polluted
area, the vertical cross sections from Wales to Luxem-bourg in Figures 8a – display the time evolution of NOx
and O3 concentrations between the surface and 3000 m,
for 12 August 1997. The criteria of 10% on turbulent kinetic energy maximum value is used to evaluate the top of the mixing boundary layer (see, for example, Stull [1988]).
[24] At 0600 UTC, pollutants are trapped within the thin
mixed boundary layer (Figure 8a) capped by strong turbu-lent kinetic energy gradients in Figure 8a. In Figure 8a, for example, NOxremains confined within the first 50 m over
England (90 ppb at London), within 50 and 200 m over northern France (30 ppb at Lille). Above the boundary layer, nonnegligible NOx values are found (1 to 5 ppb) as
residues from the previous day. Westerly flow is weak over England (Figure 8b), generally below 2 m s 1, while the flow is from the east (4 to 6 m s 1) over northern France. The simulated ozone concentrations at 0600 UTC (Figure 8b) are quite low in the boundary layer, particularly over
England owing to the combined effects of dry deposition and strong NO emissions trapped close to the surface. Moreover, reduced radiation at 0600 UTC prevents any high ozone production. Above the boundary layer, there are ozone residues from the previous day, owing to their isolation from the surface NO sources and dry deposition after nocturnal stratification. This local residue of ozone results from the stratification acting on the ozone formed the day before, according to a mechanism described by Tulet et al. [1999]. At 1200 UTC on 12 August 1997 the boundary layer has thickened by 200 m over England and up to 500 m over northern France (Figure 8c). This differ-ence in heights can be attributed to the presdiffer-ence of clouds over southeastern England and the Channel (Figure 3b), whereas the sky is clear over northern France. There, enhanced vertical mixing tends to reduce NOx
concentra-tions in the boundary layer (Figure 8c). Above the boun-dary layer, decreasing NOx concentrations have produced
ozone photochemically (Figure 8d). This ozone production is higher above than within the boundary layer, since it is not affected by NO emissions and dry deposition. Some mixing occurs at point C, between this upper level ozone and the ozone maximum. This latter maximum has further moved eastwards, then being located over northern France. At 1500 UTC the boundary layer thickness is 400 m over England compared to 900 m over northern France (Figure 8e). The combined effects of vertical mixing (associated with downward entrainment of high ozone concentrations aloft) and active photochemistry enhance ozone concen-trations in the boundary layer. At a height of 400 m over the Channel, there is an ozone maximum of 130 ppb. An ozone plume has thus developed, extending from the London area to northern France with subsequent surface ozone increase there, reaching up to 90 ppb. This is an example of regional pollution in which, due to the strong Figure 8. (continued)
concentrated horizontal wind shear between London and France, the use of an on-line model such as Meso-NH-C can be considered as an advantage for correct timing of the pollution plume arrival over northern France, which needs both correct transport and chemical transformations of ozone precursors.
4.4. Ozone Evolution Over Northern France
[25] Our purpose now is not to proceed to a detailed
comparison between observed and simulated ozone values at every ADEME station. Such comparisons could better be undertaken using a nested grid procedure with downscaling simulations from regional to local scale. Our goal here rather aims at a semiquantitative analysis, in order to unravel progressive building up of ozone concentrations observed over northern France. The evolution of ozone concentrations observed between 11 and 12 August 1997 is displayed in Figures 9a and 9b at Bethune and Lens, respectively noted B and L in the vertical cross section (Figure 8b). These stations are 30 and 25 km to the south-west and south of Lille respectively.
[26] Moderate ozone values are observed on 11 August
1997, peaking at 50 and 70 ppb at Bethune and Lens. The ozone increase on 12 August gives peaks of 87 and 95 ppb at Bethune and Lens, consistent with the transport from London discussed above. Meso-NH-C closely simulates this evolution, with quite similar values (cf. the red dashed lines in Figures 9a and 9b). There is, however, an overestimation of the peak ozone value by 10 to 15 ppb at Bethune, whereas at Lens the agreement is closer between the observed and simulated concentrations. Night time ozone values are somewhat overestimated by 10 to 15 ppb, problably owing to an underestimation of NO titration in the model. Ozone evolution between these 2 days at the two stations of Lens and Bethume, located within the London plume is also well captured by the model. In order to distinguish at these two stations between the relative
con-tributions due to pollution advection from London and local ozone production, a second simulation has been run with identical dynamics but in which the emission fluxes have been set to zero in the London area. Evolution of these newly simulated ozone concentrations at Bethune and Lens is depicted as the green curve respectively in Figure 9a and Figure 9b. This new simulation underestimates by30 ppb the observed values at these two stations on 12 August 1997. So, for this day, one can assess that nearly 30% of the maximum ozone level can be attributable to the London pollution plume, thus emphasizing the need to consider transboundary pollution fluxes.
4.5. Comparison Between the Simulated and Observed Ozone Fields
[27] We now proceed to a rough evaluation of the ability
of Meso-NH-C to give a correct overview of ozone con-centrations over France on 11 – 12 August 1997. In France, most air pollution measurements are made in urban areas. With simulations of 30 km in horizontal resolution, most of the urban and industrial areas are subgrid scales, apart from the few largest ones (Paris, Marseille, Lyon, Lille). Fur-thermore, the French urban pattern is made of a number of dispersed small subgrid cities of less than 50,000 inhab-itants. So, when averaging emissions over 30 km 30 km domains, high urban emissions are necessarily mixed with low rural ones, which results in underestimation of the simulated pollution peaks near urban sources when com-pared to observed ones, due to NOx and VOCs reduced simulated values. This is a reason why one-to-one corre-spondence between simulations and point measurements is difficult to assess. Nevertheless, in spite of this representa-tiveness problem, it can be indicative to proceed to a comparison between the ADEME network data and the ozone fields simulated by Meso-NH-C. On the basis of ozone average and maxima values for 11 – 12 August 1997, this comparison is displayed in Figure 10.
Figure 9. Ozone evolution observed (solid lines with stars) and simulated for complete simulation (red heavy dash-dotted lines) and simulation without London emission of pollutant (green plain line) for 11 and 12 August 1997 at (a) Bethune and (b) Lens. See color version of this figure at back of this issue.
[28] On 11 August 1997 the least polluted day under
study, satisfactory agreement is seen between observations and model results in Figure 10a. On 12 August 1997 a larger dispersion is observed in Figure 10b for the ozone maxima. The model underestimates the ozone maxima observed on 12 August (Figure 10b) and only infrequently captures local high ozone values in middle-sized French cities. Once again, these results describe the caution that has to be put upon urban measurements when considered in regional studies. The large dispersion observed in Figure 10 for 11 and 12 August 1997 confirms that a 30-km resolution model is not sufficient to correctly predict ozone maximum in small French cities. Nether-theless, information at regional scales (even with such a limited resolution) is important, not so much for urban
pollution itself than for pollutant transfer at the national and continental European scales.
5. Summary and Conclusions
[29] A new air quality model, Meso-NH-C, has been
presented with the capability of on-line coupling between the meteorological and chemical variables at each time step and each grid point. The model chemistry takes advantage of the detailed transport and physical parameterizations developed for the mesoscale meteorological model. The model has been applied to a complex August 1997 episode where both anticyclonic and convective conditions prevail over Europe. The episode encompasses typical clear-sky anticyclonic highly polluted areas with transboundary pol-lution fluxes, and severely perturbed cloudy convective areas with deep intense vertical transports between the tropopause and the boundary layer. In the complex situation the model has simulated the rapid ozone increase observed on 12 August over the French polluted areas. A detailed study of the ozone levels over the French ADEME network has shown quite a reasonable agreement between model and observed results for the two simulated days over the most polluted areas, in spite of the 30-km horizontal resolution; a resolution too coarse for the small subgrid French cities. A detailed analysis of a transboundary transport/chemical transformations pollution episode between the London region and northern France has been presented. There is good agreement between the ozone simulated and observed time evolutions at Bethune and Lens in northern France, while the analysis of successive vertical cross sections for NOx and ozone emphasizes the importance of considering
regional and transboundary pollution fluxes in Europe. Another simulation, using the same physics and meteoro-logical fields but in which the pollutant emissions over the London area have been set to zero has shown that30% of the ozone maximum level over northern France could be attributed to pollutant transport from the London area, against 70% for local pollution.
[30] Acknowledgments. The authors warmly acknowledge Uwe Schwartz and Pr Reiner Friedrich from IER Stuttgart for providing the GENEMIS emission data. Simulations were performed on the CNRS/ IDRIS computers.
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Figure 4. The French Agence De l’Environnement et de la Maitrise de l’Energie (ADEME) ozone network. Hourly maxima ozone concentrations displayed according to the European ozone thresholds levels 1 (55 ppb) and 2 (90 ppb). (a) 11 August 1997 and (b) 12 August 1997.
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Figure 6. Reactive odd (a) nitrogen and (b) ozone surface concentrations (in ppb) superimposed upon wind fields on 12 August 1997, at 1500 UTC (after 39 hours of simulation). NOxisocontours for 1 and 10 ppb against 55
ppb (level 1) and 90 ppb (level 2) for ozone. The heavy dashed line in Figure 6a is the surface trace of the vertical cross section in Figure 8.
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6Figure 7. Zoom on northern France of Ozone surface concentrations (in ppb) superimposed upon wind fields on 12 August 1997 at 1500 UTC (after 39 hours of simulation). O3isocontours for 55 ppb (level 1) and 90 ppb (level 2).
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7a)
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Figure 8. Vertical cross sections on 12 August 1997, along the dashed line in Figure 6b for (a) NOx, (b)
O3at 0600 UTC, (c) NOx, (d) O3at 1200 UTC and (e) NOxand (f ) O3at 1500 UTC (concentration
scales at the right). Wind vectors (scale at bottom left) are superimposed upon the ozone fields in Figure 8b, Figure 8d, and Figure 8f ). The criteria of 10% on the turbulent kinetic energy maximum value appears as a plain line depicting the top of the mixed boundary layer (0.03 m2s 2for Figure 8a, 0.12 m2
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9Figure 9. Ozone evolution observed (solid lines with stars) and simulated for complete simulation (red heavy dash-dotted lines) and simulation without London emission of pollutant (green plain line) for 11 and 12 August 1997 at (a) Bethune and (b) Lens.
![Figure 1. Meso-NH is a meteorological model jointly developed by CNRM (Meteo France) and Laboratoire d’Aerologie (Centre National de la Recherche Scientifique) [Lafore et al., 1998]](https://thumb-eu.123doks.com/thumbv2/123doknet/2346643.35033/3.888.86.430.97.401/meteorological-developed-laboratoire-aerologie-national-recherche-scientifique-lafore.webp)
