JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, D18, 8189, doi: /2000jd000046, 2002

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, D18, 8189, doi: /2000jd000046, 2002 Simulation of the circulation and related photochemical ozone dispersion in the Po plains (northern Italy): Comparison with the observations of a measuring campaign A. Dosio, S. Galmarini, and G. Graziani Environment Institute, Joint Research Center, Ispra, Italy Received 13 October 2000; revised 29 March 2001; accepted 16 April 2001; published 6 September [1] The atmospheric flow and pollutant dispersion over the Po plains in high-pressure conditions and weak synoptic circulation is investigated by means of a model simulation. This study stems from the work conducted during the Pianura Padana Production of Ozone (PIPAPO) campaign conducted in The typical mesoscale flow that establishes in the Po plains under these conditions is responsible for the diffusion of atmospheric pollutants in the prealpine valleys, their recirculation over the plain, and in general for the condition of stagnation and pollution buildup often found in the region. The model simulation presented here reproduces correctly the meteorological circulation that occurred in the central part of the Po valley during the PIPAPO first intensive observational period. The model results are compared with the measurements collected at a large number of surface observational sites and the airborne instruments deployed during the campaign. Subsequently, the diffusion and transformation process of the atmospheric pollutants is analyzed. Direct comparison with the measurements collected during the campaign indicates the good quality of the model results and the relevance of the conclusions deduced from the model simulation. This paper represents one of the few attempts made so far to simulate the circulation and photochemical ozone production over the Po plains. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution--urban and regional (0305); 0368 Atmospheric Composition and Structure: Troposphere--constituent transport and chemistry; 3329 Meteorology and Atmospheric Dynamics: Mesoscale meteorology; KEYWORDS: mesoscale flow simulation, passive tracer dispersion, photochemical ozone production, Po Valley Citation: Dosio, A., S. Galmarini, and G. Graziani, Simulation of the circulation and related photochemical ozone dispersion in the Po plains (northern Italy): Comparison with the observations of a measuring campaign, J. Geophys. Res., 107(D18), 8189, doi: /2000jd000046, Introduction [2] The recent improvements of mesoscale flow models and their coupling with atmospheric chemistry schemes have largely increased the possibility of simulating and understanding the high levels of photochemical pollution in regions of high geographical and meteorological complexity, such as the Po plains (northern Italy). The latter is the vast plain located 200 m above sea level (asl) at the Alps foothill, extending for about 200 km in the N S direction and 400 km in the W E and bounded north by the Alps and south by the Apennines. [3] In the absence of prevailing synoptic weather systems the atmospheric circulation is strongly influenced by the topography of the region that produces a mountain-valley circulation, especially during summertime when an anticyclone settles over southern Europe. These conditions are particularly significant for atmospheric dispersion because of the intense solar radiation and owing to the fact that the Po valley is the most densely populated and industrialized region of Italy. A number of studies are being and were carried out in the past Copyright 2002 by the American Geophysical Union. Paper number 2000JD /02/2000JD for this area [Prevot et al., 1997; Vecchi and Valli, 1999; Silibello et al., 1998; De Martini et al., 1998a]. However, little is known on the capacity of the local atmospheric circulation to transport and dilute the photochemical pollutants or the role of the various sources in determining the air composition of the region. [4] To better understand the pollution problems in the region, the study Pianura Padana Production of Ozone (PI- PAPO) was launched a few years ago as part of Limitation of Oxidant Production (LOOP) (EUROTRAC II) that foresaw a 2-month observational campaign (May June 1998). Information on the project can be found at the web site: www1.psi.ch/loop/pipapo/pip. As a first attempt to understand the mechanism of the photochemical pollution in the Po valley in this paper, we simulate the atmospheric flow and the dispersion of the chemicals observed during 1 day of the PIPAPO Intensive Observational Periods (IOP), 13 May 1998, which was characterized by the high values of the ozone concentration. 2. Model Description [5] The flow model used in this study is the mesoscalemeteorological Topographic Vorticity Mesoscale (TVM) model [Thunis, 1995; Schayes et al., 1996] that solves the at- LOP 2-1

2 LOP 2-2 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY mospheric conservation equations in vorticity formulation. The model has already been successfully applied in the past to other case studies [Clappier et al., 2000; Grossi et al., 2000; Bornstein et al., 1996; Thunis and Cuvelier, 2000]. The flow model is coupled [Martilli, 1996] with a chemical module. The chemistry mechanism is the condensed Lurmann Carter Coyner (LCC) mechanism [Lurman et al., 1987]. In this case, 44 gas phase species were considered, of which 35 are dynamically transforming, and 9 are in steady state. These species are coupled in 106 gas phase chemical reactions. To simulate the chemical reactions of a complex mixture of organic compounds, 12 lumped organic precursors are considered in the scheme. The gas phase chemical mechanism has been extended [Harley et al., 1993] to treat explicitly biogenic emissions and combustion activities. Though quite relevant for the air quality assessment of the region considered, particulate matter is not taken into account in this study. Advection of velocity and temperature is achieved through a third-order scheme [Colella and Woodwart, 1984] with a correction [Clappier, 1998] for multidimensional applications. Subgrid parameterization of turbulent transport is performed according to classical similarity theory in the surface layer and the TKE- scheme of [Duynkerke, 1988] in the mixed layer. For the chemistry the solver of an eulerian chemical transport model (CIT [McRae et al., 1982]) is used in which the transport of pollutant is performed with the same advection scheme used in the dynamical module; turbulent transport is calculated only in the vertical direction using the coefficients of the dynamics. To solve the chemical system, the hybrid scheme of Young and Boris [1977] is used; the photolysis rates coefficients needed for the photochemical reactions are computed by means of the STAR module [Ruggaber, 1994] Main Assumptions in the Simulation [6] An overview of the region and the numerical domains selected is presented in Figure 1a. It includes mountains (ranging from the hills 40 km north of Milan, named pre-alps, to the 4000-m-high Monte Rosa and Mount Blanc ridge), a variety of water bodies (Lake Maggiore, Lake Garda, Lake Como), rivers (Ticino, Po), and a number of important urban areas (Milan, Turin, Brescia, Verona). Two domains have been chosen for the meteorological simulation, taking into consideration the fact that the Alps are a natural barrier that surrounds the plain and that they define the main forcing driving the mesoscale atmospheric circulation. In total, three domains were used: two for the atmospheric circulation calculation and one for the dispersion and photochemistry. The finest domain selected for the meteorological simulation covers a surface of approximately km 2 centered over Milan, with a horizontal resolution of 3 km in each direction (innermost continuous rectangle in Figure 1a). A larger domain (coinciding with the external rectangle in Figure 1a), with a resolution of km 2 in the horizontal, is one-way nested to the inner domain to generate its boundary conditions. The boundary conditions of this external domain are obtained from the European Centre for Medium-Range Weather Forecasts (EC- MWF) analysis for the period considered. For both domains a vertical grid of 25 levels was selected with the first level at 25 m above the surface. The model uses the staggered Arakawa type C numerical grid, and therefore at this height the vertical component of wind and the turbulent kinetic energy are defined, whereas the horizontal components of wind and potential temperature are recalculated at 12.5 m in the first grid cell. The upper levels are distributed with a stretching factor of 1.2 up to about 9 km. The domain used for the dispersion calculations is a portion of the high-resolution one used for the flow simulation (innermost dashed rectangle in Figure 1a) since only for this area a detailed emission inventory is available. It uses only meshes of 3 3km 2 resolution in the horizontal and 15 in the vertical (up to about 5000 m). A blowup of the innermost meteorological domain and of the chemistry one is shown in Figure 1b. The figure shows the topography resolved by the model, the location of the major water basins, as well as the position of the sampling stations of meteorological variables. The curves in Figure 1b represent the trajectories of the aircraft deployed during the campaign. [7] The land use classes and soil parameters were obtained by lumping the 250 classes provided by the U.S. Geological Survey (USGS), the University of Nebraska-Lincoln (UNL), and the European Commission s Joint Research Center (JRC) (The 1-km resolution global land cover characteristics database for use in a wide range of environmental research and modeling applications, 1999, available at The flow simulation starts at 1800 LST of May 12 to initialize the flow and continued until midnight of the following day. The time step used is 30 s Emission Inventory [8] The emission inventory used in the project PIPAPO and in this study was prepared by ASL-Lecco-Regione Lombardia (Italy) in collaboration with Meteotest/BUWAL (Swiss). It includes data from the Italian regions Piemonte and Lomabardia and the Swiss cantons of Ticino and Grisons. The inventory contains 38 species, with VOC separated in 32 classes [Maffeis et al., 1999; Longoni and Maffeis, 1998a]; spatial and temporal resolution are 3 km 3 km and 1 hour, respectively. Traffic has been estimated by considering urban and extraurban roads; urban traffic emission inventory was developed in municipalities, and the extraurban traffic on all the main roads (highways, national and provincial roads, etc.) was calculated for each arc [De Martini et al., 1998b]. Industrial emissions were determined by sample analysis of 2000 industries, whereas the emission for each municipality has been computed on the basis of Italian National Statistical Institute (ISTAT) data. The biogenic emission inventory includes three NMVOC groups of compounds, such as isoprene and monoterpenes [Longoni and Maffeis, 1998b]. Six point sources (the main power plants and refineries in Lombardia) were also included in the inventory Atmospheric Chemistry Boundary and Initial Values [9] The boundary conditions for the dispersion calculation were taken from a MM5/EURAD model simulation that involves a double-nesting simulation of meteorology and chemistry starting from the European scale down to the Po plains. To verify those boundary conditions in the vertical, the values obtained with TVM model using EURAD data as boundary values were compared with the available aircraft measurements. Since a considerable difference was found in the nitrogen oxides for the upper layers, the aircraft observations were adopted for these species. This modification does not affect the boundary layer ozone concentration prediction that was found to be insensitive to the NO x upper-layer values. The O 3 background value was determined from the daily evolution of the

3 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-3 Figure 1. (a) Map of the domain of interest and the numerical domains used for the simulation. The continuous rectangle represents the domains used for the meteorological simulation (one-way nested); the dashed triangle is the domain for the dispersion of the photochemical pollutants. The diamonds indicate the major urban centres enclosed: Milan (MI), Turin (TO), Novara (NO), Brescia (BS), Parma (PR). (b) The innermost domain of the meteorological simulation with the topography resolved by the model. The dashed line shows again the subdomain used for the dispersion of the photochemical pollutants. The stars represent the locations of the meteorological sampling stations, and the curves are the trajectory of the aircraft flight. Points A and B are the locations in which the aircraft performed a vertical profile. concentration in a remote station, namely, Colma del Piano (1000 m asl). [10] The transport/chemical simulation started at 0000 LST on 13 May; to have more realistic initial chemical conditions, a prerun of 2 days with the same initialization (background and boundary conditions) was performed: the chemical concentrations obtained were then used to initialize the simulation of May 13. In this way a quasi-stationary state is reached by the chemical system before the real conditions are analyzed. 3. Results of the Flow Simulation and Qualitative Analysis 3.1. Evolution of the Surface Wind Field [11] In Figures 2a 2d the inner-domain horizontal wind fields at 12.5 m above surface (first level) are shown for 0600, 0900, 1200, and 1500 LST on 13 May At 0600 (Figure 2a) an overall drainage flow from the slopes of the Alps and Apennines toward the Po plains is observed, in particular in the

4 LOP 2-4 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 2. Time evolution of the model surface wind fields from (a) 0600, (b) 0900, (c) 1200, and (d) 1500 LST. western and central parts of the plain, whereas the eastern part is characterized by an easterly flow. A drainage flow can also be observed in the southern part from the Apennines ridge turning westward when it encounters the flow from the Alps. Though the maximum surface wind over the domain is approximately 8 m/s, in the plain it does not reach 3 m/s. [12] By 0900 (Figure 2b) the flow has reversed (sunrise occurs at 0730 LST). The surface wind in the western part of the plain is negligible. At the Alps and the Apennines foothills it started blowing toward the mountain ridges and through the valleys. An exception is observed in the region between Piacenza and Brescia where a wind from the south blows down the Apennine into the plain in direction of the Lake Garda region. [13] At 1200 (Figure 2c) a similar field is present at noon, with increased wind intensity. A local circulation at Lake Garda is quite evident with the flow moving from the lake toward the land and in general in a direction opposite to the main flow in the plain. A clear channel flow is present in

5 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-5 Figure 2. (continued) Valtellina (from Colico to Sondrio, one of the most extended and well-resolved valleys of the domain). In the southern part of the domain the flow has reversed direction also at Piacenza and is now pointing toward the Apennines ridge though the presence of a vortical motion is weaker than that calculated for 0900 LST. The flow in the plain is clearly separated into two parts (more or less half way along the Po plains), one moving toward the Alps and the other moving south. The divergence of the flow at the surface corresponds to a subsidence motion in the middle of the plain as verified by the vertical wind analysis. [14] At 1500 (Figure 2d) a reinforcement of the wind appears. A comparison with the previous time interval indicates that now a larger portion of the plain s atmosphere at the surface is affected by the Alps-induced flow. At later times the flow reverses starting to blow again toward the plain. [15] At all times the topographic complexity of the region produces very different flows in the upper portion of the domain, where valleys intersect mountains of different altitudes. Nonetheless, a diurnal cycle of the flow from the mountain regions to the valley bottoms and vice versa results whenever

6 LOP 2-6 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY the adopted resolution allows it. The winds at 2000 m show the presence of a northerly flow that basically varies only in intensity throughout the day. The results in the horizontal wind field indicate the importance of including in the simulation the eastern part of the domain since the Alpine ridge extends considerably eastward and therefore still influences the flow in that part of the domain which is the only side not bounded by mountains Vertical Cross Sections [16] A vertical section that passes through Milan-Brera in N S direction is presented in Figure 3. The figure shows the wind components (U, V, W) at 1500 LST. Arrows have been added by hand to help the interpretation of the results. The Po plains are clearly visible extending from the Apennines on the left-hand side to the Alps on the right-hand side of the figure. The southern wind component indicates the flow separation in the N S and W E direction. The southern flow observed in the surface wind plots extends for approximately 1000 m in the vertical. Above it a return current flows toward the center of the domain. A flow to the south is also produced due to the presence of the Apennines. A weaker return current moving north can be observed above the mountains. The horizontal divergence at the middle of the plain causes a descending motion visible in the W component plot. The circular motion, typical of the valley to mountain breeze, is therefore well established at this time of the day. The temperature distribution in the selected plane (not shown) gives an expected horizontal gradient of approximately 1 K/km from the middle of the plain to the Alps foothills. From 1500 m above, the atmosphere is well stratified. 4. Comparison of the Flow Results With Observations 4.1. Ground-Level Measurements [17] During the campaign a number of ground-based meteorological sites operated in addition to the existing networks of the Italian Meteorological Service (Aeronautica Militare) of the Region Lombardia and of the Electrical Supplier Industry (ENEL). In total, horizontal wind direction, intensity, and air temperature were measured at 24 sites for the day considered. Figures 4a and 4b show the time evolution of the total wind intensity and direction and of the potential temperature for a subset of the available stations, selected as the most representative of the flow in the domain. In particular, we have selected 14 stations located in valleys, one at high altitude (Alpe del Vicerè at 800 m asl), two in the prealpine region (Laveno, Verzago), and the rest in the plains (Brera, Verona, Piacenza, Novara, Leno, Seregno, Bergamo, Brescia, Bresso). [18] In general, the model results agree well with the observations for the variables considered. The model seems to better reproduce the wind direction in the plain sites than in the prealpine or alpine region where the discrepancies can be attributed to subgrid local topographical effects. Laveno, located on the shores of Lake Maggiore, is a good example in this respect. There the comparison with the observations shows a good agreement in wind intensity but a poor performance for the wind direction. This is probably due to the limited model capacity to capture local flows otherwise observed by the single meteorological station located at the village. The comparison of the model results with the measurements at Colico and Sondrio is an example of the model capability to deal with local circulation in complex terrain. These two sites are located at the two extremes of the Valtellina valley at an altitude of 1046 and 700 m asl, respectively. This valley is very deep and wide and relatively well resolved by the model resolution. At both locations, wind direction and intensity are well captured during the central hours of the day, while during the night the wind direction is off the measurement by several tens of degrees. The observations at both locations confirm the existence of a mountain wind during the night that is only partially reproduced by the model. The location of the station within the grid cell may be crucial in the comparison since during the day the valley induces a channel flow with a clear and well-defined direction. [19] The importance of the comparison of the temperature at the sites depends on the fact that temperature gradients are the driving force for the local flow and for the turbulence. The experimental values (measured at heights from 2 to 5 m depending on the station) are compared with the calculated values at ground level and at 12.5 m. The figures indicate that the model reproduces correctly the daily variation in all the cases presented On-Site and Aircraft Vertical Profiles [20] During the day a motor glider operated by MetAir out of Locarno and able to reach 6000 m asl collected meteorological variables and chemical composition data, traveling with a speed of 170 km/h during cruising and 130 km/h during ascents [Neininger and Baumle, 1998]. Two flights were conducted, one in the morning and one in the afternoon. The profiles were conducted at two locations indicated in the domain map of Figure 1b. Point A corresponds to the Lake Como region, whereas point B is located above Milan. The aircraft measurements at the two locations in the morning and in the afternoon are presented in Figures 5a and 5b, respectively. The profiles of potential temperature, horizontal and vertical wind intensity, and wind direction were collected with a frequency of 1.6 s 1. The figures also show the model results for 6-min averages, indicated by a star showing also the exact time of model output. Note that no averaging in time or space was performed on the model output. The only averaging is represented by the model time step and spatial resolution. Not surprisingly, the aircraft measurements are more scattered than the model results. However, the differences between the model data and observations are rather small (especially at point A) for all the variables. The vertical velocity is smaller than the in the observations, due to the fact that this variable is measured punctually while it is calculated on a grid volume. [21] All the information that can be deduced from the vertical profiles of potential temperature related to the atmospheric boundary layer depth shows that even in this case the model reproduces well the atmospheric structure. The potential temperature slopes and inversions are well reproduced for both ground-based soundings (not shown here) and aircraft measurements though the temperature values may be different from the observed ones. 5. Statistical Analyses [22] Taking advantage of the large number of stations, a statistical analysis was performed, to have a more condensed evaluation of the model s capability to reproduce the flow. All available sites were grouped in two classes, those located on the plain (14 stations) and those in the prealpine and alpine

7 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-7 Figure 3. Vertical cross section of the three components of the wind in the north-south direction passing through Milan (indicated in the figure as Brera). The grey arrows have been added by hand.

8 LOP 2-8 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 4a. Daily evolution of horizontal wind intensity (first panel from left to right), direction (second panel), and potential temperature (third panel) for a selection of the PIPAPO meteorological sites. The line indicates the model result. For the temperature the two curves correspond to the simulation at 0 m (dashed line) and 12.5 m (continuous line).

9 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-9 Figure 4b. Same as Figure 4a, but for another set of stations.

10 LOP 2-10 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 5a. Comparison of the aircraft vertical sounding (continuous line) and model simulation (stars). The sounding relates to the profile conducted on the morning of 13 May in zones A and B (Figure 1b). The sounding duration is indicated by the beginning and ending time at the curve extremes. The aircraft sampling is every 60 s; the model output is every 6 min (next to the star). The panels show potential temperature, horizontal wind intensity, direction, and vertical wind intensity.

11 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-11 Figure 5b. Same as Figure 5a, but for the afternoon flight.

12 LOP 2-12 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 6. Time evolution of the average wind intensity, direction, and temperature from all the surface meteorological stations (first column in three panels), a subset including only those in the Po plains (second column), and a subset of alpine and prealpine stations (third column). Continuous line, measurement; dashed line, simulation. part of the domain (10 stations). Figure 6 shows the comparison between the wind intensity and the direction for all stations, for those in the plains and for the mountain sites, respectively, in terms of bias and root-mean-square error (RMSE) as functions of time for air temperature, horizontal wind intensity, and direction. A circular statistics [Mardia, 1972] was applied to direction values. For the sites in the plain there is a good agreement during the central part of the day. The value of the bias is in the order of 0.5 m/s for all the day, and RMSE is also rather small ( 1 m/s). For the mountain sites the peak due to the breeze is on the average well reproduced both in value and time of occurrence. Owing to the topography smoothing, necessary to reduce the steepness of the mountain slopes, the model tends to reproduce the breeze development more uniformly than that observed. The largest discrepancy in wind direction can be noticed both for plain and prealpine sites during the night (about 35 ), when the wind intensity is rather low, and in the early morning for the plain sites (about 20 ). During the day the average wind direction is well reproduced by the model results. Figure 6 also shows the daily evolution of the temperature at 2 m elevation for the sites located in the plain and in the prealpine region. The daily cycle is correctly reproduced for the sites in the plains with peak temperatures somewhat lower than those observed (about 1 ). In the central part of the day, between 1200 and 1800 LST, the RMSE is also rather low, and correlation is in the range of For the sites in the mountainous zone the model tends to underestimate the daily peak by 2 3, either due to the complex topography which may induce local phenomena that cannot be reproduced with a 3 3km 2 grid, or to the soil type employed. In fact, the plain sites are located in a vast area that has similar soil characteristics from Novara to the west to Verona to the east, while land use at the locations in the pre-alps may show sudden variations not considered in the simulation. 6. Release of a Nonreactive Pollutant [23] To visualize the trajectories that pollutants experience when transported by the wind field calculated by the model, we have analyzed an ideal case that describes the evolution of a passive tracer released from the area of Milan, as the most intense source in the region. The passive tracer is released for 1 hour at surface level from Milan (12 grid points) at four time

13 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-13 intervals across 13 May The midnight release (from 0000 to 0100) helps to determine the evolution of the dispersion during the valley flow and at low wind speed. The other three releases take place in the early hours of the morning (from 0800 to 0900 LST), at midday, and midafternoon, respectively, when the valley breeze is well established. The evolution of the tracer at the surface in the first hour of the night release shows that diffusion process is very slow and takes place symmetrically around the release area. At this time of the night the boundary layer is approximately 200 m deep, thus confining the tracer close to surface. At 0800 LST the second release occurs. This is visible over Milan in Figure 7a. In the southwestern part of the domain the first one is also visible. At this time of the day the wind has already reversed its direction, and the Milan emission is being transported to the north. The vertical extension of the planetary boundary layer is now up to 500 m. The low wind intensity produces a slow transport process. At midday (Figure 7b) the night release is at the lower left corner; the second one is moving northwest and merges with the midday release that is still concentrated on the Milan area. Part of the night release stretches to the north, being now influenced by the mountain breeze. The presence of the flow divergence zone in the middle of the plain causes part of the night cloud to move south. The boundary layer is now fully developed, and the tracer extends in the vertical up to 1000 m. By the time the fourth release takes place (1500 LST, Figure 7c) the night tracer is sensibly reduced in concentration. The fourth emission is visible over Milan as well as the two previous ones. The early morning cloud has almost reached the southern tip of Lake Como, and the midday one is still in the plain moving northeast. The 1800 LST evolution of the dispersion process is visible in Figure 7d. The persistence of the flow in the northeast direction moves all the three clouds (now merged into one) to the Lake Como basin and the prealpine region to the east. An interesting feature is represented by the presence of a tracer cloud in the lower part of the domain. This is the tracer released during the night, which has been transported toward the Apennines. As the tracer has moved upward into the return current, it has been transported back over the plain and fumigated in the subsidence region of the flow. The study of the passive tracer dispersion allows us to draw the following conclusions: (1) Because of the breeze regime, any surface release from the Milan area occurring during the night has the tendency to be transported first to the southwest part of the domain; (2) any release during the day will be transported first to the north and then to the northeastern part of Milan in the Lake Como region and will mainly interest the prealpine area; (3) the dispersion process is relatively slow due to the low wind speed. This aspect produces the conditions for a long-term persistence of the cloud in the above mentioned regions; (4) there is a possibility for the night tracer to be fumigated back in the middle of the plain where the two return currents of the Alps and Apennines converge. 7. Simulation of Chemical Pollutants [24] The simulation of the chemical pollutants dispersion was carried out for the same domain used for the passive tracer dispersion. Figures 8a 8d show the simulated time evolution of the ozone concentration at ground from 0600 LST to 2200 LST on 13 May. At 0600 LST (Figure 8a), ozone is very scarce over Po plains, while in the prealpine region of the lake of Como a maximum concentration value of 140 ppb is calculated. This represents the residual of the previous days of simulation. The minimum O 3 concentration in the plain is due to the absence of solar radiation at this time of the day and to the abundance of nitrogen oxides. After sunrise, ozone starts to be produced in the plain as well as transported down to the ground from the upper layers, with concentrations rapidly reaching the background level of 70 ppb. The concentration over the prealpine and alpine region does not vary considerably due to the small emissions of ozone precursors in that zone (Figure 9a). The only variations of the concentration are mainly due to the dispersion of the ozone already present in the region. At 1200 LST (Figure 8c) the situation changes drastically. The background O 3 level in the plain reaches 80 ppb, and a plume is formed, extending from the Milan urban area to the lake of Como region. The extension of the plume south of Milan can be attributed to the presence of large VOC and NO x emissions from the intensive agricultural area south of the city (Figures 9a and 9b) and to the nitrogen oxides and VOCs produced during the night in the urban areas and transported south in a fashion identical to the transport of the passive tracer. The ozone field at 1500 LST (not shown) indicates that at this time the maximum ozone concentration is 160 ppb and is located to the northeast of Milan, as for the passive tracer emitted at Milan. A portion of the plume enters the basin of Como Lake (approximately the triangle formed by Como, Lecco, and Varenna) which is surrounded by relatively high hills that channel the flow. At 1800 LST, O 3 concentration peak is still at the same level, though much more distributed over the plain and the prealpine region. The cloud portion that extended south of Milan is now moving to the northwest as was shown by the passive tracer night release. As the evening progresses, the depletion of ozone takes place wherever nitrogen oxides are being emitted. This process clearly appears in Figure 8d (surface ozone concentration at 2200). This shows a minimum of ozone (below background level) present above the urban region in the plain, whereas the ozone produced during the day and transported over the mountain region is still present at relatively high concentrations (120 ppb). The circle is now closing, with the morning situation reappearing in the plain. The result of the daily evolution of the circulation and the ozone cycle is a net buildup of O 3 concentration in the prealpine region. [25] Figure 10 shows a north south cross section passing through Milan with the isosurfaces of ozone at three different times of the day. The vertical structure of the ozone concentration shows the presence of an elevated concentration maximum that represents the residual of the day before. At 0800 the surface O 3 concentration is decoupled from the upper levels, due to the surface depletion of nitrogen oxides. The concentration maximum moves from the north (right side of the picture) to the south under the influence of the return current (Figure 3) as the valley breeze regime is formed. At 1200 LST the convective boundary layer presence can be evinced by the distribution of ozone. This is in fact homogeneously distributed in the vertical up to about 1000 m. A maximum concentration of 130 ppb through the layer is estimated. Owing to the position of the elevated maximum shown in the previous plot, part of the concentration at this time of the day can be attributed to a fumigation process of ozone down to the surface. At 1800 LST the concentration is extended in the vertical, and the plume has moved farther to the north and is now penetrating the alpine valleys. At 2200 LST (not shown) the depletion of O 3 at the surface appears as well

14 LOP 2-14 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 7. (a) Dispersion of a passive tracer from the Milan area. Morning release from 0800 to 0900 LST on 13 May and evolution of the midnight release. Dark greys correspond to high concentration. Minimum concentration level: 100 ppb. (b) Same as (a), but at 1200 LST, including midday release from 1100 to 1200 LST on 13 May and evolution of the midnight and morning releases.

15 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-15 Figure 7. (continued) (c) Same as Figure 7, but at 1500 LST, including afternoon release from 1400 to 1500 LST on 13 May and evolution of the midnight, morning, and noon releases. (d) Final evolution (at 1800 LST) of the emissions.

16 LOP 2-16 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 8. Ozone concentration (in ppb, dark contours and grey scale) on 13 May at surface as a result of the dispersion and chemical reaction. Distribution at (a) 0600, (b) 0900, (c) 1200, and (d) 2200 LST. The topography of the region (light contours) and the chemistry surface sampling sites active during PIPAPO are also indicated.

17 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-17 Figure 8. (continued)

18 LOP 2-18 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 9. NOx concentration (in ppb, dark contours and grey scale) on 13 May at surface as a result of the dispersion and chemical reaction. Distribution at (a) 0800 and (b) 2000 LST. The topography of the region (light contours) and the chemistry surface sampling sites active during PIPAPO are also indicated.

19 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-19 Figure 10. Vertical distribution of ozone in a north-south cross section passing through Milan. Contours at 0800, 1200, and 1800 LST.

20 LOP 2-20 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 11. Comparison of the simulated time evolution of ozone (continuous line) and the measurements collected on 13 May at 20 stations indicated in Figure 8.

21 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-21 Figure 12. Comparison of the simulated time evolution of NO 2 (continuous line) and the measurements collected on 13 May at 12 stations indicated in Figure 8.

22 LOP 2-22 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY Figure 13. Comparison of simulated O 3 and NO 2 with the measurements obtained during the PIPAPO flights. The four top panels show the morning profiles comparison at points A and B. The next four panels show the afternoon profiles comparison at points A and B. The last two panels give the comparison of simulated O 3 and NO 2 with the values measured when flying from points A to B in the morning.

23 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY LOP 2-23 as the decoupling from the upper layers. A large structure remains aloft as the nocturnal boundary layer evolves. 8. Comparison With Experimental Data 8.1. Surface Measurements [26] A comparison of the simulation results with the hourly means of O 3 and NO 2 measurements collected during the campaign is presented in Figures 11 and 12, respectively, at all but two available stations. The model reproduces quite well the diurnal variation of ozone at the plain, prealpine, and alpine stations. In some cases the model fails in reproducing the ozone maximum value though catching its time of occurrence. This can be attributed to the model difficulty in simulating very local processes from both the circulation and the chemistry viewpoint and to the volume-averaged character of the concentration value. Both model results and measurements show the buildup of ozone in the prealpine region indicated in the previous section. The difference in ozone concentration between the morning and the evening is about ppb (see Magadino, Lugano, Colico). [27] The overall plume position shown in the spatial distributions of the previous section seems to be realistic as indicated by all stations. In this respect, an exception is represented by Lecco where the O 3 concentration is largely overestimated. The reason for this can be found in the NO 2 time series (Figure 11) at Lecco. The model, probably due to some inconsistencies in the emission inventory used, largely underestimates nitrogen dioxide. This explanation is corroborated by the fact that NO 2 is well reproduced in all the stations surrounding Lecco Airborne Measurements [28] Model results were also compared with the measurements collected during the flights described above. The results are shown in Figure 13 in terms of instantaneous simulated (dashed line) vertical profiles of O 3 and NO 2 with observations. The model results match well the measurements. This is particularly true for what concerns the boundary layer values. In some cases even the transition between the concentration in the boundary layer and that in the free troposphere is very well reproduced. The bottom panels of Figure 13 show the variation of O 3 and NO 2 measured at a constant level of 500 m between points A and B. The measured values are reproduced quite well both in order of magnitude and in trend by the model results (asterisks) although the fine structure (such as the peak of NO 2 at 5050 m) is underestimated by 10 to 15 ppb. The location of the peak of NO 2 is an indication of the fact that the glider has caught a portion of the city plume and that the model is able to reproduce its position. 9. Conclusion [29] The large set of observations collected during the campaign PIPAPO has allowed a full validation of the flow and pollutant dispersion in the plains of northern Italy. As expected, during the period considered, the circulation is driven by both of the mountain ridges surrounding the Po plains, the Alps and the Apennines. The flow caused by the mountaininduced circulation extends in the vertical for about 1000 m. At higher altitudes a return current can be observed that subsides in the middle of the plain. In general, the simulation obtained can be considered satisfactory since surface, surface-based soundings, and airborne measurements reveal a very good agreement with the model results. The simulation shows that other than the general mesoscale flow, a series of local circulation systems exist. Whenever the model resolution adopted allows it, these flows are resolved, as in the case of the Lake of Garda and the Valtellina valley. In other cases this is not possible as indicated by the discrepancy between the model results and the observations due to the weakness of the mesoscale flow. This is particularly true for the small lakes and the narrow alpine valleys. To compare the results of the simulation with all observations available, the 24 stations were grouped in two classes (plains and Alpine stations). The time evolution of the average value of these groups compared with the average obtained from the model reveals that in the central hours of the day all meteorological parameters are very well reproduced regardless of the region selected. [30] The analysis of a passive tracer evolution emitted from an extended area source centered in Milan has provided us with an important insight on the effect of the mesoscale circulation present during the selected period on the dispersion of a surface emission. The passive tracer experiment has revealed the following features: (1) Because of the breeze regime, any surface release from the Milan area occurring during the night has the tendency to be transported first to the southwest part of the domain; (2) any release during the day will be transported first to the north and then to the northeastern part of Milan in the Lake Como region and will mainly interest the prealpine area; (3) the dispersion process is relatively slow due to the small wind speed. This aspect and the cyclic character of the breeze regime produce the conditions for a long-term persistence of the cloud in the above mentioned regions; (4) there is a possibility for the night tracer to be fumigated back in the middle of the plain where the two return currents of the Alps and Apennines converge. Although these results are valid for the simulation period, a certain degree of generalization is allowed since cases in which the valley (mountain) breeze regime is present in the region are frequent and similar to the one observed during the campaign. [31] The use of a detailed and up-to-date emission inventory was adopted to simulate one photochemical episode of the PIPAPO campaign. The results revealed that (1) The night emission of ozone precursors has a relevant role in the ozone formation in the region south of Milan. This aspect has been confirmed by the behavior of the passive tracer dispersion during nighttime hours and the presence of a plume of ozone that extends south of Milan. (2) The early morning emission and the daytime ones influence the prealpine and alpine region. As the valley breeze establishes in fact, ozone and its precursors are transported to the north first and to northeast later in the day. (3) High concentrations of ozone are found by the end of the day in the Lake Como basin and the prealpine region nearby. (4) In the vertical, ozone is well distributed during the day and tends to persist at high concentrations in the upper layers as the daytime boundary layer evolves to the stable one. This aspect has important consequences for the evolution of the following day when ozone is fumigated down to the surface. The simulation results on the vertical distribution of ozone and its precursors were also confirmed by the aircraft measurements collected during PIPAPO. (5) The periodicity of the breeze process and the continuous emission of ozone precursors favors a net ozone buildup estimated in the order of 10 ppb/d. [32] Whether and when the large ozone production in the Po plains is NO x - or VOC-limited can result from the appli-

24 LOP 2-24 DOSIO ET AL.: SIMULATION OF CIRCULATION AND PHOTOCHEMISTRY cation of the Sillman s indicators [e.g., Sillman, 1995; Sillman et al., 1990]. Comparison of these indicators obtained by our models with those obtained from other modelers participating in the PIPAPO project has shown already a good agreement and will be a subject of a joint separate publication. Most of the model results have been endorsed by a direct comparison with the observations. [33] Although the project PIPAPO was carefully planned and involved a large effort from several groups, the present data set does not allow us to validate the model results that relate to Po plains south of Milan. In this region the model shows the presence of a city plume, but no measurements are available for confirming the results. The periodical character of the flow that slowly blows from the mountains during the night and toward them during the day identifies clear decoupling between the nighttime behavior of the dispersion and the chemical reaction and the daytime one. Therefore, although the comparison with the data reveals a good agreement with the results obtained for the region north of Milan, there is no evidence to endorse the same results obtained for the southern portion of the valley, and presently the conclusions presented are just speculations. In the future this aspect will have to be checked since it allows the assessment of the ozone pollution levels in the region south of Milan. [34] Acknowledgments. The authors would like to express their gratitude to all the staffs that have measured the flow and pollution variables during the campaign. Gratitude is also expressed to all the institutes that took part in the PIPAPO project, namely: EPFL Lausanne, Institut Suisse de Meteorologie (Payerne), IUL Liebefeld Bern, MetAir Illnau, Meteotest, BUWAL (Berne), PSI Villigen, Swiss Federal Institute of Technology Zurich, Regione Lombardia, CISE spa (Segrate), ENEL-CRAM (Milano), Provincia di Como, P.M.I.P. A.S.L. (Bergamo), PMIP ASL (Como), PMIP-USSL7, PMIP Milano; PMIP ASL Varese, Università di Milano, Università di Brescia, IFU Garmisch-Partenkirchen, IGM-K Koeln, KFA Julich, Universitadt und KfK Karlsruhe, Universitadt Heidelberg, Universitadt Stuttgart, Austrian Research Centre of Seibersdorf, IMP Wien, Norwegian Institute for Air Research, IVL Göteborg, Universitè de Paris 6, Universitè Paris 12, University of Ljubljana. We are also grateful to the following people who have contributed to the collection of information fundamental for the realization of this study: G. Maffeis, M. G. Longoni, D. Toscani, A. De Martini, M. Tamponi, U. Joss, and C. Spirig. Thanks are also due to A. Martilli and G. Favaro for the fruitful discussions had on the modeling aspects of PIPAPO. B. Voegel and A. Neftel are thanked for the constant effort put into the campaign organization. The authors are also grateful to F. Pasi, who analyzed the 1994 case and established the basis for this study. References Bornstein, R., P. Thunis, P. Grossi, and G. Schayes, Topographic Vorticity-Mode Mesoscale-b (Tvm) model, part II, Evaluation, J. Appl. Meteorol., 35, , Clappier, A., A correction method for use in multidimensional time splitting advection algorithms: Application to two- and threedimensional transport, Mon. Weather Rev., 126, , Clappier, A., A. Martilli, P. Grossi, P. Thunis, F. Pasi, B. C. Krueger, B. Calpini, G. Graziani, and H. Van den Bergh, Effect of sea breeze on air pollution in the Greater Athens Area, part I, Comparison between field measurements and mesoscale model simulation for the MEDCAPHOT campaign, J. Appl. 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