TROPOSPHERE BOUNDARY LAYER (BL) The temperature profile responds to changes in ground temperatures over a period of less than an hour, while in the.

Presentazione sul tema: "TROPOSPHERE BOUNDARY LAYER (BL) The temperature profile responds to changes in ground temperatures over a period of less than an hour, while in the."— Transcript della presentazione:

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4 TROPOSPHERE BOUNDARY LAYER (BL) The temperature profile responds to changes in ground temperatures over a period of less than an hour, while in the free troposphere it responds to changes in ground temperatures over a longer period (Stull, 1988) It extends form the surface to between 500 and 3,000m altitude Pollutants emitted near the ground accumulate in the BL. When pollutants escape the BL, they can travel horizontally long distances before they are removed from the air Variation of temperature with height during the day and the night in the atmospheric boundary layer over land under high pressure system

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7 TROPOSPHERE BOUNDARY LAYER (BL) The depth of the boundary layer varies hour by hour during the day and depends upon the time of the year. It is also influenced by meteorology and the main factors are the amount of insolation (the amount of sunshine) and the strength of the wind

8 TROPOSPHERE BOUNDARY LAYER (BL) - Diurnal profile…

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15 Allinterfaccia suolo-atmosfera si stabiliscono due sorgenti di turbolenza completamente differenti. La prima è di tipo meccanico, comune a tutti i fluidi viscosi in moto su una superficie rigida e rugosa e che dà luogo a vortici di dimensione relativamente limitata. La seconda è di tipo convettivo e quindi di origine termica, che produce vortici (thermals) di dimensione decisamente maggiore sui quali agisce la forza di Archimede dovuta alla differenza di densità dellaria contenuta nei vortici rispetto alla densità dellaria circostante. La loro forza motrice è quindi il galleggiamento (buoyancy) e le loro dimensioni sono ben maggiori di quelle di origine meccanica, raggiungendo anche molte centinaia di metri.

16 L'aria sopra superfici particolarmente calde si riscalda in maniera prevalente, dando origine ad una bolla molto più calda dell'aria circostante, appiattita al suolo (stadio 1) e che possiede un'instabilità interna che non si manifesta immediatamente, ma solo dopo che la bolla ha catturato sufficiente calore. A questo punto (stadio 2) essa inizia a contrarsi e ad assumere una forma sempre più sferica finché inizia a staccarsi dal suolo ed ad iniziarsi entro il PBL, mossa dalla forza di galleggiamento (stadio 3). Inizialmente la velocità di ascesa è elevata, tuttavia, durante lascesa, la bolla inizia il processo dinglobamento (entrainment) dell'aria fredda circostante (a temperatura inferiore) che, da un lato, produce un aumento dimensionale della stessa e dall'altro un abbassamento della sua temperatura media e quindi della spinta di galleggiamento. La diminuzione della spinta di galleggiamento e l'aumento della resistenza aerodinamica dovuta all'incremento di dimensione fanno sì che la velocità di ascesa del thermal diminuisca progressivamente fino ad arrestarsi completamente. La generazione di bolle calde o thermals ed il loro innalzamento nel PBL fa sì che il profilo verticale della temperatura potenziale media in situazioni convettive abbia una forma ben precisa, come quella riportata in figura

17 TROPOSPHERE BOUNDARY LAYER (BL) - During the day…. Surface layer (bottom 10%) strong change of wind speed with height (wind shear) wind shear occurs in the surface layer simply because wind speeds at the ground are zero and those above are not about 50 to 300m thick La temperatura potenziale diminuisce con la quota, evidenziando uno stato di instabilità statica. È questo lo strato in cui si ha la formazione e la prima evoluzione delle strutture termiche coerenti ed in cui sono concentrati la maggior parte degli effetti di shear. Ricordando quanto detto, in presenza di un gradiente negativo di temperatura potenziale, una particella daria (in questo caso un thermal), liberata nei pressi del suolo con una data velocità iniziale, abbandona la propria posizione iniziale acquisendo un moto ascensionale sempre più veloce. Questo strato rappresenta quindi una specie di catapulta per il flusso verso lalto di particelle di aria e quindi di quantità di moto, di calore, di umidità e di inquinamento.

18 TROPOSPHERE BOUNDARY LAYER (BL) - During the day…. Convective mixed layer when sunlight warms the ground during the day, some of the energy is transferred from the ground to the air just above the ground by conduction. Because the air above the ground is now warm, it rises buoyantly as a thermal Thermals originating from the surface layer rise and gain their maximum acceleration in the convective mixed layer. As thermal rise, they displace cooler air aloft downward, thus upward and downward motions occur, allowing air and pollutants to mix in this layer Al di sopra è presente un ulteriore strato caratterizzato da una temperatura potenziale praticamente costante, corrispondente ad una situazione di adiabaticità statica. Tale strato prende il nome di Strato Rimescolato (Mixed Layer, ML). È questo lo strato in cui le strutture coerenti hanno il loro massimo sviluppo. Una particella che raggiunge il ML dal SL non incontra ostacoli al proprio moto ascensionale, salvo la resistenza aerodinamica.

19 TROPOSPHERE BOUNDARY LAYER (BL) - During the day…. Entrainment zone the top is often bounded by a temperature inversion, an increase in temperature with increasing height which inhibits the rise of thermals. some mixing (entrainment) between the inversion and the mixed layer occur pollutants are trapped within an inversion, thus the closer the inversion is to the ground the higher pollutants concentration become Cloud and subcloud layers

20 During the night the grounds cools radiatively, causing air temperatures to increase with increasing height from the ground, creating a surface inversion Cooling at the top of the surface layer at nights cools the bottom of the mixed layer, reducing buoyancy and mixing. The portion of the daytime mixed layer that loses its buoyancy at night is the nocturnal boundary layer The remaining portion is called residual layer Pollutants are confined to the surface layer TROPOSPHERE BOUNDARY LAYER (BL) - During the night….

21 Traditional approach Pasquill-Gifford stability categories TROPOSPHERE BOUNDARY LAYER (BL) - Stability… The state of the boundary layer (amount of turbulence, meteorological conditions) will greatly influence the dispersion of any plume within it The ground level concentration of a pollutant can change from very low to very high as the state of the boundary layer changes from hour to hour The state of the boundary layer is conveniently called the stability Modern approach Monin-Obukhov length

22 TROPOSPHERE BOUNDARY LAYER (BL) - Stability… traditional approach Pasquill-Gifford stability categories to characterise the boundary layer 7 classes (A – G) with A being associated with the most convective (or unstable) conditions, D being associated with neutral conditions and G being associated with the most stable conditions. Concentration distribution in all stability conditions followed a normal (or Gaussian) distribution in both the horizontal and vertical directions The spread in each direction was based on experiments and was assumed to be constant throughout the boundary layer height In reality, the temperature gradient changes with height and convective eddies of turbulence take time to grow and rise and therefore the conditions will vary throughout the boundary layer

23 TROPOSPHERE BOUNDARY LAYER (BL) - Stability… modern approach Definition of Monin-Obukhov length Definition of Monin-Obukhov length (Lmo) is, in simple terms: L mo = -U* 3 /B, u*: the friction velocity, which increases with increasing wind speed and roughness length B: the buoyancy, which increases with increasing surface heat flux

24 TROPOSPHERE BOUNDARY LAYER (BL) - Comparison between traditional and modern approach The Pasquill-Gifford scheme describes the state of the atmosphere by 7 separate categories, whereas the approach based on Monin-Obukhov length uses a continuous scale. The variation of boundary layer properties with height is accounted for by the Monin- Obukhov characterisation. It is difficult to make exact comparisons between the two schemes. The following figure shows Pasquill-Gifford stability categories corresponding approximately to ranges of h/L MO. Note that, particularly in stable met conditions, the Pasquill- Gifford class is not a simple function of h/L MO.

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26 U(ms -1 )L MO (m)1/L MO (m -1 )h(m)h/L MO P-G Category 12553211255321 -2 -10 -100 100 20 5 -0.5 -0.1 -0.01 0 0.01 0.05 0.2 1300 900 850 800 400 100 -650 -90 -8.5 0 4 5 20 ABCDEFGABCDEFG Stability:Stable h/L MO 1 Neutral -0.3 h/L MO < 1 Convectiveh/L MO -0.3 TROPOSPHERE BOUNDARY LAYER (BL) - Comparison between traditional and modern approach

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28 Stability A: convective (or unstable) Very convective (unstable) conditions, Pasquill Class A, occur on hot, sunny days with light winds, when the earths surface is dry and there is strong solar radiation. Warm thermals rise from the ground creating boundary layer turbulence Such conditions are infrequent, occurring for approximately <1% of the time Effects of stability on plume dispersion

29 Unstable conditions The Monin-Obukhov length is negative. The magnitude of L MO (denoted as L MO ) is a measure of the height above the ground above which convective turbulence (that is, turbulent motions caused by heating of the surface) are more important than mechanical turbulence generated by friction at the earths surface. Typically, its value will be below 10m in convective conditions The boundary layer depth, h, is likely to be large (typically 1000-1500m or even up to 2500m) The convective turbulence dominates throughout almost the whole depth of the boundary layer and there is only a very shallow layer near the ground in which mechanical turbulence is most important

30 Stability D: neutral Neutral meteorological conditions, Pasquill Class D, commonly prevail in cloudy conditions with medium to strong wind speeds which cause vigorous mixing of the lower atmosphere Near dawn and near dusk conditions will tend to be neutral This is the broadest category and neutral conditions occur over a wide range of times of day and times of year Effects of stability on plume dispersion

31 Neutral conditions The Monin-Obukhov length may be positive or negative but its magnitude is very large Mechanical turbulence dominates throughout most or all of the depth of the boundary layer The wind speed is usually moderate to high, and it is often cloudy. The higher wind speed increases the mechanically generated turbulence, whilst the cloud cover inhibits the heating or cooling of the ground which would otherwise occur The magnitude of L MO is likely to be greater than that of the boundary layer depth (typically around 800m), therefore buoyancy does not dominate at any height in the boundary layer

32 Stability F/G: stable Very stable conditions, Pasquill Class F-G, occur on clear calm, nights with strong cooling of the ground and lower layer of the atmosphere caused by long wave radiation to space Temperature inversions usually occur; that is, the temperature increases with height above the ground Very stable conditions generally occur a few percent of the time Effects of stability on plume dispersion

33 Stable conditions The Monin-Obukhov length is positive The boundary layer tends to form into layers of different density such that the denser layers are near the ground. These layers will tend to suppress any vertical motion caused by the friction effects at the earths surface, although there will be weak turbulence generated by the action of the different layers on each other L MO is a measure of the height above ground above which vertical turbulent motion is greatly inhibited by the stable stratification. The value of L MO will be fairly small (typically less than 20m) However, as the boundary layer depth is also small (100-200m), the mechanical turbulence is still dominant throughout a significant proportion of its depth

34 Ground level source - Convective conditions will tend to mix the plume away from the ground and disperse it - Stable conditions will result in less mixing,therefore a ground level plume, of high ground level concentration, may persist for some distance downwind Effects of stability on plume dispersion

35 Elevated source - Convective conditions will bring the plume down to the ground more rapidly than neutral or stable conditions; this will lead to higher ground level concentrations nearer to the stack - Stable conditions may lead to the plume remainingwell above the ground for some considerable distance downwind. By this time, the plume may have dispersed sufficiently that ground level concentrations are below levels that may cause harm Effects of stability on plume dispersion

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38 Enhancing Dispersion with Smokestacks Pollution emitted from a taller stack has to travel a longer distance to get to the ground, so it will become more dilute.

39 Tall and Short Smoke Stacks With a tall enough smokestack, pollution is emitted within the inversion aloft, forming a fanning plume that does not pollute the area near the smokestack. If it's not tall enough, it will fumigate the countryside. Switching the layers so that the inversion is at the ground, we need the smokestack tall enough to be above the ground inversion, so that a lofting plume is formed. Architects will need to know the average depth of the nocturnal radiation inversion in order to know how tall to build the smokestack.

40 Exit Velocity The faster the smoke gushes out, the more momentum it has, and the higher it will fly before it levels out and disperses toward the ground. Methods for Increasing Exit Velocity Narrowing the smokestack's opening forces the smoke out as a faster streaming, narrower jet. Backpressure from the smaller opening may reduce the efficiency of the flow of smoke out of the chimney, however, partially offsetting the increasing in plume momentum.

41 Exit Temperature The higher the temperature, the greater the positive buoyancy in smoke streaming out of the smokestack. The smoke has to rise higher before it has adiabatically cooled to a neutral buoyancy temperature

42 dispersione degli inquinanti in atmosfera e in particolare in ambito urbano Monitoraggio inquinamento atmosferico Reti fisse di monitoraggio (recettori puntuali) superamento di valori critici di concentrazione non sono in grado di discriminare le sorgenti che contribuiscono maggiormente, in quellarea e in quella particolare situazione, ai livelli di concentrazione rilevati non sono in grado di fornire informazioni relative a tutte le aree in cui non sono disponibili i misuratori o a fornire stime di "scenario" SIMULAZIONE MODELLISTICA elaborazione dei dati, pianificazione dei sistemi di monitoraggio e supporto alla gestione degli interventi Monitoraggio e modellistica

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52 Modalità di funzionamento: continue o discontinue Dislocazione sul territorio: fisse o mobili

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73 Modelli di dispersione ADMS Modellistica dellinquinamento atmosferico Rilasci accidentali Gestione delle emissioni Flusso su terreno complesso www.cerc.co.uk

74 Modelli di dispersione ADMS

75 ADMS 4

76 Modellistica dellimpatto sulla qualità dellaria di insediamenti industriali esistenti o proposti Scenari presenti e futuri sulla qualità dellaria valutati sulla base degli standard di qualità dellaria quali EU Air Quality Directive, UK Air Quality Strategy, US National Ambient Air Quality Standards (NAAQS), Chinese Class I, II and III e WHO guidelines. Applicazioni tipiche: Permessi/autorizzazioni IPPC Determinazione dellaltezza delle sorgenti Modellizzazione delle sostanze odorose Valutazione dimpatto ambientale Sicurezza e pianificazione delle emergenze ADMS 4

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78 SETUP screen SOURCE screen ADMS 4

79 Sorgenti industriali Sorgente puntuale, es. ciminiera Sorgente areale, es. emissioni evaporative da un serbatoio Sorgente lineare, es. nastro trasportatore in una cava Sorgenti volumetriche, es. emissioni fuggitive Jet source (rilasci direzionali), es. tubo rotto Numero massimo di sorgenti: 300 Fino a 300 possono essere puntuali o jet sources, Nel limite di 300 fino a 30 sorgenti lineari, 30 areali e 30 volumetriche possono essere modellizzate contemporaneamente ADMS 4

80 Dispersione in presenza di edifici La modellizzazione della dispersione di inquinanti intorno agli edifici è complicata Fino a 25 edifici contemporaneamente Il pattern di flusso consiste in una regione di ricircolo (cavity, CR) dietro ledificio, con una zona diminuzione della scia turbolenta a valle Le concentrazioni all'interno della cavità, CR, sono uniformi e proporzionali alla frazione del rilascio che viene intrappolato ADMS 4

81 Dispersione in aree costiere Uno specifico modulo può essere attivato nel modello sotto queste condizioni: il mare è più freddo della terraferma Ci sono condizioni convettive sulla terraferma Cè un vento verso terra ADMS 4

82 Flusso su terreno complesso ADMS 4 usa modello complesso terreno, FlowStar, per calcolare il flusso e campi di turbolenza che vengono poi utilizzate per migliorare il calcolo della dispersione. Il modello calcola un flusso e un campo di turbolenza tridimensionale sopra la regione di interesse, che dipende sia dai valori di ingresso di altezza e di rugosità del terreno, sia dalle condizioni meteorologiche locali. ADMS 4 Il pennacchio è esposto a questi flussi e campi di turbolenza variabili, che si traduce in una concentrazione a livello del suolo che potrebbe essere superiore o inferiore alle previsioni corrispondenti al terreno pianeggiante.

83 METEOROLOGY screen BACKGROUND screen ADMS 4

84 GRIDS screen OUTPUT screen ADMS 4

85 Plot 3D Plot 2D ADMS 4