7. Model Pencemaran Udara

Model Pencemaran Udara 1

MODELPENCEMARAN UDARA


Air quality dispersion model

• Air quality models are used to predict ground level concentrations down point of sources.

• The object of a model is to relate mathematically the effects of source emissions on ground level concentrations, and to establish that permissible levels are, or are not, being exceeded.

• Models have been developed to meet these objectives for a variety of pollutants and time circumstances.


Penyebaran polutan di atm dari suatu sumber bergantung kpd:

1. Sifat fisik dan kimia polutan

2. Bentuk geometri sumber

3. Ketinggian emisi

4. Kondisi atmoefer: stabilitas, arah dan kecepatan angin, tekanan udara, T.


Pengaruh kecepatan angin terhadap plume


Pengaruh stabilitas atm


Plume Dispersion • In the stable atmosphere case (producing a fanning plume), there

is horizontal dispersion at a right angle to the wind due to turbulence and diffusion. In the vertical, dispersion is suppressed by the stability of the atmosphere, so pollution does not spread toward the ground. This results in very low pollution concentrations at the ground.

• In unstable air, the plume will whip up and down as the atmosphere mixes around (whenever an air parcel goes up, there must be air going down someplace else to maintain continuity, and the plume follows these air currents). This gives the plume the appearance that it is looping around.

• An inversion aloft will trap pollutants underneath it, since the stable inversion prevents vertical dispersion. Pollution released underneath the inversion layer will fumigate the mixed layer. Note that if the smokestack was high enough to release the pollution within the inversion layer, the plume would fan because the plume occurs within stable air.


• In the neutral atmosphere case, the horizontal dispersion at a right angle to the wind is due to turbulence and diffusion, which occurs at the same rate as the vertical dispersion, which is not being opposed nor encouraged by the stability (or lack of it) in the atmosphere. So, the plume spreads equally in the vertical and horizontal as it propagates downstream, forming a coning plume.

• In the lofting case, pollution dilutes upward. This produces much lower pollution concentrations at the ground at a distance downstream than the straight stable case (fanning plume), because molecular diffusion and some turbulence allow smoke to reach the ground eventually, and the fanning plume does not have the upward dispersion that the lofting plume has.


Model Bentuk Sumber:

1. Titik

2. Garis

3. Box

Gambar kanan: Model titik dan model box.


Proses penyebaran zat pencemar:

Dispersi:

• Transportasi

• Difusi


Dispersi: hasil dari 3 mekanisme yg sangat penting:1. Pergerakan udara rata2 yg mentransfer

pencemar searah arah angin.

2. Fluktuasi kecepatan turbulensi yg mendispersi zat pencemar udara ke segala arah.

3. Difusi massa yg disebabkan gradien konsentrasi.


Gaussian models, point source

• The most widely used models for predicting the impact of relative unreactive gases, such as SO2, released from smokestacks are based on Gaussian diffusion.

• In Gaussian models, the spread of a plume in vertical horizontal directions is assumed to occur by simple diffusion along the direction of the mean wind.


Asumsi model dispersi Gauss:

1. Emisi dari sumber kontinu dan konstan.

2. Kecepatan angin tidak nol, konstan, dan hanya pada arah x.

3. Tidak ada reaksi polutan di atm.

4. Difusi massa pada arah x diabaikan.

5. Dispersi dlm masing-masing arah koordinat konstan.

6. Stabilitas atm konstan.

7. Daerah sekitar relatif datar.


Coordinate system and elements of the bi-

Gaussian plume.


Dispersi plume dari sumber titik


Gaussian point source plume model:

Where:• Cx=ground level concentration at some distance x downwind

(g/m3)• Q=average emission rate (g/sec)• u=mean wind speed (m/sec)• H=effective stack height = h + h (m) • σy=standard deviation of wind direction in the horizontal (m) • σz=standard deviation of wind direction in the vertical (m)• y=off-centerline distance (m)

2

21

2

21

),,( expexp2 zyzy

zyx

Hzy

u

QC


Polutan GasUntuk polutan gas permukaan tanah dianggap sebagai reflektor yg ideal. Hal ini memungkinkan untuk mengasumsikan suatu sumber imajiner pada z-H dan sumber yg ada pd z+H.


Pasquill-Gifford model


Ground level concentration

Where:• Cx=ground level concentration at some distance x downwind

(g/m3)• Q=average emission rate (g/sec)• u=mean wind speed (m/sec)• H=effective stack height (m) • σy=standard deviation of wind direction in the horizontal (m) • σz=standard deviation of wind direction in the vertical (m)• y=off-centerline distance (m)

2

21

2

21

),,( expexpzyzy

zyx

Hy

u

QC


Gaussian models

• The parameters σy and σz describe horizontal and vertical dispersion characteristics of a plume at various distances downwind of a source as function of different atmospheric stability conditions. Values are determined from the graphs found in the figure.

• The effective stack height H is equal to the physical stack height (h) plus the height of the plume (plume rises, Δh) determined from where the plume bends over. Plume rises must be calculated from model equations before the effective stack height can be calculated.


Plume Rise

• The most common type of stationary source is a stack. Emissions from stacks may rise well above the stack height.

• The final vertical plume position depends on the temperature difference and on the exit velocity. This is an important parameter when designing stacks and air pollution control equipment.

• The main physical process causing a plume to rise is described below:


Plume Rise


Plume rise: daya apung kepulan asap:

• T > T udara lingkungan

pada saat keluar cerobong kecepatannya tinggi arah vertikal. Kenaikkan berhenti karena pencampuran udara sekitar, hilangnya kecepatan vertikal, dan T = T lingkungan

• Jika tidak ada angin plume akan mencapai elevasi yg tinggi dan C ground state rendah. Jika dibelokkan angin terjagi pengenceran.


Tinggi plume rise (Nevers, 1995):

h: kenaikkan plume di atas cerobong,m• vs : kecepatan keluar cerobong, m/s• D : diameter cerobong, m• us : kecepatan angin pd mulut cerobong, m/s• P : tekanan atm, mbar• Ts: T polutan pd cerobong, K• T : T udara ambient, K

)4..(..........1068,25,1 3

s

s

s

s

T

TTPDx

u

Dvh


Kecepatan angin • Kecepatan angin biasanya diukur pd 10 m• Kecepatan angin pada z:

…………………… pers. (5)

p

z

z

v

v

1

2

1

2

Stabilitas atm p

A 0,15

B 0,15

C 0,20

D 0,25

E 0,40

F 0,40


Parameter dispersi Kota menurut Briggs

Kelas pasquill

y, meter z, meter

A-B 0,32x(1+0,0004x)-0,5 0,24x(1+0,001x)-0,5

C 0,22x(1+0,0004x)-0,5 0,2x

D 0,16x(1+0,0004x)-0,5 0,14x(1+0,0003x)-0,5

E-F 0,11x(1+0,0004x)-0,5 0,08x(1+0,0015x)-0,5

x dalam meter


Parameter dispersi Desa menurut Briggs

Kelas pasquill

y, meter z, meter

A 0,22x(1+0,0001x)-0,5 0,20x

B 0,16x(1+0,0001x)-0,5 0,12x

C 0,11x(1+0,0001x)-0,5 0,08x(1+0,0002x)-0,5

D 0,08x(1+0,0001x)-0,5 0,08x(1+0,0015x)-0,5

E 0,06x(1+0,0001x)-0,5 0,11x(1+0,0004x)-0,5

F 0,04x(1+0,0001x)-0,5 0,11x(1+0,0004x)-0,5


Polutan Partikel

• Untuk partikel karena pd partikel bekerja gaya drag, maka H harus dikurangi dg:

• vt: terminal speed.

………………..(6)

ux

tv

g

ppt

gdv

18

2


Persamaan 1 menjadi:

)7(..........exp

2

22

21

),,(

z

ux

t

yzyzyx

vHzy

u

QC

Untuk partikel permukaan tanah tidak merefleksikan polutan.


Polutan radioaktif

• Polutan radioaktif mengalami peluruhan. Jadi harus dikoreksi (Nevers, 1995): f = exp(-t) ………………..(8)

t : wkt polutan keluar sumber sampai titik yg ditinjau = x/u.

• Jika pengukuran Q (t=0) di mulut cerobong, maka perlu ditambah faktor koreksi peluruhan dari mulut cerobong ke plume rise (selama menempuh h)

• Jadi pers. 8 menjadi f = exp {- (2 h/vs + x/u)} ………(9)


• Although the use of air quality models is the subject of considerable controversy, there's a general agreement that there a few alternatives to the use of models, particulately to make decisions on an action which is know in advance to pose potential environmental problem.

• The debate arises as to which models should be used, and the interpretation of models results. The underlying question such in debates is how well, or how accurately, does the model predict concentrations under the specific circumstances, since model accuracy may vary from 30% to a factor of 2 or more?

• The uncertainty associated with input variables, such as wind data, and source emission data. Such data are usually estimated and not well documented.


Contoh.

Suatu reaktor nuklir yg mempunyai tinggi cerobong 50 m, diameter 0,5 m mengemisikan 85Kr yg T1/2nya 3,18 menit dg laju emisi 1 µCi/s. Suhu gas keluar cerobong 30oC dan suhu udara sekitar 25oC. Tekanan udara luar 1 atm, stabilitas atmosfer C. Kecepatan angin pada 10 m adalah 3 m/s.


DOSE CALCULATIONS FROM RADIOACTIVITY IN

THE ATMOSPHERE

(Eisenbud, 1987)


EXTERNAL DOSE FROM PASSING CLOUD OF BETA EMITTERS

The dose rate from radiation to an individual located on the ground over which a cloud of radioactive gas is passing that is greater in size than the range of the haigest-energy particles can be estimated from the following:

Where:• D is the instaataneous dose rate to yhe skin (rad/sec)• E is the mean energy per disintegration (MeV/dis) is the concentration of emitting nuclide at a given

point downdwind of the source (Ci/m3)

ED 23,0


The integrated dose delivered by the passing cloud will be:

Where:

• D is the invinite dose to the skin (rads)

t is the integral of concentration x time for the intire emission, obtained by substituting Q (total curies released) for Q (Ci/sec), and has units Ci/sec per m3.

tED 23,0


• If more than one -emitting radionuclide is involved, the dose must be summed from the total of the dose from individual nuclides.

• The range of the particles in tissue is only a few millimeters, and the dose from external radiation is thus limited primarily to skin and varies with depth.

• The emitting gases are mainly isotopes of fission products Kr and Xe, for which the dose from inhalation is much less than the dose to the skin due to immersion in the cloud.


EXTERNAL DOSE FROM PASSING CLOUD

• The dose from passing cloud is based on the assumption that the individual is standing on the ground immersed in a cloud that is invinite in size, through which the total exposure is given as t.

• The dose estimate may be simplified by neglecting backscatter from the ground. This tends to reduce the dose estimate, but is somewhat offset by the error in the opposite direction that is introduced by the assumption of an invinite cloud.


The dose from a cloud containing 1 Ci sec/m3 can be estimated in this way to be:

Where:• D is the dose (rads)• E is the average energy (MeV) t is the product of concentration x time (Ci sec/m3)

calculated by substituting Q (total curies released) for Q (Ci/sec)

3

64

/0012,0.../100

/106,1sec/107,32/1

cmgradgerg

MeVergxECidxtD

EtD 25,0


• The dose from the passing cloud of fission products, as from a nuclear explosion or reactor release, will usually be much lower than either the dose received by inhalation (particularly inhalation of radioiodine) or the dose that results from the large-scale deposition of the cloud on surface.


SUMBER SESAAT (PUFF)

The PUFF program models the dispersion of volcanic ash from an eruption and provides predictions of ash particle locations (latitude/longitude/altitude) versus time given volcano, eruption characteristics, and wind field forecasts.


SUMBER SESAAT (PUFF)

Merupakan sumber sesaat jika t (wkt ledakan) < x/u.

Menentukan konsentrasi pd ttk lokasi ttt “puff downdwind concentration”

Perhatikan:

1. Interval wkt pd ledakan pertama.

2. Difusi dlm arah 3 dimensi x, y, z.


Persamaan Sutton:

• QT: total massa yg terlepas ke udara pd saat ledakan

• U: kecepatan angin

2

21

2

21

2

21

2/3 expexpexp2

2

yzxzyx

T yHutxQX


Untuk kondisi netral (Met & Atomic Energy, 1965): x=y

Y=0,8X+1,567

X=log jarak downwind, Y=log x=y

misal pada jarak 4 m:log 4=0,602, Y=0,8(0,602)+1,567=2,05

log x=2,05, jd x=112,2 m= y

z

Y=0,65X+1,33

X=log jarak downwind, Y=log z

7. Model Pencemaran Udara

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