T. Elperin, A. Fominykh and B. Krasovitov

Department of Mechanical EngineeringThe Pearlstone Center for Aeronautical Engineering

Studies Ben-Gurion University of the Negev P.O.B. 653, Beer

Sheva 84105ISRAEL

Rain Scavenging of Moderately and Highly Rain Scavenging of Moderately and Highly Soluble Gaseous Soluble Gaseous PollutantsPollutants in the Atmosphere in the Atmosphere

Motivation and goals

Fundamentals

Description of the model

Results and discussion

Conclusions

Outline of the presentation

Ben-Gurion University of the Negev11th EMS/10th ECAM Berlin 2011

Gas absorption by falling droplets

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is the species indissolved state

Henry’s Law:

Falling raindroplets

SO2, CO2, CO fossil fuels burning, forest firesNH3 agriculture

CO2, NOx – boilers, furnaces

Air

Soluble Gas

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Vertical concentration gradient of soluble gases

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Scavenging of air pollutions

Gaseous pollutants in atmosphere

– SO2 and NH3 – anthropogenic emission

– CO2 – competition between photosynthesis, respiration and thermally driven buoyant mixing

Fig. 1. Aircraft observation of vertical profiles of CO2 concentration (by Perez-Landa et al., 2007)

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Fig. 2. Vertical distribution of SO2. Solid lines - results of calculations with (1) and without (2) wet chemical reaction (Gravenhorst et al. 1978); experimental values (dashed lines) – (a) Georgii & Jost (1964); (b) Jost (1974); (c) Gravenhorst (1975); Georgii (1970); Gravenhorst (1975); (f) Jaeschke et al., (1976)

Scavenging of air pollutionsGaseous pollutants in atmosphere

SO2 and NH3 – anthropogenic emission

CO2 – competition between photosynthesis, respiration and thermally driven buoyant

mixing

Precipitation scavenging of gaseous pollutants by rain

Ben-Gurion University of the Negev11th EMS/10th ECAM Berlin 2011

Gas absorption by rain:

• Asman, 1995 – uniformly distributed soluble pollutant gas • Slinn, 1974 – wash out of plums• Zhang, 2006 – wash out of soluble pollutants by drizzle

Measurements of vertical distribution of trace gases in the atmosphere:

• SO2 – Gravenhorst et al., 1978• NH3 – Georgii & Müller, 1974• CO2 – Denning et al., 1995; Perez-Landa et al., 2007

Precipitation scavenging of gaseous pollutants by rain in inhomogeneous atmosphere:

• Elperin, Fominykh & Krasovitov 2009 – non-uniform temperature and concentration distribution in the atmosphere (single droplet)

• Elperin, Fominykh & Krasovitov 2010 – Effect of Rain Scavenging on Altitudinal Distribution of Soluble Gaseous Pollutants in the Atmosphere

Gas absorption by rain:

• Asman, 1995 – uniformly distributed soluble pollutant gas • Slinn, 1974 – wash out of plums• Zhang, 2006 – wash out of soluble pollutants by drizzle

Measurements of vertical distribution of trace gases in the atmosphere:

• SO2 – Gravenhorst et al., 1978• NH3 – Georgii & Müller, 1974• CO2 – Denning et al., 1995; Perez-Landa et al., 2007

Precipitation scavenging of gaseous pollutants by rain in inhomogeneous atmosphere:

• Elperin, Fominykh & Krasovitov 2009 – non-uniform temperature and concentration distribution in the atmosphere (single droplet)

• Elperin, Fominykh & Krasovitov 2010 – Effect of Rain Scavenging on Altitudinal Distribution of Soluble Gaseous Pollutants in the Atmosphere

Ben-Gurion University of the Negev11th EMS/10th ECAM Berlin 2011

Integral mass balance of the dissolved gas in a droplet:

characteristic diffusion time characteristic diffusion time

3

maD

LG

D

L

cmcdt

dc

1

where

m solubility parameter solubility parameter

mass transfer coefficient in a gaseous phase mass transfer coefficient in a gaseous phase

Gc concentration of a soluble gaseous pollutant in a gaseous phase concentration of a soluble gaseous pollutant in a gaseous phase

mixed-average concentration of the dissolved gas in a droplet mixed-average concentration of the dissolved gas in a droplet Lc

(1)

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For small Eq. (1) yields:

(2)

Dimensionless mass transfer coefficient for a falling droplet in a case of gaseous phase controlled mass transfer:

d

DGSh

3121 ScRe6.02Sh

D

td

cdcmc

G

DGL (3)

Total concentration of soluble gaseous pollutant in gaseous and liquid phases reads:

(4) LG ccc 1

Ben-Gurion University of the Negev11th EMS/10th ECAM Berlin 2011

Since

(5)

(6)

1

td

cd

c

G

GD

Eqs (3) (4) yield: Eqs (3)yield (4): mcc G )1(

where where volume fraction of droplets in the air. volume fraction of droplets in the air.

The total flux of the dissolved gas transferred by rain droplets: The total flux of the dissolved gas transferred by rain droplets:

Lcuqc

where u velocity of a droplet. whereu velocity of a droplet .

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where

Using Eqs. (3) and (6) we obtain :

,/ LtUT

(7)

td

cdcumq

GG

Dc

z

q

t

c c

2

2

Pe

1

GGG CC

T

C

(8)

(9)

,/ DULPe ,)1(

m

umU

,12 CTCRHm gA

Equation of mass balance for soluble trace gas in the gaseous and liquid phases:

Combining Eqs. (3) (8) we obtain :

,)1(

2

m

umD

,Uuu GGGcccC 0,

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(11)

(12)

(13)

Boundary conditions :0T )(fC G

0 1GC

1

0

GC

where 1,0, Lz

Peclet number: Peclet number:

3161

43211

251

6.026

PeGG

G

D

dc

dcm

LD

D

LU

(10)

]sm[130 121

1cwhere

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Fig. 1. Evolution of ammonia (NH3) distribution in the atmosphere due to scavenging by rain

Feingold-Levin DSD:

Fig. 3. Evolution of sulfud dioxide (SO2) distribution in the atmosphere due to scavenging by rain

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Fig. 4. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the

atmosphere) ( 20,0, 10GG

cgr cc

Fig. 5. Dependence of scavenging coefficient vs. altitude for ammonia wash out (linear initial distribution of ammonia in the

atmosphere) ( 20,0, GGcgr cc

Scavenging coefficient:Scavenging coefficient:

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Fig. 6. Dependence of scavenging coefficient vs. rain intensity for ammonia

wash out at the early stage of rain (dimensionless time T = 5 . 104)

Fig. 7. Dependence of scavenging coefficient vs. rain intensity for ammonia wash out at the early stage of rain (dimensionless time T = 0.05)

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LG

D

L

cmcdt

dc

1

Integral mass balance of the dissolved gas in a droplet:Integral mass balance of the dissolved gas in a droplet:

zcgradcc GGG 0wherewhere

1/

G

gr

G

c

dzdcuma

Criteria of equilibrium scavenging approach applicability:Criteria of equilibrium scavenging approach applicability:

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0

GG CT

C

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In this study we developed a model for scavenging of soluble trace gases in the atmosphere by rain. It is shown that gas scavenging is determined by non-stationary convective diffusion equation with the effective Peclet number that depends on droplet size distribution (DSD). The obtained equation was analyzed numerically in the case of log-normal DSD with Feingold-Levin parameterization.

It is demonstrated that scavenging coefficient for the wash out of soluble atmospheric gases by rain is time-dependent.

It is shown that scavenging coefficient in the atmosphere is height-dependent. Scavenging of soluble gas begins in the upper atmosphere and scavenging front propagates downwards with “wash down” velocity and is smeared by diffusion.

It is found that scavenging coefficient strongly depends on the initial distribution of soluble trace gas concentration in the atmosphere. Calculations performed for linear distribution of the soluble gaseous species in the atmosphere show that the scavenging coefficient increases with the increase of soluble species gradient.

Conclusions

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