ENVIRONMENTAL LETTERS, 10(1), 53-67 (1975)

CHARGED AEROSOL PARTICLES AND A I R POLLUTION

a i r p o l l u t i o n , a e r o s o l s , e l e c t r i c a l charge

Key Words:

J. R. B r o c k Department of Chemical E n g i n e e r i n g The U n i v e r s i t y o f T e x a s a t A u s t i n A u s t i n , T e x a s 78712

W. H .

Marlow

School o f P u b l i c Health U n i v e r s i t y of North C a r o l i n a C h a p e l H i l l , N o r t h C a r o l i n a 27514 ABSTRACT P o s s i b l e environmental e f f e c t s and t h e importance of c h a r g e d a e r o s o l p a r t i c l e s a r e examined. Some c a l c u l a t i o n s of t h e e v o l u t i o n of t h e charge d i s t r i b u t i o n of t h e atmospheric aerosol a r e presented. Several p o s s i b l e environmental e f f e c t s of p a r t i c l e c h a r g e a r e n o t e d i n c l u d i n g e f f e c t s on p a r t i c l e c o a g u l a t i o n , d r y d e p o s i t i o n , and d e p o s i t i o n i n t h e l u n g . F i n a l l y some p r o b l e m s i n a e r o s o l c h a r g i n g a r e r e v i e w e d a s t h e y pertain t o various technical operations i n a i r pollution control. INTRODUCTION

I n many i n s t a n c e s t h e s t a t e o f e l e c t r i c a l c h a r g e o f a e r o s o l s h a s b e e n o v e r l o o k e d b o t h i n c o n s i d e r a t i o n of e f f e c t s of p a r t i c u l a t e a i r p o l l u t a n t s and i n t e c h n i c a l o p e r a t i o n s i n volving aerosols.

We w i s h t o i n d i c a t e h e r e b r i e f l y some a r e a s

of i n v e s t i g a t i o n i n which t h e e l e c t r i c a l c h a r g e of a e r o s o l s

53 Copyright 0 1975 by hlarcel Dekker, lnc. All Rights Reserved. Kicither this work nor any part may be reproduced 01 transmitted id any form or by any means, electronic or mechanical, including photocopying, microfilming, by m y informdtion storage and retrieval system. without permission in writing from the publisher.

54

BROCK AND FfARLOW

h a s r e c e i v e d p e r h a p s i n s u f f i c i e n t a t t e n t i o n and t o i n f e r t h e p o s s i b l e i m p o r t a n c e of

t h e charged s t a t e i n sone of these areas.

I t h a s l o n g b e e n known t h a t t h e c o n d u c t i v i t y o f o u r a t m o s -

phere i s i n t i m a t e l y r e l a t e d t o t h e presence of p a r t i c l e s .

This

r e l a t i o n s h i p h a s been used t o monitor t h e o c e a n i c a e r o s o l b u r d e n a s a means o f m e a s u r i n g t h e g l o b a l b a c k g r o u n d a e r o s o l and i t s c h a n g e s ( s e e r e f e r e n c e 1 and r e f e r e n c e s t h e r e i n ) .

In

t h e u r b a n s e t t i n g where s o u r c e s o f f r e e c h a r g e a r e numerous and i o n i z a t i o n much h i g h e r t h a n i n r u r a

areas (2),

conductivity

o f t h e a i r is much l o w e r b e c a u s e o f t h e g r e a t l y e l e v a t e d l e v e l s o f a e r o s o l c o n c e n t r a t i o n owing t o man's a c t i v i t i e s .

I n addi-

t i o n , many o f m a n ' s a c t i v i t i e s p r o d u c e p a r t i c l e s p r e d o m i n a n t l y charged e i t h e r p o s i t i v e l y o r negatively..

T h e r e f o r e , i t be-

h o o v e s u s t o examine p o s s i b l e e f f e c t s of

t h e a e r o s o l charge

s t a t e on such processes a s a e r o s o l coagulation, deposition i n the lungs,

etc.

I n technical operations, c h a r g i n g a r e made.

dry deposition,

many a p p l i c a t i o n s o f a e r o s o l

For e x a m p l e , i n a i r p o l l u t i o n m o n i t o r i n g

t h e e l e c t r i c a l m o b i l i t y a n a l y z e r - is e n t i r e l y d e p e n d e n t upon a r t i f i c i a l p a r t i c l e c h a r g i n g and atmospheric c o n d u c t i v i t y m o n i t o r i n g is i m p l i c i t l y d e p e n d e n t upon n a t u r a l a e r o s o l c h a r g ing.

In a i r pollution control, electrostatic precipitation

and e l e c t r i c a l enhancement d f b o t h f i l t r a t i o n and s c r u b b i n g e f f i c i e n c y d e p e n d upon t h e a b i l i t y o f p a r t i c l e s t o c a p t u r e charges. I n t h i s n o t e we d i s c u s s f i r s t o f a l l some c a l c u l a t i o n s w e h a v e c a r r i e d o u t (3) f o r t h e c h a r g e s t a t e o f t h e a t m o s p h e r i c aerosol.

We t h e n n o t e s e v e r a l p o s s i b l e e f f e c t s o f a t m o s p h e r i c

p a r t i c l e charge i n c o n s i d e r a t i o n s of a i r p o l l u t i o n .

Finally,

CHARGED AEROSOL PARTICLE POLLUTION

55

i n s i g h t g a i n e d from o u r c a l c u l a t i o n s of a e r o s o l c h a r g e d i s t r i b u t i o n s l e a d s u s t o s u g g e s t some i m p o r t a n t c o n s i d e r a t i o n s relevant t o the aerosol charge s t a t e i n various technical operations.

THE CHARGE STATE OF THE ATMOSPHERIC AEROSOL F o r t h e p u r p o s e s of t h i s d i s c u s s i o n w e n e g l e c t t h e e f f e c t s o f a t m o s p h e r i c d i s p e r s i o n and e l e c t r i c a l f i e l d s on t h e c h a r g e

s t a t e of t h e atmospheric aerosol. s t r a t e t h a t under calm,

I t i s t h e n s i m p l e t o demon-

f i e l d free conditions,

the equations

describe t h e evolution of charge of a polydisperse a e r o s o l i n t h e p r e s e n c e o f two s p e c i e s o f s m a l l ions, o n e o f e a c h s i g n . H e r e , F i j r e p r e s e n t s t h e number d e n s l t y o f a e r o s o l p a r t i c l e s c a r r y i n g i c h a r g e s o f s i g n s a n d w h i c h a r e i n s i z e c a t e g o r y j. N'

is t h e number d e n s i t y o f s m a l l i o n s o f s i g n 1 ; U'

i j

and B

II i j

a r e t h e p r o b a b i l i t i e s p e r s m a l l i o n o f s i g n II t h a t i t w i l l .become a t t a c h e d t o a p a r t i c l e o f s i z e c l a s s j c a r r y i n g i c h a r g e s o f r e s p e c t i v e l y t h e same s i g n o r t h e o p p o s i t e s i g n .

Q is t h e v o l u m e t r i c g e n e r a t i o n r a t e o f s m a l l i o n s and R is t h e recombination c o e f f i c i e n t . By u s i n g w e l l - e s t a b l i s h e d

t is the t i m e

(3).

values f o r t h e small ion mobili-

t i e s and g e n e r a t i o n r a t e s a l o n g w i t h measured a e r o s o l d e n s i -

t i e s , u n d e r t h e a s s u m e d c o n d i t i o n s we h a v e c a l c u l a t e d n u m e r o u s examples o f s m a l l i o n d e n s i t y and p o l y d i s p e r s e a e r o s o l c h a r g e d i s t r i b u t i o n (3).

BROCK AND MARLOW

56

S t a r t i n g f r o m a n a t m o s p h e r e w i t h no i o n s i n i t i a l l y , c a l c u l a t i o n s show t h a t t h e g r e a t e r m o b i l i t y o f

our

the negative

ions l e a d s t o t h e i r f a s t e r disappearance s o t h a t the small ion d e n s i t y i s p r e d o m i n a n t l y composed of p o s i t i v e i o n s . For t h e p o l y d i s p e r s e a e r o s o l , o u r c a l c u l a t i o n s i n d i c a t e

a s l i g h t l y higher p o s i t i v e than negative charge concentration o n 0 . 0 1 pm p a r t i c l e s w h e r e a s o n l a r g e r p a r t i c l e s t h e n e g a t i v e charge dominates.

T h i s e v i d e n t d i s c r e p a n c y i s d u e t o t h e much

higher p r o b a b i l i t y of capture e x i s t i n g f o r t h e l a r g e r p a r t i c l e s By t h e t i m e t h e s m a l l e r p a r t i c l e s a r e l i k e l y t o c a p t u r e i o n s , t h e r e i s an excess of p o s i t i v e i o n s , become p o s i t i v e l y c h a r g e d .

so that these particles

T h i s example d e m o n s t r a t e s t h e

existence of e f f e c t s i n a polydisperse a e r o s o l a n t i c i p a t e d by c o n s i d e r i n g perse aerosols.

t h a t cannot be

i t s i m p l y a s t h e sum o f monodis-

F o r 0 . 1 pm p a r t i c l e s i n t h e a e r o s o l i t i s

found t h a t m u l t i p l e c h a r g i n g o c c u r s , a f a c t o f t e n overlooked. We h a v e a l s o c a l c u l a t e d t h e a p p r o a c h t o e q u i l i b r i u m o f a n i n i t i a l l y p o s i t i v e l y c h a r g e d a e r o s o l ( 4 ) u s i n g t h e same a n d Bs a s were e m p l o y e d i n r e f e r e n c e 3. c o e f f i c i e n t s Us ij il The a e r o s o l r e t a i n s a n e t p o s i t i v e c h a r g e f o r a l o n g t i m e , ostensibly contrary t o observations.

However,

t h i s occurs i n

t h e c a l c u l a t i o n b e c a u s e m i x i n g and b o u n d a r y e f f e c t s w e r e neglected.

According t o o u r c a l c u l a t i o n s ,

the steady-state

small ion density i n t h i s case is qualitatively s i m i l a r t o t h e p r e c e d i n g example,

s o t h a t t h e incomplete approach t o the

normal s t e a d y s t a t e h a s t o b e a t t r i b u t e d t o t h e s e o t h e r causes r a t h e r t h a n t o a q u a l i t a t i v e d i s t o r t i o n o f t h e s m a l l i o n dens i t y owing t o t h e i n i t i a l c h a r g e o f t h e a e r o s o l .

57

CHARGED AEROSOL PARTICLE POLLUTION E N V I R O N M E N T A L EFFECTS OF P A R T I C L E CHARGE

From n u m e r o u s m e a s u r e m e n t s i n t h e a t m o s p h e r e and s u c h c a l c u l a t i o n s a s described above, i t i s c l e a r t h a t e l e c t r i c a l charge i s a u n i v e r s a l f e a t u r e of

the "equilibrated"

atmos-

p h e r i c a e r o s o l a s w e l l a s o f t h e p o l l u t a n t a e r o s o l e m i t t e d from Up t o t h e p r e s e n t ,

various primary sources.

however,

possible

e f f e c t s of such c h a r g e have n o t r e c e i v e d wide c o n s i d e r a t i o n . W e suggest here,

and c o n s i d e r b r i e f l y ,

among p o s s i b i l i t i e s ,

t h r e e phenomena w e b e l i e v e a r e i m p o r t a n t l y d e p e n d e n t o n t h e e l e c t r i c a l charge s t a t e of t h e aerosol: tion;

( 2 ) Dry d e p o s i t i o n ;

(1) P a r t i c l e coagula-

( 3 ) Lung d e p o s i t i o n .

These a r e

discussed i n order. P a r t i c l e Coagulation I f the aerosol p a r t i c l e concentration i n t h e s i z e range a b o v e 0 . 1 pm i s n o t t o o l a r g e ,

t h e n i t i s always observed,

p a r t i c u l a r l y i n s i t u a t i o n s i n which n u c l e i are b e i n g g e n e r a t e d by g a s p h a s e r e a c t i o n s ,

t h a t c o a g u l a t i o n between p a r t i c l e s i n

t h e s i z e r a n g e b e l o w 0 . 1 pm i s t h e d o m i n a n t p r o c e s s i n e s t a b l i s h i n g t h e t o t a l number c o n c e n t r a t i o n .

These f a c t s have been

f r e q u e n t l y r e c o g n i z e d i n t h e l i t e r a t u r e ; however, r e s u l t s through a p p l i c a t i o n of

analyses of

t h e c o a g u l a t i o n e q u a t i o n have

usually neglected t h e r o l e of p a r t i c l e charge i n t h i s s i z e r a n g e on t h e c o a g u l a t i o n r a t e . Numerous f i e l d s t u d i e s i n t h e a t m o s p h e r e h a v e d e m o n s t r a t e d t h a t a s a r u l e o f t h e o r d e r o f 40-50% o f p a r t i c l e s i n t h e s i z e r a n g e 0 . 1 Pm ( 4 ) p o s s e s s e l e c t r i c a l c h a r g e a s s p e c i f i e d t h r o u g h

a bipolar equilibrium.

I n a d d i t i o n , i n urban atmospheres,

P a r t i c l e s e m i t t e d from combustion s o u r c e s f r e q u e n t l y have

fine

BROCK AND MARLOW

58

s t r o n g unipolar charges.

I n both instances,

even s i n g l e

c h a r g e s o n p a r t i c l e s b e l o w 0 . 1 pn p r o d u c e i n p o r t a n t a l t e r a t i o n s i n c o a g u l a t i o n r a t e s between charged p a r t i c l e s , a s i t i s q u i t e easy t o demonstrate (5). W e p r e s e n t h e r e o n l y a n i n d i c a t i o n of t h e s e s t a t e m e n t s .

D e t a i l s o f t h e d e v e l o p m e n t o f t h e t h e o r y o f c o a g u l a t i o n of c h a r g e d p a r t i c l e s may b e f o u n d i n (5). q u a n t i t y w h i c h we d e s i g n a t e a s B,

One e x a m i n e s h e r e a

t h e r a t i o of c o l l i s i o n f r e -

q u e n c i e s ( c o a g u l a t i o n c o e f f i c i e n t s ) f o r p a i r s of p a r t i c l e s w i t h c h a r g e s t o t h a t f o r p a i r s of uncharged p a r t i c l e s . F o r p a i r s o f c o l l i d i n g p a r t i c l e s w i t h t h e same s i g n w i t h c o l l i s i o n frequency

r a t i o 6,:

Bu

E

-

12 E

e 12-1 For o p p o s i t e s i g n s :

Where E12 Q,

= Q1Q2eL/2rkT

= n u n b e r o f c h a r g e s o n p a r t i c l e "1" o f p a i r

Q, '= n u n b e r o f c h a r g e s o n p a r t i c l e " 2 "

e

= elementary charge

r

= particle radius

kT

=

t h e r n a l energy of gas.

V a l u e s o f BU a n d table.

of p a i r

BB

a r e i n d i c a t e d i n t h e acconpanying

59

CHARGEB AEROSOL PARTICLE POLLUTION

TABLE 1 A l t e r a t i o n o f C o l l i s i o n Rate o f P a i r s o f E l e c t r i c a l l y Charged P a r t i c l e s Particle Radius, u m 0.15

0.01

-1

0.10

- 10-128 - 10-12

0.01 0.10 1.0

0.15

3

1

-1

1

300

10

30

10

3

10

I t i s c l e a r f r o m T a b l e 1 t h a t c o a g u l a t i o n o f t h e 40-50% f r a c t i o n o f c h a r g e a t n o s p h e r i c . p a r t i c l e s making up t h e condens a t i o n n u c l e i c o u n t w i l l b e s t r o n g l y i n f l u e n c e d by t h i s c h a r g e . Also,

c o a g u l a t i o n of u n i p o l a r charged s m a l l p a r t i c l e s which

i s s u e from combustion s o u r c e s cannot be d e s c r i b e d adequately w i t h o u t due c o n s i d e r a t i o n of t h e c h a r g e s t a t e . Dry D e p o s i t i o n The d e t a i l s o f v a r i o u s mechanisms b y which p a r t i c l e s a r e removed f r o m t h e a t m o s p h e r e a r e i n c o m p l e t e l y u n d e r s t o o d . of t h e s e mechanisns,

One

dry deposition, refers t o the deposition

o f a e r o s o l p a r t i c l e s on v a r i o u s s u r f a c e s n e a r t h e g r o u n d i n

t h e a b s e n c e of p r e c i p i t a t i o n s c a v e n g i n g p r o c e s s e s . Inasmuch a s t h e d r y d e p o s i t i o n p r o c e s s e n t a i l s t u r b u l e n t diffusion, of

interception,

and i m p a c t i o n ,

t h e p r o c e s s i s complex.

t h a t such deposition,

t h e complete theory

However, i t seems g e n e r a l l y a g r e e d

as i t occurs f o r f i n e p a r t i c l e s ,

as a n important s t e p t h e

involves

t r a n s f e r of p a r t i c l e s across a laminar

gaseous boundary l a y e r immediately a d j a c e n t t o t h e s u r f a c e of

60

BROCK AND MARLOW

deposition.

For l a r g e r p a r t i c l e s , t r a n s f e r o f p a r t i c l e s a c r o s s

t h e b o u n d a r y l a y e r may o c c u r t h r o u g h i n e r t i a l e f f e c t s . smaller particles, tant,

For

f o r w h i c h t h e i n e r t i a l mechanism is u n i m p o r -

t r a n s f e r a c r o s s t h e l a m i n a r b o u n d a r y l a y e r d e p e n d s , among

o t h e r f a c t o r s , on t h e B r o w n i a n d i f f u s i o n o f s u c h p a r t i c l e s . We f o c u s o n t h e s m a l l e r p a r t i c l e s and i n q u i r e a s t o how e l e c t r i c a l c h a r g e s o n s u c h p a r t i c l e s may a c t t o a u g m e n t o r d e c r e a s e t h e r a t e of t h e important s t e p of

t r a n s f e r of s u c h

p a r t i c l e s a c r o s s l a m i n a r boundary l a y e r s t o s u r f a c e s of deposition. Of a l l t h e p o s s i b l e e f f e c t s o f a t m o s p h e r i c p a r t i c l e c h a r g e on t h e r a t e of t r a n s f e r a c r o s s t h e l a m i n a r boundary l a y e r ,

the

m o s t i m p o r t a n t would a p p e a r t o b e t h e m i g r a t i o n v e l o c i t y i n p a r t e d t o a c h a r g e d p a r t i c l e by t h e a c t i o n of

the earth's

e l e c t r i c a l f i e l d w h i c h i n f a i r w e a t h e r is s u c h t h a t p o s i t i v e l y charged p a r t i c l e s tend t o m i g r a t e toward t h e e a r t h ' s

surface.

We c a n t h u s c o m p a r e t h e d e p o s i t i o n v e l o c i t i e s f o r a p o s i t i v e l y c h a r g e d p a r t i c l e i n t h e l a m i n a r boundary l a y e r of

thickness

6 owing t o B r o w n i a n d i f f u s i o n , VD, t o t h a t owing t o t h e v e r t i c a l a t m o s p h e r i c e l e c t r i c a l f i e l d , VE.

where:

Q = number o f e l e m e n t a r y

I n general:

c h a r g e s on p a r t i c l e

e = elementary charge

6 = t h i c k n e s s of l a m i n a r boundary l a y e r E = v e r t i c l e atmospheric e l e c t r i c f i e l d a t s u r f a c e

kT = t h e r m a l e n e r g y o f g a s . I n f a i r weather,

E w i l l r a n g e from 200-400

volts/meter.

The m a g n i t u d e o f 6 w i l l v a r y w i t h m e t e o r o l o g i c a l c o n d i t i o n s .

CHARGED AEROSOL PARTICLE POLLUTION

61

Under c o n d i t i o n s of s t r o n g v e r t i c a l m i x i n g 6 w i l l b e s m a l l . A t n i g h t , h o w e v e r , when t h e s u r f a c e i s c o o l e d a n d a s u r f a c e i n v e r s i o n e x i s t s 6 could be r e l a t i v e l y l a r g e .

6, therefore,

may b e e s t i m a t e d t o h a v e t h e r a n g e , 0 . 0 0 1 cm