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Graphitic carbon content of aerosols, clouds and snow, and its climatic implications
The Science of the Total Environment, 36 (1984) 117--120 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands
117
GRAPHITIC CARBON CONTENTOF AEROSOLS, CLOUDS AND SNOW, AND ITS CLIMATIC IMPLICATIONS I
PETR CHYLEK, V. RAMASWAMYAND V. SRIVASTAVA National Center f o r Atmospheric Research, Boulder, Colorado 80307 and Atmospheric Sciences Research Center, State University of New York, Albany, New York 12222
ABSTRACT Effect of g r a p h i t i c carbon on r a d i a t i v e c h a r a c t e r i s t i c s of aerosols, clouds and snow is investigated using a random internal mixture model in which most p a r t i c l e s are randomly d i s t r i b u t e d throughout the volume of i n d i v i d u a l aerosols, cloud drops and snow grains. I t is found that the specific absorption of carbon in aerosols, drops and snow grains is increased several times over the s p e c i f i c absorption of carbon in a i r . This leads to a decrease in the albedo of aerosols, clouds and snow, suggesting that g r a p h i t i c carbon could exert a nonnegligible influence on regional and global climate. INTRODUCTION Graphitic carbon (soot), which enters the atmosphere c h i e f l y through combustion processes, has been detected at widespread locations, ranging in concentrations from lOpg/m 3 in urban areas to Ipg/m 3 in rural areas.
I t has been found
in remote atmospheres ( r e f . l ) as well as in snowpacks in sparsely populated areas (ref. 2).
In comparison with the other constituents of the atmosphere,
g r a p h i t i c carbon is unique because of i t s uniformly high absorption in the v i s i b l e and the near infrared spectrum.
In f a c t , studies on the r a d i a t i v e
properties of aerosols, clouds ( r e f . 3 , ~ and snow ( r e f . 5), have i n f e r r e d that the absorption occurring in the v i s i b l e part of solar spectrum is a t t r i b u t a b l e to g r a p h i t i c carbon.
This is because the commonly occurring aerosols ( s u l f a t e s ) ,
water and ice are nonabsorbers in the v i s i b l e spectrum and so would be expected to have an albedo close to unity.
The fact that t h e i r albedos are
less than unity necessitates a study into the role of g r a p h i t i c carbon in lowering cloud and snow albedos, enhancing the absorption of solar r a d i a t i o n and thus a f f e c t i n g the climate of the planet. ABSORPTION OF SOLAR RADIATION BY SNOW, CLOUDS AND AEROSOLS To i n v e s t i g a t e the influence of g r a p h i t i c carbon on the absorption of solar r a d i a t i o n by aerosols, clouds and snow, we use a random internal mixture model in which soot is assumed to be d i s t r i b u t e d randomly throughout s u lf a t e aerosols, water drops and snow grains. 0048-9697/84/$03.00
The o pt ic a l properties of the mixture
© 1984 Elsevier Science Publishers B.V.
118 are evaluated by using the mixing r u l e d e r i v e d by Ch~lek and Srivastava ( r e f .
6)
in the frame-work o f aggregate s t r u c t u r e model f o r absorbing p a r t i c l e s embedded in a nonabsorbing medium.
M u l t i p l e s c a t t e r i n g p r o p e r t i e s are evaluated using
the d e l t a - E d d i n g t o n a p p r o x i m a t i o n ( r e f .
7).
The i m p o r t a n t consequence emerging
is the increase in the imaginary p a r t o f the r e f r a c t i v e over t h a t o f pure d i e l e c t r i c fractions
index o f the m i x t u r e
by several orders o f magnitude f o r even small soot
( r e f . 8).
Soot p a r t i c l e s
are assumed to conform to a m o d i f i e d gamma d i s t r i b u t i o n ,
w i t h the mode radius equal to 0.03 um and the w i d t h equal to 3 ( r e f . refractive
i n d i c e s are o b t a i n e d from Ackerman and Toon ( r e f .
Wiscombe ( r e f .
5).
in the v i s i b l e
region.
it
8).
The
3) and Harren and
Fig. l shows the albedo o f pure and soot-contaminated snow Comparing w i t h the data of G r e n f e l l e t . a l .
(ref.
2),
is seen t h a t a soot volume o f 3.5 x lO -8 is r e q u i r e d to e x p l a i n the measured
albedos.
Fig. 2 shows the c a l c u l a t e d r e d u c t i o n in the r e f l e c t i v i t y
s t r a t u s cloud in the v i s i b l e
spectrum due to d i f f e r e n t
the presence o f soot in small f r a c t i o n s
soot amounts.
of an a l t o Again,
lowers the albedo c o n s i d e r a b l y .
For
the case of c l o u d l e s s skies laden w i t h soot p a r t i c l e s and s u l f a t e a e r o s o l s , Fig. 3 shows t h a t , w i t h increases in the soot volume amounts, the net r e f l e c t i v i t y o f the a e r o s o l - s u r f a c e systems over most surfaces would be reduced.
,.o
1.0 0 r~ W {:D .-.I
0.8 o E3 b.I
0.6
09-
I,..... ,.#r,., ~'~
0.80.7-
..... . ........
1:3 z
0 .J u
o.z I 0.30.4
I
I
i
0.6
I
0.8
(M.m)
,
1.0
..
06
_ ..y'""
0.5 0.4
I
I
I
i
I
0.4 0.6 0.8
I
1.0
(M, rn)
Fig. I . mental albedo Dashed
Snow albedo as a f u n c t i o n o f wavelength. Crosses r e p r e s e n t e x p e r i measurements o f G r e n f e l l et a l . ( r e f . 2). S o l i d curve gives c a l c u l a t e d of soot-contaminated snow w i t h carbon volume f r a c t i o n o f 3.5 x 10 -8 . curve gives c a l c u l a t e d pure snow albedo.
Fig. 2. A l t o s t r a t u s cloud albedo as a f u n c t i o n o f wavelength. S o l i d curve represents c a l c u l a t i o n s f o r pure water cloud. A l l o t h e r curves give albedo o f cloud w i t h d e f i n i t e soot c o n c e n t r a t i o n (dashed curve represents carbon volume f r a c t i o n V=IO-7; dash-dot curve V=IO -6. dash-double dot curve V=IO-5; d o t t e d curve V=IO-4).
119
80
~ 'V
.
.
.
.
.
.
I
AA
'C\ ,
-zo 0
1
i "~"~" ~i~'-T'---t---.--1-- --.4 O.~
0.4
0.6
0.8
A
Fig. 3. Percentage changes AA in albedo A due to 1 km t h i c k s u l f a t e aerosol l a y e r c o n t a i n i n g d e f i n i t e amounts of soot. S o l i d curve represents soot volume f r a c t i o n of 10-J; dashed curve I 0 - 2 ; dash-dot curve lO - I . Cosine of s o l a r z e n i t h angle is 0.5. Aerosols' log-normal size d i s t r i b u t i o n is given by rg=0.124 ~m and Og=2.0.
Table 1 S p e c i f i c absorption in m2 per gram of carbon of a spherical snow grain with radius of I00 um, cloud d r o p l e t with radius of 5 ~m and spherical s u l f a t e aerosol p a r t i c l e with radius of 0.2 um c o n t a i n i n g a given volume f r a c t i o n of graphi-t i c carbon in a form of monodispersed carbon spheres with radius of 0.08 ~m d i s t r i b u t e d randomly throughout the volume of the snow g r a i n , cloud drop or aerosol p a r t i c l e . For comparison the value of s p e c i f i c absorption of 0.08 um radius carbon sphere in a i r is about 10.6 m2/g. soot volume fraction l .E-08 5.E-08 l.E-07 5.E-07 l .E-06 5.E-06 l .E-05 5. E-05 l .E-04 5. E-04 I. E-03 5. E-03 l .E-02 5. E-02 l.E-Ol
snow grain r=100 ~m 23.2 18.3 17.7 17.2 17.1 17.0 16.9 16.0 15.0 9.7 6.4 l .4 0.7 O.l ,0.1
cloud drop r=5 ~m
s u l f a t e aerosol r=O.2 um
30.7 27.6 27.2 26.9 26.8 26.8 26.8 26.7 26.6 25.6 24.1 16.6 I I .9 3.03 l .51
40.9 36.1 32.3 31.8 31.4 31.3 31.3 31.2 30.9 30.6 27.8 24.6
120
To i l l u s t r a t e
the role of soot in the internal mixtures, Table 1 l i s t s the
s p e c i f i c absorption (absorption per u n i t mass of carbon) of r=lO0 um snow grain, a r=5 ~m cloud drop and r=O.2 um s u l f a t e aerosol for d i f f e r e n t soot volume f r a c t i o n s .
Each contains monodispersed soot p a r t i c l e s of radius 0.08 um
d i s t r i b u t e d randomly throughout i t s volume.
These values, reveal the e f f i c i e n c y
with which soot in an internal mixture behaves, as compared to i t s e f f i c i e n c y in a i r (10.6 m2/gm). CLIMATIC EFFECT According to r a d i a t i v e - c o n v e c t i v e models (ref. 9, lO, l l )
the global
average surface temperature is a s e n s i t i v e function of clouds albedo.
Global
average albedo decrease of about 3% can produce surface heating around IK. ACKNOWLEDGEMENTS Reported research was supported in part by the National Science Foundation grant ATM-8007443.
National Center for Atmospheric Research is sponsored by
the National Science Foundation. REFERENCES l
H. Rosen, T. Novakov and B. A. Bodhaine, Atmos. Environ., 15 (1981) 1371-1374. 2 T. C. G r e n f e l l , D. K. Perovich and T. A. Ogren, Cold Reg. Sci. Technol., 4 (1981) 121-127. 3 T. Ackerman and O. B. Toon, Appl. Opt., 20 (1981) 3661-3667. 4 R. E. Danielson, D. R. Moore and H. C. Van de Hulst, J. Atmos. S c i . , 26 (1969) I078-I087. 5 S. G. Warren and W. J. Wiscombe, J. Atmos. S c i . , 37 (1980) 2734-2745. 6 P. Ch~lek and V. Srivastava, Phys. Rev. B, 27 (1983) 5098-5106. 7 J. H. Joseph, W. J. Wiscombe and J. A. Weinman, J. Atmos. S c i . , 33 (1976) 2452-2459. 8 P. Ch~lek, V. Ramaswamy and V. Srivastava, J. Geophys. Res., submitted for publidation. 9 V. Ramanathan, J. Atmos. S c i . , 33 (1976) 1330-1346. lO V. Ramanathan and J. Coakley, Rev. Geophys. Space Phys., 16 (1978) 465-489. I I P . Ch~lek and J. Kiehl, J. Atmos. S c i . , 38 (1981) If05-1110.
117
GRAPHITIC CARBON CONTENTOF AEROSOLS, CLOUDS AND SNOW, AND ITS CLIMATIC IMPLICATIONS I
PETR CHYLEK, V. RAMASWAMYAND V. SRIVASTAVA National Center f o r Atmospheric Research, Boulder, Colorado 80307 and Atmospheric Sciences Research Center, State University of New York, Albany, New York 12222
ABSTRACT Effect of g r a p h i t i c carbon on r a d i a t i v e c h a r a c t e r i s t i c s of aerosols, clouds and snow is investigated using a random internal mixture model in which most p a r t i c l e s are randomly d i s t r i b u t e d throughout the volume of i n d i v i d u a l aerosols, cloud drops and snow grains. I t is found that the specific absorption of carbon in aerosols, drops and snow grains is increased several times over the s p e c i f i c absorption of carbon in a i r . This leads to a decrease in the albedo of aerosols, clouds and snow, suggesting that g r a p h i t i c carbon could exert a nonnegligible influence on regional and global climate. INTRODUCTION Graphitic carbon (soot), which enters the atmosphere c h i e f l y through combustion processes, has been detected at widespread locations, ranging in concentrations from lOpg/m 3 in urban areas to Ipg/m 3 in rural areas.
I t has been found
in remote atmospheres ( r e f . l ) as well as in snowpacks in sparsely populated areas (ref. 2).
In comparison with the other constituents of the atmosphere,
g r a p h i t i c carbon is unique because of i t s uniformly high absorption in the v i s i b l e and the near infrared spectrum.
In f a c t , studies on the r a d i a t i v e
properties of aerosols, clouds ( r e f . 3 , ~ and snow ( r e f . 5), have i n f e r r e d that the absorption occurring in the v i s i b l e part of solar spectrum is a t t r i b u t a b l e to g r a p h i t i c carbon.
This is because the commonly occurring aerosols ( s u l f a t e s ) ,
water and ice are nonabsorbers in the v i s i b l e spectrum and so would be expected to have an albedo close to unity.
The fact that t h e i r albedos are
less than unity necessitates a study into the role of g r a p h i t i c carbon in lowering cloud and snow albedos, enhancing the absorption of solar r a d i a t i o n and thus a f f e c t i n g the climate of the planet. ABSORPTION OF SOLAR RADIATION BY SNOW, CLOUDS AND AEROSOLS To i n v e s t i g a t e the influence of g r a p h i t i c carbon on the absorption of solar r a d i a t i o n by aerosols, clouds and snow, we use a random internal mixture model in which soot is assumed to be d i s t r i b u t e d randomly throughout s u lf a t e aerosols, water drops and snow grains. 0048-9697/84/$03.00
The o pt ic a l properties of the mixture
© 1984 Elsevier Science Publishers B.V.
118 are evaluated by using the mixing r u l e d e r i v e d by Ch~lek and Srivastava ( r e f .
6)
in the frame-work o f aggregate s t r u c t u r e model f o r absorbing p a r t i c l e s embedded in a nonabsorbing medium.
M u l t i p l e s c a t t e r i n g p r o p e r t i e s are evaluated using
the d e l t a - E d d i n g t o n a p p r o x i m a t i o n ( r e f .
7).
The i m p o r t a n t consequence emerging
is the increase in the imaginary p a r t o f the r e f r a c t i v e over t h a t o f pure d i e l e c t r i c fractions
index o f the m i x t u r e
by several orders o f magnitude f o r even small soot
( r e f . 8).
Soot p a r t i c l e s
are assumed to conform to a m o d i f i e d gamma d i s t r i b u t i o n ,
w i t h the mode radius equal to 0.03 um and the w i d t h equal to 3 ( r e f . refractive
i n d i c e s are o b t a i n e d from Ackerman and Toon ( r e f .
Wiscombe ( r e f .
5).
in the v i s i b l e
region.
it
8).
The
3) and Harren and
Fig. l shows the albedo o f pure and soot-contaminated snow Comparing w i t h the data of G r e n f e l l e t . a l .
(ref.
2),
is seen t h a t a soot volume o f 3.5 x lO -8 is r e q u i r e d to e x p l a i n the measured
albedos.
Fig. 2 shows the c a l c u l a t e d r e d u c t i o n in the r e f l e c t i v i t y
s t r a t u s cloud in the v i s i b l e
spectrum due to d i f f e r e n t
the presence o f soot in small f r a c t i o n s
soot amounts.
of an a l t o Again,
lowers the albedo c o n s i d e r a b l y .
For
the case of c l o u d l e s s skies laden w i t h soot p a r t i c l e s and s u l f a t e a e r o s o l s , Fig. 3 shows t h a t , w i t h increases in the soot volume amounts, the net r e f l e c t i v i t y o f the a e r o s o l - s u r f a c e systems over most surfaces would be reduced.
,.o
1.0 0 r~ W {:D .-.I
0.8 o E3 b.I
0.6
09-
I,..... ,.#r,., ~'~
0.80.7-
..... . ........
1:3 z
0 .J u
o.z I 0.30.4
I
I
i
0.6
I
0.8
(M.m)
,
1.0
..
06
_ ..y'""
0.5 0.4
I
I
I
i
I
0.4 0.6 0.8
I
1.0
(M, rn)
Fig. I . mental albedo Dashed
Snow albedo as a f u n c t i o n o f wavelength. Crosses r e p r e s e n t e x p e r i measurements o f G r e n f e l l et a l . ( r e f . 2). S o l i d curve gives c a l c u l a t e d of soot-contaminated snow w i t h carbon volume f r a c t i o n o f 3.5 x 10 -8 . curve gives c a l c u l a t e d pure snow albedo.
Fig. 2. A l t o s t r a t u s cloud albedo as a f u n c t i o n o f wavelength. S o l i d curve represents c a l c u l a t i o n s f o r pure water cloud. A l l o t h e r curves give albedo o f cloud w i t h d e f i n i t e soot c o n c e n t r a t i o n (dashed curve represents carbon volume f r a c t i o n V=IO-7; dash-dot curve V=IO -6. dash-double dot curve V=IO-5; d o t t e d curve V=IO-4).
119
80
~ 'V
.
.
.
.
.
.
I
AA
'C\ ,
-zo 0
1
i "~"~" ~i~'-T'---t---.--1-- --.4 O.~
0.4
0.6
0.8
A
Fig. 3. Percentage changes AA in albedo A due to 1 km t h i c k s u l f a t e aerosol l a y e r c o n t a i n i n g d e f i n i t e amounts of soot. S o l i d curve represents soot volume f r a c t i o n of 10-J; dashed curve I 0 - 2 ; dash-dot curve lO - I . Cosine of s o l a r z e n i t h angle is 0.5. Aerosols' log-normal size d i s t r i b u t i o n is given by rg=0.124 ~m and Og=2.0.
Table 1 S p e c i f i c absorption in m2 per gram of carbon of a spherical snow grain with radius of I00 um, cloud d r o p l e t with radius of 5 ~m and spherical s u l f a t e aerosol p a r t i c l e with radius of 0.2 um c o n t a i n i n g a given volume f r a c t i o n of graphi-t i c carbon in a form of monodispersed carbon spheres with radius of 0.08 ~m d i s t r i b u t e d randomly throughout the volume of the snow g r a i n , cloud drop or aerosol p a r t i c l e . For comparison the value of s p e c i f i c absorption of 0.08 um radius carbon sphere in a i r is about 10.6 m2/g. soot volume fraction l .E-08 5.E-08 l.E-07 5.E-07 l .E-06 5.E-06 l .E-05 5. E-05 l .E-04 5. E-04 I. E-03 5. E-03 l .E-02 5. E-02 l.E-Ol
snow grain r=100 ~m 23.2 18.3 17.7 17.2 17.1 17.0 16.9 16.0 15.0 9.7 6.4 l .4 0.7 O.l ,0.1
cloud drop r=5 ~m
s u l f a t e aerosol r=O.2 um
30.7 27.6 27.2 26.9 26.8 26.8 26.8 26.7 26.6 25.6 24.1 16.6 I I .9 3.03 l .51
40.9 36.1 32.3 31.8 31.4 31.3 31.3 31.2 30.9 30.6 27.8 24.6
120
To i l l u s t r a t e
the role of soot in the internal mixtures, Table 1 l i s t s the
s p e c i f i c absorption (absorption per u n i t mass of carbon) of r=lO0 um snow grain, a r=5 ~m cloud drop and r=O.2 um s u l f a t e aerosol for d i f f e r e n t soot volume f r a c t i o n s .
Each contains monodispersed soot p a r t i c l e s of radius 0.08 um
d i s t r i b u t e d randomly throughout i t s volume.
These values, reveal the e f f i c i e n c y
with which soot in an internal mixture behaves, as compared to i t s e f f i c i e n c y in a i r (10.6 m2/gm). CLIMATIC EFFECT According to r a d i a t i v e - c o n v e c t i v e models (ref. 9, lO, l l )
the global
average surface temperature is a s e n s i t i v e function of clouds albedo.
Global
average albedo decrease of about 3% can produce surface heating around IK. ACKNOWLEDGEMENTS Reported research was supported in part by the National Science Foundation grant ATM-8007443.
National Center for Atmospheric Research is sponsored by
the National Science Foundation. REFERENCES l
H. Rosen, T. Novakov and B. A. Bodhaine, Atmos. Environ., 15 (1981) 1371-1374. 2 T. C. G r e n f e l l , D. K. Perovich and T. A. Ogren, Cold Reg. Sci. Technol., 4 (1981) 121-127. 3 T. Ackerman and O. B. Toon, Appl. Opt., 20 (1981) 3661-3667. 4 R. E. Danielson, D. R. Moore and H. C. Van de Hulst, J. Atmos. S c i . , 26 (1969) I078-I087. 5 S. G. Warren and W. J. Wiscombe, J. Atmos. S c i . , 37 (1980) 2734-2745. 6 P. Ch~lek and V. Srivastava, Phys. Rev. B, 27 (1983) 5098-5106. 7 J. H. Joseph, W. J. Wiscombe and J. A. Weinman, J. Atmos. S c i . , 33 (1976) 2452-2459. 8 P. Ch~lek, V. Ramaswamy and V. Srivastava, J. Geophys. Res., submitted for publidation. 9 V. Ramanathan, J. Atmos. S c i . , 33 (1976) 1330-1346. lO V. Ramanathan and J. Coakley, Rev. Geophys. Space Phys., 16 (1978) 465-489. I I P . Ch~lek and J. Kiehl, J. Atmos. S c i . , 38 (1981) If05-1110.