The Science of the Total Environment, 36 (1984) 1--10 Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
±
THE ROLE OF SOOT AND PRIMARY OXIDANTS IN ATMOSPHERIC CHEMISTRY*
T. NOVAKOV Applied Science Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720, USA
ABSTRACT This paper discusses the role of soot in atmospheric sulfur chemistry. A methodology for estimating the primary and secondary particulate carbon concentrations is outlined. The relationship between ambient S02, sulfate, and primary carbon is examined; and certain regularities in their behavior are defined. An explanation of these regularities is proposed, based on a mechanism involving oxygen chemisorbed on combustion-generated carbon particles and its subsequent reaction with aqueous sulfite.
INTRODUCTION Carbon is the largest elemental fraction of atmospheric aerosol particles and is found in many different chemical and physical forms.
These carbonaceous par-
ticles play important roles in atmospheric chemistry and physics and are crucially involved in a number of ecological and environmental effects, including human health.
These may be broadly classified according to their origin or
structure, using the following definitions: Total carbon, C - Total particulate carbonaceous material. Black carbon, C b - Combustion-produced black particulate carbon having a graphitic microstructure. Primary carbon, C'
Particulate carbon derived directly from sources, being
the sum of primary organic species, C'o, and black carbon.
The term "soot," Cs,
is synonymous with primary carbon derived from combustion. Secondary carbon, C" - Organic particulate carbon formed by atmospheric reactions from gaseous precursors, defined as C" = C - C'. The units of all these quantities are micrograms of carbon, either per cubic meter of air or per square centimeter of filter. This paper discusses i) methods for determining the concentrations of primary and secondary carbonaceous material, 2) an examination of evidence that suggests a chemical link between the carbonaceous fraction and other aerosol species such as sulfates, and 3) the role of primary carbonaceous species in the atmospheric *This work was supported by the Director, Office of Energy Research, Office of Health and Environmental Research, Physical and Technological Research Division of the U.S. Department of Energy under contract DE-ACO3-76SFO0098 and by the National Science Foundation under contract ATM 82-10343.
oxidation of sulfur dioxide to particulate mostly to polluted
atmospheres
direct source of carbonaceous
DETE~IINATION
is the principal
the relative abundances
will be discussed.
(black carbon/total (ref.l).
carbon)
Cb. and compares the ambientCb/C
to the ratio Cb/C for sources and source-dominated
(EGA) to obtain characteristic
secondary carbon components
the black carbon concentration has been shown to be linear, (ref.4).
C is determined
of the optical
absorption.
The method
for ambient
attenuation
ATN = OCb,
of black carbon on a filter.
by a conventional
combustion
The total
technique.
contain black carbon and primary organics
and have a specific attenuation of Z' = O.Cb/(Cb+C,o). contribute
produce black carbon.
to measure
Z = ATN/C = O.Cb/C gives the ratio of black to total
Primary particulates
sion mechanisms
o£
of primary and
(LTM)(ref.3)
and free of interferences
It gives a measurement
Specific attenuation
method
on a filter by its optical
sensitive,
to the surface concentration
carbon concentration
"signatures"
(ref.2).
The first appraoch uses the laser transmission
carbon.
of primary and secondary
The second approach relies on the more elaborate methodology
evolved gas analysis
samples
direct or in-
The first uses a fast and simple
to measure the black carbon component,
proportional
are limited
aerosol particles.
for determining
carbon in ambient particles
sites
where combustion
The discussions
OF PRIMARY AND SECONDARY CARBON
Two methods
technique
sulfate.
secondary carbon,
The specific
sample is then Z = O'Cb/(Cb+C'o+C,,).
Gas-to-particle
(C'o) conver-
C", to the particles but cannot
attenuation
of such a mixed carbon aerosol
The inclusion of C" in the denominator
re-
duces Z, so that the fraction of primary carbon may be determined by the relation
[C'/(C'+C")]
= Z/Z'
The validity of this approach depends on the constan-
cy of Z' for primary ambient carbonaceous
Manhattan, t
--
'
jj-~~l"
particles.
In Fig. 1 we show the
NY
---7
~
i[ 2'057 + . ~RangeforLsources Fig. I. Monthly average specific attenuation values, Z, for Manhattan. ~' is the adopted value for primary carbon. The range of Z values for automotive sources is also shown.
_
~
1979 DJ
I
1980
J
D
range of ~ w~lues for samples from automotive values for ambient samples collected heavily impacted by vehicular
and local
pect
to be dominantly
its carbonaceous
aerosol
show this constancy, ~alue for sources.
without
seasonal
carbon,
them
California).
stationary
and calculated
New York. sources,
This location
Fig.
5.7 * 0.5, will be adopted
2 shows monthl'," average E/r'
primary carbon data For samples from Anaheim
also tend to show lower values sults are consistent
production
with lowest
California (ref.5).
samples
These re-
of secondary carbonaceous
CA
]
..4j
0.5
Figure 2. bbnthIy average Z/Z' ratios, total carbon C, and primary carbon C' concentrations for Anaheim, California.
0 30
b d
(sou-
matter.
J
1.0
IE
Time-resolved
in the daytime than at night
with the photochemical
Anaheim, 1.5
ratios,
It is clear that the Z values for Anaheim are significantly
(i.e., highest C"/C) during the summer.
particulate
ex
The data
effects and with a mean similar to the mean
lower than for New York and that they" show a seasona[ variability values
is
and we therefore
primary" in character.
The mean Z value for Manhattan,
as the E' value for this discussion. total
sources and the monthly average
in Hanhattan,
20
lO
1977 D J
1978 D J
The method described of samples,
above is suitable
with the objective
For more detailed analysis
1979 D,
for the rapid study of large numbers
of determining
studies of the composition
(EGA) can be used
(refs.2,6,7).
average compositional of individual
In this method,
from a quartz fiber filter sample is heated at a constant 102) or neutral catalyst
(N2) atmosphere.
samples, evolved gas a small punch taken rate in an oxidizing
The evolved gases are passed over an oxidizing
to convert all carbonaceous
CO 2 analyzer.
properties.
The carbon evolution
ature gives the carbon "thermogram."
gases to CO2, which
is then detected by a
rate plotted as a function of sample temperBy simultaneous
measurement
of the optical
attenuation, a high-temperature peak corresponding to the decomposition o£ the b~ack carbon component can be identified.
The EGA-based approach to the differ-
entiation of primary and secondary fractions depends on the construction of an average thermogram for primary carbonaceous material.
After normalization o£
the black carbon peak (because this is unchanged by secondary mechanisms), the difference between the thermogram of an ambient sample and the primary thermogram is due to the secondary carbon.
This method was used to estimate the pri-
mary and secondary carbon in a number of samples from the Los Angeles area, collected during intense smog episodes when the highest concentrations of secondary carbon are expected (ref.8).
The samples were collected simultaneously at
a receptor site (Duarte) and a source-dominated site (Lennox). The potential of the EGA method is illustrated in Fig. 3, with thermograms of a daytime and a nighttime sample from the receptor site.
The common features of
both are the black carbon peak, 5, and the group of volatile organic species, ~. However, only the daytime sample shows the presence of B and y peaks.
Because
thermograms of the nighttime samples from this location were found to be similar to those at the source-dominated site, the B and y peaks have been identified as belonging to secondary carbon.
Detailed descriptions of the procedure and dis-
cussions of the chemical nature of secondary carbon are presented in ref. 8. The results of these analyses agree well with theoretical source apportionment calculations
II
(ref.9).
~ ! I
1 ~1
I
"(3
Daytime~
0
~NJqhttime-~I/~
I
I
I
Fig. 3. Carbon thermograms for a daytime and a nighttime sample collected at a receptor site (Duarte) in southern Califor-
!
11 l I i I f 100 200 300 400 500
nia.~represenvtoslatislpeecieBand s, y have been assigned to secondary carbon, and ~ is the black carbon peak.
600
Temperature °C
FIlE ROLE OF SOOT AND PRIMARY OXIDANTS IN SO2 OXIDATION Correlations tration fornia
(ref.lI),
is st r i k i n g understand ative
between sulfate
concentration
have heen o b s e r v e d f i r s t
to find such similar
and I , j u b l j a n a ,
were a n a l y z e d
for total
at
(ref.14).
carbon,
will
a n a l y z e d by" FGA.
The s a m p l e s from L j u b l j a n a
and b l i l l i p o r e
present
In l , j u b l j a n a ,
It
To b e t t e r
several
collabor-
t h e most p e r t i n e n t (ref.13);
a m b i e n t s a m p l e s were c o l filters.
These samples
and p a r t i c u l a t e
are particularly
of pollutants
sulfur.
A number
a v e r a g e 2 4 - h o u r SO2 c o n c e n -
interesting
during winter,
c a r b o n i s p r i m a r y and p h o t o c h e m i c a l p r o c e s s e s of particulate
shown i n F i g s . tions
so d i f f e r e n t .
Anaheim, ( : a l i f o r n i a
Twenty-four-hour
black carbon,
in (Tall fref.12).
were a l s o d e t e r m i n e d .
high concentrations
tions
smog c o n d i t i o n s
locations
(ref.13);
fiber
carbon concen-
and s u b s e q u e n t l y
we h a v e i n i t i a t e d
This discussion
,17-ram q u a r t z
o f s a m p l e s were a l s o trations
behavior
i n New York C i t y
Yugoslavia
on o p e n - f a c e
photochemical
correlations,
sampling programs.
from s t u d i e s
lected
intense
the causes of these
field
results
even d u r i n g
and s o o t o r t o t a l
i n Sweden ( r e f . 1 0 )
exhibit
4a, b,
sulfur
(mostly present
a similar
pattern,
suggesting
are negligible.
as s ulfa te ),
and c, w h i c h show c l e a r l y
because there
when a l m o s t a l ]
that
are
the particulate
Daily concentra-
carbon,
and SO2 a r e
SO2 :md c a r b o n c o n c e n t r a -
common s o u r c e s .
It
is also
evident
/~g/m 3
20 l-"
I
1
nll-
I
.hi
0'3
150
--
b
I
-
b F i g . 4. T w e n t y - f o u r - h o u r a v e r a g e conc e n t r a t i o n s o f a) p a r t i c u l a t e s u l f u r S, b) p r i m a r y c a r b o n C ' , and SO2. S/(I' r a t i o s a r e a l s o shown.
50O ff
250 0
O.4O 0,30 Or)
0.20 0.10 0
October November December
1981
6 that the variations
of particulate
ine these similarities C = C', ref.14),
sulfur and carbon are similar.
more quantitatively
by means of the S/C' ratios
centration
reaches ~ 30 ~g/m 3.
the average
Chere,
which are shown in Fig. 4d.
Figure 4d shows that the S/C' ratio is relatively
by approximately
We may exam-
constant after the C' con-
At lower C' concentrations,
a factor of 2.
This behavior
this ratio is higher
is shown in Table I, which gives
Ljubljana S/C' ratio for several C' concentrations.
Average S/C'
TABLE l Average sulfur to primary carbon ratios.
C' (ug/m 3)
LJB a
<5 20
~.23±o.o3
6030
t~ 0 . 1 4 ± 0 . 0 5
>60
S/C' ANAc
NYCb
f
o.19±o.o7
BRWd
SWEe
0.5±0.1
fo 8 o,o
0.40
•
0.14±0.03
aLJB - Ljubljana; 70 samples taken Sept., Oct., Nov., Dec. 1981. bNYC - New York City; 56 samples taken Feb., March, April, May, .June 1979. CANA - Anaheim; 48 samples taken Sept., Oct., Dec. 1977. dBRW - Barrow; 12 samples taken Jan., Feb., March 1983. eSWE - Sweden; samples taken Oct., Nov., Dec. 1974, Jan. Feb., March 1975. fFrom ref. 16. ratios corresponding
to different
cations are also presented. heim
(ref.13),
and Barrow,
taken from ref. 16.
C' concentration
Alaska
(ref. 15).
For Manhattan,
from several other lo(ref.13),
Ana-
Swedish data for S/soot ratios are
it was assumed that C = C', while Anaheim
and Barrow data use values of C' calculated ratios
ranges
Included are our data for Manhattan
from these entirely different
by methods described
locations
vior with respect to C' concentrations
show remarkably
earlier.
The
consistent beha-
(see Fig. 5).
Detailed analysis of these data shows the following general regularities: o For the same range of C' concentration, the same at all locations
the S/C' ratio is approximately
studied.
o The S/C' ratio increases at low concentrations. o Only a fraction of the available o Once particulate
sulfur is converted
sulfur has formed,
to sulfate.
there is no evidence
for significant
further oxidation of SO 2 to sulfate. Most atmospheric
oxidation processes dependent
lead to concentrations conditions emission, sulfur,
of S uncorrelated
of meteorology,
stagnation,
on SO 2 concentrations
with those of C', depending
solar radiation,
if strictly defined as the high-temperature
is inadequate
in magnitude
sion to explain the data.
and insufficiently
etc.
would
instead on
Primary sulfate
oxidation product of fuel correlated
The remainder of this paper outlines
with soot emisa theory that
0.8 0.7
I
I
I
I
I
I
L
~z B R W A SWE [] ANA • LJB o NYC
0.6 0.5 0.4
_
F i g . 5. Average S / £ ' r a t i o s f o r d i f f e r e n t C' c o n c e n t r a t i o n s , q'he d a t a a r e f o r b I a n h a t t u n (NYC); L j u b l j a n a , Y u g o s l a v i a (LJBi; Anaheim, C a l i f o r n i a (ANA); Sweden (%Ix'E)(from r e f . 1(~); and B a r r o w , A l a s k a (BRI~')( f r o m r e f . . . . . .
0.3
0.2
It
0.15
I 0
I
I
I
I
I
10 20 30 40 50 60 70 80 9 0 1 0 0
C' ~g/m 3) can s e m i q u a n t i t a t i v e l y fate.
This theory
explain
involves
the observed behavior
two m e c h a n i s m s :
o x y g e n c h e m i s o r b e d on w a t e r - c o v e r e d rite
studied
of soot in the oxidation
in o u r l a b o r a t o r y
carbon
( r e f . 17).
trated
in F i g .
of time after solution. in Fig.
that
of tMs
linear
that ([I
sulfate
illus-
c a r b o n i s added t o a d i l u t e i n two s t a g e s .
of sulfite
in Fig.
sulfitc
The f i r s t
(I
and t h e f o r m a t i o n o f s u l -
6) i s a s l o w e r p r o c e s s ,
to s ulfa te .
have been studied
cannot fully
are
as a func tion
c o u l d n o t be f o l l o w e d by t h e a n a l y t i c a l
conversion of sulfite
in a t m ospheric
process
concentrations
proceeds
disappearance
process
II process
second stage process
has been extensively
oxidation
and s u l f a t e
the oxidation
The s e c o n d s t a g e
of the stage
nificance
features
a known amount o f d r y a c t i v a t e d
used.
in a g r a d u a l tics
The p r i n c i p a l
This is a very fast
techniques
with of sul-
peroxides.
o f SO2 t o s u l f a t e
6, w h i c h shows s u l f i t e
It is clear
of sulfite
and 2) t h e r e a c t i o n
sul-
u s i n g b o t h c o m b u s t i o n - p r o d u c e d s o o t s and a c t i v a t e d
6) c a u s e s t h e i n i t i a l
fate.
1) t h e r e a c t i o n
soot particles
with primary combustion-generated The r o l e
of ambient particulate
The mechanism and k i n e -
in d e t a i l
(ref.
f o r m a t i o n h a s been a s s e s s e d e x p l a i n by i t s e l f
resulting
17),
and i t s
sig-
(refs.18,19).
This
the observed regularities
for ambient sulfate. The f i r s t those
stage process,
inferred
amount o f s u l f i t e bon;
Fig.
oxidized
6c shows t h a t
produced saturates process
however, has features
from t h e a n a l y s i s is
linearly
proportional
S/C'
despite
ratio,
Fig.
a considerable
to the of car-
t h e amount o f s u l f a t e
excess of aw~ilable in proportion
and would consume o n l y p a r t
similar
6b shows t h a t
to the c o n c e n t r a t i o n
for a fixed carbon concentration,
in t h e a t m o s p h e r e would produce s u l f a t e
a constant
qualitatively
of ambient samples.
of the total
sulfite. to carbon, available
Such a i.e., SO,.
8
>.
I
I
P
i
T
"~
f
¢..
--
/~omplete
4
2
l
/
6 0.. ~ 0 I 0
f
C
b
Of
e l~t
O~ ,4 0
x"-'
4
• L 2
//~
2
•
j 1
I 3
Figure 6. a) Sootcatalyzed SO 2 oxidation, showing the fast (stage I) and slower (stage II) processes. b) Initial drop in sulfite concentration vs. carbon for constant sulfite concentration, c) Initial drop in sulfite concentration vs. sulfite concentration for constant carbon concentration.
/
/
t
f
-
//~conversion
0 _ ::~--e] 4
2
i
I
I
I
4
6
8
10
(10-4M)
,.01/
10
20 30 T i m e (rain)
0
50
Based on the known mechanism of carbon-catalyzed lowing scenario
is proposed
onto the surface and becomes cools,
porarily
sulfate.
are exposed to air, oxygen is chemisorbed
"activated"
by the adsorption process.
When this aqueous
active oxygen at the solid-liquid The product
the fol-
As the com-
a film of liquid water will condense on the particles,
in which SO 2 can dissolve.
sulfate.
(ref.17),
to explain the formation of particulate
When freshly formed soot particles
bustion effluent
reaction
interface,
sulfate dissolves
free of oxygen.
sulfite gets in contact with the it will immediately react to form
in the water,
This constitutes
leaving the surface tem-
the rapid stage I process.
Dissolved
oxygen from the water will slowly diffuse to the carbon surface to f o r m a s i m i l a r surface oxygen complex, cess is diffusion
which will also react with aqueous sulfite.
controlled
and is responsible
for the slower
This pro-
(stage II) cata-
lytic oxidation process.
Because these processes occur on the liquid-solid
terface,
of condensed
a few monolayers
water are sufficient.
[) process will lead to sulfate formation the condensation
The rapid
in-
(stage
in the plume as soon as conditions
of liquid water and solution of SO 2 have been reached.
for
Parti-
culate sulfate will be formed by this mechanism relatively close to the source and could therefore be termed quasi-primary Soots produced by combustion
of propane,
soot have been found to behave qualitatively
sulfate. natural
gas, and automotive
like activated
like the one in Fig. 6b, S/C' ratios for the fast oxidation for different and natural
carbons:
0.004,
gas, respectively.
of S/C' ratios determined
carbon.
(tunnel)
From graphs
stage are derived
0.05, and O.1 for activated carbon,
tunnel
soot,
The latter two agree well with the lower ranges
from ambient measurements
(c.f. Fig. 4).
The following arguments
show the consistency of these results with the pro-
posed surface chemical mechanism.
Assuming that each sulfite ion occupies
area of 3 x i0 -15 cm 2 on the surface at the time of reaction, vated carbon must have a solid-liquid in order to accommodate
interfacial
0.004 grams of sulfur.
surface area obtained by nitrogen adsorption. most of the gas-solid available
an
one gram of acti-
area of approximately
22 m 2
This area is about 5% of the This result seems correct because
surface area is the internal pore area that may not be
to reactions
of the liquid-solid
face areas calculated
for tunnel
560 m2/g C, which are reasonable mation have diameters
interface.
soot and natural
The liquid-solid
inter-
gas soot are 280 m2/g C and
because soot particles
immediately
from about 50 to a few thousand ~ngstr~ms.
after for-
Such particles
have surface areas of hundreds of meters square per gram. During the short time when sulfate formation occurs, surface area is available
for reaction.
most of the particle
After residing
in the atmosphere,
surfaces become covered with sulfate as well as other species, surface that is chemically and physically Recent research in contrast oxidation
different
in a
less active.
in our laboratory has shown that aged ambient aerosol particles,
to source particles,
are practically
inactive as catalysts
for SOo
(ref.20).
From the foregoing discussion, particles
it is plausible
can produce enough particulate
er limit of the S/C' ratios observed possible explanations
that surface reactions
sulfur to account
in ambient air.
for the higher S/C' ratios.
lack of size segregation
There are at least two
more likely to be present at high carbon concentrations, surface areas.
be produced after longer reaction
cess, which could also be enhanced by nitrogen oxides oxidation mechanisms
involving peroxide-type
The concept of primary oxidants new because secondary
organic peroxides incomplete
However,
(ref.21),
in the context of atmospheric
it is known
(ref.22)
or by the SO 2
chemistry
is
including peroxides,
are
reactions, and under conditions of
strated that propane flames and internal combustion
We have recently demonengines emit substantial
amounts of hydrogen peroxide and organic hydroperoxides, in water
pro-
that hydrogen peroxide and
could appear in the effluent.
sulfite to sulfate when dissolved
sulfur can
primary oxidants.
are formed in precombustion
combustion,
particulate
(stage II) oxidation
it is generally assumed that all oxidants,
in origin.
Large particles,
have lower S/C' ratios
Additional
times by the slower
systematic
could be related to the
of the samples used in our studies.
because they have low specific
on soot
for at least the low-
The apparently
increase of the S/C' ratio at low carbon concentrations
(ref.23).
unit of fuel consumed depend greatly on the combustion a very turbulent propane flame produces more peroxide flame.
resulting
and generally
the
which rapidly oxidize
Their emission rates per conditions.
For example,
than a steady diffusion
This in turn produces more than a premixed propane-air
flame,
and a
I0 premixed propane-oxygen flame produces the least.
These results imply that more
primary peroxides will be produced by sooty flames than by nonsooty ones.
The
sulfate produced by the reaction of primary peroxides would therefore generally follow the soot concentrations,
thereby retaining the S-C' correlation.
The
reactions involving primary oxidants, in addition to forming particulate sulfates, may play a role in cloud and fog chemistry, and thus contribute to the overall atmospheric sulfate burden.
REFERENCES
1
2 3 4 5
6 7
8 9 I0 ii
12 13 14 15 16
17 18 19 20 21 22 23
T. Novakov, in H. Malissa, M. Grasserbauer and R. Belcher (Eds.), Nature, Aim and Methods of ~icrochemistry, Springer-Verlag, New York, 1981, pp. 141165. E.C. Ellis and T. Novakov, Sci. of Total Environ., 23 (1982) 227-238. H. Rosen and T. Novakov, Appl. Opt., 22 (1983) 1265-1267. L.A. Gundel, R.L. Dod, H. Rosen and T. Novakov, these Proceedings. A.D.A. Hansen, H. Rosen, R.L. Dod and T. Novakov, in T. Novakov (Ed.), Proceedings, Conference on Carbonaceous Particles in the Atmosphere, Lawrence Berkeley Laboratory report LBL-9037, Berkeley, 1979, pp. I16-121. H. Malissa, H. Puxbaum and E. Pell, Z. Anal. Chem., 282 (1976) i09-i13. R.L. Dod, H. Rosen and T. Novakov, in Atmospheric Aerosol Research Annual Report 1977-78, Lawrence Berkeley Laboratory report LBL-8696, Berkeley, 1979, pp. 2-I0. E.C. Ellis, M. Zeldin and T. Novakov, these Proceedings. S.K. Friedlander, Ibid. C. Brosset, Ambio, 2 (1973) 2-9. T. Novakov, in Proceedings, Second Joint Conference on Sensing of Environmental Pollutants, Instrument Society of America, Pittsburgh, 1973, pp. 197204; and T. Novakov, S.C. Chang and A.B. Harker, Science, 186 (1974) 259-261. B.R. Appel, E.L. Kothny, E.M. Hoffer, G.M. Hidy and J. Wesolowski, Environ. Sci. Technol., 12 (1978) 418-425. T. Novakov, unpublished data. M. Bizjak, V. Hudnik, S. Comiscek, A.D.A. Hansen and T. Novakov, these Proceedings. H. Rosen, unpublished data. C. Brosset, in T. Novakov (Ed.), Proceedings, Conference on Carbonaceous Particles in the Atmosphere, Lawrence Berkeley Laboratory report LBL-9037, Berkeley, 1979, pp. 95-I01. R. Brodzinsky, S.G. Chang, S.S. Markowitz and T. Novakov, J. Phys. Chem., 84 (1980) 3354-3358. P. Middleton, C.S. Kiang and V.A. Mohnen, Atmos. Environ., 14 (1980) 463-472. S.C. Chang, R. Toossi and T. Novakov, Atmos. Environ., 15 (1981) 1287-1292. W.H. Benner and T. Novakov, in Environmental Research Program Annual Report FY 1982, Lawrence Berkeley Laboratory report LBL-15298 (in press). W.R. Corer III, D.R. Schryer and R.S. Rogowski, Atmos. Environ., 15 (1981) 1281-1280. See, for example, D.J. Patterson and N.A. Henein, Emissions from Combustion Engines and Their Control, Ann Arbor Science, Ann Arbor, 1972, 347 pp. M. McKinney, W.H. Benner and T. Novakov, in Environmental Research Program Annual Report FY 1982, Lawrence Berkeley Laboratory Report LBL-15298 (in press).