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OCR for page 107
6
Chemistry
The production of nitric oxide by nuclear explosions and the production
of soot and gaseous pollutants by fires ignited by nuclear explosions
would pose chemical threats to the atmosphere in the postnuclear war
period. This chapter first discusses the generation of the polluting
substances and then assesses their impacts in the atmosphere.
GASEOUS EMISSIONS FROM NUCLEAR FIREBALLS AND NUCLEAR WAR FIRES
Nitric Oxide
Nuclear explosions produce nitric oxide (NO) by heating air to very
high temperatures both in the interior of the fireball and in the
accompanying shock wave. At temperatures above about 2000 K the
equilibrium
N2 + O2 = 2NO
is rapidly established, the amount of NO increasing with increasing
temperature. As hot air containing large amounts of NO is cooled, the
above equilibrium is maintained until a temperature is reached where
the rates of the reactions maintaining the equilibrium become slow in
comparison with the cooling rate. For cooling times of seconds to
milliseconds, the NO concentration "freezes" (becomes fixed) at
temperatures between 1700 K and 2500 K, corresponding to NO
concentrations of 0.3 and 2.0 percent by volume, respectively.
There have been numerous estimates of the amount of NO produced per
megaton of explosion energy, and these have been reviewed by Gilmore
(19751. The spherical shock wave is estimated to produce 0.8 x 1032
NO molecules per megaton of explosive yield, as a result of the rapid
heating of air in the shock front followed by rapid cooling due to
expansion and radiative emission.
The shock wave calculation of NO production does not take into
account the fact that air remaining within the fireball center contains
approximately one-sixth of the initial explosion energy. This air
cools on a time scale of several seconds by further radiative emission,
entrainment of cold air, and expansion as it rises to higher
107
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108
altitudes. These mechanisms are sufficiently complex that one can only
estimate upper and lower limits to the quantity of NO finally produced.
A lower limit to the amount of NO finally produced may be obtained
by assuming that all of the shock-heated air is entrained by the
fireball and (again) heated to a temperature high enough to reach
equilibrium. This is possible since the thickness of the ~shell. of
shock-heated air containing NO is smaller than the radius of the
fireball. To minimize the cooling rate, and thus the freeze-out
temperature, it is assumed that this air mass cools only by adiabatic
expansion as the fireball rises and by using a minimum rise velocity.
The resulting lower limit to NO production is 0.4 x 1032 molecules
per megaton.
Since the interior of the fireball is much hotter than the
surrounding shock-heated air, it will rise much faster and possibly
pierce the shell of shock-heated air to mix with the cold, undisturbed
air above it. Thus an upper limit to NO production may be obtained by
assuming that none of the 0.8 x 1032 NO molecules per megaton
produced in the shock wave are entrained by the hot fireball and that
the interior is cooled totally by entrainment of cold, undisturbed air
to produce additional NO. The under limit to total NO Production is
then estimated to be 1.5 x 1032 molecules per megaton. One can make
strong arguments that both the lower and the upper limits are extremely
unlikely. For the purposes of this assessment, a NO yield of
1.0 x 1032 molecules per megaton (0.005 Tg/Mt) is assumed.
It should be emphasized that the emission factor for nitrogen
oxides produced in nuclear explosions is based wholly on theoretical
considerations and that there has not yet been any attempt at
experimental verification of the amounts produced. Sedlacek et al.
(1983), analyzing samples for HNO3 in the stratosphere, infer that
the Chinese 4-Mt nuclear device of 1976 produced about 10 times the
amount of NO as expected from the theoretical calculations discussed
above. The discrepancy remains unexplained. Table 6.1 gives the
calculated total amounts of NO injected by the two scenarios of this
study as well as amounts used in other studies.
Fire Emissions
Uncontrolled fires result in incomplete combustion with emissions of
copious quantities of both particulate and gaseous matter. The
per tabulate emissions and their effects on the physical properties of
the atmosphere are dealt with in other sections of this report. Among
the gaseous emissions from fires are carbon monoxide, nitrogen oxides,
and a large number of hydrocarbons and other organic compounds. These
compounds, together with sunlight, are the necessary ingredients for
photochemical smog formation.
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109
TABLE 6.1 Recent Estimates of Maximal Ozone Depletion Resulting from a
Nuclear War
NO (1032 molecules) Maximum
Ozone
Depletion
(percent) Note
Yield Below Above
Scenario (Mt) 12 km 12 km
Baseline 6,5002,665 3,835 17 a
Excursion 8,5002,665 5,835 43 b
Chang Case A 10,600560 6,540 51 ~
Chang Case B 5,300280 3,270 32 d
Chang Case C 5,6700 3,800 42 e
Chang Case D 4,930560 2,740 16 f
Chang Case E 6,720180 4,340 39 g
Chang Case F 3,890390 2,220 20 h
Ambio 5,7404,510 1,230 ~0 i
Ambio Excursion 10,0001,375 8,625 65 (45°N) j
l
Turco et al. 10,0001,200 8,400 50 k
(1983)
aNo weapons larger than 1.5 Mt. See Chapter 3 for details.
bBaseline scenario plus 100 weapons of 20-Mt yield.
CAll strategic weapons in the United States and USSR arsenals
successfully detonated.
dHalf of the weapons of each type in the strategic arsenals of the
United States and USSR.
eAll weapons with individual yields greater than 0.8 Mt in the
strategic arsenals of the United States and USSR.
fall weapons with individual yields less than or equal to 0.8 Mt in
the strategic arsenals of the United States and USSR.
gall weapons in the Soviet strategic arsenal.
hall weapons in the U.S. strategic arsenal.
Then the troposphere is included, the Ambio scenario actually
results in a slight ozone increase. The Chang model also gives ~ _
result for the Ambio scenario.
.
The Ambio excursion scenario consists of 5000 1-Mt detonations plus
500 10-Mt detonations and is identical to the NRC (1975) scenario.
kThe blocking of sunlight by nuclear dust and soot was accounted for,
but the resulting heating of the stratosphere was not.
Carbon Monoxide
Carbon monoxide (CO) is the most abundant air pollutant from fires.
The emission factor may be quite high, depending on the degree of
aeration. For example, Sandberg et al. (1975) measured CO emissions
the range of 25 to 40 percent in very low intensity laboratory fires
and Ryan and McMahon (1976) state that CO emissions may approach 25
OCR for page 110
110
percent for smoldering fires in damp fuels.* Emissions from prescribed
forest fires fall in the range of 1 to 25 percent according to Tangren
et al. (19761. A review by Chi et al. (1979) recommends an emission
factor of 5.6 + 1.6 percent for prescribed fires where the indicated
error is the 95 percent confidence interval of the mean. The committee
has adopted an emission factor for CO of 5 percent for its
calculations. For the baseline scenario in which 4500 Tg of fuel is
consumed by fire, the CO emission is 225 Tg. Mixed uniformly
throughout half of the northern hemisphere troposphere, the
concentration of CO is increased from the present level of about 100
ppbv {parts per billion by volume) to about 300 ppbv.
Hydrocarbons
Hydrocarbons are an extremely diverse class of organic compounds
consisting only of carbon and hydrogen. They include aliphatic
hydrocarbons (alkanes) such as methane, ethane, and propane; olefins
(alkenes) such as ethylene and propylene; alkynes such as acetylene;
and aromatic compounds such as benzene, toluene, and the xylenes. In
addition to the hydrocarbons, numerous oxygen-containing compounds such
as alcohols, ethers, aldehydes, ketones, and carboxylic acids have been
identified in fire emissions. In fact, more than 200 individual
compounds have been identified in forest fire emissions, and
considering the results of recent studies of cigarette smoke, it is
likely that the actual number of compounds emitted is in the many
thousands. Figures 6.1 and 6.2 are chromatograms of air collected
above slash burning in the tropical forests of Brazil (Greenberg et
al., 1984~. These chromatograms illustrate the numerous compounds
typically found in fire emissions.
Total hydrocarbon emissions have been reported to be in the range
of 0.2 to 3.2 percent in laboratory fires and 1.4 to 5.4 percent in a
limited number of field fires (McMahon, 1983~. In recent measurements
of biomass burning in Brazil (Greenberg et al., 1984), emission factors
of 2.0 percent for grassland fires and 2.7 percent (expressed as
percent of carbon dioxide by volume) for forest fires were obtained.
Methane made up 35 percent of the grassland emissions and 50 percent of
the forest emissions.
As an estimate for nuclear war fires, the committee has adopted an
emission factor of 2 percent (based on weight of fuel burned) for total
hydrocarbons and further has assumed that half of these emissions are
due to methane, which is relatively less reactive than the higher
hydrocarbons. For the baseline scenario, this results in a total
emission of 45 Tg of methane and 45 Tg of other hydrocarbons. The
methane emission results in an increase in its concentration at
mid-latitudes of 70 ppbv. This represents only a minor increase in the
ambient methane concentration of 1650 ppbv. As a result, the methane
*Unless otherwise noted, all emission factors quoted refer to the mass
of a particular chemical species produced per mass of fuel consumed.
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111
a,
ax
~ ~ -
cN l ~ , . , .
-
4 -
c
Q
o
Q
(~
C)
O
(
O ~
_ ~
a)
Q
o
Cot
m '
a, Z ~
~ m
Len
m O ' 0' I
a Q C
I// ~ ~
_. ULA~' i,
I ~ I I I I ~ I I I I I I
0 2 4 6 8 10 12 14 16 18 20 22 24 min
FIGURE 6.1 Light hydrocarbon chromatogram of air collected above slash
burning in the tropical forests of Brazil (Greenberg et al., 1984~.
input may be ignored in the calculations of tropospheric
photochemistry.
To represent the nonmethane hydrocarbons, the committee has chosen
to distribute the emissions as 25 percent ethane, 25 percent propane,
and 50 percent ethylene, as these are the major compounds observed in
fire emissions, approximately 50 percent of which are alkenes.
Oxides of Nitrogen
Data for nitrogen oxide emissions in forest and urban fires are still
limited. The flame temperatures are generally not high enough in
isolated fires to produce nitric oxide directly from air. The fixed
nitrogen in the urban and forest fuels represents another fire-produced
source of oxides of nitrogen. For example, for ponderosa pine the
nitrogen content ranges from 0.1 percent in boles to 1 percent in
growing needles (Tangren et al., 1976~. The Environmental Protection
Agency has assigned an emission factor of 0.2 percent (as nitrogen
dioxide) based on laboratory burning of landscape refuse (McMahon,
1983~. Ward et al. (1982} recently reported a value of 0.18 percent
from burning forest materials in a field study. Another study
(DeAngelis et al., 1980) found a nitrogen dioxide emission factor of
0.19 percent for the burning of wood in fireplaces. Burning of wood,
bark, and limbs at temperatures below 1000°C gave an average emission
factor of 0.15 percent, compared to 0.75 percent for pine needles and
other forest foliage (Clements and McMahon, 1980~. Considering that
lumber makes up most of the urban fuels, the committee has adopted a
conservative emission factor of 0.15 percent. This results in a total
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112
a'
c
x
I
1
a) C
~Q
c I
m
a,
C
X
c
0
a)
a,
a)
~m
c
a, ~ 4 -
0 ~
x
G)
~a)
X N
__, ' 1 1 1 1 1 l I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ~ I ' I
~ _
0 4 8 12
-
c
~ O
x -.
-
16 20 24 28 32 36 40 min
FIGURE 6.2 Heavy hydrocarbon chromatogram of air collected above slash
burning in the tropical forests of Brazil (Greenberg et al., 1984~.
emission to the atmosphere of 6.8 Tg of nitrogen dioxide (equivalent to
4.4 Tg of nitric oxide) from fires for the baseline scenario.
EFFECTS OF EMISSIONS
Ozone Shield Reduction
The first perceived threat of stratospheric ozone by pollutants
implicated the oxides of nitrogen (NO and NO2, known collectively as
NOX). At that time, the early 1970s, it was the prospect of
supersonic flight that caused concern (see, e.g., NRC, 19731. Threats
to the ozone layer from emissions of chlorofluorocarbons and from
increases in nitrous oxide (N2O) concentrations (caused by the
increased application of nitrogen fertilizers) have been recognized and
assessed (see, e.g., NRC, 1982~. The problem of ozone reduction by
N2O increases is in essence the same as that of reduction by adding
NOX, since N2O is converted to NO in the stratosphere. In 1975 the
NRC conducted a workshop for the purpose of studying effects of
large-scale nuclear detonations. Of all of the aspects addressed, that
concerning the effects of NOX injection received the most detailed
treatment because of the recent awareness brought about by the SST
studies (Crutzen, 1971; Johnston, 1971) and the work of Foley and
Ruderman (1973), who pointed out that the NOX produced in the
fireballs of nuclear weapons should lead to ozone reduction (see also
Johnston et al., 1973~. Recently, estimates have been made of ozone
reductions from NOX injections for various nuclear war
(Chang and Wuebbles, 1982; Crutzen and Birks, 1982~.
all of these studies and their respective scenarios
Table 6.1.
scenarios
The results of
are summarized in
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113
The list of chemical reactions thought to describe the behavior of
ozone in the stratosphere is long and imposing. The interactions of
the various atoms and molecules among themselves and with sunlight and
their further dependency upon atmospheric transport make up a very
complicated system. Though much is known about this system and the
ability to model it has increased considerably in the last decade, much
uncertainty still remains attendant to the application of the models to
such drastic perturbations as those in the baseline scenario. However,
there is now a large body of evidence that concentrations of ozone in
the present stratosphere are principally controlled by NOX from
natural sources. For this reason alone, it is expected that a large
perturbation in the stratospheric burden of NOX, particularly in the
upper regions of the stratosphere, would result in a large decrease in
the ozone column.
The committee attempts here to give only a brief explanation of the
manner in which NOX causes ozone reduction in the baseline and
excursion cases. (For a thorough review of the chemistry of
stratospheric ozone, the reader is referred to Logan et al. (1978~. An
update is available in the appendixes by Wofay and Logan and by
Anderson in NRC (1982~.) Figure 6.3 shows a "normal" ozone
concentration vertical profile and the altitude ranges into which the
NOX would be deposited in the 6500-Mt baseline scenario and the
8500-Mt excursion scenario. The ozone concentrations are controlled by
balances of production and loss reactions and transport. There are
several sets of photochemical reactions, some of which form cycles that
can explain much of the observed behavior of ozone. These cycles
include catalytic destruction of odd oxygen (O3 and O atoms) by the
oxides of nitrogen, the odd hydrogen radicals tHO and HO2), and the
chlorine radicals (C1 and C1O). The pertinent cycle for ozone
destruction by NOX is the set of reactions:
NO + O3 ~ NO2 + O2
NO2 + O ~ NO + O2
O3 + O ~ 2O2
At mid-latitudes in the normal atmosphere, this reaction cycle
provides the principal means of odd oxygen destruction above about 23
km. Although the cycle also provides most of the chemical loss of odd
oxygen at lower altitudes, the rate of the NO2 + O reaction (which
limits the rate of the cycle) slows in relation to the rate of
transport as the altitude decreases below about 23 km. The amount of
ozone reduction caused by injection of NO into the stratosphere depends
on the amounts of NO and their distribution with altitude, which in the
case of a nuclear bomb depend upon the yield and height of burst.
Figure 4.3 shows the distribution of nuclear cloud tops and bottoms
used to calculate the distributions of injected NO in model
calculations of ozone reduction. Thus the estimate of the ozone
reduction that would result from a nuclear war depends on the yield,
type of burst, and latitude, for each weapon of the scenario used. For
the baseline scenario, concentrations of NOX would be greatly
enhanced in the lower stratosphere up to about 19 km.
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114
nor
50
40
L1J
~ 30
-
20
10
o
-
-
Regions of NOx I ejection
Baseline l o O O ° 0 o
Excursion ~ +
1 o 1 1
1 \ 1 1
1 o1 2
O3 DENSITY (cm~3)
1013
FIGURE 6.3 Concentration (solid line) of ozone in the unperturbed
atmosphere at regions of NOX injection.
The ozone reduction at these elevations would occur because the
rate of the catalytic cycle shown above would be enhanced relative to
removal by transport. The ozone destruction rate would also increase
as the oxides of nitrogen mixed upward, where the ozone concentrations
are higher and the photochemical reaction rates are faster. It is a
technically noteworthy point that for this massive low-level injection
of NOX, below the level of the ozone maximum at 25 km, the overlying
ozone would prevent compensating odd oxygen production resulting from
photolysis of O2 at the lower elevations.
The Lawrence Livermore National Laboratory (LLNL) one-dimensional
eddy diffusion and chemical reaction model was used to estimate the
amounts of ozone reduction with time corresponding to the present two
scenarios (J.E. Penner and P.S. Connell, Lawrence Livermore National
Laboratory, private communication, 1984~. The results are shown in
Figure 6.4, which illustrates that for the baseline case the maximum
ozone reduction of 17 percent (average over the northern hemisphere)
would be reached 1 year after the war and recovery to one-half of the
peak reduction would require an additional 2 years. The relatively
slow development of the ozone minimum reflects primarily the slow
upward transport of NOX to regions where the odd oxygen destruction
rates are greater.
The 8500-Mt excursion scenario would place additional large amounts
of NOX at elevations up to about 37 km due to the use of much larger
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115
~ -10 ~
LL \ /
I -20 ~
~ 1 ~
o -30 ~ /
~ /
Z \ /
O E /
O ~40 \ /
, _ ~
0 2
-20
-40
-50
Computed Hemispheric Average Ozone
Column Change as a Function of Time
B = Baseline Scenario (6500 Mt.)
E = Excursion Scenario
(Baseline plus 100 x 20 Mt)
1 1 1 1 1 1 1 1
4 6 8 10
TIM E (yr)
FIGURE 6.4 Hemispherically averaged percent ozone depletion estimated
in a one-dimensional eddy diffusion and chemical reaction model (J.E.
Penner and P.S. Connell, LLNL, private communication, 1984~.
weapons. This would cause very rapid reduction of ozone in the region
where its concentrations are the highest. This is reflected in the
shorter time to achieve maximum reduction, namely about 8 months to
reach 43 percent reduction. Recovery to one-half that value would
occur after 4 years.
The complex set of chemical reactions that control stratospheric
ozone concentrations constitutes a system in which the dependency of
ozone reductions amounts on NO injection amounts is somewhat
nonlinear. These effects were discussed in the NRC (1975) report (see
particularly Figure 1.9~. Though there are differences in details
between the model used then and the present model, the plot is still
approximately applicable to the present model for the purpose of rough
guidance.
The results of this study are consistent with those of other
studies using the LLNL model. Comparison of these results with those
reported by Chang and Wuebbles (1982) shows the same shapes for the
ozone versus time curves. Table 6.1 presents a comparison of the
present scenarios, NOx injections, and maximum ozone (03)
reductions with those of Chang and Wuebbles and of the Ambio scenario.
The details of time to maximum reduction, the value of maximum ozone
reduction, and time to recover to one-half the maximum depletion are
scenario-dependent. That is, the amount and distribution of NOx and
thus the model-derived details depend on the numbers and yields of
individual weapons. (The results for the excursion scenario are
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116
somewhat similar to those reported in NRC (1975~. However, there are
substantial differences in several of the reaction rate parameters as
well as yields and numbers of weapons.)
The Ambio scenario gave no ozone reduction and is consistent with
the results of the present study for the reason that the preponderance
of the NO was injected below 12-km altitude. There the ozone
destruction by the NOx cycle is offset by the creation of ozone by
the smog cycle (see next section).
The calculated ozone changes discussed above were obtained with the
use of models in which the stratospheric transport properties are those
that represent the present (unperturbed) atmosphere. The perturbation
of the atmosphere by smoke and dust could affect the circulation of the
stratosphere and thus provide a circumstance different from that upon
which the present ozone calculations were based. As discussed in
Chapter 7, the committee has no sound qualitative notions as to how
stratospheric circulation would be altered. In the absence of further
information, the committee believes that the use of the unperturbed
atmospheric transport characteristics provides the best basis for
assessing the ozone reductions caused by NO injections from nuclear
bursts.
Since the model used in this study considers transport only in the
vertical dimension, it cannot provide an estimate of the amounts of
NOx transported into the southern hemisphere. The ability of the
atmosphere to transport trace substances across the equator in the
stratosphere was demonstrated by many observations of radioactive
debris from nuclear weapons testing in the atmosphere. The nature of
this phenomenon was delineated by Mahlman and Moxim (1978) using a
general circulation model. Their study, using a single mid-latitude
tracer injection, showed that the maximum burden in the southern
hemisphere occurred about 9 months after the injection and was less
than 10 percent of the initial amount injected. Crutzen and Birks
(1982) calculated southern hemisphere ozone reduction to be of the
order Of 15 percent occurring after the injection of somewhat higher
amounts of NOx than in the excursion case.
Ozone Holes and Effects of NO2 Radiation Absorption
Luther (1983) has studied short-term chemical and radiative effects of
injections of NO into the stratosphere by nuclear weapons. The
particular problem he addresses is the Ozone hole." Rapid heating of
portions of the stratosphere containing high concentrations of NO2,
with subsequent mixing throughout the heated and destabilized volume,
causes the ozone hole, which is a large reduction in the ozone column
abundance distributed over most of the vertical extent of the
stratosphere, but confined laterally. Ozone holes would permit a very
large increase in irradiance of ultraviolet light at the top of the
troposphere, which, in the absence of smoke or clouds, would result in
life-damaging effects at the surface. Luther's study assumed that the
cloud remained cylindrical throughout the depths of the stratosphere
and that horizontal mixing could be represented by eddy diffusion.
OCR for page 117
117
These assumptions are probably not realistic, since the "fillings of
the holes by shear in the vertical is likely to be rapid and
effective. Thus, it is considered that the ozone holes would exist for
no more than a few hours and their effects would be less severe than
those from global-scale reduction.
Effects on Ozone calf Past Nuclear Weapons Tests
In accordance with the committee's estimates, the approximately 300 Mt
of total bomb yield in multimegaton atmospheric bursts by the United
States and USSR in 1961 and 1962 introduced about 3 x 1034 additional
molecules of nitric oxide into the stratosphere. Thus one might ask
whether these tests resulted in a depletion of the ozone layer. Using
a one-dimensional model, Chang et al. (1979) estimated that these
nuclear weapons tests should have resulted in a maximum ozone column
depletion in the northern hemisphere of about 4 percent in 1963.
Analysis of the ground ozone observational data by Johnston et al.
(1973) showed a decrease of 2.2 percent for the period 1960-1962
followed by an increase of 4.4 percent in 1963-1970. Although these
data are consistent with the magnitude of the ozone depletion expected,
by no means is a cause and effect relationship established. Angell and
Kor shover (1973) attribute these observed ozone column changes to
meteorological factors. The ozone decrease began before most of the
large weapons had been detonated and persisted for too long a period to
be totally attributed to recovery from bomb-induced ozone depletion.
Unfortunately, because of the large scatter in the ground-based ozone
observational data and our lack of understanding of all of the natural
causes of ozone fluctuations, one cannot draw definite conclusions
about the effects of nuclear explosions on stratospheric ozone on the
basis of previous tests of nuclear weapons in the atmosphere.
Uncertainty in Model Results
Normally, a scientific study using a model to "predict" a result should
be accompanied by an analysis of uncertainties ending with a set of
error limits on that result. The assessment of global effects of
perturbing trace substances on stratospheric ozone has caused much
effort to be expended in attempting to estimate error limits on
calculated ozone reductions. Yet after more that a decade of
experience in this exercise the most recent assessment by the NRC
(1984a) states, The detailed treatments often leave the wrong
impression that the actual sources of uncertainty are well defined.
. . . [O] nly a qualitative statement of uncertainty is made here. n The
perturbation of the stratosphere by NO and smoke emissions from a
large-scale nuclear war is likely to be so large that effects not
considered could well play an important role. Certainly, the models
used in the present assessment were not constructed to handle such
perturbations. Further, the present problem is complicated by the many
injections of NO in the vicinity of the tropopause by low-yield
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118
weapons. The estimated ozone depletion for the baseline case is quite
sensitive to the height of the tropopause relevant to the particular
bursts. As discussed in Chapter 4, there is uncertainty associated
with the estimates of cloud tops and bottoms. All of these factors
combine to make of any rational estimate of error limits in ozone
reduction a virtual impossibility. The numbers calculated here, though
given to two figures, should be viewed as plausible values that are
based upon the best methods available to the committee.
Tropospheric Composition Changes
Because the troposphere is in direct contact with the biosphere, it is
especially important to understand the chemical changes that would take
place in this region of the atmosphere following a nuclear war. The
many fires ignited by the nuclear explosions would inject large
quantities of carbon monoxide, hydrocarbons, and many other organic
compounds into the atmosphere. Both fires and the nuclear explosions
themselves would produce large quantities of oxides of nitrogen. In
the presence of sunlight, these compounds react to form strong
oxidants, particularly ozone and organic peroxides such as peroxyacetyl
nitrate (PAN). PAN and related compounds have strong phytotoxic
effects. Ozone, while being necessary in the stratosphere to serve as
a shield against solar ultraviolet radiation, is considered undesirable
at ground level because of its toxic effects on both plants and animals.
Whether or not a dense photochemical smog with high oxidant
concentrations would form in the wake of a nuclear war is difficult to
evaluate for several reasons. Perhaps the largest uncertainties are
associated with (1) the extent and duration of the darkening caused by
the smoke and dust, and (2) changes in tropospheric dynamics and
precipitation rates, which in turn affect the lifetimes of the relevant
chemical species. The generalized mechanism of photochemical smog
formation includes the critical reaction sequence
ROO + NO ~ NO2 + RO
NO2 + he ~ NO + O
O + O2 + M ~ O3 + M
where R can be a hydrogen atom or any organic radical and M is any
molecule. This sequence of reactions requires sunlight (photon, ho)
and oxides of nitrogen (NO and NO27. Sunlight is also necessary to
the formation of the hydroxyl radical, OH, as follows,
O3 + he ~ O2 + O(1D2)
O(1D2) + H2O ~ 2 OH
where 0~1D2) is an electronically excited oxygen atom. The OH
radical is an important initiator of chain reactions in the atmosphere
via reactions such as
CO ~ OH ~ CO2 + H
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119
followed by
H + O2 + M ~ HOO + M
and
RH + OH ~ H2O + R
which is followed by
R + O2 + M ~ ROO + M
J.E. Penner and P.S. Connell (Lawrence Livermore National
Laboratory, private communication, 1983) have investigated the
tropospheric composition changes associated with the baseline scenario
using a one-dimensional model of tropospheric photochemistry. Because
most of the oxides of nitrogen in the troposphere are removed in this
model by natural processes of dry deposition and rainout during the
first few weeks, while the sunlight is greatly attenuated by suspended
smoke and dust, the average concentration of ozone in the troposphere
increases by less than a factor of 2. After several more weeks, the
ozone concentration is expected to have decreased to near-ambient
levels as the many chemical pollutants are removed from the atmosphere.
The high loading of particulate matter in the troposphere may be
significant not only in blocking sunlight, but also in promoting
heterogeneous reactions. Assuming all smoke particles are perfect
spheres of radius 0.05 um with a density of 1 g/cm3, the specific
surface area is 60 m2/g. If the 200 Tg of smoke aerosol of the
baseline case is uniformly distributed with a constant mixing ratio
(aerosol particles/molecules of air), then every atmospheric molecule
collides with a particle on the average about 4 times every second.
This collision lifetime is shorter that the lifetime of many highly
reactive atmospheric species. Birks and Staehelin (1984) have
investigated the possible role of reactions on particulate surfaces in
further reducing tropospheric oxidant concentrations. They found for
the baseline case that oxidant formation in the troposphere is
significantly inhibited when the efficiencies (y) of reaction upon
collision with aerosol surfaces exceed 10-6 for O3, 10~1 for OH
and/or 10-2 for HO2. The variations in values of y that result
in significant reduction in oxidant formation simply reflect the
relative lifetimes of oxidant species in the atmosphere. Whereas a
small value of To is required for ozone, a relatively long-lived
species, a value Of YoH > 10-1 is required for hydroxyl
radicals, which have a very short atmospheric lifetime.
The reaction efficiencies for atmospheric species with smoke
aerosol have not been measured. However, the reaction of OH, one of
the most important oxidants in the atmosphere, with a graphite surface
has been studied (Mulcahy and Young, 19751. Because the rate of the
reaction was sufficiently fast to be diffusion limited in the
experimental apparatus, only a lower limit for YOH of 5 x 10-2
was obtained. Although Mall effects" for other labile atmospheric
species such as O3, O. and HO2 are well known because of the
OCR for page 120
120
difficulties they pose in measurements of their homogeneous reaction
rates, no ~ values for reactions with atmospheric aerosol have been
obtained.
It is not possible to make quantitative predictions of all the
chemical composition changes of the troposphere following a nuclear
war. However, it seems likely that the rate of oxidation of
tropospheric species would be greatly decreased, particularly near the
surface of the earth, for the period of time that the particulate
matter resides in the atmosphere. Although oxidants in the atmosphere
are usually looked upon as undesirable because of the damage they cause
to plants and animals, oxidants serve an important function in
cleansing the atmosphere of many anthropogenic and biogenic
In fact, the lifetimes of nearly all compounds released to the
atmosphere are determined by the rates of reaction with the hydroxyl
.
-".
· ~
emlss cons .
radical. The source of OH radicals in the troposphere is photolysis of
ozone, as discussed above. In addition to the reduced sunlight and
loss of OH on particulate surfaces, OH concentrations would be reduced
by combination with NO2 to form nitric acid:
OH + NO2 ~ M ~ HNO3 + M
In addition to the increased burden of toxic chemicals as the
result nuclear war fires, one would expect large increases in the
concentrations of many reduced compounds for two reasons: (1) the
lifetimes of many compounds would be increased by large factors due to
reduced concentrations of OH and other oxidants, and (2) biogenic
emissions of some compounds might increase by large factors following a
nuclear war. For example, compounds such as hydrogen sulfide and
dimethyl sulfide are thought to have large biogenic emissions estimated
at about 50 To S of each Her vear (Adams et al. 1981: Andreas and
Raemdonck, 19831. However, their atmospheric concentrations are
limited by short lifetimes of one or two days owing to reactions with
the OH radical (Hatakeyama and Akimoto, 1983~. It is difficult to
predict the changes in biogenic emission rates that would follow a
r.~1 ~' ~' rnh~ e - ~^C!C,~= ~ ~ he '~' arm ~ he lo; ~=r~h^~^ ; ^~1 ''A; r`^ a
1lU~ ~ ="L W"L · -l-1l" O~L=~=e. V, bile W"L ~1 ~= ~V=~ICL=' .11~ ~ U" Ally
long period of darkness and freezing temperatures, would be expected to
result in the death of many plants and animals, which in turn might
lead to an increase in the rate of release of many reduced compounds.
On the other hand, the low temperatures over land surfaces could
decrease the rate of bacterial degradation of organic matter, and
frozen freshwater systems could delay the escape of gaseous compounds
to the atmosphere.
Because of the large heat capacity of the mixed layer of the ocean,
the temperature of the ocean would be little changed. The principal
effect of a nuclear war on biogenic emissions from the ocean would
probably result from periods of low light intensity. Photosynthesis in
the ocean takes place to a critical depth where the sunlight is
attenuated to about 1 percent of its normal incident light flux. The
darkness following a nuclear war would shift this critical depth much
closer to the surface. As a result, one might expect the death of a
OCR for page 121
121
.
significant fraction of the phytoplankton and zooplankton of the
northern hemisphere ocean following a nuclear war (Milne and McKay,
1982).
Despite the large uncertainties, it is possible to place reasonable
bounds on the concentrations of reduced sulfur compounds that would
accumulate in the atmosphere. As a result of the rapid oxidation rate
of dimethyl sulfide (DMS) in the normal atmosphere, the concentration
of DMS in marine air is at least 2 orders of magnitude below the
concentration that would be in equilibrium with seawater (Andreas and
Raemdonck, 1983~. As an upper bound, we may assume that the
atmospheric concentration of DMS comes into equilibrium with surface
water, resulting in an atmospheric mixing ratio of 21 ppbv. As a lower
bound, we assume release of DMS at the present average sea-to-air flux
(290 ug S/m2 per day) for a period of 1 month ~
mixing to an altitude of 10 km. This results in a mixing ratio of 0.8
ppbv. Considering that biogenic emissions of hydrogen sulfide are
comparable in magnitude to DMS and that there would also be emissions
from dimethyl disulfide and methyl mercaptan, for which emission
factors are not well known, it appears likely that following nuclear
war, the total concentration of reduced sulfur compounds in the
troposphere would accumulate to a few parts-per-billion by volume.
Although these are not toxic levels, at least for short-term exposure
to humans, it is noteworthy that the threshold for smell in humans has
been found to be in the ranges 0.9 to 8.5 ppbv for H2S and 0.1 to 3.6
ppbv for (CHRIS.
_ _ ~ _ ~
and allow uniform
Toxic Chemical Releases
In addition to the emissions of carbon monoxide, nitrogen oxides, and
organic compounds produced by the pyrolysis and partial combustion of
wood, several million tons of noxious chemicals would be released to
the atmosphere as a result of the pyrolysis and partial combustion of
synthetic polymers such as rubber, plastics, and synthetic fibers
located in urban areas, and chemicals in industrial storage. These
chemical releases could have severe local consequences in and near the
heavily populated urban areas. Occasional accidental releases of
noxious chemicals have resulted in temporary evacuations of large
areas. Contamination of the ground at very low levels (one part per
million and below) by some particularly toxic chemicals has caused the
permanent evacuation of some areas (e.g., Love Canal, New York, and
Times Beach, Missouri). Recent attention has been drawn particularly
to the polychlorinated biphenyls (PCBs), dioxins, and chlorine-
substituted dibenzofurans. In the United States alone, more than
300,000 tons of PCBs are in use in electrical equipment and
approximately 10,000 tons in storage (S. Miller, 1983~. A large
fraction of this toxic chemical could be released to the environment in
a nuclear war. Apparently, dioxins and dibenzofurans may be produced
in large quantities in the combustion of fuels containing chlorine,
although this is currently a matter of considerable controversy (J.A.
Miller, 1979; Bumb et al., 1980; Chemical and Engineering News, 1983~.
OCR for page 122
122
Annual production in
chemicals is provided in
percent of these amounts
the United States of some important industrial
Table 6.2. On the average, perhaps 5 to 10
are in storage at any particular time.
Pyrolysis and partial combustion of these and less abundant chemicals
would result in the deposition of thousands of chemical species in the
atmosphere and ultimately in the soil and water. The chlorine
compounds would be expected to account for a large fraction of the more
toxic, mutagenic, teratogenic, and carcinogenic compounds.
The problem of toxic chemicals released in a nuclear war is highly
specific to locality and does not lend itself readily to general
analysis. It seems likely, however, that portions of most of the urban
areas affected would be seriously contaminated, at least in the smoky
air during and immediately following burning. The possibility of
serious local contamination of the ground and water for long times
after the war cannot be ruled out.
Among the toxic materials released to the environment would be
asbestos. The current world production of asbestos fibers amounts to
about 4 million metric tons per year. More than 30 million tons (30
Tg) of asbestos has been accumulated in the United States alone.
Accumulation by industrialized nations is in excess of 100 Tg. These
fibers are bound in a wide variety of construction materials and other
products. Much asbestos contained in the nonflammable materials would
be released as the result of pulverization by the nuclear blast. Since
asbestos fibers are nonflammable, they would also be released to the
atmosphere upon combustion of materials such as floor tile and asphalt
shingles.
It is difficult to estimate how much asbestos would be released to
the atmosphere as the result of a nuclear war. However, when mixed
uniformly throughout the lower 9 km of the atmosphere and over half of
the northern hemisphere, the atmospheric concentration of asbestos is
calculated to be about 0.3 fibers per cubic centimeter for each
teragram of asbestos released. This calculation uses the conversion
factor used in epidemiological studies in which it is assumed that 1
fiber would be detected by phase contrast light microscopy for every 30
x 10-12 g of suspended asbestos. An optical fiber is defined as any
particle longer than 5 Em, having a length-to-diameter ratio of at
least 3-to-1 and a maximum diameter of 5 um. Of course, the actual
number of fibers is much larger, owing to the preponderance of smaller
fibers not counted. The present Occupational Safety and Health
Administration (OSHA) standard for exposure to asbestos is a
time-weighted average of 2.0 fibers per cubic centimeter over an 8-h
period, and OSHA announced a decision to lower it to 0.5 fiber per
cubic centimeter in November 1983. A recent NRC study (NRC, 1984b)
estimated the average nonoccupational exposure in the United States to
asbestos to be 0.0004 fibers per cubic centimeter. Five teragrams
(less than 5 percent of the world accumulation) of asbestos released to
the atmosphere would increase the general population exposure to
asbestos by a factor of about 4000 for the period of time that the
particles are suspended and uniformly distributed. Of course, the
fibers would be subject to resuspension and would be concentrated in
the boundary layer of the atmosphere.
OCR for page 123
123
TABLE 6.2 U.S. Production of Some Major Chemicals in 1982
Millions of Tons
Sulfuric acid
Ammonia
Ethylene
Chlorine
Phosphoric acid
Toluene
Nitric acid
Propylene
Ethylene dichloride
Xylenes
Benzene
Methanol
Ethylbenzene
Vinyl chloride
Styrene
Hydrochloric acid
Terephthalic acid
Ethylene oxide
Ethylene glycol
Acetic acid
Cumene
Phenol
Acrylonitrile
Vinyl acetate
Butadiene
Acetone
Formaldehyde
Propylene oxide
Isopropanol
Cyclohexane
Adipic acid
Acetic anhydride
Ethanol
29~4
14~1
11~2
8~3
7~8
6~9
6~9
5~6
4e5
3~8
3~6
3~3
3~0
3~0
2~7
2~4
2~3
2~2
1~8
1~2
1~2
0~96
0~92
0~85
0~83
0~80
0~76
0~67
0~59
0~58
0~54
0~48
0~46
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Representative terms from entire chapter:
ozone reduction
