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Symposium
rom the chemistry
of
responsibile environmentalism
to environmentally responsible chemistry
The Environmental Chemistry of Trace tmospheric Gases
William C. Trogler
University of California, San Diego, La Jolla, CA 92093-0358
Concentrat ions and Lifetimes
of Trace Atmospheric Ga se s
Excluding water vapor, about 99.93 of the Earth's atmos-
phere consists of nitrogen, oxygen, and argon. Human activi-
ties, called anthropogenic, do not s d l c a n t l y perturb the con-
centrations of these species. The situation is different for trace
components ofthe atmosphere. Anthropogenic emissions have
noticeably increased theconcentration of several trace gases,
which were set at a steadv state bv natural for mation-d est~c-
tion cycles.
Air
samples ;an
be
obtained from bubbles trapped
in glacial ice from the Antarctica or Greenland. Yearly bands
are visible in cores drilled out of the ice, because of the annual
surface thawing and refreezing. This provides an atmospheric
time record tha t spans over 150,000years. Within the past 100
years, the average concentration of methane in the atmos-
phere has nearly doubled, and tha t of carbon dioxide has in-
creased by about 20 1).New compounds, such as chlo-
rofluorocarbons (CFCs), did not exist 100 years ago but now
make up about ppbv (part per billion by volume) of the at-
mosphere. For many trace gases, concentrations have in-
creased a t unprecedented rates. The effect of such increases on
the Earth's climate and ecosystemisan area of active scientific
study and political concern.
In studying issues related to th e environment, i t is im-
portant to separate scientific fact from hypothesis. The
chemical composition and changes in t he concentrations of
atmospheric gases can be measured with high precision. I t
is difficult, however, to predict quantitatively how these
changes will al ter t he climate and ecosystem. In t he popn-
la r press, environmental debates often intermingle scien-
tific fact s wit h hypotheses. This clouds objectivity Fac ts
can also be presented in different ways. For example, one
person might arg ue tha t a species present a t only
1
ppbv is
of little significance. On the other hand , another individua l
might emphasize that a liter of air you breathe a t sea level
contains 27 trillion molecules of a
1
pbv component. Some
key facts about important t race atmospheric constituents
appear i n Table 1.
Concentrations for atmospheric species ar e tabulated on
a per volume basis in a dimensionless unit, which is called
th e mixing ratio. For long-lived atmospheric components,
the mixine ratio remains constant. even thoueh t he ores-
sure decreases with increasing altitud e in t he troposphere.
The troposphere is the region 0-15 km above th e Earth's
surface; it contains about 85 of th e atmosphere's mass.
Above th e troposphere t he mixing ratios of gases change,
depending on photo-decomposition eff~ciencies, s th e flux
of high energy ultraviolet radiation increases. The average
atmospheric lifetime (or residence time) given in Table
1
reflects the average time spent in the atmosphere by an
individual molecule. I t can be calculated from th e atmos-
pheric concentration of a compound divided by its esti-
mated rat e of supply Atmospheric lifetimes are extremely
important because they represent how long it take s to re-
store a perturbed atmospheric concentration.
Volume 72 Number 11 November 1995
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Table 1. Trace Atmospheric Species of Environmental Concern 1 4 ,
13
Constituent Avg. Concentration Avg. Atmospheric Rate of increase
( P P ~ V ) Residence Time per Year
COz (carbon dioxide) 355.000 5&2W yr
2600 ppbv/yr
CH4 (methane) 1.700 10 yr 1&13 ppbvlyr
N 2 0
(nitrous oxide) 310 150yr
0.6-0.9 ppbvlyr
CFCs (avg.)
3 (per Ci atom) 6C-150
y r
-0.05
CCi2Fz (CFC-12)
0.484 130
y r
-0.017
CC13F (CFC-11)
0.28 65 y r -0.01
NO,(nitrogen oxides)
0.001-50 1-3 days
SO2
sulfur
dioxide)
0.01-50 14 days
Developed countries have agreed to phase out CFCs by
1996, because CFCs deplete s tratospheric ozone. However,
it will be decades before CFC concentrations in t he strato-
sphere will decrease significantly, The reductions in emis-
sions necessary to stabilize atmospheric concentrations a t
thei r present values ar e >60 for COz, 15-20 for meth-
ane, 1 5 8 5 for CFC-12, and 70-80 for NzO (2, 3). In
contrast , short-lived species, such as NO, (refers to reac-
tive oxides of nitrogen, suc h as NO, NOz, NOa, an d NzOd
and SOz, are unlikely to become unbalanced on a global
scale because they are removed rapidly from the atmos-
phere by oxidation a nd r ain out. I n thi s case the deposited
secondary products, nitric acid and sulfuric acid, may
cause regional imbalances in sensitive ecosystems. So, al-
though anthropogenic emissions of NO, an d SOz ar e large,
elevated atmospheric concentrations only occur locally
nea r th eir sources (e.g., cities).
The Greenhouse Effect
A
p1ant.t'~atmosphere plays an important role in deter-
minine its su tia re temperature. Scatte rmg of solar mdia-
tiou by a tmospher ic gaHes a nd clouds decreases the energy
flux to a planet's surface. The fraction of solar radiation
scattered , called the albedo, is 0.29 for Eart h. Mars, whose
surface atmospheric pressure i s only 5 torr, has an albedo
of 0.15 and Venus (surface pressure of 92 at m) has an al-
bedo of 0.77 (4). If a planet behaves a s blackbody, th is re-
quires thar the emitted ritdiatlon equals the absorbed solar
r:tdia~ion t it s equilihr~um urface temperatu n,
rT,].
Thls
obeys th e simple relation:
black-body radiative
fraction
o
radiation not
energy emitted per
scattered by the atmosphere
area
times the solar radiation flux
4rr Rp2 s Te4
=
aRP2
1
-Ap)Fs
surface area
\disk area of Earth
of Earth
exposed to the Sun
The albedo Ap),Stephen-Boltzmann constant
s),
solar
lux Fa ) t the edge of the
la net s
atmosphere, an d the
planet's radius R ~ )an be used to solve for
T
his equa-
tion underestimates the surface t em ~e ra tu re f Mars by
onlv 6 K, the Earth's tempe rature by 32 K, and t hat of vL
nu s by 5'00 K.
The enhanced surface warmina th at correlates with the
pressure of a planet's atmosphere is called radiative forc-
ing. I t is also referred to a s thegreenhouse effect. 4 ) .The
effect results from the atmosphere's ability to absorb the
blackbody radiation emit ted from a planet's surface. The
peak in the Earth's blackbody emission occurs at 16 Fm
wavelength (or 625 em-'), which lies in th e
infrared spectral region. Greenhouse warm-
ing occurs because th e atmosphere transm its
visible and ultraviolet light tha t th e surface
absorbs, but t he greenhouse gas es tra p much
of the infrared radiation em anated from the
surface. Without t he greenhouse effect, t he
Earth would likely be below 0
OC,
and most
wate r would ex ist a s ice.
The main constituents of the dr y atmos-
phere (Nz, Oz, and Ar) ar e either infrared
inactive (th e N-N an d
0 0
stretching vi-
brations cannot absorb infrared radiation
by a n electric dipole allowed process) or do
not Dossess a vibration Ar).Water vapor is
the hominant species th at absorbs
(-8b
of
thegreenhouseeffect)infraredadiation from the Eart h's
surface. Th at is why night-time cooling occurs rapidly in
dry climates (deser ts and Antarctica). Besides water, th e
infrared absorbing trace gases (COz, CH4, CFCs, and
NzO) cont ribu te significantly to radia tiv e forcing.
Carbon dioxide is the sewnd most important greenhouse
gas. Numerical studies suggest th at i t accounts for about 55
of the increased radiative forcing by anthropogenic emissions
to th e atmosphere (2,3). It ha s two vibrations nu OCO bend-
ing) and
+
OCO asymmetric stretch) capable of directly
absorbing infrared radiatiou, whereas
Zg
(OCO symmetric
stretch) is inactive. Molecules of higher complexity and lower
symmetry possess more vibrations that are capable of ab-
sorbing infrared radiation. The relative absorbing strength
averaged over a given atmospheric lifetime is given by the
global warming parameter (GWP). These parameters are
normalized to COzwith
a
GWP of 1.Values for selected trace
gases are given in Table 2. The net wntribution
to
global
warming depends on the product of the GWP and the amount
of anthropogenic emissions for a specific gas. These values
appear in t he last column of Table 2.
The high global warming potential of the CFCs reflects
their complex molecular structure. They contain low fre-
quency vibrational modes th at absorb infrared radiation i n
a region where water and COz do not absorb. Nitrous oxide
lacks a center of symmetry. Unlike COz, all three of it s fun-
damental vibrations (n,
+,
nd Xgt) ar e infrared absorb-
ing. This and the greater intrinsic dipole moment change
for N20 vibrations result in a.GWP 270 times t ha t of COz.
Th e GWP values have impor tant implications for pro-
grams directed toward the reduction of greenhouse gas
emissions. Fo r example, t he removal of one mole of CFC-11
emissions is as effective a s the removal of 3400 moles of
carbon dioxide.
Table 2. Global Warming Potentials GWP) Integrated
Over a 100-year Period for Key Trace Gases and the
Relative Anthropogenic Contributions to the Increased
Greenhouse Warming
1-3).
Gas Major Human Sources
G WP
Est.
Contribution to
Increase
Con combustion, deforestation
1
55
CH, rice fields, cattle, landfills,oil 11 15
production
N 2 0 fertilizers, deforestation,nitric 270 6
acid and adipic acid synthesis
CFC-11 plastic foam blowing solvent, 3400 7
electronic circuit board
cleaning solvent
CFC-12 refrigeration compressor fluid 7100 10
974
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Prospects or the Future
There is little douht th at th e concentra-
tions of trace atmospheric gases are in-
creasing. There is little douht t ha t these
gases tra p infrared radiation and will en-
hance t he greenhouse effect of th e Earth's
atmosphere. Quantitative estimates of the
amoun t of future greenhouse warming
have sparked vigorous debate. Tempera-
ture measurements during this century
suggest th at a warming of about 0.5 OC ha s
occurred, but the scatter in th e dat a is
nearly as large as the estimated increase
(2, 3).There is also debate about how one
measures a n accurate average tempera-
ture given the Earth's different climates.
Experiments have been proposed to obta in
average global oceanic temperatures by
measuring the way sound waves propagate
across t he oceans
5-8).
Without conclusive temperature records,
computer simulations have been used ex-
tensively to estimate futu re global warm-
Table 3. Estimated S ou rc es and Sinks of Methane and Nitrous Oxide 3).
~ ~~~~
Methane
T4 CH4
per y e T ~ i t r & sxide Tq
N
per yea?
Natural Sources Natural Sources
oceans
freshwater
Hn hvdrate
.
Anthrooooen~c ources Anthrooosen~c ources
atmospheric
oxdn 450-520
stratospheric
removal 7-13
so11 emoval 1 545
.
petroleum/gas 70-120
rlce paddies 20-150
cattle flatulence 65-100
animal waste 20-30
sewa e treatment -25
landfifs 20-70
biomass burning 20-80
Net Atmospheric Increase
28-37 Net Atmospheric Increase 3-4.5
.
1 Tg
(termgram) 10 g
.
agriculture 0.03-3.0
blomass burning 0.2-1 .O
power plants 0.1-0.3
combustion engines 0.2-0.6
adipic acid prod. 0.4-0.6
nitrlc acid prod. 0.1-0.3
iug. couple d atmosphere-ocean general circulation mod-
els (GCM) predict a 1.5-4.5 OC warming if the present at -
mospheric COz level doubles during the next 100 years (2,
3).
The are many uncertainties in th e parameterized GCM
models, s uch a s th e diff~culty f modeling cloud processes,
ocean currents, polar ice sheet melting, coupling to plant
growth, an d other factors. These models as sume a n equi-
librium sta te an d th e time to reach equilibrium (some-
times thousands of years in the simulations) remains un-
certain. There a re also fundamenta l uncertainties in COz
sources and sinks, and new factors continue to he discov-
ered. Recent estimates of the effect of sulfate aerosol par-
ticles (acid rai n particles from SOz pollution) suggest that
they may increase t he local albedo over heavily populated
areas and lead to cooling th at will partially counteract the
increased greenhouse effect (9, 10). In discussing futu re
scenarios with students, i t is importan t to distinguish facts
from estimates and hypotheses.
Global Scale Budget s
The prediction of future atmospheric concentrations re-
quir es a knowledge of all sources a nd s inks of a given spe-
cies. When th e sources exceed t he sinks, then t he concen-
tration should increase. The global budget for CFCs is
defined accurately because thei r sources derive only from
industrial production, which is known well. Atmospheric
concentrations of CFCs and their trends have also been
measured accurately. This information has facilitated the
development of treaties limiting CFC emissions, where the
future outcome can be predicted with confidence.
Other trace eas es have manv natu ral and anthropogenic
sources and sinks. Predict ing [he net concrntrdtior;rl;Age
from the hudmt de~endsn calculating m a l l dtfltrencrs
between large numbers, which ma y no tb e well defined. At
present th e carbon dioxide budget is lacking enough sink s,
because i ts experimentally observed atmospheric concen-
tration is increasing slower than expected based on the
known sources and sinks. In fact, th e growth rate'of COz
ha s unexpectedly slowed from 2.5 ppmvlyr in 1988 to 0.6
ppmv a s of 1992 11). decrease in fossil fuel emissions
cannot explain th is large change. The nitrous oxide budget
is uncertain in both sources and sinks. The methane
budget is only approximately balanced. A summary of th e
budgets for NzO an d CH4 appears in Tahle 3. Notice th at
the n et atmospheric increase of these gases is less than t he
estimated ra nges for several of the component sources and
sinks. An illustra tion of the problem is provided by recent
data shwmng large decrease in methane' s atmosphsr ic
growth rate in 1992
12I.
'l'hv reason for thts obser \mio n is
&der debate (13, 14). Much remains to be done in obtain-
ing reliable values for global sources and sinks.
Nitrous Oxide
Our specific research interes t in nitrous oxide stemmed
from using i t to illustrate reaction mechanisms in a n intro-
ductory chemistry course a t UCSD. Measurements of th e
amount of NzO entrapped i n polar ice show th at t he global
concentration of nitrous oxide remained constant a t about
285 ppbv until th e mid 1700s. Then i ts concentration in-
creased to 310 pphv a t present (2, 3). The increasing con-
centration of atmospheric nitrous oxide (0.2-0.3 %/yr), its
long atmo;iphcrtc lifetime
1
150 yr,, its conrrihution ds :I
~ ~ t v m h ~ , u i eas tG117'270 ,a nd its rd a
as
a it~;itosphcric
ozone sink make i t a n mportant trace atmospheric compo-
nent. The ozone-depleting role of NzO arises from its re-
markable atmosnheric stahilitv.
After 3-5 yea& a long-liveri gas emit ted a t the Earth's
surface is tran s~o rte do the s tratos~herev diffusion and
atmospheric circulation. I t i s important to recognize t ha t
the predominance of turbulent mixing prevents apprecia-
ble mass separation of the atmosphere with altitude. Ex-
perimental proof for this has been obtained by the meas-
urement of times elapsed between the emission of
radioactive aerosols and gases a t ground level and the ir
detection later in the stratosphere (15). This debunks a
fact often cited by tal k show hosts and the popular press,
that heavy molecules (like CFCs) can't rise to the strato-
sphere. The only known sinks for NzO are stratospheric
photolysis (eq 1)and reaction with oxygen atoms (eq 2)
tha t a re produced in th e strato sphere by 0 3 and Oz pho-
tolysis. About 80% of th e stratospher ic sin k of NzO occurs
by eq
1
and t he remaining 20% by eq 2
(16,
17). The most
significant reaction p ath i s that of eq 2, which produces NO
in th e stratosphere. Astratospheric source of NO is impor-
tan t. Most of the NO, produced in the troposphere cannot
reach the stratosphere directly, because of its short life-
time (Tahle 1).
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Nitric oxide is a free radical t ha t catalyzes stratospheric
ozone destruction bv a radical chain mechanism (eqs 3-51
analogous to one of the cycles by which chlorine radicals
(produced by photolysis of CFCs in the stra tosphere) de-
plete ozone (1,2 , 4, 15, 18-20),
0 3 + N O + 0 2 + N 0 2 0 3 + C 1 +
0 2 + C 1 0 3 )
N O , + O +
N 0 + 0 2
C 1 0 + 0 + C 1 + 0 2 4)
net 0 3 + 202 O 3 + O - t 2 O 2
5 )
Natural sources of NzO primarily arise a s byproducts of
biological nitrification (oxidation of NH4') and denitrifica-
tion (reduction of N03-) (19, 21-23). Since about 1982, the
sequence of eqs 3-5 was thought to be the main si nk
(-60%) for removal of 0 8 n the lower stratosphere (4).
Recent in
s i tu
spectroscopic meas uremen ts suggest a
lesser role for NO in the lower stratosphere (-20% of the
ozone sink), and a more important role for the peroxy radi-
cal HOO (24). The reduced role of NO is due in part to the
mitigating effect tha t NO an d NOz have on the ozone de-
pleting potential of HOO and C10. Nitric oxide intercepts
the peroxy radical by eq 6.
H 0 2
NO
t OH
+ NO,
6 )
This short circuits its ozone depleting mechanism (eqs
7-91,
O , +O H + H 0 2 + 0 2
7)
H 0 2 + 0 3 + O H + 2 0 2 8)
net 20,
+
30,
9)
In the halogen loss cycles for ozone (e.g., eqs 3-51, NOz
can intercept the key intermediate C10 to form stable
ClONOz (chlorine nitrate). This removes chlorine from the
radical forms necessary to cause ozone loss. The coupling
of NO and NOz to the ozone loss mechanisms for oxygen
and chlorine radicals can even lead to a n inverse depend-
ence of ozone loss on the NOx concentration a t low concen-
trations. Such behavior contributes to formation of the
Antarctic ozone hole. Here the conversion of ClONOz and
NO2 to HN 03 is catalyzed by the surfaces of polar strato-
spheric cloud aerosol particles. This enhances t he contri-
bution of s tr atos~he ri c hlorine to ozone depletion, a s i t
htwmes conven&l into reactive forms, whilethe nitrogen
oxides are conver~ed nto Icis reactive HNO3.
Identification of an Indus trial Sourc e of Nitrous Oxide
The estimated excess NzO emissions of 1 Tg Nlyr in the
Northern Hemisphere suggested possible industrial
sources, since more industrialized cities lie in the North-
ern Hemisphere (23). This was puzzling because nitrous
oxide's uses as a dental anesthe tic (laughing gas) and as a
propellant in canned whipping cream were too small to be
significant. In 1990, we became aware that NzO is pro-
duced as a byproduct from the manufacture of adipic acid.
Large amounts of adipic acid are synthesized yearly, pri-
marily as a component of 6,6-nylon. The imbalance be-
tween the estimated atmospheric emissions of NzO in the
two hemispheres, and its role as a greenhouse gas and
ozone depletion agent, led us t o study the stoichiometry of
the overall reaction (25). The oxidation of cyclohexanoWcy-
clohexanone mixtures with 50% nitric acid charged with
VOs-, Cu2+catalysts evolved about 1mole of NzO gas for
each mole of adipic acid isolated. The gas was collected
quantitatively by Toepler pumping, analyzed by infrared
spectroscopy an d mass spectrometry, and separated from
NO and COz byproducts. The yearly production of adipic
acid requires tha t 0.4-0.6 Tg Nlyr are produced by this
process. Since nitrous oxide is a relatively nontoxic natura l
atmospheric component, there were no restr ictions on its
emissions. We estimated that most NzO produced by
manufacturing adipic acid escaped to the atmosphere.
These emissions can account for about
half
the difference
seen in NzO emission rates from the Northern and South-
ern Hemispheres. Publication of the first large indust rial
source of NzO received media coverage. Within two months
worldwide nylon producers voluntarily committed to a
five-year phase out of NzO emissions. The EPAC'GreenNy-
lon program will meet 10% of the target U S. greenhouse
gas emission reduction goals by placing controls on adipic
acid manufacturing facilities. I t is much more effective to
control NzO emissions (GWP 270) than COz (GWP 1).
Summary
The atmospheric chemistry of trace gases illustra tes fun-
damental principles in kinetics, bonding, reaction mecha-
nisms, photochemistry, spectroscopy, and geochemical cy-
cling. Selected topics are useful in introductory chemistry,
analyt ical chemistry, physical chemistry, and spectroscopy
courses. The rapidly developing aspect of the science can
be used to show how new experiments may contradict ear-
lier ones. The corresponding evolution of hypotheses pro-
vides insight into how the scientific process works. Per-
haps most important, this approach relates fundamental
chemical facts and concepts to environmental issues of
widespread interest. Students readily participate in class
debates about what should be done (if anything) about the
increasing atmospheric concentrations of trace gases.
Acknowledgment
I than k the undergraduate students i n introductory
chemistry a t UCSD, whose lack pf enthusiasm for kinetics
motivated me to make the topic more appealing with an
atmospheric approach. The National Science Foundation
is acknowledged for supporting research in my group re-
lated to inorganic atmospheric problems.
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