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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.

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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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