* v

C 0 C0 2 0 2 H20 CF2CI

^O-v *Ν ^

^

Chemistry in the , troposphere

Special Report

William L. Chameides and Douglas D. Davis, Georgia Institute of Technology The troposphere is the layer of the atmosphere extending from Earth's surface to an altitude of 10 to 16 km. It contains the air we breathe. From it falls the water we drink. And through it passes the life-producing energy of the sun.

Since the days of pre-Socratic Greek philosophers, scholars have puzzled over the mysteries of the atmo­sphere. However, the first significant scientific advances in atmospheric chemistry date back only to the 18th-century studies of Joseph Priestley, Antoine-Laurent Lavoisier, and Henry Cavendish. It was these scientists who first established that the major constituents of Earth's atmosphere are nitrogen (79% by volume) and oxygen (20%), with lesser amounts of water and carbon dioxide.

During the 19th and early 20th centuries, many prominent chemists and physicists further advanced our knowledge in this area. Sir William Ramsey documented the presence of the noble gases argon, helium, krypton, neon, and xenon in the atmosphere. Other investigators found evidence for small quantities of methane, hydro­gen, ozone, carbon monoxide, sulfur dioxide, and hy­drogen sulfide in the air and nitrate, sulfate, chloride, and ammonium ions dissolved in rainwater. Thus, by the middle of the 20th century, a good deal of qualitative information on the chemical composition of the atmo­sphere had been gathered.

Since the 1950s, rapid advancement in chemical ana­lytical techniques and high-speed computers has led to an explosion of knowledge concerning atmospheric composition. The atmosphere has been found to be a reservoir for a myriad of trace gaseous and aerosol species with concentrations below 1 ppmv (1 part per million per volume of air). And, in spite of their relatively low con­centrations, these atmospheric trace gas and aerosol species often can have a major impact on the environ­ment. Some gases, for instance, because of their toxicity, can affect plant and animal life; others can affect climate via the "atmospheric greenhouse effect." Aerosols, on the other hand, can have a significant effect upon cloud for­mation and precipitation patterns.

Recognizing the potential of atmospheric trace species for altering or influencing the global environment, it may reasonably be asked: Why are these species present in the atmosphere? Can their presence, for example, be ex­plained directly in terms of planetary thermody­namics?

In fact, simple calculations show that most trace species are present in the atmosphere at much higher levels than would be predicted solely on the basis of thermodynamic equilibrium with nitrogen, oxygen, car­bon dioxide, and water. Clearly, kinetic mechanisms in­volving a combination of chemical, physical, and bio­logical processes are controlling the trace gas composition of the atmosphere. It is the identity of these kinetic mechanisms and their relationship to natural life cycles that have been of fundamental interest to atmospheric chemists in recent decades.

We will focus here on the global troposphere, specifi­cally that part of the tropospheric system far removed from the perturbing influences of urban areas. This does not mean, however, that global tropospheric chemistry is unconcerned with air pollution; on the contrary, since pollutants can move from urban sources to remote loca­tions, tropospheric chemists must consider their effect upon the natural global system. In some instances, this can be quite significant.

Although it would be convenient if tropospheric chemistry could be studied from a purely chemical viewpoint, a multidisciplinary approach often is neces­sary. The trace gas and aerosol composition of the at­mosphere is influenced strongly by a variety of physical, biological, and geological processes, including solar ra­diation, atmospheric winds, and interactions at the Earth-atmosphere interface.

Solar radiation and winds

Chemical reactions between the major constituents of the troposphere occur at negligible rates. However, the formation of more chemically active species under the influence of light gives rise to rapid reaction chains that play a major role in controlling the trace composition of the atmosphere. Because it is driven by the absorption

Changes in temperature mark atmosphere's four layers Pressure (atm) 1 0 -13

10~8

ΙΟ"3

ΙΟ"1

1 -100 -50 0 +50 +100

Temperature (°C)

Altitude (km)

1000

500

200

100

50

20

10

5

2

1

0

Oct. 4, 1982 C&EN 39

Thermosphère

Mesopause-

Mesosphere

Stratopause —

Ozone region Stratosphere

• Tropopause — ·

Mt. Everest

Troposphere*

Cirrus cloud

Altocumulus cloud

Cumulus cloud V

Stratus cloud

Special Report

Lab kinetic studies elucidate atmospheric reactions

Neal Savage of Ford Motor's Dearborn Scientific Research Laboratory makes final adjustments on a long-path/Fourier transform Infrared kinetics system that provides direct, In-situ, muhlspecles detection under simulated atmospheric conditions and has proved especially valuable In Identifying Intermediates and final reaction products from atmospheric processes Involving polyatomic molecules

One of the most significant advances in atmospheric chemistry during the past 15 years has been an improved under­standing of gas-phase atmospheric re­action mechanisms. This has come about partly as a result of the availability of high-speed computers that can sim­ulate atmospheric transformations. Probably the most important factor, however, has been the development of a wide assortment of highly sensitive and selective gas kinetic techniques for

of photons, the chemistry of the lower atmosphere often is referred to as tropospheric photochemistry.

In one of the classic works on atmospheric chemistry, "The Photochemistry of Air Pollution" (1961), Philip A. Leighton noted the need to characterize carefully the flux of solar photons as a function of wavelength as these photons pass through the atmosphere and are absorbed and scattered. For instance, most ultraviolet photons from the sun do not reach Earth's surface. Photons with wavelengths shorter than 240 nm are absorbed by oxygen and nitrogen molecules in the thermosphère, and ozone, found primarily in the stratosphere, is a key absorber of photons in the spectral region between 240 and 300 nm. Thus, for the troposphere we need consider only those chemical reactions that are activated by photons with wavelengths of 300 nm or longer.

One of the key distinguishing features between the chemistry of the troposphere and that of the stratosphere is that ultraviolet photons sufficiently energetic to break an oxygen-oxygen bond are present in the stratosphere but not in the troposphere. As first noted by William J. Humphreys in 1910, this photodissociation of strato­spheric oxygen molecules leads to the generation of ozone

40 C&EN Oct. 4, 1982

Georgia Tech's Paul Wine prepares to Initiate a set of kinetic measurements using a laser flash-photolysis/resonance fluorescence system In which atoms or free radicals formed from the pulsed laser are followed by time-resolved fluo­rescence spectroscopy; this system Is particularly suited to very fast reactions Involving atomic species or diatomic free radicals, such as hydroxy! radicals

made in the more traditional detection techniques of ultraviolet absorption spectroscopy and mass spectrometry.

This new generation of techniques makes it possible to isolate and study individual elementary reactions under controlled laboratory conditions. As a result, gas-kinetic rate constants for more than 200 elementary atmospheric reactions now are known as a function of temperature and, where appropriate, as a function of pressure.

and, as a result, much higher concentrations of ozone are found in the stratosphere than in the troposphere.

Strictly speaking, the study of winds in the atmosphere falls within the domain of meteorology. Nevertheless, because atmospheric winds can transport chemicals from one region to another, they must be considered by at­mospheric chemists. A major feature of winds is their highly variable and turbulent nature. Much as rapidly shaking a bottle of oil and vinegar causes the two liquids to mix, the net long-term effect of the winds' variability is to mix atmospheric constituents.

The characteristic mixing time for the winds to mix a species uniformly throughout a given region is a measure of the intensity of this atmospheric mixing process. From a variety of meteorological tracer experiments, it has been estimated that one to two months is required to mix a species throughout a hemisphere; however, about one to two years is needed to mix a species throughout both hemispheres. The relatively long interhemispheric mixing time arises from the presence of the intertropical con­vergence zone (ITCZ) located near the equator. Because air rises in the ITCZ, this region is characterized by considerable cloudiness and rain; the absence of strong

studying elementary atmospheric re­actions.

Pivotal advances have been made in the detection of atomic species, as well as diatomic and polyatomic free radi­cals. These new techniques include: resonance fluorescence detection using both conventional and laser light sources, laser-magnetic resonance, electron spin resonance, and Fourier transform infrared monitoring. In addi­tion, major improvements have been

north-south winds across the ITCZ tends to minimize the mixing of trace gases between the northern and southern hemispheres.

Whereas winds tend to produce a uniform distribution, processes that generate, transform, and/or remove a species from the atmosphere, such as photochemical re­actions, often are highly variable as a function of space and time. As a result, these processes oppose the effects of turbulent mixing and tend to produce a nonuniform distribution. To determine whether a given species' dis­tribution is dominated by mixing or photochemistry, the species' atmospheric lifetime must be compared with its atmospheric mixing time. (Atmospheric lifetime is a measure of the time a species typically resides in the at­mosphere before being removed; it is mathematically defined as the time for the species to decrease in con­centration by a factor of 1/2.72 of its initial abundance, if all sources of the gas were turned off suddenly.)

Species with lifetimes of a few years or more stay in the atmosphere long enough for winds to mix them thor­oughly. Hence, they have fairly uniform tropospheric distributions in spite of heterogeneities in their sources and sinks. Species with lifetimes of a few months may be well mixed within each hemisphere but exhibit variations between hemispheres. Extremely reactive species having lifetimes of less than an hour are controlled almost ex­clusively by local production and destruction mecha­nisms. To a first approximation, these species are in photochemical equilibrium because their rate of pro­duction equals their rate of destruction. Species that are in photochemical equilibrium adjust rapidly to changes in their rates of production and destruction and thus may vary greatly over time and space.

Perhaps the most difficult species to study are gases with lifetimes too short for a well-mixed distribution but too long for photochemical equilibrium. In this case also, distribution may be highly variable. However, the vari­ability can reflect changes in wind patterns as well as changes in rates of production or loss.

The Earth-atmosphere interface

A variety of biological and geological processes can result in the emission of gases from Earth's surface to the atmosphere. These surface emissions, which can have a major effect on atmospheric composition, appear to be highly variable in both the types of species released and the rates at which they are emitted into the atmosphere. Although data are quite limited, those available suggest that different biotic provinces, such as tropical evergreen forests, deciduous midaltitude forests, swamps and wetlands, alpine tundra, and deserts each emit a unique array of gases.

Even the oceans should not be looked upon as having a homogeneous interface with the atmosphere; there is growing evidence that the biological diversity of the oceans is quite large. Given the diversity of ecosystems on Earth, it is quite likely that we will continue to find that the "natural" troposphere is highly inhomogeneous in its chemical composition.

In spite of the complexity of biogeospheric surface emissions, one generalization is in order: The vast ma­jority of trace species emitted into the atmosphere are in reduced oxidation states (such as hydrogen sulfide, am­monia, and methane). By contrast, materials returned to the surface from the atmosphere, usually by dissolution in raindrops or by dry deposition, are highly oxidized (such as sulfuric acid, nitric acid, and carbon dioxide). The link between these reduced and oxidized species is

supplied by atmospheric photochemical reactions, thus giving rise to a chemical cycle whereby reduced gases are emitted into the atmosphere, photochemically oxidized, and then removed from the atmosphere.

That reduced atmospheric gases are oxidized is not surprising; the large atmospheric abundance of oxygen thermodynamically favors their oxidation. However, molecular oxygen rarely is directly responsible for oxi­dizing these reduced gases. The oxygen-oxygen bond is relatively strong (120 kcal/mole) and thus molecular oxygen does not react readily with most reduced gases at atmospheric pressures and temperatures.

In the early 1900s, reduced atmospheric species were believed to be oxidized by ozone and hydrogen peroxide; today, highly reactive free radical species (present in the atmosphere at levels of 10 ppt per volume of air or less) are recognized as being responsible for a large part of this oxidation. The recognition of the importance of these free radical species has constituted the major scientific breakthrough in tropospheric photochemistry during the past decade.

Tropospheric free-radical photochemistry

Because free radicals have an unpaired electron in their outer shell, and thus an affinity for adding a second electron, they can act as strong oxidizers of atmospheric trace gases. Of the free radicals present in the atmo­sphere, the hydroxyl species appears to be the most piv­otal in tropospheric photochemistry. The chemical mechanism responsible for the presence of hydroxyl radicals in the troposphere was proposed by Hiram Levy II, currently at the Geophysical Fluid Dynamics Labo­ratory in Princeton, N.J.

Oct. 4, 1982 C&EN 41

Most solar radiation below 290 nm does not reach the troposphere Radiation flux, c m - 2 sec- 1 (100 A ) - 1

1016

1015

1014

1013

200 300 400 500 Wavelength, nm

At top of atmosphere

" • In troposphere, for solar zenith angle of 0°a

In troposphere, for solar zenith angle of 45° *

a Local solar zenith angle is the angle the sun makes with a line pointing di­rectly upward; at noon, at the spring or fall equinox, the solar zenith angle is equal to the latitude at that location. Note: Most of the biologically harmful ultraviolet portion of the solar spectrum is absorbed by nitrogen, oxygen, and ozone and does not reach Earth's surface.

Special Report

New instruments improve nitric oxide detection Perhaps no obstacle to progress in at­mospheric chemistry is greater than that imposed by limitations in field sampling technology. Efforts to detect nitric oxide under natural conditions—at concen­trations of a few parts per trillion—il­lustrate this point. As with many other trace gases, detection of tropospheric NO requires sensitivity while avoiding interference signals from numerous other species. This interference can be particularly acute for instruments mounted on airborne sampling plat­forms. Investigation of these potential interference problems, as well as other systematic errors in the detection of NO and such other trace gases as hydroxyl radicals, currently is an active area of research by several government agen­cies.

A new laser-induced fluorescence (LIF) sensor, designed at Georgia Insti­tute of Technology, will be used for both ground-based and airborne sampling of NO. The sensor involves two ap­proaches: single-photon NO excitation and sequential two-photon excitation.

In the single-photon approach, an ultraviolet photon with a wavelength of 226 nm is used to excite ground-state NO into its lowest excited electronic state (Α2Σ+). A small fraction of this initially excited NO escapes quenching by atmospheric oxygen and falls back to the ground (Χ2π) state with the release of fluorescent photons. Since several spectral components of this fluores­cence are shifted toward longer wave­lengths, relative to the laser excitation wavelength, extensive discrimination is possible against scattered laser pump­ing radiation. The most prominent noise in this type of system, therefore, is white fluorescence (unstructured) generated by the laser pumping radiation hitting the walls of the fluorescence chamber and interacting with atmospheric aerosols. Because this noise source usually is spectrally broadband in nature, it can be reduced significantly, but not eliminated totally, by the use of narrow-bandpass optical filters.

In the two-photon LIF approach, the same initial step occurs as in the one-photon system. However, a second photon (spectrally located in the infrared

Georgia Tech's John Bradshaw and Michael Rodgers engage In laboratory cali­bration exercises on a new laser-Induced fluorescence sensor for nitric oxide

at 1060 nm) pumps NO from the low-lying Α 2 Σ + excited state into the upper D22 excited state. Thus, the final fluo­rescence occurs in the near-vacuum ultraviolet (187- to 200-nm range) and is blue-shifted relative to the shortest ex­citation wavelength (226 nm) employed. Use of a solar-blind multiplier tube in conjunction with long wavelength cutoff filters results in noise levels that are several orders of magnitude lower than with the single-photon system. Labora­tory tests suggest this system should have a detection sensitivity in the field of less than 1 ppt per volume. And dou­ble spectroscopic selectivity in pumping NO into its final excited state should greatly reduce interference signals from other tropospheric species.

Another improved nitric oxide de­tection system, developed originally at York University, Ont., by Harold Schiff and Brian Ridley, is based on sensing the light emitted from electronically excited nitrogen dioxide produced from the re­action of nitric oxide with ozone. NO contained within an air sample is chan­neled into a flow reactor, where ozone produced by the instrument is mixed with the air sample to react with NO.

Only about 10% of the total nitrogen dioxide generated is formed in an elec­tronic-excited state. Most of this excited N02 is quenched by collisions; however, 0.1 to 0.001 % of the excited N02 (de­pending on the pressure in the reactor) emits photons in the 630- to 2000-nm spectral region. This chemilumines-cence is monitored by a red-sensitive photomultiplier tube operated in the photon-counting mode.

This chemiluminescence NO instru­ment has several unique features as a field instrument. It is highly sensitive, lightweight, and portable. Sensitivities as high as 6000 counts per second per ppb of NO have been achieved by Mack McFarland of the National Oceanic & Atmospheric Administration's Aero-nomy Laboratory and Ridley at the National Center for Atmospheric Research. This represents a detection limit in air at atmospheric pressure of better than one part of NO in 1012 parts of air. Systems of this type have been used in a wide variety of sampling con­figurations, including remote ground-based field sampling sites, ship plat­forms, aircraft, and high-altitude balloon platforms.

The production of hydroxyl radicals is initiated by the photolysis of ozone. Ozone, present in the troposphere at concentrations ranging from 10 to 100 ppbv, has a bond energy of 26 kcal/mole. Solar photons having wavelengths between 315 and 1200 nm can dissociate ozone and pro­duce an oxygen atom in its ground electronic state:

0 3 + hi> (1200 > λ > 315 nm) -* 0(3P) + 0 2

The 0(3P) atom rapidly reforms ozone by combining with O2 in a three-body reaction:

0(3P) + 0 2 + M — 0 3 + M (M = N2 or 02)

Thus, the sequence of these two reactions is a null cycle with no net chemical effect.

When ozone absorbs a photon in the near-ultraviolet

42 C&EN Oct. 4, 1982

Photochemistry of OH radical controls trace gas concentration

XO HCI N O

\ t / XO I Several i \* I — Ar CH2-

ecu NH,

HNO^ \ CH3CCL / u o ^ ^Sev<

λ ι ns Several

HSO3 steps

N02 \ • / S 0 7 S e4

V e r a l

z ^ S 2 steps < 2 H20 OH - H 2 S - ^ HS ο υ 2

Several .^steps

CO

Removal in 2 ^ O g precipitation

The photochemistry of the free hydroxyl radical controls the rate at which many trace gases are oxidized and removed from the atmosphere. Processes that are of primary impor­tance in controlling the concentration of OH in the tropo­sphere are indicated in red in the schematic diagram; those that have a negligible effect on OH levels but are important because they control the concentrations of the associated reactants and products are indicated in blue. Circles indi­cate reservoirs of species in the atmosphere; arrows indi­cate reactions that convert one species to another, with the reactant or photon needed for each reaction indicated along each arrow. Multistep reactions actually consist of two or more sequential elementary reactions. HX = HCI, HBr, HI, or HF. CxHy denotes hydrocarbons

with a wavelength shorter than 315 nm, an electronically excited oxygen atom is produced:

0 3 + hi/ (λ < 315 nm) — 0(XD) + 0 2

The 0(XD) -* 0(3P) transition is forbidden and therefore OOD) has a relatively long radiative lifetime of 110 sec­onds. In the lower atmosphere, rather than relax radia-tively, 0(XD) most often collides with nitrogen or oxygen molecules:

0(XD) + M — 0(3P) + M (M = N2 or 02) This ultimately leads to the regeneration of ozone by the previous three-body reaction, resulting in another null cycle. Occasionally, however, 0(XD) collides with water to generate two hydroxyl radicals:

0(XD) + H20 — 20H This reaction sequence is the primary source of tropo-spheric hydroxyl radicals.

The removal of hydroxyl from the atmosphere occurs most frequently as a result of reactions with carbon monoxide or methane:

CO + OH - C02 + H CH4 + OH -> CH3 + H20

However, several complications arise in hydroxyl chem­istry when one considers the ultimate fate of the hydro­gen and methyl radicals formed in these two reactions. Both radicals combine rapidly with molecular oxygen to form hydroperoxyl and methylperoxyl radicals. But the hydroperoxyl radical can regenerate OH:

H02 + NO — N0 2 + OH H02 + 0 3 -> 202 + OH

It also can lead to chain termination: H02 + OH — H20 + 0 2

H02 + H02 — H202 + 0 2

This is followed by the removal of hydrogen peroxide in rain.

The chemistry of the methylperoxyl radical (CH302) and its products is quite complicated and the kinetics of many of the reactions still are quite tentative. The work of Princeton's Levy, Paul Crutzen of Max-Planck Insti­tute for Chemistry in Mainz, West Germany, and Michael McElroy and Stephen Wofsy of Harvard University in­dicates that it is likely that CH302 is oxidized completely, resulting in the production of carbon monoxide. However, depending upon the exact chemical pathway taken, the generation of carbon monoxide from CH302 can lead ei­ther to further generation or to consumption of OH and H02 radicals.

By using a mathematical model to simulate these photochemical processes, the atmospheric concentration of OH can be estimated. Such calculations suggest a seasonally, diurnally, and globally averaged OH con­centration of about 2 X 105 to 106 per cc, with the highest levels in the tropics. The OH maximum in the tropics arises from the high humidity and large 0(1D)-producing actinic flux present in tropical regions, which causes a large rate of OH primary production.

Another important feature of the calculated global OH distribution is its asymmetry between the hemispheres; model calculations predict that about 20% more OH should be found in the southern hemisphere than in the north. This OH asymmetry is believed to be caused by the large amounts of carbon monoxide produced by

human-related activities in the northern hemisphere. This asymmetry in OH may have significant climatic implications.

Model-predicted OH concentration levels of 105 to 106

per cc, together with laboratory data demonstrating the highly reactive nature of this radical, imply that OH controls the rate at which a large number of other atmo­spheric species are oxidized and removed from the at­mosphere. However, until these model-predicted levels are confirmed by direct atmospheric measurements, this conclusion must remain tentative.

Thus far, two approaches have been taken to verify the accuracy of the photochemical theory. One involves a direct comparison of measured OH levels with those predicted by a photochemical model. For this comparison to be meaningful, all parameters used in the model to calculate OH must be measured simultaneously with OH. These parameters include temperature and solar flux, as well as the concentrations of H202, CH3OOH, CH20, H20, O3, CO, CH4, and NO. Unfortunately, performing this experiment has proven to be quite difficult. At this time, several of the above species either have not been

Oct. 4, 1982 C&EN 43

Special Report

Field sampling platforms range from ships to satellites Field sampling requirements for studying global tropospheric chemistry are so extensive and complex that several types of sampling platforms are used for collecting data. These platforms may consist of ground-based stations, ship platforms, airborne platforms, or high-altitude space platforms. Each type has a unique capability for data collection, and all are required to achieve a com­prehensive understanding of tropo­spheric chemistry.

Aircraft platforms, in particular, can transsect the entire global troposphere at different altitudes in a relatively short time, providing what might be called a long-term snapshot of a slice of the troposphere. Several turboprop and jet aircraft owned and operated by both the National Center for Atmospheric Re­search and the National Aeronautics & Space Administration now are in use as

airborne platforms for chemical sam­pling.

In the long run, certainly, the most comprehensive global data base for a given trace gas will be that collected from orbiting space platforms. Great technical difficulties must be overcome to achieve this, however, and mea­surements probably can be made for only a limited number of species. How­ever few in number, though, they un­doubtedly will prove to be very signifi­cant in developing a complete chemical picture of the troposphere. A start in developing an orbiting tropospheric monitoring capability was initiated by NASA in last November's launch of the space shuttle Columbia, which carried, in its bay, a tropospheric carbon mon­oxide sensor designed by Henry G. Reichle and colleagues at NASA's Langley Research Center.

Gas finer correlation radiometer, de­signed by Henry G. Reichle and col­leagues of the National Aeronautics Λ Space Administration's Langley Research Center, was carried on the space shuttle Columbia last November to measure tropospheric levels of carbon monoxide

Boom-mounted meteorological probe on an L- 188c Electra turboprop plane operated by the National Center for Atmospheric Research Is used to determine chemical fluxes In the lower troposphere

measured at all, or, if they have been measured, serious questions remain about the accuracy of the measure­ments.

The measurement of OH itself has presented a major challenge to atmospheric chemists. Even though mea­surements have been reported in the literature that tend to be in qualitative agreement with model calculations, the accuracy of the data continues to be the subject of much scientific debate. Currently, at least seven inde­pendent groups in the U.S. and Europe are addressing this problem. Thus, direct comparison of measured OH levels with model-predicted values continues to be one of the challenging frontiers of global tropospheric chemistry.

An alternate, indirect test of OH photochemical theory, originally proposed by Hanwant Singh of SRI Interna­tional, involves the global distribution of methylchloro-

form (CH3CCI3). This chemical currently is used as a cleaning and degreasing agent; its release into the tro­posphere is believed to be its only atmospheric source. Since methylchloroform is removed from the atmosphere by reaction with hydroxyl radicals, the concentration of hydroxyl determines its lifetime. Using the history of methylchloroform emissions and its present atmospheric distribution and abundance, it is possible to infer its lifetime. This lifetime then can be compared to the life­time obtained from model-calculated OH levels in the atmosphere. At present, the lifetime estimates from both methods agree within a factor of two or less. Thus, it ap­pears that hydroxyl photochemical theory is at least qualitatively correct. However, large uncertainties are associated with both estimates and this agreement still could prove to be fortuitous.

The central role of hydroxyl radical in atmospheric

44 C&ENOct. 4, 1982

chemistry is well illustrated by examining the atmo­spheric cycles of methane and carbon monoxide. A quantitative assessment of both of these species was carried out in the 1920s in Belgium by Marcel Migeotte, who detected their absorption lines in the spectrum of infrared solar radiation reaching Earth's surface.

The major sources of atmospheric methane are bio­genic. They include the anaerobic fermentation of organic material in swamps, tropical rain forests, paddies, and in the digestive systems of livestock. Patrick Zimmerman of the .National Center for Atmospheric Research in Boulder, Colo., recently found evidence that also suggests that methane emitted by termites could be another im­portant atmospheric source. The major sink or loss pro­cess for methane is its reaction with hydroxyl radicals. This is an important example of how tropospheric free radicals affect the environment; by limiting atmospheric levels of methane (a greenhouse gas), hydroxyl radicals play a role in controlling climate.

In addition to destroying methane, the reaction of hydroxyl radicals with methane initiates a series of sub­sequent reactions that ultimately lead to the generation of carbon monoxide. Model calculations indicate that methane oxidation produces about 20 to 50% of the car­bon monoxide found in the atmosphere. The remainder is believed to arise from three other processes: the OH-initiated oxidation of nonmethane hydrocarbons, such as isoprenes and terpenes emitted by trees and other vegetation; the incomplete combustion (principally au­tomotive) of fossil fuels; and the burning of biomass, such as wood, agricultural wastes, and forests. As with meth­ane, the major sink for carbon monoxide is reaction with hydroxyl radicals.

It is interesting to note that the sources of both meth­ane and carbon monoxide are mostly continental rather than oceanic. Because most of the world's land mass is in the northern hemisphere, the largest sources of both compounds are there. One might expect, therefore, to find an interhemispheric asymmetry for both methane and carbon monoxide. Whereas considerably more carbon monoxide is found in the northern hemisphere, methane has a nearly constant mixing ratio of 1.65 ppmv with only a slight change in abundance across the ITCZ near the equator.

Differences in the latitudinal distributions of methane and carbon monoxide result from their respective at­mospheric lifetimes. Based on the globally averaged hydroxyl radical concentration of about 7 X 105 per cc and the appropriate rate constant, the atmospheric life­time of methane is estimated to be about seven years and that of carbon monoxide about 65 days. Because long-lived species tend to be mixed uniformly throughout the troposphere by the winds, the uniform distribution of methane is consistent with its long lifetime. The lifetime of carbon monoxide is sufficiently short to prevent it from being mixed thoroughly throughout the troposphere. As a result, its distribution more closely resembles the dis­tribution of its sources.

Because methane is long-lived and exhibits relatively small random fluctuations as a function of space and time, it is ideally suited for studies of the long-term variability of the atmosphere. And, in fact, growing evi­dence suggests that its atmospheric concentration is in­creasing slowly. For instance, Rai Rasmussen of the Or­egon Graduate Center has concluded, on the basis of measurements taken since 1965, that methane levels have increased about 2% per year. Wolfgang Seiler of Max-Planck Institute for Chemistry independently has come to a similar conclusion.

Should this rate of increase be sustained over a period of 40 to 50 years, methane could contribute to a signifi­cant global warming as a result of the greenhouse effect. This increase could be due to an increase in the rate at which methane is emitted from Earth's surface. For ex­ample, in the past few decades cultivation of rice and the populations of domesticated cattle have increased rap­idly, which could have resulted in a significant increase in emission of methane.

The increase in methane also may be related to in­creases in emissions of carbon monoxide. For example, carbon monoxide from combustion sources has been in­creasing at an estimated average rate of about 4 to 5% per year for several decades. Although it is well documented that pollution has caused a significant increase in levels of carbon monoxide in urban areas, its impact on carbon monoxide concentrations in remote regions is more un­certain. Because carbon monoxide levels are so variable as a function of time and space, a temporal trend in background levels is difficult to see in the present data base of observations.

Nevertheless, an analysis of the carbon monoxide budget, such as that carried out by Seiler, suggests that levels in the northern hemisphere may have been en­hanced by a factor of 1.5 to two because of man-made emissions. Since the reaction of hydroxyl radicals with carbon monoxide is the major process for removing hy­droxyl, this northern hemispheric enhancement in carbon monoxide could have caused a corresponding decrease in hydroxyl radicals. Any decrease in hydroxyl presum­ably would be accompanied by an increase in methane and the myriad of other species that are scavenged by hydroxyl radicals. This scenario, at present, is highly speculative, but serves to underscore the need to refine our understanding of tropospheric photochemistry.

Atmospheric lifetimes of trace gases vary from a second to a century

NLO 2 H2

CH3CCI3 CH4

OCS CO O3

cs2 so2

HN03 N02

H202

H2CO H02

OH Interhemispheric troposphere Intrahemispheric troposphere

_ Atmospheric mixing time

- Atmospheric _ lifetime

_ --

— · ----

- · "~ · -

_

"1 _J 1 1 1 1 I

Atmospheric concentrations of species with lifetimes that are long compared to atmospheric mixing times are dom­inated by turbulent transport and are fairly uniform through­out the troposphere; concentrations of species with life­times comparable to or shorter than the mixing times often are highly variable as a function of time and location, re­flecting variations ir Ί their rates of production and loss

Special Report

Atmospheric trace gas composition is controlled by a

Species

CH4

CO 0 3

NO + N02

HNO3 NH3

N20 H2

OH H02

H202

H2CO S02

CS2

OCS

CH3CCI3

Thermodynamically equilibrated

concentration* (Mole fraction)

1 0 - 1 4 5

6 Χ ΙΟ"49

3 Χ ΙΟ - 3 0

2 x 1er10

1 χ 10-9

2 Χ 10"·°

2 X 10"19

2 X 1(T42

5 X 10"28

4 Χ ΙΟ""28

1 X 10"24

9 Χ 10"9e

0

0 0

0

Actual atmospheric

concentration (Mole fraction)

1.6 X 10"e

(0.5-2) Χ ΚΓ 7

1 0 - e _ 1 0 - 7

(1CT12 - 10"8)b

(i<r11 - io-9)b

(i<r10 - io~9)b

3 Χ ΙΟ"7

5 Χ ΙΟ"7

10"15 - 10"12c

10~ 1 3 ~ 10~11c

i < r 1 0 - i<r8

1 0 - 1 0 _ 1 0 - 9

(1(T11 - 10"10)b

(1<r11 - 10"10)b

1 0 - 1 0

(0.7-2) X 10~10

Reliability with which

global distri­bution Is known

High Fair Fair Low

Low Low

High High Very low Very low Very low Low Fair

Low Fair

Fair

complex combination of processes

Principal sources

Biogenic Photochemical, anthropogenic Photochemical Lightning, anthropogenic,

photochemical Photochemical Biogenic

Biogenic Biogenic, photochemical Photochemical Photochemical Photochemical Photochemical Anthropogenic, photochemical,

volcanic Anthropogenic, biogenic Anthropogenic, biogenic,

Photochemical Anthropogenic

Principal sinks

Photochemical Photochemical Photochemical Photochemical

Rainout Photochemical,

rainout Photochemical Photochemical Photochemical Photochemical Rainout Photochemical Photochemical

Photochemical Photochemical

Photochemical

a Thermodynamically equilibrated concentration is that calculated by assuming thermodynamic equilibrium with a gas mixture containing 0.78 atm N2, 0.21 atm 0 2 , 0.01 atm H20. and 3.3 X 10"4 atm C02 at a temperature of 298 K. b Lower concentration limits cited may not necessarily reflect actual minimum concentrations but rather the lower limit on detection sensitivity of current instrumentation, c Concentrations of these transient species become much lower at night.

Long-lived methane is nearly uniformly distributed between hemispheres... Parts per million by volume

2.00

1.75

+ Boundary layer3

fc Free troposphereb

L I I L· 1 L 1 ITCZ° 1^1 1 1 I I I 1.50

70 60 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 10 20 30 40 50 60 70 Latitude

... but reactive carbon monoxide is more concentrated near its sources in the north Parts per billion by volume

200

150

100

50

- 7 0 - 6 0 - 5 0 - 4 0 - 3 0 - 2 0 - 1 0 0 10 20 30 40 50 60 70 Latitude

a Troposphere up to 1 km above Earth's surface, b Troposphere higher than 1 km; measurements primarily 5 to 7 km above the surface, c Intertropical convergence zone. Source: Leroy Heidt and colleagues, National Center for Atmospheric Research

+ Boundary layer3

• Free troposphere15 ^ J ·

?r • •

• · ' * · · ITCZC

1 l- J J - 1 1 1 1 1 I 1 I I I

The principal reactive nitrogen oxide species found in the lower atmosphere are nitric oxide, nitrogen dioxide, and nitric acid, with nitric acid being more abundant by a factor of about two to 10. These three species are cou­pled chemically by a series of reactions that act to cycle them among each other. Of special interest is the reaction of nitric oxide with hydroperoxyl radicals:

NO + H02 — N02 + OH Not only does this reaction regenerate OH from Η02,

but it leads to the generation of ozone when followed by the sequence:

N02 + hi/ — NO + Ο Ο + 0 2 + M — 0 3 + M (M = N2 or 02)

As with carbon monoxide, large quantities of NO are produced as a by-product of burning fossil fuels and wood. Although a buildup of CO causes OH to decrease, NO in the remote atmosphere acts to enhance OH levels. Thus, if anthropogenic production of NO is large enough to cause a rise in NO levels in remote regions, it could offset the OH-depleting tendencies of CO pollution. Measurements of tropospheric NO, N02, and HNO3 are quite sparse, but those that are available indicate that the distribution of nitrogen oxides is not so strongly affected by man-made urban sources as that of CO.

The reason for this apparent difference is the relatively rapid rate at which nitrogen oxides are removed from the atmosphere. After being emitted into the atmosphere, NO is converted rapidly to N02 by reaction with ozone; N0 2 reacts with the critically important OH radical to form nitric acid. And nitric acid, which is highly soluble, is removed by such heterogeneous processes as dissolu­tion in rain, attachment to aerosols and suspended par­ticulates, and dry deposition on Earth's surface. In the

46 C&EN Oct. 4, 1982

Increases in trace gases could affect climate In the absence of an atmosphere, Earth's surface would need a blackbody temperature of only 255 Κ to maintain a thermal balance with incoming solar energy. The actual surface temperature is about 288 K. This temperature in­crease arises from the absorption by gases within the atmosphere of infrared radiation emitted from Earth's surface and the reradiation of a fraction of this absorbed energy back to the surface. As a result, radiation from both the sun and the atmosphere impinges upon the surface and its temperature in­creases.

Because the homonuclear diatomic molecules of nitrogen and oxygen do not absorb in the infrared, the "atmospheric greenhouse effect" arises primarily from water, carbon dioxide, and ozone. (It is interesting to note that Svante Arrhenius in 1896 was one of the first scientists to attempt to quantify the ef­fect of carbon dioxide on the atmo­sphere.)

Trace species such as methane, ni­trous oxide, ammonia, nitric acid, sulfur dioxide, dichlorodifluoromethane. and

trichlorofluoromethane also contribute to this effect. A change in the atmo­spheric concentration of any of these gases, therefore, can influence climate. The sensitivity of climate to each of these trace gases can be determined by calculations, using radiative transfer models, that indicate how the surface temperature changes as a result of an assumed change in the concentration of any species (see table).

Because of uncertainties in these models, such calculations must be viewed as only a rough estimate of cli­matic response to changes in the chemical composition of the tropo­sphere. Nevertheless, they do point to a significant environmental problem. For example, the continued burning of fossil fuels and wood is predicted to cause a doubling of carbon dioxide levels by the early part of the 21st century and result in major global warming. Furthermore, recent observations indicate that trace gases such as methane, nitrous oxide, and the chlorofluorocarbons also may be rising, which could further exacer­bate the climatic effect of carbon diox­

ide. Ironically, however, this warming trend could be offset by a simultaneous decrease in stratospheric ozone due to the release of chlorofluorocarbons.

Clearly, however, the coupling be­tween atmospheric chemistry and the climate will require continued re­search.

Trace gas

C02

Stratospheric H20

o3 N20 NH3

HN03 CH4

S02 CF2CI2

+ CFCI3

Assumed Increase

In concen­tration

2 2

0.75 2 2 2 2 2

20

Resulting change

In surface temperature*

+2.6 Κ +0.65

-0 .4 +0.65 +0.12 +0.08 +0.26 +0.03 +0.65

a Based on calculations of V. Ramanathan and col­leagues at the National Center for Atmospheric Re­search and James Hansen and colleagues at Goddard Institute for Soace Studies.

lower troposphere, this process leads to a net nitrogen oxide lifetime of only a few days to a week—considerably shorter than the lifetime of carbon monoxide.

Thus, although NO and N02 may reach levels of 500 ppbv in urban centers and levels of 1 ppbv in much of the continental U.S., their removal is so rapid that little is transported to remote regions. For instance, observations by Mack McFarland of the National Oceanic & Atmo­spheric Administration's Environmental Research Laboratory in Boulder, Colo., indicate that NO levels in the remote Pacific are in the 1- to 20-pptv range.

Thus, it appears likely that remote levels of nitrogen oxides are dominated by natural rather than anthropo­genic sources; these natural sources are believed to in­clude generation of NO by lightning, OH oxidation of biogenically produced ammonia, and the direct release of NO and NO2 from soils and marine waters. In the upper troposphere, the downward movement of strato­spheric nitric acid (produced from the photochemical oxidation of N20) also may be an important source. However, because natural NO sources are poorly un­derstood, quantifying the size of each source and locating them geographically mark an important research fron­tier.

Another interesting facet of the nitrogen system is the cycling of ammonia through the atmosphere. Because it acts as a base when in solution, ammonia neutralizes acidic species, such as nitric and sulfuric acids, in rain­water. Its concentration in the lower troposphere is es­timated to range from less than 0.1 ppbv to a few ppbv and appears to vary significantly with season and loca­tion. Ammonia is lost from the atmosphere by physical absorption and/or chemical interaction with atmospheric aerosols and cloud droplets. In addition, some loss occurs by reaction with hydroxyl radicals; laboratory experi­ments indicate that this reaction leads to NO production.

Known sources of atmospheric ammonia include its direct volatilization from soils, its production during the breakdown of urea, and release during coal combustion and other industrial processes.

Unfortunately, the ammonia cycle is understood only qualitatively at present. Observations of the concentra­tion of ammonium ions in rain indicates that the rate of removal of ammonia from the atmosphere is larger than our best estimates of its rate of emission into the atmo­sphere. It is conceivable, therefore, that large unknown

Major reactive nitrogen species in troposphere are NO, N02, and HN03

HNO

Lightning NH3 oxidation Emissions from soils Combustion Transport from stratosphere

N205

NO or hv Rainout, dry deposition

Primary reactive pathway

Secondary reactive pathway

Oct. 4, 1982C&EN 47

Special Report

Distribution of ozone and water vapor may vary greatly with altitude Altitude, km

w —

Ι ι 1 J

| - H * A ^ — S = ? ^ ^ _ _ L

• " " • « Dewpoint 1^^^^m Ozone

J 1 1 1 1 J 1 1 1 1

- 4 0

10 20 30 40 50 60 70 Ozone, ppb by volume

J L

80 90 100 110 120

- 3 5 -30 - 2 5 -20 -15 - 1 0 - 5 Dewpoint, °C

10 15 20

Over the subtropical and tropical Pacific Ocean, ozone and water vapor frequently have a layered distribution. The strong neg­ative correlation shown above between ozone concentration and the air's dew-point temperature (a direct measure of the water content of air) suggests the presence of dynamic processes involving the downward movement of ozone-rich air from the upper troposphere and the upward movement of water-laden air from the lower troposphere Source: Frank Routhier and Douglas D. Davis, Georgia Institute of Technology

sources of atmospheric ammonia remain to be identi­fied.

Ozone is of major interest to tropospheric chemists for two reasons: It leads to the production of hydroxyl radi­cals, and it is a greenhouse gas. For many years, tropo­spheric ozone was believed to be chemically inert. Sci­entists held that the ozone present in the troposphere was formed initially in the stratosphere—where ultraviolet radiation is of high enough energy to dissociate oxy­gen—and mixed down into the troposphere. It was pos­tulated further that ozone was removed from the atmo­sphere primarily by reacting with Earth's surface.

Investigators such as Christian Junge of Max-Planck Institute for Chemistry and Edwin Danielsen of the National Aeronautics & Space Administration's Ames Research Center, Moffett Field, Calif., have shown that several aspects of the global ozone distribution support this "transport theory." For one thine, tropospheric ozone concentrations increase with altitude, presumably indi­cating a stratospheric source. Positive correlations be­tween ozone and species of stratospheric origin, such as the radionuclide 7Be, also have been pointed to as evi­dence in support of the transport theory. Similarly, nu­merous observations showing that levels of tropospheric ozone above 1 km frequently are correlated negatively with water (such as those reported by Frank Routhier and Douglas Davis of Georgia Institute of Technology) strongly suggest a stratospheric source for ozone.

However, growing awareness in the early 1970s that free radicals are a ubiquitous component of the atmo­sphere led researchers to realize that ozone, rather than being chemically inert, is produced and destroyed by tropospheric photochemical processes. In 1973, William Chameides of Georgia Institute of Technology and James

Walker (now at the University of Michigan) used a pho­tochemical model to demonstrate that photochemical processes may play a major role in the overall tropo­spheric ozone budget. One of the primary factors af­fecting ozone photochemistry is the concentration of nitric oxide. In regions such as the remote Pacific where NO levels are extremely low, tropospheric photochem­istry results in a net loss of ozone. But in regions with larger NO concentrations, the oxidation of methane and carbon monoxide generally leads to generation of ozone.

In fact, evidence gathered by Wolfgang Seiler of Max-Planck Institute and Jack Fishman of NASA's Langley Research Center in Hampton, Va., appears to support the hypothesis that ozone can be produced photochemically in the remote troposphere. They found, for instance, that in several cases measurements in the free troposphere (the region above 1 km) show concen­trations of ozone that are correlated positively with levels of carbon monoxide, a photochemical precursor of ozone.

The dichotomy between the evidence supporting the transport theory and the photochemical theory has led to intense debate. Although many more measurements of ozone, carbon monoxide, nitric oxide, and water will be needed to resolve this question completely, the picture that is beginning to emerge is that both transport from the stratosphere and photochemistry are important and that neither can be neglected.

It is interesting to note the striking difference between nitrogen oxide/ozone photochemistry in the remote, clean atmosphere and the polluted atmosphere. In regions significantly affected by NO pollution, especially when accompanied by hydrocarbon emissions, levels of ozone

48 C&EN Oct. 4, 1982

Heterogeneous chemistry is important in removing sulfur dioxide

Heterogeneous reactions are those in which more than one phase is involved; in the atmosphere, these most often are gas and liquid or gas and solid phases. A special class of heterogeneous re­actions is that referred to as gas-to-particle conversion processes. These reactions cause the transfer of a chemical species from the gas phase to an aerosol or a liquid droplet suspended in the atmosphere. Because aerosols and droplets are removed efficiently from the atmosphere in rain, gas-to-particle conversion often is one of the final steps by which the atmosphere rids itself of a species.

Of the myriad of gas-to-particle con­version processes believed to occur in the atmosphere, one of the most inter­esting is the conversion of gas-phase sulfur dioxide to sulfate dissolved in a cloud droplet (see diagram). Because this process also generates two hydro­gen ions, it often is responsible for

producing acid rain in regions containing high levels of sulfur dioxide.

As with most gas-to-particle pro­cesses, the conversion of sulfur dioxide to dissolved sulfate actually consists of a sequence of steps and, in fact, can follow several different paths. It can be initiated by a gas-phase reaction with hydroxyl radicals.

Most sulfur dioxide oxidation within a cloud occurs in the liquid phase. Gas­eous sulfur dioxide diffuses to the sur­face of a droplet, dissolves, and hydro-lyzes to form HS03~ and S0 3

2 ~ ions. These ions then are oxidized to form S0 4

2 ~ by reacting with ozone and hy­drogen peroxide, which also is formed in the gas phase and dissolved into the cloud droplet.

Very recently, the authors proposed a mechanism by which free radical species also can take part in the heter­ogeneous oxidation of sulfur dioxide. For instance, gaseous OH and H0 2 radicals,

generated within a cloud by gas-phase photochemical reactions, may be dis­solved in droplets within the cloud. The dissolved H0 2 then is transformed into hydrogen peroxide, which in turn oxi­dizes dissolved sulfur dioxide to S04

2~. Aqueous-phase OH also reacts with HS03~ and S0 3

2 ~ ions, forming S 0 42 " .

Recent model calculations indicate that this aqueous-phase, free-radical mechanism may be a major pathway for the oxidation and removal of atmo­spheric sulfur dioxide.

Although the individual steps that convert sulfur dioxide to S0 4

2 ~ can be defined in general terms, the detailed mechanisms of each step, especially those occurring at the droplet surface, are uncertain. This generally is true of most gas-to-particle conversion pro­cesses. As a result, innovative field and laboratory experiments are needed to better define the kinetics and thermo­dynamics of these processes.

Gas-phase reactions

r-so,

Aqueous-phase reactions (liquid cloud drop)

k - O H

White Face Mountain Atmospheric Laboratory, located at an elevation of 4800 feet near Lake Placid, Ν. Y., and often immersed in clouds, is used by scientists from the State University of New York, Albany, to collect and analyze cloud water for studies of heterogeneous oxidation pro­cesses within clouds

SO, •HSO, • S 0 3 2 -

{hv + H20)

OH

(CO + 02)

H09 HO,

(H02) (H02)

Several steps H202·

S0 42-

Note: Arrows crossing liquid cloud drop barrier signify heterogeneous reactions that transfer a species from the gas phase to the aqueous phase

in excess of the national air quality standard of 0.12 ppm often are encountered. In the remote troposphere, how­ever, ground-level nitrogen oxide levels generally are quite low and, as a result, photochemical ozone concen­trations in the lower remote troposphere usually remain relatively small.

The low abundances of NO found in remote loca­tions—in spite of its large urban source—is testimony to the significant role photochemistry can play in cleansing the atmosphere of pollutants. In the case of nitrogen oxides, the conversion of nitrogen dioxide to nitric acid by hydroxyl radicals and its removal in precipitation acts as a highly efficient scavenging mechanism and ulti­mately prevents the photochemical generation of po­tentially harmful levels of ozone.

Atmospheric sulfur cycle

The concern that sulfur species will reduce air quality and enhance the acidity of rain has led to intense interest in the atmospheric sulfur cycle. And, although much of the detailed chemistry of atmospheric sulfur is not yet fully understood, some of the major components of this system have been identified.

Biogenic processes generally emit sulfur species in reduced form (for example, CS2, CH3SCH3, CH3SSCH3, H2S, OCS). These reduced species are believed to be oxidized to an intermediate oxidation state in the form of sulfur dioxide. In the case of H2S and CH3SCH3, ki­netic evidence suggests that the initial oxidation step most likely involves reaction with hydroxyl radicals.

Oct. 4, 1982C&EN 49

Hydroxyl radicals are most abundant in the lower troposphere of the tropics Altitude, km OH contours χ 105 per cc 10

Contour lines on this diurnally and seasonally averaged profile of the global distribution of hydroxyl radical north and south of the equator indicate concentration of the radical χ 105 per cc, as calculated from a two-dimensional mathematical model of photo­chemical processes. The overall global average concentration is 7 χ 105 molecules per cc, with somewhat higher concentrations in the southern hemisphere than north of the equator. Uncertainties in the model's input data, such as kinetic rate constants and concentrations of ozone, water, carbon monoxide, methane, and nitrogen oxides, suggest an uncertainty in the calculated levels of hydroxyl radical of a factor of ±3

Source: William L. Chameides, Georgia Institute of Technology

Sulfur cycle oxidizes and removes reduced sulfur species from atmosphere

hi / , Ο

Several steps so l ­

Stratosphere

CS,

Troposphere

SO,

( ocs ) / 1

(CH3SCH3) V etc. 7

X 1 r

( S042- J

Rainout, wet and dry depositions

Surface sources

Atmospheric sulfur dioxide may be emitted directly into the atmosphere or produced from the oxidation of reduced at­mospheric sulfur species, such as CS2, CH3SCH3, and H2S. Its oxidation results in the production of sulfate in aerosols and cloud droplets, often in the form of sulfuric acid, to pro­duce acid rain. OCS, which is emitted by natural and anthro­pogenic processes, is chemically inert in the troposphere but is mixed into the stratosphere where it is oxidized to form particles of sulfuric acid. These particles can inhibit the penetration of solar radiation into the lower atmosphere and thus influence the climate.

However, the subsequent chemical reactions that lead to sulfur dioxide have yet to be confirmed by laboratory experiment. More important, because of large uncer­tainties in the emission rates and atmospheric abun­dances of many reduced sulfur species, the production rate of sulfur dioxide from these species cannot yet be established firmly.

In addition to being formed from reduced sulfur sources, sulfur dioxide may be injected directly into the atmosphere as a pollutant. Although pollution sources may be a major cause for high levels of sulfur dioxide in and around cities, their influence falls off significantly in more remote areas. Thus, whereas free-tropospheric levels of sulfur dioxide in the continental U.S. and Eu­rope, as well as over remote areas of Canada, generally range from 0.1 to a few ppbv, Alan Bandy and coworkers at Drexel University, Philadelphia, have found that in the South Pacific, the concentration of sulfur dioxide is only about 0.04 to 0.12 ppbv.

The lifetime or removal time for atmospheric sulfur dioxide appears to be somewhere between that of the nitrogen oxides (a few days) and carbon monoxide (two months). Removal can be accomplished by a variety of chemical mechanisms, all of which involve the oxidation of sulfur dioxide to sulfuric acid and the incorporation of the acid into cloud droplets and aerosols. This oxida­tion can be initiated either by gas-phase oxidation of sulfur dioxide by hydroxyl radicals (followed by the in­clusion of the oxidized product into cloud droplets), by liquid-phase reactions within cloud droplets involving dissolved sulfur dioxide and oxidizing species, such as hydrogen peroxide or ozone, or by its reactions on solid aerosol surfaces. The removal of aerosols and cloud droplets containing sulfuric acid in precipitation returns sulfur to Earth's surface and closes the atmospheric sulfur cycle.

Oxidation of sulfur dioxide, followed by the incorpo­ration of sulfuric acid into cloud droplets, can enhance the acidity of precipitation, as can the oxidation and re­moval of nitrogen oxides. In fact, in regions containing high levels of sulfur and nitrogen oxides, this effect often produces acid rain, which is precipitation with a pH below 5.6 (the pH of water in equilibrium with atmospheric carbon dioxide). The term apparently was coined in 1872

50 C&EN Oct. 4, 1982

oes

8

6

4

2

0 -75° -60° -45° -30° -15° 0° 15° 30° 45° 60° 75°

Latitude

by Angus Smith, working in London, who found evidence for acidity in rain in urban areas. Numerous measure­ments in the past few decades document the widespread occurrence of acid rain in the U.S., Canada, and Europe and its harmful effects on terrestrial ecosystems. Today, acid rain has attained the dubious distinction of being a major environmental problem. However, acid rain is not limited to regions of intense anthropogenic emissions. It also has been found to occur in fairly remote regions of the globe. This now appears to be caused by organic acids, as well as sulfuric and nitric acids, in precipitation.

Biogeochemical cycles

The atmospheric chemical cycles we have just dis­cussed actually are components of a larger set of cycles known as biogeochemical cycles. In these larger cycles, the key biological elements—carbon, oxygen, nitrogen, sulfur, and phosphorus—circulate through the biosphere, lithosphère, and oceans, as well as in the atmosphere. By circulating these elements continuously, nature recycles the chemicals needed to support life on Earth. In the absence of biogeochemical cycles, it is not hard to envi­sion the atmosphere becoming depleted in oxygen and overrich in biologically toxic chemicals or the soils and oceans becoming too nutrient-poor to support plant life.

Perhaps the most familiar of the biogeochemical cycles is that whereby carbon dioxide is converted to oxygen and organic carbon during photosynthesis and carbon dioxide is reformed from oxygen and organic carbon by respira­tion and decay. It is estimated that this process results in the transfer of about 2.4 Χ 1010 metric tons of carbon and 6.4 Χ 1010 metric tons of oxygen annually between the biosphere and atmosphere. A small fraction of this conversion (about 5 Χ 108 metric tons of carbon per year) occurs by the release to the atmosphere of methane and higher-molecular-weight hydrocarbons, which then are oxidized photochemically.

Approximately 0.3% of the organic carbon produced during photosynthesis escapes immediate decay and oxidation and is buried as organic sediment on the ocean floor. This actually represents a net source of oxygen to the atmosphere. In fact, the large atmospheric abundance of oxygen is related directly to the huge reservoir of or­ganic carbon currently stored in these sediments. The cycle finally is closed when the organic carbon sediments are lifted back to Earth's surface during the formation of mountains and then are oxidized or weathered.

The burning of fossil fuels and the clearing of forests accelerate the rate at which organic carbon is cycled back to the atmosphere, adding an estimated 5 X 106 to 107

metric tons of carbon, as carbon dioxide, to the atmo­sphere annually. This process appears to be responsible for the approximate 10% increase in observed atmo­spheric levels of carbon dioxide during the past 40 years. Because of carbon dioxide's important role as a green­house gas, a significant global warming is predicted if the present trend continues.

The biogeochemical cycle of nitrogen centers on the flow of nitrogen into and out of the "fixed" nitrogen reservoir. Fixed nitrogen species are compounds in which nitrogen atoms are not bonded to another nitrogen atom. Thus, in the atmosphere, nitric oxide, nitrogen dioxide, nitric acid, and ammonia are in fixed form, whereas mo­lecular nitrogen and nitrous oxide are not; species such as nitrite, nitrate, ammonium ions, and organic amines make up the fixed nitrogen within soils and marine water. The availability of fixed nitrogen is important because

Samples of trace gases and aerosols are collected by Volker Mohnen of State University of New York, Albany, and Paul Sperry of the National Center for Atmospheric Research aboard an NCAR aircraft during an acid precipitation field study

the atmosphere contains almost 80% N2 but most living organisms can utilize nitrogen only when it is fixed. Thus, fixed nitrogen is one of the critical nutrients limiting the productivity of terrestrial and marine plants.

The cycling of nitrogen is driven by a few select or­ganisms that convert molecular nitrogen to a fixed form, and by organisms that convert nitrate and ammonium ions in soil and seawater to molecular nitrogen, nitrous oxide, and nitric oxide. Collectively, these organisms cycle approximately 2 X 108 metric tons of nitrogen per year through the fixed nitrogen reservoir. Abiotic processes in the atmosphere that convert nitrogen, nitrous oxide, and oxygen to nitric oxide include lightning discharges, cosmic rays, and ultraviolet radiation. These processes may produce an additional 106 to 107 metric tons of fixed nitrogen annually.

Industry also appears to have a significant role in the nitrogen cycle. Industrial processes, such as the produc­tion of fertilizer, currently fix about 3 X 107 metric tons of nitrogen per year, and combustion fixes and releases about 1 to 2 X 107 metric tons to the atmosphere annually as nitric oxide.

The major cycling of sulfur occurs between reduced and oxidized compounds. In anaerobic environments, for instance, some forms of bacteria reduce sulfate ion to hydrogen sulfide as a means of obtaining oxygen. Other bacteria break down sulfur contained in amino acids in dead organic material. In the ocean, many phytoplankton form reduced sulfur compounds as a defense against predators and fungi. Each of these processes can lead to the emission of reduced sulfur compounds to the atmo­sphere. These compounds then are oxidized and returned to Earth's surface. Scientists estimate that approximately 108 metric tons of sulfur are cycled through the atmo­sphere each year. Anthropogenic processes, by emitting about 6.5 X 107 metric tons annually, as sulfur dioxide, apparently are beginning to overtake the natural global sulfur budget. In regions of intense industrial activity, these emissions cause highly elevated levels of sulfur dioxide, particulate sulfate, and acidic rain.

This brief discussion of biogeochemical cycles em­phasizes the close coupling between biological processes and atmospheric chemistry. On the one hand, biospheric emissions have a major effect upon the trace composition

Oct. 4, 1982 C&EN 51

Special Report

of the atmosphere. On the other hand, atmospheric chemistry, because of the greenhouse effect, the scav­enging and generation of toxic species, and the recycling of elements back to the biosphere in oxidized form, can have a significant impact upon life systems.

Lynn Margulis of Boston University and James Love­lock of the University of Reading in England have pro­posed that this coupling between life and the atmosphere is not coincidental but rather comprises a complex net­work of feedback mechanisms by which the biosphere acts to maintain an atmospheric-biospheric homeostasis. Thus, they hypothesize, critical environmental param­eters such as temperature have been maintained at levels favorable to life through the course of geologic time by means of biospheric-atmospheric interactions. Exploring the validity of this theory, referred to as the Gaia hy­pothesis by its authors in deference to classical Greek philosophy, is one of the challenging research frontiers of atmospheric chemistry and biology. Even as this hy­pothesis is explored, however, the possibility that man-made emissions may perturb global ecology must not be overlooked. For this reason, we must continue to study the impact of these emissions on the environment.

During the past decade, understanding of the chem­istry of the troposphere has grown significantly. New insights into the role of free radicals make it possible to identify many of the fundamental chemical transfor­mations that occur in the atmosphere. And, with new field sampling instrumentation now emerging, progress is being made in defining the concentration of numerous trace atmospheric species.

Nevertheless, many research frontiers remain. The area of heterogeneous chemistry, gas-to-particle con­version, and gas-phase/aqueous-phase interactions within clouds is one example where much innovative re­search will be needed in the future. Another challenging research area concerns the interaction between the bio­sphere and atmosphere. Certainly, the full extent to which natural biological and anthropogenic emissions affect the atmospheric environment needs to be eluci­dated fully. However, no less important is the need to investigate the effects of reactive atmospheric species upon processes in the biosphere. It is possible, for in­stance, that tropospheric free radicals act to modulate the levels of viruses and bacteria as they disperse in the en­vironment. This, and other possible biospheric-atmo­spheric interactions, should not be overlooked.

From a technological point of view, we must continue to make significant improvements in field sampling hardware. Instruments with greater sensitivity, greater selectivity, and faster time resolution are needed. These instruments must be capable of performing on a wide variety of sampling platforms and must encompass both in-situ and remote detection techniques.

The challenges we face in tropospheric chemistry are multidisciplinary in scope and of international concern. As a result, tropospheric chemistry offers a multitude of exciting research opportunities for scientists and engi­neers trained in a wide diversity of fields. D

Suggested readings For a general overview: Broecker, W. S., Li, Y.-H., Peng, T.-H., "Carbon Dioxide—Man's Unseen Artifact," in "Impingement of Man on the Oceans," Hood, D. W., Ed., Wiley Interscience, New York, p. 287 (1971). Rowland, F. S., Molina, M. J., "Chlorofluoromethanes in the En­vironment," Rev. Geophys. Space Phys., 13, 1 (1975). Calvert, J. G., McQuigg, R. D., "Computer Simulation of the Rates

52 C&EN Oct. 4, 1982

and Mechanisms of Photochemical Smog Formation," Int. J. Chem. Kinetics, Symp. /, 113 (1975). "Nitrogen, Phosphorus, and Sulphur—Global Cycles," SCOPE Report No. 7, Ecological Bulletin 22, Svensson, B. H., Soderlund, R., Ed., Orundsboro, Sweden (1975). Levy, H. II, "Photochemistry of the Troposphere," Advan. Pho-toc/wn., 9, 364 (1974). For some of the most recent findings: GAMETAG Papers, series of 12 papers published in J. of Geophys. Res., 85, 7285(1980). Logan, J. Α., Prather, M. J., Wofsy, S. C, McElroy, M. B., "Tro­pospheric Chemistry: A Global Perspective," J. Geophys. Res., 86,7210(1981). Chameides, W. L, Davis, D. D., "The Free Radical Chemistry of Cloud Droplets and Its Impact Upon the Composition of Rain," J. Geophys. Res., 87, 4863 (1982).

Douglas D. Davis (left) is professor and William L. Chameides (right) is associate professor of geophysical sciences at Georgia Institute of Technology.

Davis received a Ph.D. in 1966 from the University of Florida. As a postdoctoral fellow at the National Bureau of Standards, he was one of a three-man team that de­veloped the technique of flash photolysis-resonance fluorescence y used in studies of elementary gas-phase reactions. He further developed this technique and its applications to studies of atmospheric processes at the University of Maryland, where he was assistant pro­fessor from 1972 to 1976. Davis joined the faculty at Georgia Tech in 1976 and was instrumental in estab­lishing the division of atmospheric sciences within the school of geophysical sciences there. His research has involved global measurements of atmospheric trace gases and the development of new laser techniques ca­pable of detecting atmospheric species.

Chameides earned a Ph.D. in physics from Yale in 1974. He spent two years as a research scientist at the University of Michigan and four years as an assistant professor at the University of Florida before joining Georgia Tech in 1980. Chameides* research has focused on processes that control the composition of the atmo­sphere, especially photochemical generation of ozone, production of nitrogen oxides from lightning, and the interface of homogeneous gas-phase photochemistry with heterogeneous processes within clouds.

The authors acknowledge the help of C. S. Kiang of Georgia Tech in preparing this article.

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