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Vol. 21, No. 3 OCTOBER 2003

ChemMatters, OCTOBER 2003 3

COVER PHOTOGRAPH COURTESY OF NASA JSC STSO77

Welcome to This Special Issue of ChemMatters 2

Question From the Classroom 4Would a vacuum cleaner work in a vacuum? You move to planet X. It has noatmosphere. Our challenge to you: Which of the 50 activities on our list will work?

Whose Air Is It Anyway? 6The atmosphere was the first to go global, baby! Do you know where your air has been?

Alien Atmospheres: There’s No Place Like Home 9Venus will crush, suffocate, and burn you with hot acidic gas. Mars is no better—it’s too thin and cold. Maybe we’ve got a good thing going here?

Clouds 12They’re mostly water, surprisingly complex, and very different from one another.

Activity: Cloud in a Bottle 16Make your own cloud! Enough said.

Life in a Greenhouse 18A close look at how some gases keep our planet warm, maybe too warm.

Chemistry in the Sunlight 22Some chemicals just have a little too much fun in the sun. The result is smog for you and me.

Beefing Up Atmospheric Models 25They’re difficult to work with, but many find them attractive. They’re mathematical models! Chemists use them to study processes from local pollution to global warming.

Nobel Prize Winner Sherwood Rowland: A Conversation 29Would you give up a shot at a baseball career to be a chemist? A Nobel Prize winner reflects back on growing up smart and athletic.

The GLOBE Program: Science in the Sunshine 31Imagine presenting your science project at an international conference in Croatia.

Dedication 32We dedicate this issue to the crew and the legacy of Shuttle Mission STS 107.

Production TeamFrank Cardulla, Special Editions EditorKevin McCue, EditorDavid Harwell, Manager NCW programCornithia Harris, Art DirectorLeona Kanaskie, Copy Editor

Administrative TeamHelen Herlocker, Administrative EditorSandra Barlow, Senior Program Associate

Technical Review TeamErnest Hilsenrath, NASAJeannie Allen, SSAI at NASAStephanie Stockman, SSAI at NASASeth Brown, University of Notre Dame

Teacher’s GuideFrank Cardulla, Teacher’s Guide Editor

Division of Education and International ActivitiesSylvia Ware, DirectorMichael Tinnesand, Assistant Director for Academic Programs

Policy BoardSusan Cooper, Chair, LaBelle High School, LaBelle, FLLois Fruen, The Breck School, Minneapolis, MNDoris Kimbrough, University of Colorado–DenverRon Perkins, Educational Innovations, Inc. Norwalk, CTClaudia Vanderborght, Swanton, VT

Frank Purcell, Classroom Reviewer

ChemMatters (ISSN 0736–4687) is published four times ayear (Oct., Dec., Feb., and Apr.) by the American ChemicalSociety at 1155 16th St., NW, Washington, DC 20036–4800.Periodicals postage paid at Washington, DC, and additionalmailing offices. POSTMASTER: Send address changes toChemMatters Magazine, ACS Office of Society Services,1155 16th Street, NW, Washington, DC 20036.

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The American Chemical Society assumes no responsibilityfor the statements and opinions advanced by contributors.Views expressed are those of the authors and do not neces-sarily represent the official position of the American ChemicalSociety.

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CMTEACHERS!

FIND YOUR COMPLETE TEACHER’S GUIDE FOR THIS ISSUE AT

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4 ChemMatters, OCTOBER 2003

A. Good question! If you think of a vacuum cleaner as somethingthat pulls or sucks the dirt into it, then it should probably work in avacuum. This way of thinking, however, is based on a belief in a forcecalled “suction”. Unfortunately, most people don’t know that such aforce does not exist.

Consider a suction cup—stick one on top of a table and ask 100randomly chosen people what is holding it down. You will probablyget the same answer: “Suction”. It is a suction cup after all. If youquestioned them further about the nature of the force, they will mostlikely attribute it to some kind of pulling force created by the vacuumbeneath the suction cup.

But when you ask, “What is a vacuum?” many willrespond correctly, “A vacuum is nothing. It’s the absence of any mat-ter.” “So if it’s nothing”, you say, “what’s doing the pulling?” At thispoint you will probably hear a whole lot of “Hmmm’s”—that’s thesound of people reexamining their ideas about how the worldworks.The simple truth is that a vacuum is nothing and can thereforedo nothing! So what supplies the force that keeps a suction cupstuck to the table? It’s not a pulling force from within; it’s a pushingforce from without. That’s right! Our atmosphere pushes with a forceof 14.7 pounds per square inch. That’s like having the weight of abowling ball pushing on every square inch, of every surface aroundyou, all the time. It is caused by an almost unimaginably large num-ber of molecules moving at hundreds of miles per hour, collidingconstantly with each other and with every object on our planet. Wetake that pressure for granted. We forget it’s even there ... until it’sgone, that is, and then we start attributing strange pulling forces toits absence! Hmmm ...

But if you simply place a suction cup on the table and try to liftit up, it’s comes off easily. Why is it easy with allthat pressure pushing down on it? The explana-tion is simple: there’s also air beneath the suctioncup pushing upward with an equal force. But whenyou push the suction cup firmly to the table, youforce that air out. In this instance, when you try tolift it, you fight against atmospheric pressure butreceive no help from beneath. It’s just you againstthe atmosphere. If it’s a small suction cup with asurface area of only one square inch, then you willneed to pull upward with a force of about 14.7 poundsto break it lose. That’s assuming that you’ve created aperfect vacuum beneath the cup. If it’s a larger suctioncup with a surface area of 10 square inches ... good luck.

Let’s get back to the vacuum cleaner. A dirt particle sitting onyour carpet is pushed equally from all sides by atmospheric pres-sure. Normally, these forces cancel each other out, so the particle

just sits there. A vacuum cleaner works because it cre-ates an area of lower pressure above the parti-

cle. The dirt particle thus gets pushed (notpulled) into the vacuum cleaner by the greater air

pressure on the underside of it. This means that everyphenomenon you attribute to a vacuum pulling, you now haveto rethink.

For example, what about a drinking straw? Certainly, thatliquid is being pulled into our mouths by the suction we create.Right? Guess again! It’s being pushed up the straw by theatmospheric pressure pushing down on the liquid surface out-side the straw.

So would a vacuum cleaner work inside a vacuum? Not achance. The same goes for suction cups and drinking straws.Would anything work inside a vacuum? Most definitely! Somethings would even work better becauseair hinders their performance. Sosome things depend on air and somethings don’t, but how good are you at

Question From the ClassroomBy Bob Becker

Q. Would a vacuum cleaner still work inside a vacuum?

www.chemistry.org/education/chemmatters.html

telling which is which? Try the following activity, and findout. The first few should be easy based on what you’vejust read. For the rest, you’re on your own. Hint: there’smore to air than just pressure; just think of all the thingsit does that we take for granted.

So just how important is our atmos-phere? Imagine you relocate to planet X, a planet justlike Earth, with the exact same gravitational pull, but with

no atmosphere at all. Which of the following 50 items will still work onplanet X, and which ones will not? Don’t use the cop-out answer andsay none of them will work because we’d all be dead. Imagine you andyour friends have pressurized space suits with oxygen tanks andeverything you will need to survive.

For the things that will work, will they work exactly the same, orperhaps even better? For the things that will not work, can you think ofpossible modifications that could enable them to work?

Getting around ...31. a bicycle (and tire pump)32. a hang glider33. a golf cart34. a helicopter35. a parachute36. a hovercraft37. an automobile (with air bag)38. a hot air balloon39. the Goodyear blimp40. the space shuttle

We’re talking major energy ...41. a shot gun42. an emergency road flare43. dynamite (and fuse)44. lightning (and thunder)45. a coal-burning power plant46. a nuclear power plant47. a nuclear warhead48. a shooting star49. our sun

50. a beautiful sunset

Around the house ...1. a suction cup2. a vacuum cleaner3. a drinking straw4. a siphon5. a light stick6. a flashlight7. a magnet8. a broom9. an aerosol spray can10. an alarm clock

Heating it up ...11. a match12. a candle13. a blow dryer14. an electric stove15. a gas stove16. a pressure cooker 17. a microwave oven 18. a convection oven19. a smoke detector


Outdoor fun ...20. a soap bubble maker21. a boomerang22. a yo-yo23. a Frisbee24. a kite25. a swing26. a pogo stick

27. a bow and arrow28. the game of golf29. the game of baseball30. a flower garden


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6 ChemMatters, OCTOBER 2003 www.chemistry.org/education/chemmatters.html

A ir is arguably the closest and mostimportant biological connection wehave with the rest of the world. We

breathe air into our bodies every few sec-onds. Like it or not, we share the air webreathe with the people around us, whetherfriends or strangers.

Yuck, you may say! But it’s how ourworld is constructed. We don’t get a new airsupply every day. We simply “recycle” thesame air over and over. As one atmosphericchemist pointed out, “Air may not look likemuch, but try breathing something else!”

We share the air we breathe not onlywith other people but also with the rest of ourenvironment—cars, trucks, buses, factories,airports, trees, grass, livestock and wildlife,lakes and oceans—you name it. Air is inconstant motion, ever blowing and breezingfrom one place to another. The air we breatheand the stuff that’s in it have come fromsomewhere else.

Moving air carries some interestingbaggage, including moisture, dust, bacteria,fungal spores, viruses, and varying tracesof chemical constituents worthy of ourattention, like ozone (O3), nitrogen oxides(NOx), and sulfuric acid (H2SO4). As for anyair travel, some passport checking is in

order. For our health and safety, we need toknow where our air comes from, what it’scarrying, and what happens to it before itreaches us.

Curious about where your air has been?So is NASA. Scientists working with datafrom NASA’s Earth-observing satellites havebeen discovering, often to their amazement,that air pollution is quite an inter-continentaltraveler! Dust from the Sahara has turned upon coral reefs in Florida, and dust from theAsian Gobi Desert has appeared as far awayas the East Coast of North America. Air pollu-tion from the northeastern United Statessometimes reaches Europe, and, occasion-ally, European pollution travels the oppositedirection in return.

Sea salt and frozen plankton that tra-versed the Pacific Ocean via Hurricane Noraappeared over the midwestern United Statesin 1997. Satellite photos have revealed smokefrom Asian fires crossing the Pacific to reachsouthern California. Air pollution from Chinaoften drifts directly over Japan.

All of this air travel has significant politi-cal impacts. It’s becoming obvious that coun-tries, despite their own efforts to curbpollution, won’t have clean air until theirneighbors do.

Aura will map the sources andtransport of aerosols and chemical

pollution on regional and super-regional scales.

It’s a fact of life. We live on the Earth and inthe air. As for your share of that air—do you

know where it’s been?

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By Jeannie Allen

ChemMatters, OCTOBER 2003 7www.chemistry.org/education/chemmatters.html

Tracking ozoneOzone in the lower atmosphere is one of

the pollutants of greatest concern to NASA.Ozone in the upper atmosphere (stratosphere)protects us against harmful ultraviolet radia-tion from the sun, but ozone where webreathe harms us. The effect of breathingozone is somewhat akin to a slow burn. Ozoneis an oxidizer, readily donating one atom ofoxygen to other “willing” molecules. Overtime, breathing too much ozone can perma-nently reduce our lung capacity.

Hate getting up early? If you’re intosports, you might prefer doing your summerworkouts in the cooler morning hours. Youmay even find breathing a little easier at thattime. Ozone levels tend to be low in the morn-ing, before the chemical soup of nitrogenoxides and hydrocarbons has time to build upfrom vehicle traffic and other sources. They’rethe chemical ingredients for ozone formation,a group of chemical reactions that really getgoing as sunlight becomes more intense laterin the day. You can read more about that in“Chemistry in the Sunlight” on pages 22–24.

Ozone concentrations vary widely aroundthe globe. With recent improvements in satel-lite technology and the availability of moredata, atmospheric scientists are only now rec-ognizing the extent of ozone’s travels.

Anne Thompson is one of the trailblazersin ozone tracking at NASA Goddard SpaceFlight Center. She and her colleagues want tobe able to predict ozone concentrations anddistribution. She explains, “What we’re tryingto do is to parse out which ozone comes fromnatural causes and which ozone comes fromhuman activity. It’s extremely hard to sepa-rate natural and man-made sources of gases.

The precursors of ozone such as nitrogenoxides are not labeled ‘I came from an aircraftengine’, ‘I came from the stratosphere’, ‘Icame from the ground’, or ‘I came from light-ning’. You have to measure other relatedchemicals that fingerprint the source.”

Thompson and her colleagues recently

focused on a region off the West Coast ofAfrica. She describes the research in thesewords:

“We would combine all the data we couldget from various sources. We would get real-time weather data from many sources. Wewould get real-time satellite data on ozoneconcentrations from the Total Ozone MappingSpectrometer (TOMS satellite) and the OpticalTransient Detector (OTD) Lightning Sensor.Then, we would combine all of them to makeour best forecast of where the ozone, dust,biomass burning, and lightning would maketheir impact, and so where we should directthe NASA research aircraft to make observa-tions for us. We would forecast and fly, ana-lyze our observations, and then forecast andfly again to extend our understanding.”

Thompson notes that she has developeda healthy respect for how variable the atmos-phere is. “Weather systems in the tropos-phere are constantly moving and mixing theair, and at the same time, chemical reactionsare changing the air’s chemical composition.”

Major patterns of air circulation haveenormous impacts on ozone and its precur-sors, the airborne chemicals that react to formit. Thompson’s group was particularly inter-ested in an anticyclone in the South AtlanticOcean that delivers pollution from bothAfrican and South American fires over the

Atlantic. By this process,ozone from fires allacross southern Africarises up and mixes intothe circulating air, whichmay carry the pollutionhundreds of miles.Sometimes parcels ofpolluted air spin off,streaming out over theAtlantic, the IndianOcean, and as far awayas the Pacific.

A complete,detailed, global pictureof the travels of ozoneand its precursorsrequires more research,but some trends are

becoming clear. Background concentrationsof ozone—amounts that are usually there—generally range from about 25–55 parts ofozone per billion parts of air (ppb) in surfaceair over the United States. Much of this ozoneprobably comes from outside the UnitedStates.

Keeping tabs on ozone levels is impor-tant because the U.S. National Air QualityStandard is 80 ppb over 8 hours, an amountnot to be exceeded more than 3 times a year.But even if people in North America succeeded

SeaWiFS satellite image of the West Coast ofNorth America on April 25, 1998, shows thearrival of airborne dust from China. The dust isvisible in the clouds at the center of the left edgeof the image, and as streaks of light brown hazeover Cape Mendocino on the California coast.

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Anne Thompson NASA scientist, and AgnesPhahlane, a meteorologist of the South AfricanWeather Service, prepare for a balloon launchduring the SAFARI-2000 campaign in Zambia.The balloon carried both an ozonesondeinstrument to measure ozone and a radiosonde tomeasure temperature, pressure, and relativehumidity—conditions that can affect ozoneconcentrations and distribution.

The ever-changing and complex nature oftroposphere, the lower atmosphere, makes itdifficult to trace the chemical reactions thatproduce ozone.

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As often occurs in science, Sarah Mims’s discovery happened when she wasn’t reallylooking for it. Sarah had a good idea for a science fair project. After reading abouttraveling air currents and the particles they carry, she decided to sample the air near

her home in Seguin, TX. Her plan was to search for fungal spores she suspected were arriv-ing with dust from Asia.

But when she looked at a NASA SeaWiFS satellite image, Sarah found that on the dayof her sampling, a large cloud of smoke from fires in Mexico had passed over Seguin. Couldthe smoke have delivered the living fungal spores she collected that day? ”I was really sur-prised,” Sarah said. She had to be sure. “Did the fungal spores I captured come from CentralAmerica?”

Sarah decided to do an experiment tofind out if fungal spores could actuallysurvive burning. She burned several kindsof plants—making sure to follow local firedepartment rules. Then she exposedpieces of a water-soluble gel, Petrifilm, tothe smoke. She placed the film in culturecontainers and stored them at conditionsfavorable to fungal growth. In two or threedays, she had her answer. There werecolonies of at least 10 different species offungi growing in the dishes. Clearly, fungalspores are tough enough to withstandtoasting!

What makes Sarah’s research so important is that some fungal spores spread plantdiseases. Farmers in tropical countries regularly set fires to clear fields of old crop residues,making room for new plantings. Sarah demonstrated that infectedplants may release living fungal disease spores when they burn.Because the lightweight spores can travel vast distances on air cur-rents, the risk of fungal infection in healthy crops seems obvious.

After reading about the project recently, Eugene Shinn, a scien-tist with the U.S. Geological Survey, wrote this to Sarah: “I thinkthere is great potential in what you are doing. We have been identify-ing microbes including fungi in African dust that crosses the Atlantic.And, believe it or not, it reaches where you are living. One thing thathas puzzled us for some time is why the number of fungal spores …in dust fluctuates over time. We have always thought it had to domainly with the source area. Now I wonder if it is related to fires inthe Congo ... Could it be the smoke is the main source of the fungi? Iencourage you to continue [your work].”

Sarah plans to submit her findings to a scientific journal. If herresearch is corroborated, it will provide strong evidence that agricultural diseases canspread from one continent to another—in smoke!

For Sarah, science research is an exciting challenge. “The reason I do science projectsis because the benefits are incredible,” says Sarah. “I enter competitions such as the JuniorAcademy of Science. You really have to understand the science ideas well to answer thejudges’ questions.”

And it doesn’t hurt to be organized! Sarah advises, “Doing science projects alsorequires a great deal of discipline. You can't wait till the last minute to do the experiment andthrow a report together.”

A Texas student shows us that living sporescan go up in smoke and survive.

With help from friends, Sarah exposes Petrifilm to smoke from a grass fire.

The presence ofcarbon told Sarah thatthe fungal spores wereaccompanying smokefrom biomass burning.

in reducing their emissions of nitrogen oxidesand hydrocarbons by 25%, the ozone problemwould persist. Emissions expected from Asiaby 2010 could completely wipe out that gainin clean air.

“For the cooperation required to controlair pollution, we need an international agree-ment,” explains Guy Brasseur, an expert inmaking mathematical models of the atmos-phere at the Max Planck Institute for Meteo-rology and the National Center forAtmospheric Research. “But before we canmove to such an agreement, we need tounderstand the problem scientifically. Satel-lites are key to our research.”

“NASA’s Aura spacecraft will provide uswith the first truly global view of troposphericozone… .We’ll be able to track ozone bothregionally within continents and from onecontinent to another,” explains Reinhold Beer,principal investigator for one of Aura’s fourinstruments.

Daniel Jacob, atmospheric scientist atHarvard University notes that satellite observa-tions are changing the way atmospheric chem-istry is done. “Our field is undergoing arevolution, because my community is havingto think about satellite observations. Interpret-ing satellite observations is a very difficult task… . But we need to understand what satelliteobservations are saying to us, because weneed that global scale [perspective].”

Jeannie Allen is a science writer at the NASAGoddard Space Flight Center in Greenbelt, MD. Sheis a frequent contributor to the Earth Observatory,NASA’s award-winning website at http://earthobservatory.nasa.gov.

Space Shuttle astronauts photographed thesesmoke plumes from the Amazon rain forest inWestern Brazil in September 1984. "Slash-and-burn" techniques are sometimes used to clearforest land for agricultural purposes—for raisingcrops or for developing pastureland for cattle.Peak burning periods occur during the dryseason, which for this Southern Hemisphereregion is June through September.

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continued from page 7.


Why has Mars so captured ourinterest? The answer might bethat Earth, Mars, and Venus,

along with sun-loving Mercury, are all neigh-bors. Also called the inner planets, these arethe planets closest to the sun. With theirsolid, rocky surfaces, they’re also called theterrestrial planets. That makes them differentfrom Jupiter, Saturn, Uranus, and Neptune,which are the gas planets.

Like Earth, Venus and Mars have atmos-pheres. They’re nice places for a probe tovisit, but you probably wouldn’t want to gothere. How do we know? Although astronautshave yet to walk on their surfaces, roboticprobes from the United States and Russiahave circled and landed. They’ve gathered significant data and sent back pictures.

VenusBoth Venus and Mars are alien, but in

very different ways. Let’s start with Venus.Despite being named after the Roman god-dess of love and beauty, up close Venus ishardly inviting. Superman might even find theclimate more than he could bear.

Let’s start with air pressure. On Venus,it’s crushing! It’s about 90 times the pressureyou’re experiencing right now. You’d have todive to an ocean depth of more than a half-mile to duplicate the effect. In fact, when theRussian probe Venera 13 landed on the sur-face of Venus, it lasted about one hour beforebeing crushed by the atmosphere.

On Venus, if the pressure doesn’t killyou, the chemistry will. Consisting of 96%carbon dioxide (CO2), about 3.5% nitrogen(N2), no oxygen (O2), and just trace amountsof argon (Ar) and water vapor, the Venusianatmosphere offers little chance that any kindof life as we know it could exist, let alonethrive. Do you still want to put Venus onyour travel list?

Here on Earth, global warming is blamedon the increase of various greenhouse gasesin the atmosphere. On Venus, the overwhelm-ing presence of CO2 sets up a runaway green-house effect. Here’s why. Planets receiveenergy from the Sun. Much of this energy isradiated back from the surface and escapesinto space. But gases like CO2 absorb some ofthe energy preventing its escape.

The greenhouse effect warms the atmos-phere and therefore the planet. Earth’s atmos-phere is only about 0.036% CO2, yet there is

concern that even small increases due tohuman activity may be causing the averagetemperature on Earth to rise.

Now imagine the greenhouse effect onVenus. It’s closer to the Sun with about260,000 times as much CO2! We don’t have toimagine. We know. The average temperatureon the surface of Venus is about 467 °C (873 °F)—higher than the melting point of lead(Pb).

Like Earth, Venus has clouds. In fact, it isso covered with clouds that they completelyobscure its surface. But don’t look for ourfluffy, white, water-droplet variety. Scientiststhink that at some point in its past, water wasplentiful on Venus, but it all boiled away. Theclouds on Venus are mostly sulfuric acid(H2SO4)! Huge volcanic eruptions release sul-fur dioxide (SO2) gas that is converted to thestrong acid by photochemical reactions in theupper atmosphere.

MarsCompared to Venus, Mars looks down-

right inviting. But hold off making your travelplans just yet until you’ve heard more

about its atmosphere.

A L I E N A T M O S P H E R E S :

When the Mars Pathfinder mission reached Mars on July 4, 1997,

more than 100 million hits were made on NASA’s Mars Pathfinder

website in a single day. That’s more than the number of people

who watched Ruben beat Clay for the American Idol crown.

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EOS Aura will focus on Earth’s atmosphere. Studying the

atmospheres of other planets provides insight into the unique

chemical and physical properties of our own.

Astronomers classify Mars as a planetthat has an atmosphere, but compared toEarth, you might not notice. Atmosphericpressure on Mars is only about 1/100 that ofEarth. Similar to Venus, Mars has an atmos-phere of mostly CO2 (95%) plus some N2

(2.7%), Ar (1.6%), and small amounts of O2

(0.15%), and water vapor (0.03%).But Mars is much colder than Venus. It’s

farther from the Sun, and it has a lower totalconcentration of CO2—that adds up to almostno greenhouse effect at all. It has an averagetemperature around –63 °C (–81 °F) thatvaries with the seasons. Mars has winters andsummers like Earth. During summer, localtemperatures can get high enough to meltwater, but it would quickly evaporate away atthe low atmospheric pressure. Essentially, anywater present on the surface is in solid form(see “The Search for Martian Water”, Chem-Matters, Oct. 2002).

Mars is a cold and desolate place. Andthen there are the storms! The worst stormson Earth are only small disturbances whencompared to what happens on Mars. Even amajor storm on Earth only affects a specificregion. Storms on Mars can encompass theentire planet. There is no escape! Huge duststorms scour the planet. Temperatures soar,then plunge as a storm subsides. Brilliantclouds of ice reappear.

The atmosphere of Mars won’t squishyou like a bug on a windshield—recallVenus—but it’s no picnic either. At firstglance, the Martian atmosphere seems soalien and different from Earth’s that it hardlyseems relevant.

Why do we studyother planets’ atmospheres?

It turns out that despite their dif-ferences, the atmospheres of Marsand Earth share some similarities.Both are mostly transparent to sun-light and are heated in similar ways—largely by infrared radiation thatemanates from their surfaces. Theyexchange energy in a similar manner.But the climate on Mars changesmore rapidly and drastically than itdoes on Earth. Its orbit is more ellipti-cal than that of Earth. Its distancefrom the Sun varies by about 20% asit orbits once every 687 “Earth days”,about every two Earthyears. It receives 40%

more sunlight at its closestapproach compared to whenit is most distant from theSun. This causes rapid andsignificant changes in globaltemperatures, which pro-duces those incrediblestorms.

Mars does not haveoceans, unlike Earth, andbecause of this, study of theMartian atmosphere and cli-mate is made much simpler.Oceans exert an effect on aplanet’s climate that is very complex and diffi-cult to predict. These complexities do notexist on Mars, making it easier for scientiststo reach confident conclusions about whathas caused various phenomena to occur.

In one sense, Mars serves as a simplernatural laboratory where we can study andtest various climate theories free from someof the complexities that affect the Earth’satmosphere.

Mars of oldIn the distant past, Mars may have had a

more substantial atmosphere. Evidence forthis comes from two different sources—geo-logical and geochemical. The surface of Marshas erosional features that are similar to thosecreated by rivers on Earth. It also has valleynetworks that look like river drainage systems.If that’s the case, then at some point, Marsmust have been warm enough to supportflowing water on its surface. The most proba-

ble explanation for this evidence of water isthat Mars may have had more atmosphere atone time.

Mars is also dotted with many impactcraters. The older craters appear to be moreheavily eroded than the younger ones. This iswhat we would expect if Mars had a moresubstantial atmosphere long ago than at pre-sent. Alternatively, the great amount of crater“erosion” may have actually resulted from“resurfacing” by lava flows and dust deposits,material that was ejected from later impacts.The jury is still out.

The geochemical evidence for an ancientatmosphere comes from isotope ratios in thepresent Martian atmosphere. You probablyknow that isotopes are atoms of the same ele-ment that differ in the number of neutrons. Anatom with an extra neutron is a bit heavier.Mars is less massive than Earth. With lessgravity, it is difficult for the planet to hold ontoits atmosphere. If atmospheric gases were toescape into space, atoms and molecules of


Above: Without oceans,Martian weatherpatterns are easier totrack. Here rapidheating and coolingproduce incrediblestorms!

Left: Mars GlobalSurveyor captured thisimage of gullies in aMars crater at 42.4degrees S, 158.2degrees W. Patches offrost are visible on thecrater wall

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Wrapped in a dense layer of atmosphere, Venus gives newmeaning to the term “greenhouse effect”. NASA’s 1990Magellan mission used radar to penetrate the clouds and mapthese surface features. Colors used in this image and in theMars image at the right are used to enhance the features; theydo not represent the colors you would see using an ordinarylight telescope.


CO2 (%) 96 0.036 95

N2 (%) 3.5 78 2.7

H2O (%) 0.01 ~1 0.03

O2 (%) Negligible 20.9 0.13

Ar (%) 0.007 0.93 1.6

Surface temperature (°C) 464 15 –63

Atmospheric pressure (atm) 90 1 0.01

Wind speed range (km/hr) 1.1–3.6 0–360 7.2–108

Total mass (1024 kg) 4.87 5.97 0.64

Equatorial radius (km) 6052 6378 3397

Surface gravity (m/s2) 8.9 9.8 3.7

Source: NASA Planetary Fact Sheets at http://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html

Venus Earth Mars

the lighter isotopes would tend to escape a bitmore readily. This would mean that theremaining gases would contain a higher ratioof the heavier isotopes than is found on Earth.Such is the case for Mars. Isotopic ratios indi-cate that perhaps 90% of the Martian atmos-phere has escaped.

There is still much to learn about theMartian atmosphere. In fact, as you are read-ing this, spacecraft are on their way to theRed Planet. The European Space Agency’sfirst mission, Mars Express, is scheduled toenter a Martian orbit in December 2003. Partof the mission is to search the planet forwater and probe the atmosphere for thepresence of methane (CH4). Becausemethane is destroyed by solar ultravioletradiation and reaction with hydroxyl (OH)groups, the atmospheric lifetime of methaneis only about 300 years. The presence ofmethane in the atmosphere would supportthe possibility that life exists there now.

NASA also has two rovers scheduled toland on the surface in early and late January2004. These two 400-lb. mobile laboratorieswill land at very different locations to searchfor water and study the geological record.They can travel several meters across theMartian surface each day. These laboratoriesare expected to explore the surface of Marsuntil April 2004, or perhaps even longer.

Frank Cardulla is special editions editor ofChemMatters.

The air we breathe is mostly nitrogen (78%), oxygen (21%), some argon (<1%), andsmall but critical amounts of water vapor and carbon dioxide. But it wasn’t always

that way. The composition of Earth’s atmosphere has changed dramatically over the past5 billion years of its existence. But exactly how, when, and why remains a subject ofdebate and research.

Over the past 4 billion years or so, theamount of carbon dioxide appears to havedropped dramatically, while the amount of oxy-gen increased greatly. Indirect evidence for thechange comes from what we know about stellarevolution. The Sun was about 30% dimmer 4billion years ago. If the Earth’s atmosphere hadthe same amount of CO2 that it does today, theEarth should have frozen over. It didn’t, somaybe CO2 levels were higher.

Of course, other greenhouse gases suchas methane (CH4) or ammonia (NH3) couldalso have provided this warming effect. Onecurrent theory argues that methane-producingancient bacteria provided a continuous supply

of methane to Earth’s early atmosphere. Methane could have produced a significantgreenhouse effect given that the greenhouse effect of methane is 23 times greater thanthat of carbon dioxide.

How did oxygen become so abundant? One theory credits an increase in photosyn-thesis—as plant life developed, carbon dioxide was consumed and oxygen generated.Another theory argues that early bacteria were capable of separating water into hydrogenand oxygen. The hydrogen escaped to space, whereas the oxygen was left behind. Who’sright? We aren’t sure. Perhaps neither theory is correct. But trying to find the answer ispart of what makes science fun.

How do Venusand Mars

compare toEarth?

The evolution of Earth’s atmosphere

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Clouds may blanket hundreds ofsquare kilometers of marine sky orbillow tens of thousands of feet above

the earth. Despite these vast proportions,cloud formation actually depends on the pres-ence of microscopic airborne particles.

In order to understand clouds, we needto think about the properties of one of earth’smost abundant compounds: water. Within ouratmosphere, water is the only substance thatexists naturally as a gas, liquid, and solid.When it changes from water vapor—thegaseous form—to make liquid water dropletsor solid ice crystals, the water molecules relyon help from tiny suspended particles, whichserve as condensation nuclei.

On the surfaces of these tiny suspendedparticles, H2O meets H2O. As molecules con-tinue to gather, weak hydrogen bonds formbetween them. The result of all this gatheringand ordering is liquid water. Or if it’s reallycold, solid ice crystals or snowflakes form.

The temperature at which water vaporturns to droplets or crystals depends on howmuch water vapor there is in the air. For agiven amount of saturation, water condensesat a temperature called the dew point. Some-times, however, even though the dew pointhas been reached, nucleation particles may belacking to initiate the phase change. As aresult, nothing visible happens.

If more and more water vapor enters theair, for example by evaporation from a moun-tain lake, the air may become saturated. Asthe air cools, it may reach supersaturation, anoverloaded state. Then, provide the water-

laden air with minute particles, perhaps theexhaust from a passing vehicle and—PRESTO!—a cloud appears.

What types of particles contribute tocloud formation and exactly how minute arethey? Sea salts are a major source of nuclei.Because of their water-attracting or hygro-scopic quality, they can induce precipitation attemperatures above the dew point. Smoke,exhaust, soil, and even meteoritic dust con-tribute to cloud formation. Most of these parti-cles have diameters less than onemicrometer—that’s about 0.0001 cm or0.00004 inches.

Meteorologists know that increasing icecrystal formation within supercooled cloudsresults in increased precipitation. Weathermodification companies apply a synthetic ver-sion of this phenomenon by seeding cloudswith dry ice (solid carbon dioxide) or silveriodide (AgI) crystals. For relieving droughtconditions, the process has met with only lim-ited success; however, it has proved useful indissipating cold fogs that might otherwiseshut down airports.

When you’re lying back in the grass on abalmy day, clouds passing by might resembleponies or even monsters—“cloud illusions”,as the song says. However, it’s helpful toknow the scientific names too, since eachtype of cloud has its own interesting charac-teristics.

While scientists differ on exactly howclouds should be categorized, here’s a classi-fication system that is often used.

Blanket-like clouds that form thick layersare called stratus clouds; when they occur atground level, we call them fog. As rain clouds,the prefix “nimbo” meaning “precipitating” isadded, resulting in the term “nimbostratus.”

With four instruments on board, Aura will orbit the Earth from pole

to pole, gathering ozone data 24 hours a day.

By Anne M. Rosenthal

Finding your way out of a cave can be hard enough.

Imagine if a dense “pea-soup” fog suddenly appeared! That’s what

happened to the famous cloud scientist Vincent J. Schaefer as he

was enjoying the refreshing waters of a cave pool.

In order to conserve flashlight batteries, his companion decided

to light a lamp. But the air above the lake was supersaturated with

water vapor! As soon as the match was struck, the water vapor con-

densed into a thick fog. Smoke particles from the match flame

served as nuclei on which the fog droplets formed. Schaefer is given

credit for inventing cloud seeding, a method for coaxing more rain or

snow from clouds by dusting them with tiny particles.

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Associated with dreary days of continuousrainfall or snowfall, nimbostratus cloudstend to be dark, low storm clouds.

Cumulus clouds, also known as heapor lump clouds, may be small fair-weatherpuffs dotting the sky as far as the horizon.They can also become the immense black-ened cumulonimbus clouds jutting intothe upper reaches of the troposphere, thelayer of Earth’s atmosphere closest to thesurface. The bases of these giant stormclouds, usually at 4000-5000 feet, definethe point where the air is cold enough forwater to condense; the tops of theseclouds touch the stratosphere, wherewinds may further sculpt their crests intocharacteristic anvil shapes.

Within these cumulonimbus giants lienumerous convection cells that feed on theheat produced when water vapor condenses.Think for a moment on how “hot-and-sweaty”after a workout quickly becomes “cold-and-clammy” as your sweat evaporates into watervapor. The opposite process occurs whenwater vapor changes to droplets or crystals:Heat is produced. Since hot air rises, pocketsof air where condensation is occurring rapidlymove upward within a cloud.

These updrafts are the beginning of achain reaction. As the air pocket rises, it

expands in the thinner air and cools.This cooling triggers further

condensation, which inturn heats the pocket.

Heating ensures thepocket’s continuedbuoyancy. As itcontinues its climb

skyward, the pocketcools, more water

condenses, and thecloud builds further.

Storm clouds andlightning bolts

Lightning sizzles throughout these largeclouds as static electricity produced by theconstant movement of materials within the

cloud discharges. When a large negativecharge collects on the underside of the thun-dercloud, it induces an opposite chargeknown as an “electric shadow” on the groundbelow. As negative charges move downwardfrom the cloud on a zigzagging path or“stepped leader”, positive charges begin totravel upward from the earth’s surface, oftenmaking use of a conductor such as a tree or ametal antenna—or even a person. Lightningflickers as opposite charges meet, and elec-tricity travels along the stepped leader. So hotis a lightning bolt that it heats the surroundingair to temperatures greater than those at thesun’s surface. Thunder isthe shock wave producedas the air quickly expandsfrom the heat and thencontracts once the light-ning has passed. On asingle day, the thunder-storm-prone state of Col-orado may receive10,000–15,000 lightningstrikes!

Besides lightning,thunderclouds often bearnumerous other hazards.Ice, repeatedly coated withwater and tossed to freezing heights withinthe cloud, drops as hail when it becomes tooheavy for upward drafts to bear. Fierce down-bursts can produce winds blowing over 100miles per hour when cooler air drops sud-denly from the storm cloud toward theground. In the United States, about 1% ofsevere storms—those producing winds atleast 58 miles/hour, hail at least 3/4 inch indiameter, or heavy rainfall—spawn torna-does, rapidly spinning columns of air thatbecome visible when they carry dirt anddebris.

Besides the nimbostratus andnimbocumulus clouds, another pre-cipitating cloud is the icy web or cir-rus cloud. These feathery clouds,generally composed of ice crystals,form at altitudes over 23,000 feet.Their precipitating tails are called vir-gas, which means that the precipita-tion never reaches the ground, butinstead evaporates, changing backinto water vapor.

Clouds may take particularlyinteresting shapes. Some of the fran-

tic reports about aliens from outerspace about to land spaceships on

earth may well be related to sightings oflenticular clouds—clouds typically occurringleeward of mountain ranges when fast-mov-ing winds tumbling over the peaks developisolated air pockets. As the pockets moveupward and cool off, “flying saucer” shapescan condense out.

Contrails—Streaks in the sky

In today’s world, not all clouds resultfrom natural causes. A significant contributorof clouds are jet airplanes, which leave linear

clouds called condensation trails, or “con-trails”, in their wake. Contrails form whenexhaust from jets flying above 30,000 feetcools rapidly in the subzero reaches of theupper troposphere and lower stratosphere.Water vapor and liquid water droplets withinthe jet exhaust precipitate almost instantly intoice crystals. Additionally, the tiny particlespresent in the exhaust seed clouds from watervapor in the surrounding atmosphere.

Although contrails often fade quicklyfrom the sky, the opposite is also true: Somecontrails have staying power. The narrow


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m both sides now,

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w clouds, at all.

s I recall,

Joni Mitchell (1969)

Nimbostratus clouds mean dreary days.


linear cloud tracing the airliner’s path may tearinto wispy cirrus or widen to a sheet of cirro-stratus clouds. Observing contrails helps us tounderstand a portion of our atmospherewhere it is difficult to use weather instru-ments. Contrails are more likely to form andlinger in air already saturated with watervapor.

An important aspect of clouds is theirreflectivity of sunlight. Like cirrus clouds, con-trails block rays that would otherwise warmthe earth’s surface. On the other hand, theytrap radiant energy emitted by the ground,retaining heat in the atmosphere. Scientistsaren’t sure exactly how the equation addsup—whether contrails help cool or heat theearth’s atmosphere.

A research opportunity arrived in Sep-tember 2001 when commercial jets were tem-porarily grounded in the United Statesfollowing the terrorist attacks. Atmosphericscientist David J. Travis at the University ofWisconsin-Whitewater reported in the journalNature (August 8, 2002) that the average dif-ference between daytime and nighttime tem-peratures during the three-day period whenjets and their contrails were absent was oneCelsius degree larger than normal. Thisoccurred even though ranges for the three-dayperiods immediately preceding and followingthe hiatus were smaller than normal. The databolstered the view that contrails may beaffecting the earth’s climate.

Cloud researchTo help answer questions about cloud

reflectivity and the trapping of infrared heat,NASA has launched two instruments called

CERES (Cloud and the Earth’s Radiant EnergySystem). The first instrument orbiting theearth on NASA’s EOS Terra satellite consistsof three telescopes. The first telescope mea-sures how much solar radiation is reflected,while the other two are sensitive to longer-wavelength infrared radiation.

The second instrument launched onNASA’s EOS Aqua satellite carries similar tele-scopes. Because Terra flies over the equatorat about 10:30 a.m. and Aqua flies over at1:30 p.m., tropical clouds are observed at twodifferent times of day.The separated timesprovide an opportunity for scientists toobserve how clouds build.

Then at night, Terra and Aqua cross theequator at 10:30 p.m. and 1:30 a.m. respec-tively. In the darkness, they continue to mea-sure infrared radiation.Taken together, the twoinstruments provide better coverage of theplanet than a single instrument and make iteasier to study variations in earth’s energybalance between day and night.

Another important area of NASA cloudresearch is related to ozone depletion or“ozone holes” over the two polar areas, whichhas left high-latitude populations especiallyvulnerable to increases in UV radiation andskin cancer. It is the ozone layer in thestratosphere that absorbs much of the dam-aging ultraviolet light and makes life on earthpossible.

The primary culprits in ozone destruc-tion are CFCs, or chlorofluorocarbons. Thesehuman-made chemicals were produced formany years as effective spray-can propel-lants, refrigerants, solvents, and blowingagents for plastic foams. CFCs were initially

promoted because they were stable and,therefore, safe for use at the ground level.However, it was this very stability that madethem dangerous over the long term. Theirpersistence made it possible for CFCs to riseinto the stratosphere. There, solar ultravioletradiation splits the CFC molecules, releasingozone-destroying chlorine. Mounting evi-dence of the role of CFCs in destroying ozoneled to unprecedented international coopera-tion to phase out the use and production ofthe chemicals by industrial nations in the1987 Montreal Protocol and its ammend-ments.

But why is ozone most likely to bedepleted over the Earth’s poles? And why do“holes” appear and disappear? The answersto both questions may be in the clouds.

The complete story of ozone depletioninvolves a type of cloud called a PSC, or polarstratospheric cloud. Normally, the air over thepoles is so dry that clouds don’t form. Butduring the polar winters, temperaturesbecome extremely low and ice crystal cloudsform from the minute amounts of water vaporpresent in the stratosphere. The surfaces ofthese ice crystals are sites for chemical reac-tions that produce free radicals. Chemistsdefine these as atoms or molecules that con-tain a single unpaired electron—a feature thatcauses them to be extremely reactive. Unfor-tunately, some of these reactions result in thedestruction of ozone.

The seasonal appearance of ozone holesat the South Pole is further explained by longperiods of light and darkness. Since ozonedestruction is dependent on UV radiation, itdoesn’t occur until daylight reappears.

High, feathery cirrus clouds reflect heat back to the surface.


Land masses tend to deflect and divertwinds into vertical north-south pathways.With little land-mass interference in theSouthern Hemisphere, air circulates in astrong circumpolar or "horizontal" fashion. Atthe South Pole, the strong circulation createsa vortex, a whirlpool of air which preventswarm northerly air from reaching the pole.As a result, the stratosphere over the SouthPole becomes very cold—cold enough toallow the formation of PSCs which acceleratethe catalytic destruction of ozone. When thevortex weakens in the late spring, the ozonedepleted air disperses in the atmosphere.

Ozone depletion in the Southern Hemi-sphere was the first to capture scientists’

attention. Then, during the 1999–2000 sea-son, scientists observed record ozone lossesof 70% over the Arctic. NASA’s Arctic cam-paign, involving over 350 scientists fromaround the world, studied the problem during

the winter of 2003 with acombination of satelliteinstruments, measurementsfrom aircraft, remote sens-ing, and research balloons,as well as ground-basedinstrumentation.

What interests atmos-pheric scientists the most iswhether we’re making anyprogress with our efforts tosave Earth’s fragile ozonelayer. Is the Montreal Proto-col having any effect? Or arethere already so manyozone-destroying chemicalson the loose that the risk isspreading? Finding theseanswers may be the focus ofmany NASA missions tocome. NASA’s EOS Auramission scheduled to launch

in early 2004 will gather the most accurateinformation on chemistry and dynamics todate. With four instruments on board, Aurawill orbit the Earth from pole to pole gather-ing ozone data 24 hours a day.

PHOTODISC

This map shows the August 2003 SouthernHemisphere total ozone from the SolarBackscattering UltraViolet (SBUV/2)instrument on board the NOAA polar orbitingsatellite. In austral spring the analysis showsthe "ozone hole" (values below 220 DobsonUnits) over Antarctica and the AntarcticOcean. This area of low ozone is confined bythe polar vortex. Usually circular in Augustand September, the vortex tends to elongatein October, stretching toward inhabited areasof South America. By November, the polarvortex begins to weaken and ozone-rich airbegins to mix with the air in the "ozone hole"region. The "ozone hole" is usually gone bylate November/early December.The SBUV/2 instrument cannot makeobservations in the polar night regionbecause it relies upon backscatteredsunlight. The blackened area centered overthe pole represents the latitudes in which noobservations can be made.

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Anne M. Rosenthal is a science writer from the San Francisco Bay Area. Her most recent ChemMattersarticle, “Nanotechnology—The World of the Super Small”, appeared in the December 2002 issue.

REFERENCES Anthes, R. Meteorology, 7th edition; Prentice Hall: Upper Saddle River, NJ, 1997.Atmospheric Chemistry and Global Change; G. Brasseur, J. Orlando, and G. Tyndal, Eds.;

Oxford University Press: New York, 1999.Perkins, S. September’s science: Shutdown of airlines aided contrail studies. Science News,

May 11, 2002, p 291.“Cloudman” (John Day) website, www.cloudman.com (accessed July 2003).NASA’s Earth Observatory Reference Library, http://earthobservatory.nasa.gov/Library

(accessed July 2003).


Activity

W e’ve all done it: lainback on a grassy hill-side staring up at a

multitude of puffy white clouds—one looking like an elephant, thenext like Abraham Lincoln. Buthow often do we stop and con-sider why clouds form in the firstplace?

We know that clouds com-prise small suspended dropletsof water and that they have agreat influence on weather pat-terns. But what causes theirappearance and subsequent dis-appearance in the sky overhead?The following activity will enableyou to make your own clouds ina plastic bottle and then toexplore some of the factorsresponsible for their formation.

You will need:■ One empty 2-L soda

bottle, preferably color-less, rinsed out, andallowed to dry

■ 50 mL of room-temperaturetap water

■ One match■ One dark-colored backdrop

such as a black tabletop ornotebook cover

■ Safety goggles

What to do:1. Remove the label from the

bottle to ensure an unob-structed view. Screw the capon to the bottle securely.Then, using a dark backdropto provide greater contrast,squeeze the bottle andrelease three to five timesnear the bottom as youobserve the air space in theupper portion of the bottle.Since nothing has beenadded to the bottle, this canserve as a control for futureobservations.

2. Now remove the cap, andpour in 50 mL of water.Screw the cap back on

securely and swirl the wateraround inside the bottle for10–15 seconds. This shouldensure that the air inside thebottle is well saturated withwater vapor. Or, in otherwords, that the inside humid-ity is 100%. Now, repeat thesqueezing technique used instep 1. What do you observeinside the bottle after therepeated squeezing?

3. Remove the cap again. Inone hand, hold the bottlesideways with a slight upwardtilt. In the other hand, take alit wooden match and insert itpartially into the bottle.Immediately give the bottle aquick squeeze to extinguishthe match. You should see asmall amount of smoke fromthe match trapped in the bot-tle. Withdraw the match,screw the cap back on, andset the bottle upright. Repeat

the squeezing technique usedin step 1. What do youobserve inside the bottlewhen you squeeze? Whenyou release the squeeze?

What’s going on?Although you couldn’t tell

from looking at it, there were twoimportant changes occurringwhen the air-filled bottle wassqueezed in the first trial: (1) asubstantial increase in pressure,which should be obvious, becauseyou were decreasing the volumeof the bottle by squeezing it, and(2) a slight increase in tempera-ture, although you probably didn’tobserve this change. This happenswhenever a gas is compressed inthis fashion.

When the squeeze isreleased, the gas molecules sud-denly occupy a larger volume.Again you probably didn’t notice it,but there was a slight decrease in

By Bob Becker

“Nature is a mutable cloud,which is always and never

the same.”Ralph Waldo Emerson (1803–1882)

Safety:Use standard precau-

tions for any use of openflames. Strike match onsafety strip; be sure the areais free of flammable material;wear safety goggles in thelaboratory; be sure to havefire extinguishing equipmenthandy.

Assemble materials.


temperature. This type of coolingis quite noticeable whenever youlet the air out of a pressurized tire.It cools off quite substantially, andthe valve stem can become quitecold. When you increase the pres-sure in a tire, the exact oppositeoccurs.

During the second trial, youintroduced water vapor. As youmight imagine, the amount ofmoisture that air can hold greatlydepends on the temperature; thehigher the temperature, the morewater evaporates. 100% humidityat 32 °C ( 90 ºF) translates intotwice as much moisture content inthe air as 100% humidity at 20 °C(68 °F). Thus, when the bottle wassqueezed and the temperatureincreased slightly, so did themoisture content of the air as thewater at the bottom and on thesides of the bottle evaporated a bitmore. When the squeeze wasreleased and the temperature

dropped slightly, that extra vaporhad to condense back into a liquid.

Water condenses best whenit has a place to condense. Theonly surfaces available were thesides of the bottle and the waterlayer at the bottom. The totalamount of moisture condensingwould have been just a fraction ofa drop, so this evaporating andcondensing went pretty muchunnoticed.

Then in the third trial, youintroduced smoke into the bottle:not much—probably only a fewmillionths of a gram—but enoughto create microscopic condensa-tion sites throughout the bottle.This time when the squeeze wasreleased and the temperaturedropped, the water could con-dense onto the smoke particlesand form miniscule water dropletssuspended throughout the bottle.In other words, it formed a cloud.When the bottle was resqueezed

and the temperature went backup, these droplets evaporated, butthe smoke particles were stillthere, and so the whole processcould be repeated. Eventually, thesmoke precipitates out—onto thesides of the bottle or into the liq-uid layer below—and the cloudeffect wears off.

In our atmosphere, clouds

can form whenever warm mois-ture-rich air comes in contact withcooler air. There, the tiny dis-persed solid particles—referred toas aerosols—act as nucleationsites for cloud formation.Although they are quite small,these particles can have hugeeffects on global climates andweather patterns.

in a Bottle

Further investigationThe water you used in this activity was at room tempera-

ture. Experiment with water at a variety of temperatures andsee what effect it has on the cloud formation. Also try othersources of condensation sites such as smoke from a candle,chalk dust, talcum powder, etc. Does the size and type of par-ticle make a difference in cloud formation?

Using a slide projector or strong flashlight, shine somebright light through the bottle. Have the room as dark as pos-sible and view the bottle from various angles. The scatteringand diffraction may cause different colors to emerge, andthese colors can change over time as the clouds in the bottlestart to thin out.

Try making a cloud.First, use dry air,then humid air.

Next, light a match ... ... and insert it into thebottle. Squeeze the bottleto extinguish the matchand capture the smoke.Now try making a cloud.

Reprints of this activity may be purchased by calling 1–800–635–7181, ext. 8158.

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Y ou’ve probably heard about the greenhouse effect andhow it has something to do with global warming. Anddespite your thoughts last January, all of that sounds like

bad news. The greenhouse effect on the planetary scale is actually a

good thing—a very good thing. Scientists estimate that withoutsome surrounding greenhouse gases, the Earth’s average temper-ature would be a freezing –18 °C (–0.4 °F). Tucked inside oursecurity blanket of heat-absorbing gas molecules, we enjoy anaverage global temperature of 15 °C (59 °F), and it’s rising!

“Greenhouse” is an interesting term for our planetary arrange-ment. If you’ve ever wandered through one of those glassed-ingreenhouses on a bright sunny day, you were probably happywhen the tour was over. Plants appeared to be thriving, but the airtemperatures you felt were well above your comfort zone.

The explanation behind a greenhouse effect is actually prettyclear—transparent, in fact. Greenhouse gases allow light to enter,but are far less transparent to the lower-frequency light reflectedback toward the atmosphere by objects warmed by the sun.

Blame it on thegreenhouse

effect.

Aura will quantify and map the variability of upper tropo-spheric ozone, water vapor,and aerosols to help under-

stand climate change.

Hot out?Hot out?


Light is described as waves of radiant energy with various frequen-cies and wavelengths. The most abundant molecules in the atmosphere,nitrogen (N2), oxygen (O2), and argon (Ar), compose 99% of the atmos-phere and offer only minimal obstacles to the passage of radiant energy.As far as those molecules are concerned, radiant energy can passthrough in either direction.

When radiant energy strikes the earth, much of it is absorbed andthe surface gets hotter as a result. Warm objects emit radiant energywith a set of wavelengths that are collectively called infrared (IR).

For certain molecules in the atmosphere, the frequency of the radi-ant energy they encounter makes an enormous difference. For these so-called greenhouse gases, some frequencies in the IR region of thespectrum are absorbed temporarily before being re-emitted, often in adirection that sends the IR right back where it came from—the warmsurface of the earth.

What determines whether an atmospheric gas is an IR energyabsorber? You might be able to come up with a hypothesis yourself ifyou take a close look at the formulas for these non-greenhouse gases—N2, O2, and Ar; and then a look at the formulas for a few greenhousegases—CO2, O3, H2O, and CH4. Notice something interesting? Hold thatthought!

The fact that molecules are in motion is nothing new to you. That’swhat explains liquid rising in a thermometer, the smell of fresh breadcoming out of the oven, and steam rising from a kettle. But what youmight not know is that individual molecules move in another way. Theystretch and bend with a kind of vibration unique for each molecule type.

All molecules vibrate, greenhouse and non-greenhouse gas mole-cules included. Likewise, all molecules are made up of atoms with posi-tive charges centered in the nuclei. When atoms bond together, theircollective electrons form a negatively charged cloud surrounding thewhole molecule.

When a molecule consists of only two atoms, the only way it canvibrate is for the bond connecting those two atoms to expand and con-tract. If the two atoms are the same, as in N2 or O2, then this symmetri-cal stretching motion leaves the positive and negative charges evenlydistributed. Isolated atoms, like Ar, cannot vibrate at all.

But for molecules with more than two atoms, there are lots of pos-sible ways that the molecule can stretch, bend, or wiggle. Althoughsome of these vibrations may not distort the charges, there are alwayssome that do. In these, the electron cloud first concentrates more nega-tive charge in one direction before swinging the negative charge inanother direction—and then back again.

What does this have to do with energy capture? The shiftingcharges for each of these mixed molecules occurs at a certain fre-quency. If the frequency happens to match that of radiant energy in theregion, the molecule, with its charges already oscillating at that fre-quency, absorbs that energy in much the same way that someone push-ing a swing with just the right frequency adds to the motion of thatobject.

The earth’s warm surface emits the right frequencies of IR for ourgreenhouse gas molecules. Although they differ slightly in their prefer-ences, these molecules absorb and re-emit IR energy as they stretchand bend.

So, if the gases re-emit the energy, why is there a net warmingeffect at the earth’s surface? Think about a game of ice hockey. Thepuck skims off in one direction only to be struck with equal force byanother player. It may maintain its movement at the same speed, but thedirection has changed.

The energy that the greenhouse gas re-emits has a good chance ofbeing directed back down to the earth’s surface, or in any other randomdirection for that matter.

All but about 30% of the solar energy striking our planet gets through theatmosphere to the surface. The other 30% is either reflected back into spaceby clouds, or in the case of ultraviolet light, absorbed by our fragile layer ofatmospheric ozone. Even at the surface, some light is reflected, but much ofthe energy is absorbed and later radiated as heat.

© COOPERATIVE PROGRAM FOR OPERATIONAL METEOROLOGY/NCAR/UCAR/NWS HTTP://METED.UCAR.EDU


Greenhouse gasesAny molecule that vibrates as it absorbs IR is a potential green-

house gas. But that’s where the similarities end and the differencesbegin. Greenhouse gases vary widely in their effectiveness at absorbingIR. Some excellent absorbers are, fortunately, not very abundant in theatmosphere. But their presence bears some careful watching since a lit-tle goes a long way toward retaining heat at the earth’s surface.

The table above compares both the effectiveness and the relativeabundance of some well-known greenhouse gases in the earth’s tropo-sphere—the lowest atmospheric layer in which we live and breathe. Forthe sake of comparison, we’ll assign a “1” to the effectiveness of carbondioxide (CO2). Then, we’ll assign a “1” to the abundance of water (H2O),since it is the greenhouse gas that makes up nearly one percent of thetropospheric mix.

Now, let’s take a look at each of these greenhouse gases, realizingas we do so that they act together to form a climate-warming effect asthey interact with the earth’s systems.

WaterWe mentioned water making up about 1% of the troposphere, but

we didn’t mention that it is unevenly distributed around our planet—more concentrated over warm bodies of water and equatorial forests,less over the poles and stretches of deserts. Although gaseous water isan effective IR absorber, its total presence in the atmosphere gives us amixed bag of effects. Water droplets in clouds can actually work in twoways. Depending on the location and the type of cloud, water in lower-altitude clouds is good at reflecting incoming light of all wavelengthsback into space, thus shielding the earth. The opposite is true of higherclouds. Their net effect is to trap outgoing IR radiation on its way out ofthe atmosphere.

Carbon dioxideCarbon dioxide may not be the most effective greenhouse gas on

the chart, but its collective abundance in the atmosphere results in thecapture and retention of nearly half of the outgoing energy in the peakIR wavelength region of the spectrum. Carbon dioxide does not typicallyreact with other molecules in the atmosphere. As a result, it forms a sta-ble gaseous mantle, its concentration tapering off gradually withincreased altitude.

Like water, carbon dioxide is intimately involved with all living andformerly living matter on the surface of the planet. In preindustrial eras,atmospheric CO2 mainly cycled in and out of this biosphere, as plantstook in CO2 to make complex carbon compounds and all living thingsreturned the gas as the organic carbon molecules were consumed. Addforest fires and the occasional volcano eruption to the picture, and youhave the historical outlines of the earth’s carbon cycle.

Not all carbon dioxide returned directly to the atmosphere. Overmany eons of earth’s history, deposits of plant and animal remains set-tled in bogs and other areas where decay organisms failed to thrive. Asmillions of years passed, these remains formed vast energy-abundantdeposits. Today, we’re completing that interrupted carbon cycling byburning these deposits of oil, natural gas, and coal as fossil fuels. Indoing so, we rapidly reload the atmosphere with enormous quantities ofcarbon. In fact, current estimates show that industrial countries releaseone metric ton (1000 kg or 2200 lb.) of carbon per person, per year as aresult of fossil fuel consumption. In developing countries, the releaserate is about one-tenth as large, but it is growing.

Searching for the sinksTo learn more about the rates at which the earth releases and

sequesters (stores) its carbon, scientists are actively measuring the car-bon dioxide levels at numerous global sites. Measuring concentrationsduring various growing seasons and at various temperatures, lightintensities, altitudes, and humidities, they are beginning to uncoversome interesting riddles.The earth as a whole isactually sequesteringmore carbon thanexpected.

The search for nat-ural carbon sinks is on.A carbon sink is a loca-tion at which the neteffect is in favor ofremoving more carbonfrom the atmospherethan is being released.Although tropical rainforests are known toabsorb enormousamounts of CO2 as theabundant plants carryout photosynthesis,they are not proving to bethe carbon sinks that sci-entists once predicted.

Rain forest scientists Deborah and David Clark and their researchteam recently reported some unexpected findings in the April 25, 2003,online version of the Proceedings of the National Academies of Science(PNAS). They shared data showing that when equatorial temperaturessurpass a certain mean, tree growth and CO2 intake actually slow down.

If tropical forests are not the important sinks once thought, wheremight others be located? Currently, northern forests called borealforests and colder areas of the ocean are under study for their contribu-tions—now thought to be significant.

Greenhouse gas Relative Abundance ineffectiveness troposphere (%)

Carbon dioxide (CO2) 1 (assigned value) 3.6 × 10–2

Methane (CH4) 30 1.7 × 10–3

Nitrous oxide (N2O) 160 3 × 10–4

Water (H2O) 0.1 1Ozone (O3) 2000 4 × 10–6

Trichlorofluoromethane 21,000 2.8 × 10–8(CCl3F)

Dichlorodifluoromethane (CCl2F2)

25,000 4.8 × 10–8

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SOURCE: CHEMISTRY IN CONTEXT: APPLYING CHEMISTRY TO SOCIETY, AMERICAN CHEMICAL SOCIETY © 2000

Tropical forests are CO2 absorbers. But are they “sinks”?

Can we make our own sinks? A more radical line of research inves-tigates ways to use technology for sequestering carbon dioxide. Forthe past 30 years, oil companies have been injecting pressurized CO2

into wells in order to enhance their pumping capacities. Although muchof this CO2 comes from carbon dioxide-filled pockets that are alreadyunderground, the technology might be applied toward devising strate-gies for draining off some excess carbon from the atmosphere.

The United States Department of Energy is conducting geologicalsurveys for locating rock formations with underlying briny waterdeposits into which CO2 might be injected. But extreme care must betaken to find stable sites for this potential use. An abrupt release of CO2

like the one from volcanic Lake Nyos in the African country ofCameroon in August of 1986 can have deadly consequences. The LakeNyos disaster released 1 billion cubic meters of carbon dioxide in oneblast, silently killing 1724 people and countless cattle and other animallife over a 24-hour period. (See “The Lake Nyos Disaster” in the Febru-ary 1996 issue of ChemMatters.)

Other greenhouse gasesCarbon dioxide and water are literally facts of life on earth. Their

cycling in and out of the atmosphere is only partially under humancontrol. The remaining greenhouse gases appearing in the table maybe on the increase, but at least they have this going for them: Becausewe know how they got there, we can probably do something to con-trol their rate of increase in the troposphere.

Methane, currently present in the atmosphere at 1.7 ppm,has increased to more than twice its level of preindustrial times. Apart of the natural “exhaust” from the digestive systems of animals,additional methane is generated by our modern human activities.Petroleum refining releases vast quantities; decaying organic matterin garbage dumps, and large herds of grazing animals are all sourcesover which we have some control.

Ozone, a molecule known for its dual reputation in the atmos-phere is also an effective greenhouse gas. Most of the news aboutozone is discouraging. In the stratosphere where it is needed forabsorbing incoming UV light, it is decreasing. At the same time, it isincreasing at ground level as a result of the complex chemical interac-

tions of transportation exhaust chemicals on hot sunny days. Its pres-ence in the resulting photochemical smog is blamed for various healthproblems and material damages.

As if that isn’t bad enough, ozone’s effectiveness as a greenhousegas, particularly noticed in the higher troposphere, is 2000 times greaterthan CO2. Scientists hope that with the widespread use of alternativefuels and better transportation options, tropospheric ozone can be con-trolled. In fact, many of these measures are already stemming the crisis.Modest attempts to reduce ozone pollution in the United States haveresulted in small reductions over the past few decades despite the hugeincrease in total vehicle miles driven.

The two chlorofluorocarbons (CFCs) listed in the tableare extremely effective greenhouse gases and are clearly of human ori-gin. Unfortunately, once CFCs are released into the atmosphere, theystay there for a long time. Add that to their IR-absorbing capacities, andyou have a dangerous greenhouse gas, even at low concentrations.Today, the net effect of CFCs on global warming is small. Their collec-tive greenhouse gas effect is partially balanced by their infamous

appetite for stratospheric ozone. By reducing ozone’s greenhouse effect,they in turn partially cancel their own contribution to global warming.But their ozone destruction comes at a terrible price. Without Earth’s

thin layer of protective stratospheric ozone, people are at risk forskin cancers and other ailments caused by increased exposure todamaging UV radiation.

Controlling CFCs is already well under way, as the result ofthe Montreal Protocol, which banned their production in devel-oped countries after 1995. Once valued as effective coolants forrefrigeration, CFCs have been replaced by less hazardous alterna-tives. As a result, they pose much less of a threat to global cli-mate than they did 10 years ago.

As our global population continues to increase, the humancontributions to these gases, largely from the burning of fossilfuels, continues to rise. At this time, scientists are resigned to awarming earth over the next century regardless of how we limitour use of fossil fuels. Public policy attention is shifting in thedirection of coping with all-but-certain climate changes at thesame time that we attempt to slow the rate of warming.

Helen Herlocker is administrative editor of ChemMatters.


Reprints of this article may be purchased by calling 1–800–635–7181 x8158

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Clouds have different effects depending on their altitudes. Near the surface,they reflect light. High above, they reflect heat back to the surface.


Aura will provide global measurementsof ozone and ozone-destroying

radicals from nitrogen, hydrogen, and chlorine compounds.

Our lives are wholly entwined with the sun. We dependutterly on sunlight for light and warmth. Changes in the

sun’s energy output kick off a myriad of important biological changes, from a monarch butterfly’s

migration to the ripening of fruit on an apple tree.

By Jeannie Allen

Our lives are wholly entwined with the sun. We dependutterly on sunlight for light and warmth. Changes in the

sun’s energy output kick off a myriad of important biological changes, from a monarch butterfly’s

migration to the ripening of fruit on an apple tree.

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he awesome power in sunlightstreams down on us at a vari-ety of wavelengths. Differentwavelengths of light carry dif-ferent amounts of energy and

so impact us in different ways. The shorterwavelengths carry more energy, and some areenergetic enough to initiate chemical changesin the air we breathe.

Different wavelengths of sunlight impactus in different ways. For example, in the upperatmosphere, ultraviolet radiation at wave-lengths smaller than 242 nanometers can splitmolecular oxygen (two atoms bondedtogether) into atomic oxygen (individualatoms). Then when some of these energeti-cally excited individual oxygen atomsencounter molecular oxygen, they can bond toform three-oxygen molecules called ozone.

Ozone packs a punch in our lives that’sout of proportion to its low concentrations inthe atmosphere. It acts as hero or villaindepending on where it is. Ozone far above usin the upper atmosphere (stratosphere)absorbs and protects us from deadly ultravio-let radiation; however, in the lower atmos-phere (troposphere), where we live, thetoxicity of ozone makes its presence undesir-able. It is the active ingredient of air pollutioncommonly known as smog. Ozone reacts eas-ily with biological tissue, donating oxygenatoms in the process known as oxidation—not a good thing!

Breathing too much ozone over timeresults in something like a slow burn insideour lungs. Over time, it lowers human lungcapacity and causes illness and, for a few peo-ple, it causes death. Whole forests and fieldsof crops respond to ozone overexposure withreduced productivity.

If having too much ozone around is sobad, why do we put up with it? Unfortunately,one by-product of our dependence on the useof fossil fuels such as coal and oil is the pro-duction of tropospheric ozone. If we wantcleaner air, we have to control our consump-tion of these fuels, either by using them moreefficiently, switching to alternative sources ofenergy, or both.

To control anything, it helps to knowhow it works. Most of the ozone in the tropo-sphere forms with two groups of chemicalcompounds: nitrogen oxides (NOx) andvolatile organic compounds (VOCs). VOCsconsist of carbon-containing gases and thegaseous vapors emitted from volatile materi-als such as gasoline. NOx and VOCs occurboth naturally and as by-products of fossil

fuel combustion. Carbon monoxide also playsa role in some ozone formation reactions.Sunlight must be present for ozone to form,hence the term, photochemical smog.

Nitrogen oxides (NOx). Nitric oxide (NO)and nitrogen dioxide (NO2) are togetherknown as NOx and often pronounced “nox.”Sources of NOx include lightning, chemicalprocesses in soils, forest fires, and the inten-

Left photo: Solar ultraviolet (UV) radiation(invisible to us) drives the chemical reactionsthat produce ozone in both earth’s upperatmosphere (stratosphere) and its loweratmosphere (troposphere).

The amount of ozone in the air around you depends, in part, on where you areon the earth’s surface. Assuming the presence of NOx and VOCs, the moredirect the angle of sunlight, the greater its intensity, so the more ozone tends

to form. Places closer to the equator tend to harbor more ozone in the lower atmos-phere. Another thing to think about is that ozone formation takes place over time, andwinds can carry air parcels far downwind of NOx and VOC sources. If you live in arural area, you may be breathing more ozone than people in some urban areas. It alldepends on what’s going on upwind from you (where your air comes from).

Ozone con-centrations alsovary through time.The highest con-centrations gener-ally occur duringsummer, whensunlight is mostintense. On a 24-hour timescale, asindustrial andmotor vehicle activ-ity rises throughoutthe morning, con-centrations of NOx

and VOCs alsorise. Ozone con-centrations there-fore reach theirmaximum shortlyafter the peak in vehicle traffic, about noon or soon thereafter. But downwind fromurban areas, ozone may peak later in the afternoon or even after dark. After sunset,no sunlight initiates ozone formation, so ozone concentrations fall as ozone reactswith other chemicals and settles onto various surfaces.

Ozone amounts vary in space and time

Measurements of nitrogen dioxide (NO2) [in blue] and ozone (O3) [inred] show rise and fall over a 48-hour period. Because NO2participates in O3 formation, its peak concentration does not coincidewith peak O3 concentrations. Ozone concentration peaks during hoursof maximum sunlight, around the middle of the day.

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tional burning of vegetation to makeway for new crops (biomass burning).NOx emissions also come from smoke-stacks and tailpipes as by-products ofthe combustion of fossil fuels (coal, oil,and natural gas) at high temperatures.Coal-fired power plants are the primarysources of NOx in the United States.Automobiles, diesel trucks and buses,and nonroad engines (farming andconstruction equipment, boats, andtrains) also produce NOx.

Volatile organic compounds(VOCs). Volatile organic compounds,such as hydrocarbons, vaporize easily.Some plants emit VOCs, and somebacterial processes in soils also pro-duce VOCs. The smell of a pine forestcomes from a hydrocarbon calledalpha-pinene. VOCs also come fromgasoline combustion and from the evapo-ration of liquid fuels, solvents, andorganic chemicals, such as those in somepaints, cleaners, barbecue starter, andnail polish remover.

Atmospheric chemists should be givenhero awards for even being willing to tackleozone research, because it is maddeninglycomplex. NOx and VOCs together includeabout 120 different chemical compounds,and hundreds of chemical reactions can takeplace. Some of the participating chemicalsmay be intercepted part of the way throughthe process by reactions with other chemicalsin the atmosphere and may form intermediatecompounds that act as temporary reservoirsfor varying amounts of time. Many of thechemicals involved have very short lifetimesbefore they react with other chemicals toform new compounds.

An additional challenge arising for any-one seeking to track ozone-forming reactionsis that they involve interactions between dif-

ferent phases of matter (gases, liquids, andparticles known as aerosols) and can occuron various kinds of aerosol surfaces in theatmosphere. Changing environmental condi-tions such as air temperature and humidityalso affect ozone chemistry.

Formation of ozone at ground level hassome parallels with stratospheric ozone for-mation. Again, the key step involves photo-chemical formation of oxygen atoms, thistime from NO2. Because the N–O bond in

NO2 is weaker than the O–O bond inO2, longer-wavelength UV light is suffi-ciently energetic to lead to formation ofatomic oxygen, which can then com-bine with molecular oxygen to formozone.

VOCs are not directly involved inozone formation, but they play a crucialrole in regenerating NO2 from NO andoxygen, in a complex series of reac-tions, some of which are catalyzed bythe hydroxyl radical (OH). The details ofthese reactions vary with the exactnature of the VOCs present. The ratio ofNOx to VOCs determines the efficiencyof the ozone formation process.

As if it wasn’t complicated enough,scientists don’t have a good under-standing of the chemical reactions tak-ing place after the sun goes down.Without sunlight initiating ozone forma-tion, ozone levels fall as it reacts withother chemicals and settles onto varioussurfaces. Data from some recent night-time sampling missions over Boston

and Portland suggest that some pollutantsmight significantly decrease nighttime ozonelevels.

"We don't have a good idea about what'sgoing on at night," said one lead researcher."And if we don't understand what goes on atnight, we can't have confidence that we knowthe best way to fight smog during the day."

For scientists willing to tackle ozoneresearch, the problem promises to be mad-deningly complex for years to come.

NO2 + longer-wavelength UV light ➞ NO + O

O + O2 ➞ O3

Jeannie Allen is a writer for NASA Goddard Space Flight Center in Greenbelt, MD. She is a frequent contrib-utor to the Earth Observatory, NASA’s award-winning website at http://earthobservatory.nasa.gov.

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The STS-92 Space Shuttle astronauts photographed upstate NewYork at sunset on October 21, 2000. The view looks toward thesouthwest from southern Canada and captures a regional smoglayer extending across central New York, western Lake Erie andOhio, and further west. Winds bring ozone and some chemicalsthat participate in its formation to rural areas downwind ofemission sources. Ozone itself is invisible.

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Without sunlight initiating ozone formation, nighttime ozone levels fall as it reacts with other compounds and settles on various surfaces.

Models. Now there’s a word that might figure into oneof those personality-type tests. Read “model”, andwhat do you think of? Tall, slim, very fashionable

types “walking the walk” on a runway? The very latest version ofthe Corvette? Small rocket kits you assemble and launch? Theextra-credit project you made in eighth grade?

Let’s add another idea to the model mix: mathematicalmodels created by scientists to describe atmosphericprocesses. Although they seem complex to most of us, scien-tists depend on them to simplify their understanding of atmos-pheric processes. Describing how a great many factors

combine to affect our global climate, models allow scientiststo make predictions.

How good is a model at making predictions? It is only asgood as the information on which it is based. Until recently,models were weakened by the limited means for collecting mea-surements and by a lack of a computing power.

In fact, it wasn’t too long ago that atmospheric chemistrymodels were the scientific equivalent of 98-pound weaklings.The other sciences with more easily obtained laboratory datawould kick sand in their face at the beach—it wasn’t pretty. Butsatellite observations and supercomputers are changing all that.


ATMOS P H E R IC MO D E LS“A model is just a mathematical description

of our physical understanding”—Daniel Jacob, atmospheric chemist at Harvard University

Aura’s measurements will givescientists new data for testing

and refining global atmospheric models.

Mathematical modelsSo, what does a mathematical model look like? You probably rely

on mathematical models without thinking about it. Just imagine what ittakes to estimate the time a long car trip will take. Let’s say your desti-nation is 1000 kilometers away and your average speed will be about100 kilometers per hour. You don’t anticipate traffic. You plan to makethree stops lasting about 20 minutes each. After a little calculation, yourealize the trip will take 11 hours. This is a simple mathematical model.

If you insist that a model has to be something with moving partsthat you construct with materials, you could have built an elaboratesmall-scale physical model—complete with a moving car. But in thiscase, why bother? The effort would have been time consuming, and youhad your information without lifting a paintbrush or a bottle of glue.

Now let’s suppose that instead of estimating the time it takes foryour car trip, you’re wondering about other things like, How warm willthe Earth be in 25 years? What will be the status of the ozone layer in 10years? Will smog increase in my hometown? Is annual rainfall likely toincrease?

Even if you haven’t asked these questions, you are probably gladthat someone is asking them. They’re important! Concerned about theenvironment, you understand the value of recycling and the need to pro-tect the local water supply from potential hazards of a new landfill. Butwhen it comes to adding relatively small amounts of odorless and invisi-ble gases to the atmosphere, it’s hard for us to see the harm.

Models help us make the connections between our behavior todayand long-term global consequences.

A BIG questionChemistry is the study of matter and its transformations. So it

makes sense that the fundamental question atmospheric chemists askis “What is the chemical composition of the atmosphere and what kindsof transformations is it undergoing?”

Ultimately, many climate and pollution issues are reduced down toa question of how the concentrations of certain chemical species varyover time. What is the collective concentration of greenhouse gases?What will their concentration be in 50 years? What is the concentrationof ozone in the stratosphere and on the ground? What is the concentra-

tion of pollutant x?Before constructing an atmospheric model, scien-

tists have to consider factors that will affect the concen-trations of chemical species in the atmosphere. Whatgoes into the air? What comes out of the air? How does itmove through the air? Does it undergo any chemicaltransformations?

Some key factors getting a lot of attention fromatmospheric modelers are these:

Emissions. Every process that adds chemical speciesto the atmosphere is an emission. Boiling water on a stoveand evaporation from the ocean add water to the atmos-phere. Burning fossil fuels for energy and decomposingplant materials add CO2. Notice that some emissions arethe result of human activity. Our ability and willingness tocontrol these anthropogenic (human-made) emissions isthe subject of many public policy debates.

Deposition. This time, we are talking about takingchemicals out of the atmosphere. Examples include uptakeof CO2 by plants during photosynthesis and a variety ofchemical processes, some under human control, that trapand deliver some chemical species—including wateritself—back to land and sea.

Transport. How does a species travel in the system?Like most gaseous mixtures, the atmosphere is not ahomogenous or static system. Chemical species can anddo travel at different speeds. As a result, they areunequally distributed throughout our atmosphere.

www.chemistry.org/education/chemmatters.html26 ChemMatters, OCTOBER 2003

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Chemistry. Think of theatmosphere as a huge reac-tion vessel, a reaction vesselfilled with nitrogen, oxygen,water, carbon dioxide, andhundreds of trace chemicalspecies. Chemical species canreact with each other to formnew products. The products,in turn, can react to create stillmore compounds.

What transformationsare taking place in the atmos-phere? To answer that, you’dneed to know the chemicalspresent and the conditionsunder which they are reacting.Chemists are familiar withthese descriptive models.First, they make a modelbased on what is known. Thenthey compare its predictionswith experimental observa-tions—laboratory results oreven spectral data observedby satellite. If the results donot match the predictions, themodel requires some adjust-ment. The goal of any good model is to improve the agreement of itspredictions with observations.

Some of the simplest models that atmospheric chemists prepareare those that describe and predict the concentration of trace gasesover small regions and short timescales. But even these models canrequire a computer to solve anywhere from a few dozen to a hundredsimultaneous equations that include as many as a hundred chemicalspecies.

Mathematical models quickly become even more complicated asadditional factors are brought into the picture and as the region andtimescale examined are expanded. These systems and the equationsused to describe them are frequently chaotic, meaning that arbitrarilysmall differences in the initial state can lead to arbitrarily large differ-ences in the final state. Perhaps the most complex of these models arethe coupled atmosphere–ocean general circulation models (AOGCMs).AOGCMs are three-dimensional global models that represent thetoughest problem of all—climate. They include equations that take intoaccount the role of oceans, land vegetation, weather, and many otherfactors.

Models that predict future climate changes are arguably the mostcomplex. They are also the ones with the greatest potential value. Pre-dicting the future is extremely important, but it is a much trickier exer-cise than describing the present. For one thing, you don’t have data tocheck your predictions against—they are, after all, in the future. Fur-thermore, any small errors in the model are potentially magnified bythe number of years you project into the future. Inevitable problemsarise when you base your predictions of the future on the data yougather today.

Hungry for dataSo what “beefs up” a model and makes it useful? Harvard

atmospheric chemist Randall Martin puts it this way: “We are VERYhungry for data.” That’s because global measurements, especially inremote areas, have been hard to come by. Even when they’ve beentaken, it is difficult to get a feeling for how the various componentschange over time.

Having plenty of reliable data is also important in reducing both thenumber and type of assumptions made in the models. For example,when modelers use the model to forecast the concentration and locationof chemical x on a particular date, it is helpful to know the actual con-centration of x at the present time. With good data, they minimize errorsby refining the model.

Beefing up! It’s the computers …When making a forecast, modelers test their models by running

simulations on powerful computers. In many ways, these simulationsare similar to video games like SimEarth or WarCraft that let you changeconditions and then run automatically. Whether a village or fighting unit,elements of the game respond based on the mathematical results ofequations. It’s really just a mathematical model built into the program-ming game. Of course, the models behind games are built more forentertainment value than for their descriptive or predictive ability. Oneother thing, if you haven’t noticed it, these games have gotten waycooler in the past 10 years. Games are faster and can include moredetailed elements, in part, because computers have way more memoryand speed.

The first atmospheric modeler?

Better known by chemists and some chemistry students for theArrhenius concept of acids and bases and his work on thebehavior of solutions, Svante August Arrhenius also took aninterest in the role of CO2 in global warming.

A visionary of his time, he realized that our increasing reliance on fos-sil fuels would continue to increase atmospheric CO2 levels. He was alsoaware of the role of CO2 as a greenhouse gas. From this idea, he devel-oped a simple mathematical model that predicted that doubling theamount of CO2 in the atmosphere would raise global temperatures by 5.49 °C. Taking into account the CO2 output trends at the time, heassumed it would take 1000 years to double CO2 levels.

The year was 1896. Would his prediction hold up over time?Over the next 100 years, CO2 levels increased 23%—a much a larger

increase than he had foreseen. One reason: in 1900, there were fewer than 10,000 cars on the roadworldwide and many were electric. Today, there are hundreds of millions of vehicles on the road.

Given this new CO2 concentration, Arrhenius’s model predicts about 1.5 °C increase in global tem-perature. Average temperatures went up by only 0.59 °C. What happened?

Although his calculations were fairly accurate for just CO2, his model was just too simple. For one,it didn’t include complex processes such as increased cloud formation that cool the earth and modifythe effect.

Don’t be too hard on Arrhenius for his “weak” model—if he had been given access to a supercom-puter and today’s satellite and directly measured data, who knows what he might have come up with?

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Until a few years ago, it was both difficult and slow tomake predictions based on complex atmospheric models.Using computers with limited speed, scientists were forcedto oversimplify their models. Using the speed available ontoday’s supercomputers, climate and atmospheric chem-istry models are dramatically better at delivering solutionsto complicated problems.

Some of the supercomputers now running modelscan perform trillions of simple calculations per second. Atpeak performance, Japan’s new Earth Simulator supercom-puter—modeling global environmental problems such asglobal warming, El Niño, and atmospheric pollution—canperform over 40 trillion operations per second (teraflops).

… and satellites that aredriving the, uh, weightgain!

When NASA launches the Earth Observatory SystemAura in early 2004, atmospheric modeling scientists willhave a new tool for refining their models. The Aura satellitewill carry an instrument package for making quantitativemeasurements of atmospheric ozone, water vapor, andother chemical species with state-of-the-art accuracy.

Among the fundamental questions Aura is designed toanswer is “What is happening to the stratospheric ozone?”Currently, several models suggest that stratospheric ozone—that’s the good ozone—is recovering, but it has not beenconfirmed by measurements. Are they correct? Aura will beable to track the global distribution of ozone and also thechemicals that participate in ozone formation.

“If we want to monitor compliance with international agreementson emissions,” says Daniel Jacob, an atmospheric chemist also at Har-vard, “It would be nice to observe the gases from space and detectrogue nations that are emitting more than they should.”

Models are key to interpreting the data stream delivered by the

satellites. “A satellite is seeing a spectrum [of light] and you need tointerpret this spectrum in terms of chemical concentrations in theatmosphere,” says Jacob, “so what that means is that you have to say ‘Ithink the atmosphere should look like this’ and then I look at the spec-trum. Does it fit with my knowledge?’”

Particularly helpful are the two Aura instruments, the Ozone Moni-toring Instrument (OMI) and the TroposphericEmission Spectrometer (TES), which will viewthe same scene at the same time. “That’s crit-ical,” says Martin, “because they’ll both beusing different techniques to monitor thesame pollutant in some cases.” Comparingthe results will give the scientists a betterhandle on what the actual concentrations are.

“There’s always a point in a science”says Jacob, “where the models can explainthe observations. At that point, you say ‘wedon’t need more observations’. But we’re veryfar from that in atmospheric chemistry. Weprobably won’t be there in my lifetime.”

With many challenges ahead, it’s anexciting time to become an atmosphericchemist.

Kevin McCue is the editor of ChemMatters

The GEOS-CHEM mathematical model of atmospheric chemistry successfully predictsconcentrations of nitrogen dioxide, a compound involved in ozone’s formation. In acomparison of actual observations for July 1996 by the European Space Agency’s GlobalOzone Monitoring Experiment (GOME, above) with the GEOS-CHEM model (below),geographic regions with high concentrations of nitrogen dioxide appear in yellows,oranges, and reds.

Taking on such problems as global warming, El Niño, and atmospheric pollution—the Earth Simulator canperform over 40 trillion operations per second.

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Nobel Prize Winner Sherwood Rowland:

DH: You started high school when you were only twelve! What was that like?

SR: A few years ago, after the Nobel award, someone sent me anold clipping from the hometown newspaper, the Delaware,Ohio, Gazette. It was mostly about my older brother who haddone exceptionally well as a high school freshman on theOhio State test in first year latin and had essentially aced thesecond year latin test as well. A footnote mentioned that his7-year old brother had scored the highest mark among third

graders in the city. When schoolresumed in the fall, I was placed in the5th grade.

I was always among the tallestin my age group, so I was not physi-cally smaller than my classmates inhigh school—although they were onthe average 2.5 years older. By mysenior year, I was the tallest person inthe school, but you must realize thatthis was when somebody 6’1” in aschool with roughly 100–150 studentsin each grade could be the tallest.Then I grew another four inches in thenext two years while in college!

DH: What sports did you playgrowing up?

SR: I played basketball and softball from a young age; I was agood player for my age group in basketball, but not for myclass in school, so I was a nonplaying “scrub” for two years.I played JV in my junior year and varsity in my senior year,but was definitely not a star—just a player.

The basketball coach was a math teacher. In fact, hemajored in math under my father at the local college, OhioWesleyan University. The coach persuaded me to take uptennis in the spring of my sophomore year, and I lettered intennis in junior and senior years. I also lettered in debate inmy senior year. I took part in extracurricular activities butwas not socially active until after I entered college.

I think athletics were important for me in high school,although I was still in many ways a loner. After high school,I organized, managed, and played third base on a softballteam. This was long before our present “Little League” era inwhich the adults organize and supervise what the kids dofrom pre-kindergarten on. During the summer after my sec-ond year in graduate school, I was the playing manager ofthe Oshawa Merchants, which won the semi-pro baseballchampionship of Canada that year.

People who play sports get a unique “non-ivory tower”experience when what you do every day is printed in the

A C o n v e r s a t i o n

Nobel Prize-winning chemistF. Sherwood Rowlandprobably never intended to

be an environmental whistle-blower.But in the 1970s, when he and hispostdoctoral associate Mario Molinaat the University of California-Irvinestudied a set of compounds calledchlorofluorocarbons, or CFCs, theyrealized that they had uncovered aproblem. They found that, if leftunchecked, CFCs posed a seriousthreat to life on earth.

In 1974, they published their findings in the journal Nature. They reported that long-lived volatile CFCs, unreactive compounds here at ground level, gradually rise into thestratosphere. There in the presence of ultraviolet radiation, they set about destroying our thinprotective shield of ozone molecules with an alarming chemicalappetite. In fact, a single chlorine atom released from one of thesemolecules could destroy tens of thousands of ozone molecules.

Scientists from other countries extended the research. PaulCrutzen of the Max Planck Institutes in Germany studied nitrogenoxide emissions and warned of properties similar to CFCs. Finally,in 1987, world leaders ratified the Montreal Protocol, which effec-tively eliminated the use of CFCs after 1995—coincidentally, theyear that Rowland, Molina, and Crutzen shared the Nobel Prize fortheir work in atmospheric chemistry.

Recently, National Chemistry Week Manager David Harwellasked Dr. Rowland to reflect on how his career in science devel-oped. Was he always focused on science alone? His answersmight surprise you.

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newspaper the next day, along with comments about whereyou succeeded or failed or where your opponent did like-wise—not to mention hearing it all on the local radio news.Question asked of a sports-playing scientist whose namehad appeared in the newspaper: “What did it feel like whensomeone asked you for an autograph?” My response: “Doyou mean, after I became a scientist?”

DH: Did you always want to be an atmosphericchemist?

SR: Atmospheric chemistry didn’t exist as a subject when I wasin graduate school. Radiochemistry—application ofradioisotopes to chemistry—was very new in the immediatepost-World War II period, and it had many applications.

As for my classes in atmospheric chemistry—I didn’t takethem; I taught them! I taught chemical kinetics in the 1950sand atmospheric chemistry in the 1970s. Radiochemistrywas given at the University of Chicago by Professor WillardLibby, my research supervisor and a subsequent Nobel Prizewinner (Chemistry, 1960) for the invention of carbon-14 dat-ing. Since carbon-14 is produced in the atmosphere by cos-mic ray bombardment, I became aware of the possibilities ofapplications of radioactivity outside the laboratory.

DH: Why did you focus on chlorofluorocarbonsas the topic of your research?

SR: My research group had for several years used CFCs in thelaboratory as “inert” targets for making radioactive chlorineand radioactive fluorine—although the fact that we usedthem as targets indicates they weren’t completely inert. Sowhen they were discovered in the atmosphere, I wonderedwhat might happen to them there. The only chemicals mypostdoctoral associate Mario Molina and I looked at initiallywere CCl3F and CCl2F2, two of the CFCs. Later on, of course,we looked at many others.

DH: Is teamwork important in your research? Is itever more “comfortable” to work alone?

SR: One of the primary reasons, maybe the only one, for being aresearch scientist is to push yourself beyond your “comfortzone”. When you are asking questions for which you havereasonably good answers, then you aren’t really doingexperiments—you are just applying what you already knowin a situation slightly different from before. Clearly, having anew idea sometimes arises from solitary thinking aboutareas of uncertainty. And sometimes it arises from interac-tions between colleagues.

Partnerships and collaborations bring together people withdifferent backgrounds—or “comfort zones” if you want—and these different viewpoints may very well help you zero inon the correct answer to your new problem.

I had a small bit of experience in atmospheric chemistry,and Mario Molina basically had none when we began collab-orating on the fate of chlorofluorocarbons. The problemswere well outside both of our comfort zones.

DH: Did your life change after winning the NobelPrize?

SR: Certainly! Every scientist grows up hearing about theachievements of Nobel Prize winners. But I think that it mayhave changed less for Crutzen, Molina, and myself than formany Nobel Prize winners because each of us had alreadybecome “public scientists”—scientists whose work hadbeen described regularly on the front pages of the New YorkTimes, Time Magazine, the New Yorker, People Magazine,and Rolling Stone; and on the TV networks. Because of theglobal importance of our work, we began receiving extensivemedia attention in the early 1970s, 20 years before beingawarded the Nobel.

DH: Has the ban on the release of CFCs had anyeffect on the atmosphere?

SR: The first controls on the release of chlorofluorocarbons tothe atmosphere were enacted in Oregon in 1975, and in thenext year, for the entire United States, and thereafter forCanada and Scandinavia. After the discovery of the AntarcticOzone Hole in 1985, the controls became international withthe Montreal Protocol in 1987, and then, as modified later,with a total ban on the further manufacture and use of CFCs.

We know from the continuing measurements in theatmosphere that the countries of the world are obeying thisban, and the growth in CFC concentrations has stopped andhas begun to reverse. However, because these moleculeshave long lifetimes in the atmosphere, they will still be mea-surably present for many decades. But the reversal hastaken place already.

In fact, there are many simultaneous stories going on inthe atmosphere, and if you study the amounts and locationsof various gases, you can tune in on these stories.

David Harwell is the manager of National Chemistry Week, a program of theMembership Division of the American Chemical Society.


It was the perfect idea for a science fair project, and Gianna D’Emiliohad no trouble convincing several of her friends at the EdmundBurke School in Washington, DC, to help. Basically, they agreed to

go out on the lawn and take air quality readings. They would go outevery sunny day at a time when the NASA satellite Terra passed over thearea to assess the amount of sunlight reaching the ground.

The commitment paid off when the group was 1 of 12 teamsselected to present their findings at the 2003 GLOBE Learning Expedi-tion held in Sibenik, Croatia, in early July. There, they met students andteachers from 22 countries who gathered to present their projects andto think about ways they could work together collecting more data forenvironmental research.

Gianna’s initial idea was to find out how pollution affects the colorof the sky. Her science teacher, Frank Niepold suggested that she beginby studying how aerosols, tiny airborne particles from a variety of natu-ral and human-generated sources, affect not only sky color, but also

weather and climate.When Dr. David Brooks, a GLOBE

scientist at Drexel University in Philadel-phia heard about the project, he sug-gested another idea to Gianna and herteam. How about validating aerosol meas-urements taken by a NASA satellite?While the satellite measured from the skydownward, simultaneous measurementsmade from the ground up would addvalue and meaning to the data.

GLOBE (Global Learning and Obser-vation to Benefit the Environment) is aninternational organization of students andteachers who collect and share data

about the health of the environment. In the United States, GLOBE issponsored by several scientific and government agencies, includingNASA, the National Science Foundation, the Environmental ProtectionAgency, and the State Department.

Brooks heads NASA’s GLOBE Aerosols Monitoring Project in theUnited States. The project has developed an instrument called theGLOBE Sun Photometer and some protocols, or standard operating pro-cedures, for its use in collecting Aerosol Optical Thickness (AOT) data.

Dr. Brooks explained how the instrument works in “Postcard from theNetherlands”, a GLOBE report appearing in the September 2002 issue ofChemMatters.

The team, including fellow student Jordan Glist and two recentBurke School grads, Chris Hanawalt and Melanie Benatato, met first totake the daily readings, and later to analyze data and write a report.

In the report “Assessing Satellite-Based Aerosol Retrievals andGround Truth Validation for Terra’s MODIS Sensor Over Urban Areasusing the GLOBE Program’s Handheld Sun Photometers”, theyexplained that although the aerosol-measuring instruments on the EOSTerra satellite were able to make accurate readings over homogenoussurfaces like oceans, they were less successful in making readings overvariable terrain like urban areas. Thus, the nine months of simultaneousground readings over Washington, while important, varied in theiragreement with Terra’s. The team sees their greatest accomplishment assuccessfully testing the instrument and establishing effective protocols.Another Burke team is being formed to continue the project whenNASA’s EOS Aura satellite launches in early 2004. With instruments onboard designed to measure the vertical profile of aerosols using differ-ent wavelengths of light, Aura will add new and valuable data aboutglobal aerosol distribution.

Burke students hope to enlist the help of students whom they metat the recent 2003 GLOBE conference. A school in Bremen, Germany, isplanning to take similar data this summer, and several other GLOBE par-ticipants are exploring partnership opportunities in the future.

You can read more about GLOBE and its many internationalresearch opportunities at the website www.globe.gov.

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The Burke team traveled to Croatia to present their findings at aninternational GLOBE conference. Pictured are Chris Hanawalt, Frank Niepold(teacher), Melanie Benetato, and Gianna D’Emilio.

Gianna D’Emilio and ChrisHanawalt take aerosolmeasurements using a hand-held sun photometer.

Science in the Sunshine

1155 Sixteenth Street, NWWashington, DC 20036-4800

Reach Us on the Web at chemistry.org/education/chemmatters.html

NASA shuttle mission STS 107 launched into a cloudless Florida sky on themorning of January 16, 2003. On board was a flight crew of seven shuttle astro-nauts ready to deploy more than 80 science experiments, many designed in partby students in the STARS program, a hands-on program involving students ages11–21 in the adventure of space exploration. In fact, many STARS students were inthe viewing stands, cheering as the clock ticked away the seconds before liftoff.

Less than two weeks later, the mission ended tragically with the loss ofseven lives and much of the science that the crew worked so hard to bring backfor sharing. A portion of the science was sent to ground in midflight.These valu-able data are now part of the legacy of STS 107.

Among the many instruments on board, Columbia carried the Shuttle OzoneLimb Sounding Experiment-2 (SOLSE-2).The plan was to use this imaging spec-

trometer to test a new strategy for monitoring ozone change.Currently, spectrometers flying on NOAA and NASA satellites look directly

downward toward the Earth, which limits their ability to accurately measure ozone inthe lower layers of the stratosphere.The new technology called “limb viewing”allows observation of the atmosphere from the side rather than straight down. Fromthis perspective, the layers of the atmosphere appear like layers in a cake, givinginstruments an especially good view of the lower stratosphere—the thin layer of

atmosphere containing Earth’s fragile supply of protective ozone.The measurements made by the SOLSE-2 mission on the Space Shuttle Columbia demonstrated

that the limb-sounding technique will work very well for monitoring ozone in next-generation satellites.Although much of the data was destroyed, the crew did send data to Earth for about 15 minutes during each orbitof the mission. About 40% of the SOLSE-2 data is available—enough to demonstrate that the limb viewing tech-

nique was successful.President George W. Bush honored the Columbia crew at a memorial service in Houston on February 4, 2003. His speech con-

tained these words: “This cause of exploration and discovery is not an option we choose; it is a desire written in the human heart. Weare part of the creation which seeks to understand all creation. We find the best amongst us, send them forth into unmapped dark-ness and pray they will return.They go in peace for all mankind, and mankind is in their debt”.

The October 2003 issue of ChemMatters is dedicated to the memory and the legacy of the Columbia crew:

Rick D. Husband, William C. McCool, Michael P. Anderson, David M. Brown, Kalpana Chawla,Laurel Blair, Salton Clark, and Ilan Ramon

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