though OH levels are expected to depend on acomplex mixture of variables. The crucial para-meter is a measure of the frequency with whicha given molecule of ozone is broken up by solarultraviolet radiation, yielding atomic oxygen in an excited electronic state — a parameterdenoted as J(O1D) by atmospheric chemists,where 1D is the excited state (Fig. 1). This fre-quency is determined by the intensity of ultra-violet radiation in a narrow spectral windowbetween 305 and 330 nm (ref. 4) — the samelight that causes sunburn. The solar intensity at these wavelengths is inversely related to the amounts of ozone, clouds and particulatematter between the Sun and the ground.

It is not surprising that the troposphericconcentration of OH depends on J(O1D).Many molecules are fragmented by ultravioletradiation at these wavelengths, so this para-meter can be thought of as a broad measure ofphotochemical activity. What is surprising isthat the concentration of OH seems to dependsolely on a factor that is not directly related tothe local chemical and physical environment.Even more remarkably, the measured concen-tration of OH correlates substantially betterwith J(O1D) than it does with the predictedconcentration calculated from a state-of-the-art model. The model includes as constraintsnot only J(O1D), but also the measured con-centrations of most of the gases shown in Fig-ure 1. This is disheartening, as it clearlyillustrates that we still do not have an adequatedescription of tropospheric OH chemistry.

It is possible to read too much into the strongcorrelation between OH concentration andJ(O1D). As Rohrer and Berresheim’s analysisshows1, this correlation probably results fromcorrelated changes in other controlling factors,which balance each other out. For example, theatmospheric lifetime of OH is determined bythe rate of the radical’s destruction in its reac-tions with hydrocarbons (here, primarily bio-genic compounds) and with certain pollutants.In the air at Hohenpeissenberg, the concentra-tions of biogenic compounds are higher insummer, whereas the concentrations of reactivepollutants, such as carbon monoxide and nitro-gen dioxide (NO2), are higher in winter. As aresult, the seasonal variability in the atmos-pheric lifetime of OH at this site is very small.Similarly, the amount of OH produced fromatomic oxygen in an excited state (O1D) is highin summer because of the increased humidity,whereas the recycling of OH from related radi-cals is more efficient in winter because of thehigher nitric oxide (NO) levels (Fig. 1).

The findings at Hohenpeissenberg are rem-iniscent of studies made in the lower stratos-phere, the layer of Earth’s atmosphere that sitsabove the troposphere. Here, OH concentra-tions were found to be highly dependent uponanother single variable — the height of the Sunabove the horizon5. Just as at Hohenpeis-senberg, serendipitous correlations betweenthe controlling factors of OH abundancereduced the apparent influence of these

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1. Wolf, S. A. et al. Science 294, 1488–1495 (2001).2. Valenzuela, S. O. & Tinkham, M. Nature 442, 176–179

(2006).3. Dyakonav, M. I. & Perel, V. I. JETP Lett. 13, 467–469 (1971).4. Hirsch, J. E. Phys. Rev. Lett. 83, 1834–1837 (1999).5. Zhang, S. Phys. Rev. Lett. 85, 393–396 (2000).6. Kato, Y. K., Myers, R. C., Gossard, A. C. & Awschalom, D. D.

Science 306, 1910–1913 (2004).7. Wunderlich, J., Kaestner, B.,Sinova, J. & Jungwirth, T.

Phys. Rev. Lett. 94, 047204 (2005).8. Jedema, F. J., Heersche, H. B., Filip, A. T., Baselmans, J. J. A.

& van Wees, B. J. Nature 416, 713–716 (2002).9. Johnson, M. & Silsbee, R. H. Phys. Rev. Lett. 55, 1790–1793

(1985).10. The Nation 44 (25 December 1879).11. Murakami, S., Nagaosa, N. & Zhang, S.-C. Science 301,

1348–1351 (2003).12. Sinova, J. et al. Phys. Rev. Lett. 92, 126603 (2004).

ATMOSPHERIC CHEMISTRY

Radicals follow the SunPaul O. Wennberg

Hydroxyl free radicals are part of a complex network of atmosphericchemical reactions. But a long-term study shows that their concentrationcan be predicted by the intensity of ultraviolet sunlight alone.

In an impressive feat of endurance and analyti-cal skill, Rohrer and Berresheim have obtaineda five-year record of the atmospheric con-centration of the hydroxyl free radical, OH, at a research station in southern Germany (page 184 of this issue)1. This is remarkablebecause, although OH controls the rate of oxi-dation of many gases in Earth’s atmosphere,the volume mixing ratio of this highly reactive(and thus short-lived) free radical is less thanone part per trillion. Measuring its concentra-tion, even over short periods, is a tremendoustechnical challenge.

The hydroxyl radical is central to Earth’satmospheric chemistry. For example, the largeamounts of methane (CH4) and carbonmonoxide (CO) added to the atmosphere (0.6billion and 2.5 billion tonnes per year, respec-tively) are offset by equally large sinks of thesecompounds from their reactions with OH (Fig.1). In the absence of OH, the concentrations ofthese greenhouse gases would be more than anorder of magnitude greater. The reactions ofOH with carbon monoxide and with hydro-carbons are also responsible for producing mostof the ozone (O3) present in Earth’s troposphere(the lowest part of Earth’s atmosphere thatextends 8–17 km from Earth’s surface).

It was first suggested in 1971 that OH controls Earth’s tropospheric chemistry2.Researchers immediately set out to measuretropospheric OH concentrations and theabundances of the various chemicals thoughtto affect OH levels. However, measuring OHconcentration was much more challengingthan expected, and it is only in the past fewyears that analytical methods of sufficient pre-cision and accuracy have become available todo this3. These measurements have typicallybeen made in ‘campaign mode’, where aircraftor ground-based sites are equipped withinstruments for a short period of time. Thedata set described by Rohrer and Berresheim1

adds a long-term view of atmospheric chemistry, and was collected at a well-equipped and established research site — the

Hohenpeissenberg Meteorological Observa-tory, 1,000 m above sea level.

Analysis of these data1 brings the authors toa striking conclusion — that variations in theconcentration of OH may be explained, in astatistical sense, by a single parameter, even

Figure 1 | The formation and chemistry ofatmospheric hydroxyl radicals. The main routefor hydroxyl-radical (OH) generation is shown in the green ovals. Ozone is broken up byultraviolet light to form atomic oxygen in the 1D excited electronic state; this occurs with afrequency denoted by J(O1D). The excited oxygenatoms react with water vapour to form OHradicals. Alternatively, the excited atoms may be recycled back to ozone, via the 3P electronicground state. The concentration of OH is usuallyless than 1 part per trillion per volume of gas.This reflects the rate of OH formation asdescribed above; the rate of OH losses inreactions with, for example, nitrogen dioxide(NO2), carbon monoxide (CO) and hydrocarbonssuch as methane; and the efficiency with whichOH is recycled from its photochemical siblingsHO2 and RO2 (where R is any hydrocarbon chain)through reactions with the common pollutantnitric oxide (NO). Recycling with NO is themechanism responsible for the formation ofozone in the lower atmosphere. J(H2O2) andJ(HCHO) are the frequencies with which therespective molecules are fragmented byultraviolet radiation.

O3 O(3P)

O(1D)

O2

AirJ(O1D)

J(H2O2)

NO

NO

J(HCHO)

NO2

Hydrocarbons

Severalsteps

Severalsteps

CO

H2O

RainRain

OH HNO3

RO2HO2

CO

H2O2

HCHO

HO2

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factors6. Moreover, at the time of the measure-ments, the observed OH concentrations correlated better with the position of the Sunthan with predicted concentrations calculatedfrom the existing state-of-the-art photo-chemical model7. The inability to accuratelycalculate stratospheric OH levels spurred a re-examination of many of the pathways leadingto OH production8,9 — including the ultra-violet-induced destruction of ozone, describedby J(O1D) (refs 4, 10). Descriptions of strato-spheric chemistry were much improved bythese studies, and eventually led to quantita-tive agreement between the observed and calculated OH concentrations11.

The history of the stratospheric investiga-tion provides a template for further research inthe troposphere12. As illustrated by Rohrer andBerresheim1, the chemical drivers of tropo-spheric OH production and loss are often correlated, masking their individual influence.Nevertheless, to define the global distributionof OH, its dependence on climate change, andthe effect of industrial and agricultural emis-sions upon it, we must unmask the effects of

these processes. Measurements such as thoseobtained by Rohrer and Berresheim will beessential for identifying errors and omissionsin our theories of tropospheric oxidationchemistry. ■

Paul O. Wennberg is at the California Institute ofTechnology, Division of Geological and PlanetarySciences and Division of Engineering and AppliedScience, 1200 East California Boulevard,Pasadena, California 91125, USA.e-mail: wennberg@caltech.edu

1. Rohrer, F. & Berresheim, H. Nature 442, 184–187 (2006).2. Levy, H. Science 173, 141–143 (1971).3. Heard, D. E. & Pilling, M. J. Chem. Rev. 103, 5163–5198

(2003).4. Matsumi, Y. et al. J. Geophys. Res. 107, 4024 (2002).5. Wennberg, P. O. et al. Science 266, 398–404 (1994).6. Hanisco, T. F. et al. J. Phys. Chem. A 105, 1543–1553 (2001).7. Salawitch, R. J. et al. Geophys. Res. Lett. 21, 2551–2554 (1994).8. Hanson, D. R. & Ravishankara, A. R. Geophys. Res. Lett. 22,

385–388 (1995). 9. Roehl, C. M., Nizkorodov, S. A., Zhang, H., Blake, G. A. &

Wennberg, P. O. J. Phys. Chem. A 106, 3766–3772 (2002).10. Michelsen, H. A. et al. Geophys. Res. Lett. 21, 2227–2230

(1994).11. Salawitch, R. J., Wennberg, P. O., Toon, G. C., Sen, B. &

7Blavier, J. F. Geophys. Res. Lett. 29, 1762 (2002).12. Olson, J. R. et al. J. Geophys. Res. 111, D10301 (2006).

parallel-sided body, about 4 cm long, blunt atboth ends, with transverse stripes over most ofthe surface and with a body thickness only afraction of its width.

The distinctive feature, however, was astructure like a figure of eight near the blunter end of the body. The suggestion was that this

structure resembled a lophophore,the tentacle apparatus characteris-

tic of a group known as thelophophorates,

which otherwise incorporate such differentbeasts as the bivalved brachiopods (‘lamp-shells’), the tube-dwelling phoronids and thecolonial bryozoans. Lophophorate animalsfeed by filtering water drawn into this appara-tus by cilia.

Odontogriphus did not display any obvioustentacles, but, given the vagaries of fossili-zation, such a deficiency need not stop apalaeontologist. Where the tentacles were tohave been expected, there were tooth-likestructures, which could be interpreted as stiffsupports for tentacles that had failed to be preserved. The conodont animal, a classicalenigma of palaeontology, then entered the pic-ture. The isolated, tooth-shaped fossils knownas conodonts had been proposed to be notteeth, but the mineralized supports of tenta-cles. Could Odontogriphus provide the link byshowing conodont-like ‘teeth’ arranged in theshape of a lophophore?

Fortunately for our navigation in this sea ofred herrings, convincing fossils of conodontanimals were soon thereafter identified3, shred-ding any notions of resemblance to Odonto-griphus. The latter still hovered around as a possible lophophorate, however, though morelike a ghost than a real animal. There simplyweren’t enough data to make it materialize.

Other discoveries of wonderfully preservedCambrian fossils then started to compete forattention4–6, and by the early 1990s the zestseemed to have largely evaporated from stud-ies of the Burgess Shale. Even the substantialcollections made by Walcott early in the twen-tieth century and kept at the SmithsonianInstitution in Washington DC had been poredover to such an extent that the law of dimin-ishing returns started to take effect.

That tide was turned by palaeontologistDesmond Collins and his co-workers. Overseveral years, their tireless work in the moun-tains of British Columbia extracted new and wonderful material from the fabled shale, which they brought home to the Royal

Ontario Museumin Toronto.

The

PALAEONTOLOGY

A ghost with a bite Stefan Bengtson

Witness a snail scraping microbial films from the inside of an aquarium. Goback 505 million years, and this looks to have been the way an enigmaticearly animal made its living (but without the aquarium).

A faint shade on a piece of shale from BritishColumbia, Canada, has haunted palaeontol-ogy for 30 years. This is the fossil of Odonto-griphus, the half-a-billion-year-old ‘toothedriddle’ from the Cambrian Burgess Shale,which has never really found peace within theevolutionary scheme of animals. With a poorlypreserved body, it was mainly known throughpeculiarly arranged tooth-like spines, hypoth-esized to be the stiff supports of a cluster oftentacles. On page 159 of this issue, Caron etal.1 present new fossils of the ‘toothed riddle’.Odontogriphus at last comes into clear view,with a firm body and prominent teeth. Thisview suggests that it is allied to primitive molluscs, and so provides fresh insight into theearly evolution of animals.

Odontogriphus was brought to the world’sattention in 1976 by Simon Conway Morris2.The single specimen had been collected some60 years earlier by Charles Walcott, the discov-erer of that famous spot in the Burgess Passwhere organisms from the dawn of animal lifeare found with soft tissues preserved in fullflattened splendour. Even taking into accountthe flattening resulting from compression ofthe embedding sediment, Odontogriphus asreconstructed looked like a roadkill: a roughly

Figure 1 | Bared teeth — amodern snail displays its radula.

A. W

ARÉ

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