Research▼Watch

© 2003 American Chemical Society AUGUST 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 281 A

Land-use changes over the past 50years are responsible for about 40%of the earth’s surface warming trend,according to research published inthe May 29 issue of Nature. The re-sults suggest that urbanization andagriculture have contributed almostas much as greenhouse gases to therise in global temperatures.

Two methods are commonly usedto estimate the effects of urbanizationon surface temperature trends—onebased on population data and theother on satellite measurements ofnight light. The two approaches, how-ever, have yielded significantly differ-ent estimates (0.06 and 0.15 °C percentury, respectively). Using a newapproach, meteorologists EugeniaKalnay and Ming Cai of the Universityof Maryland–College Park estimatedthat land-use changes have causedthe mean surface temperature to riseby 0.27 °C per century.The new estimate showsa larger effect, presum-ably because it coversall land-use changes,not just the effects ofurbanization.

In the new approach,the researchers com-pared trends in observedsurface temperaturestaken at nearly 2000weather stations acrossthe United States sincethe 1960s with a 50-yearreanalysis of satellite andweather balloon dataconducted by the Nation-al Centers for Environmental Predic-tion (NCEP) and the National Centerfor Atmospheric Research (NCAR).Because the NCEP–NCAR data are notsensitive to surface observations, thedifference between the surface tem-perature trends and those derivedfrom the NCEP–NCAR reanalysis re-flects the effects of urbanization aswell as other land-use changes.

The results suggest that convertingundeveloped land into agriculturalareas is just as important as urbaniza-tion and needs to be factored in whenestimating the impact of land-usechanges on climate. The researchersplan to expand their work beyond thecontinental United States to deter-mine if their estimate of surfacewarming from land-use changesholds true on a global scale. (Nature2003, 423, 528–531)

Consequences of a hydrogeneconomyA hydrogen fuel-cell economy couldhave unintended environmental im-pacts, including delaying the recoveryof the stratospheric ozone layer, ac-cording to modeling results publishedin the June 13 issue of Science. Thenew predictions point to the need fora better understanding of emissionsfrom hydrogen-based technologiesand possible hydrogen sinks.

Y. L. Yung and colleagues at theCalifornia Institute of Technologyused an atmospheric chemistry andtransport model called Caltech/JPL2-D to estimate the effects of increasedhydrogen levels on temperatures andwater and ozone concentrations inthe stratosphere. In one case, the hy-drogen level was set equal to the cur-rent global surface concentration,

and in another, the hydrogen concen-tration was raised to 2.3 ppmv, whichis about 4 times the current value.The researchers estimate that if allcurrent gasoline and oil combustiontechnologies were replaced by hydro-gen fuel cells, hydrogen emissionswould increase between a factor of 4and 8.

The modeling results predict thata 4-fold increase in hydrogen con-centration will lead to an increasein stratospheric water vapor levels,which would cause the lower stratos-phere to cool and would enhance thechemistry that destroys ozone. Theresult would be an ozone hole thatis deeper, larger in area, and longerlasting in the spring. Increased hy-drogen emissions could thereforejeopardize efforts to recover theozone layer through regulation ofchlorofluorocarbons.

The model also predicts that a risein hydrogen emissions could lead tochanges in tropospheric chemistry,

such as altering levelsof hydroxyl radicals,methane, and carbonmonoxide. Because hy-drogen is a microbialnutrient, increased emis-sions could also haveunintended effects onmicroorganisms. Suchemissions, however,could be negligible if soilturns out to be a domi-nant hydrogen sink. Fur-ther research is neededto determine how an in-crease in hydrogen emis-sions would impact theuptake of hydrogen by

soil. (Science 2003, 300, 1740–1742)

Atmospheric mercury trendsAtmospheric mercury concentrationspeaked in the 1980s, then declineduntil 1996, and have been relativelyconstant ever since, according to re-search published on May 22 in Geo-physical Research Letters. The findings

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are inconsistent with inventories ofanthropogenic mercury emissionsand suggest that the contributionsof mercury from both natural andanthropogenic sources need to bereevaluated.

A team of international scientists,led by Franz Slemr of the Max-PlanckInstitut für Chemie in Mainz, Germ-any, reconstructed global atmos-pheric mercury trends using directmeasurements taken during the peri-od 1977–2001. The long-term mea-surements were obtained at six sitesin the Northern Hemisphere, twosites in the Southern Hemisphere,and on eight cruises in the AtlanticOcean.

Total gaseous mercury concentra-tions in the Southern Hemispherewere approximately one-third lowerthan those in the Northern Hemis-phere, but the same trends were ob-served in both hemispheres. Higherlevels in the Northern Hemisphereare consistent with mercury sourcesbeing primarily in the north. Mercurytrends derived from sediments, peatbogs, and ice cores show similar tem-poral profiles.

Atmospheric mercury levels de-creased during the period 1990–1996by a larger amount than predicted inpublished inventories of natural andanthropogenic emissions. The re-searchers suggest that the discrepan-cy could be due to large temporalvariations in natural emissions or asubstantially underestimated ratio ofanthropogenic to natural emissions.Because gaseous mercury concentra-tions have been constant since 1996,they believe that the latter explana-tion is more likely. (Geophys. Res. Lett.2003, 30, doi:10.1029/2003GL016954)

Airborne metals exacerbateasthmaHeavy metals attached to fine airborneparticles are likely to be importantaggravators of allergic asthma, ac-cording to research published in theMay 27 online version of Environ-mental Health Perspectives. The newstudy, in which mice are exposed tofine particulate matter (PM) from aheavily industrialized city in Germany,shows the importance of PM’s chemi-cal composition in exacerbating res-piratory problems.

Although levels of common air pol-lutants, such as PM10 (particles lessthan 10 micrometers (µm) in diame-

ter), ozone, and sulfur dioxide, havedeclined considerably in developedcountries in the last 20 years, allergicasthma is considered an epidemicthat afflicts 15 million people in theUnited States alone. The number ofasthma cases, especially in childrenunder five, has continued to rise since1980, possibly indicating that PM’sphysical and chemical properties havechanged over time and are criticalfactors in the enhancement of preex-isting allergic respiratory diseases.

Stephen H. Gavett and colleaguesat the U.S. EPA’s National Health andEnvironmental Effects Research Lab-oratory in Research Triangle Park,N.C., and the Research Center forEnvironment and Health (GSF) inNeuherberg, Germany, followed up aprevious epidemiological study show-ing that children living in Hettstedt,a city in eastern Germany heavilyimpacted by industrial emissions,have higher rates of bronchitis andallergic diseases than children livingin the less polluted neighboring cityof Zerbst. In the new study, the re-searchers exposed mice to PM2.5(particles less than 2.5 µm in diame-ter) collected in Hettstedt as well asto PM2.5 from Zerbst.

Consistent with the human studies,mice exposed to the Hettstedt parti-cles displayed stronger symptoms ofpulmonary inflammation and devel-oped stronger allergic responses thanmice exposed to equal amounts ofPM2.5 from Zerbst. Chemical analysisshowed that PM2.5 from Hettstedthad several-fold higher concentra-tions of lead, copper, cadmium, tin,strontium, arsenic, and zinc than par-ticles from Zerbst. This new link be-tween human epidemiological dataand animal data suggests that metalsin soot are the primary cause of en-hanced allergic asthma. (Environ.Health Perspect. 2003, doi:10.1289/ehp.6300)

Bioavailability of soil-boundatrazineAtrazine can be bioavailable to mi-croorganisms even when it is sorbedto soil organic matter and clay miner-als, according to research publishedin the June issue of Applied andEnvironmental Microbiology. Thefindings have implications for biore-mediation of soils contaminated withthe most commonly used herbicidein the United States.

Most organic contaminants andpesticides that are bound to soilparticles have been considered un-available to microorganisms forbiodegradation. To degrade soil-sorbed pesticides, bacteria musteither desorb the chemicals fromthe soil or somehow use the sorbedmolecules directly.

Yucheng Feng and colleagues atAuburn University and MichiganState University evaluated the bio-availability of sorbed atrazine usingthree different atrazine-degradingbacteria and six different soils. Theyperformed sorption and desorptionexperiments and fit the data to adesorption, biodegradation, andmineralization (DBM) model, whichaccounted for sorption/desorptionprocesses and biodegradation of dis-solved, but not sorbed, atrazine.

In 11 out of 18 cases, the data fitthe DBM model, indicating that min-eralization was limited to the amountof dissolved atrazine initially presentplus any that was desorbed duringthe experiments. This suggests thatsorbed atrazine was not bioavailableto the bacteria in most cases. In fivecases, however, the data did not fitthe model and indicated enhancedbioavailability, suggesting that insome cases, bacteria can use atrazineas a nitrogen source even when it issorbed to soil. One explanation is thatbacteria access localized regionswhere the atrazine is bound.

For all three bacteria tested, en-hanced bioavailability was observedfor the soil with the highest organiccarbon content. The results indicatethat physicochemical properties ofsoils, such as organic carbon content,influence the bioavailability of sorbedatrazine. In addition, the researchersfound evidence that bacterial cell char-acteristics, such as the way the cells at-tach to soil particles, also play animportant role. (Appl. Environ.Microbiol. 2003, 69, 3288–3298)

Metals in polluted air may be the culpritbehind increasing cases of asthma.

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