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cence, the intensity of which is proportional to concentration. The laser wavelength, tuned to 282 nm, excites only the fluorescence ffom the eydroxyl lOH) )adical, the most important oxidizing species in the atmosphere. The key, says Heard, is the expansion of the sample as it enters the FAGE chamber, which extends the OHfluorescencelifetime to several hunLABORATORY dred nanoseconds and allows scattered laser light to be discriminated against by a PROFILE gated photomultiplier detector. The redshifted fluorescence is measured at 307315 nm. High winds, light The system is being used to gather rain, and radicals information about the condition of the atmosphere. "The atmosphere contains later many trace gases, such as methane and Dwayne Heard is a scientist who likes other hydrocarbons, CO, NOx and to travel. Recently he and his team HCFCs. Unless some natural cleansing took a portable lab packed with equipprocess takes place, these gases would ment, which they call the "HOxBOx", build up and cause problems," explains to the barren westerly shores of Eire, Heard. "The ability of the atmosphere to 50 miles west of Galway. What poscontrol the levels of these gases—its oxisessed Heard—an atmospheric chemdizing capacity—is controlled by the conist at the University of Leeds in the centration of hydroxyl radicals." Undernorth of England—to take his crew to standing this cleansing process depends such a wild place? on knowing just how many radicals are The answer is literally blowing in present in "clean" air, found at remote the wind. Heard and his team speciallocations such as the Atlantic coast. "Our ize in sampling and analyzing trace atmospheric materials and their effects experiments seek through local measurements of OH to allow a thorough underon pollutant levels. To avoid complicastanding of the basic photochemical cytions from too many pollutants, the cles which can then be used by modelers best place to do this has to be remote. to predict the future behavior of the planSo the Atlantic coast it is. et's atmosphere " "Mace Head was chosen as a good The Ireland trip tested the validity of spot to assess the atmosphere, because the air usually comes straight off current atmospheric models. The chemistry of OH and its chemical cousin, the the Atlantic and has not passed over land for more than five days," explains H02 radical, which are also measured using FAGE, are inextricably linked to Heard. The prevailing westerlies are virtually every tropospheric species. Their almost as pure as is possible in the concentrations are very dependent on lonorthern hemisphere. cal conditions. If an atmospheric model So what is in the HOxBOx? "The predicts these concentrations accurately, essential items," says Heard, "a shipthe underlying chemistry is likely to be ping container, an expensive laser, a correct, and our understanding of tropobig pump, lots of optics and electronspheric oxidation processes is validated. ics, and an electronic lab book—an Until now few experimental outdated 486 PC," he confesses, ments of OH or H02 have been made in "which comprise the FAGE (fluoresthe marine boundary layer cence assay by gas expansion) system to measure atmospheric radicals." In their initial experiments last year, B In the FAGE system, the air sample the team reached a sensitivity of 2 x 10 3 molecules/cm for OH. "Atmospheric is sucked into a detection chamber pressure is 2.5 x 1019 molecules per cubic from 18 ft above ground. The sample centimeter at 298 K," Heard says, "so expands to about 0.7 torr, forming a supersonic molecular beam expansion, we're looking at a very low fraction." For 7 and is then subjected to UV laser light. H02 the sensitivity was ~10 molecules under the same conditions. The team The excitation of free radicals in the sample produces a measurable fluores- measures H02 by chemically converting it
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Analytical Chemistry News & Features, September 1, 1997
Analyzing clean air. HOxBOx measures OH radicals.
to OH with added NO and then detecting the OH using the same fluorescence technique. For their latest venture, Heard's team boosted laser power and finetuned their homegrown software and, despite rather muddy conditions, which gave the HOxBOx a distinct tilt at the site, achieved much better sensitivities of 3 x 105 molecules/cm3 for OH and about 106 for HOz. With these improvements, they measured typical daytime maxima for these species of a few million and 108 molecules, respectively, allowing them to collect daily profiles. "In almost every instance, some new feature of atmospheric science has emerged from a comparison of field and theoretical data," says Heard, so the work is well justified. "Accurate predictive models are vital to assessing the impact on the atmosphere caused by changes in anthropogenic emissions, for example, from motor cars or from industry. A reduction may cause the problems associated with urban pollution, global warming, and acid rain to be alleviated," says Heard. "Clearly it is important to validate these models to ensure their output is accurate, and our field measurements are the key." "Future campaigns are planned in various places, including clean background, semirural, and urban sites," adds Heard, depending on funding of course and whether his team can put up with the wind. David Bradley
