easily reconciled with current retical predictions. These fin often indicate that key aspects chemical and physical couplings the atmosphere have yet to be
Georgia Institute of Tech Atlanta, GA 30332
John D. Bradshaw School of Earth and Atmospheric Sciences Georgia Institute of Technology Atlanta, GA 30332
Understanding the fundamenta physical and chemical processes oc tional measurements within he Earth’s atmosphere continue to ANALYTICAL CHEMISTR
1
Sulfatc aerosc growth
Ldditional oxidant
\
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fiormation
ation typically remains in the
Hydrocc -- -I-
concentrations that are agnitude lower t h other compounds of ospheric interest. I n this REPORT we will describ
0, and t
ues, and discuss the future of 0 urement technology.
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anthropogeni compounds
Conversion into more readily removed compounds
r
cise ( e +lo%) measurements short time intervals ( e 10 s) a 10- 20 - pptv mixing desired. This is nece spatial resolution to which atmospheric can occur (5 2 km
posphere a n d t h e stratosphere, where mixing ratios span 4 orders of
its low concentration, preeminent role in cont e oxidation, and thereby sequent removal, of many natura and manmade compounds emitted into the Earth’s atmosphere (3, 4). The types of processes in which OH is involved are shown in imary production of OH e photolysis of O3 (0, OID + 0,) followed by the reaction OID+ H i 0 + 20H. Numerous ot coupled secondary reaction pa ways, such as H,O, + hu + 20H an NO + NO, + OH, also co to the production of spheric OH. Because it i active with a variety of the OH produced via the quickly disappears. In cleaner regions of the Earth’s troposphere, where many compounds emoved oxidatively by OH, the ical loss of OH is controlled priarily by the reactions OH + CO + + H and CH, + OH + CH3 + In regions affected by the input her natural or manmade emis-
tic and chemical that are referred the “fast photothe atmosphere.
ine OH concentrations.Mult
lasers is allowed
other optically active s cal attenuation effects broad and unstructu however, particularly near urban
cimer lasers (6) or in regions of the X211 (v” = 0) + A2Z (v’ = 0) OH rotationally resolved transitions near 308 nm. I n relatively clean air masses, additio credibility is lent to this techni ntly and simultane
e 2. Typical long- path absorption measurement. directly and with goo
not t h e same as t h e ressure. The laser used
down to the mid- lo6 range have been ding the laser beam
ir masses, multiline abs easurements are needed to deconvolute the OH absorption spectrum from that of other in species (7). There is also an absorption t nique in which the optical been folded back and forth about 100 times i n a 6 - m open s t r u c t u r e d White cell. In this system the air be studied passes nearly unobstructed between the end mirrors (8). This application of the absorption technique offers both a direct and an in situ measurement capability OH, but it requires the measurem of even lower absorbances because of the relatively short overall p a t h length. The instrument also depends largely on one or two a b lines to determine OH co tions, using the rapid scan -band laser, and may to spectral interferences. ugh long-path absorptio techniques can be used to determine OH concentrations more direct1
separated points (many kilometers apart) is required. Even the smallest than that induced by atmospheric concentrations of OH. Thus the characterization of I, as the optical power entering the path of absorbers ca not be used for these techniques. Additional complications can aris because the Radioassa!
Figure 6. Photostationa
tion with the enrichment action rate constant of CO and the reaction time
surement requires about a 10 tion time a n d because t
ave not yet been published. asurement in the future
). An
LIF system successfully use
lifetime of OH. This method thus i
m3.It is unlikely that a1 ruments are function
eing measured is, howe Nonmembers** (institutional) Oneyear $444 $464 Member subscriptionrates for person *Includes air service. **2- and %yearrates available; call for detail Editor-in-Chief,W illiam H. Glaze, University North Carolina, Chapel Hill.
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NALYTICAL CHEMISTRY, V
h the method wa
gh several steps (som ere mentioned above), some cases have dividual resear
spherically shaped silks gel for HPLC and GPC Enantiomer separation of mecoprop
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bH3 CI
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TICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1,1993
937 A
REPORT
include NEW Steel Cartridge :olumns in 4.6 x 150 mrn and rC.6 x 250 rnm dimensions. These
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tions can be accurate
in the near future several OH instruments will be tested in informal insubmission of measure probably be made two or t down the road. time. Some OH are beginning to inspire rese to work on newly identified p areas. For some time, longsorption studies conducted ban European sites that measured OH are consistently lo predicted by fast model calculations. i n 1991 the intercomparison in the Colorado mountains of the long-path absorption and the ion-assisted chemical i n struments mentioned earlier was also part of a limited photochemistry study. Whereas agreement between the two instruments was reasonably 938 A
ANALYTICAL CHEMISTRY, VOL.
good, the OH concentrations measured were three to five times lower than calculated values (27).This result has led to the belief that not all OH losses were being accounted for and t h a t unmeasured compounds might be responsible a t least in part for the discrepancy. An intercomparison was made during this past summer at the same location and with the same investigators as in the latter intercomparison, but with the addition of an in situ low-pressure LIF OH measurement technique. This intercompari son contained a photochemistry study portion that emphasized the identification and quantification of hydrocarbon species that could be responsible for OH loss. Thus, OH measurements are already helping identify areas where additional investigation is needed, and OH measurements will gain more credibility with each successful intercomparison. What next?
The measurement of OH by itself of little value; in order for a n OH measurement to provide new insight, it must be imbedded in a reasona complete photochemistry study. realistic goal for such studies wou be t o further our understanding of the natural photochemical processes that control the chemical and physical properties of the Earth’s a t sphere. In addition, although photoche istry is often considered to be primarily a mechanism for producing the strong oxidants that help cleanse the atmosphere, these same oxidants also react with atmospheric constituents, such as sulfur compounds, t o form some very nonvolatile products, such as sulfuric and methane sulfonic acid. gas pha resultin der the new particle formation. The result ing aerosol growth/production can then have both direct and indirect effects on the scattering of incident solar radiation and thus affect climate lthough sulfur may not p major role in determining OH centrations, OH and related a spheric oxidants probably do major roles in controlling s u aerosol formation. As these types processes in the natural atmosphere are better understood, the effects o natural and anthropogenic emissio must be studied on both a local an global basis. Eventually, the effe of future emission scenarios will be not only understood but also accurately predicted. The location, times, and types of future emissions might be chosen in such a way as to minimize their detrimental effects or even optimize their positive effects on the environment. AlthougkOH has largely eluded measurement since its importance was recognized more than two decades ago, continued advances in a number of technologies over the next decade will make its measurement not only possible but also well tested and available using a variety of different techniques. To take the next few steps along the road to a better understanding of the natural tropospheric photochem ical process, several other measurement advances are also required. Improvements must be made in our ability to measure HO,, which can act as a reservoir for OH and is also a strong atmospheric oxid cies. Less emphasis has bee on the measurement of HO, OH and, as a result (despite
phys. Res. 1990,95,16427.
centration), its accurate measuren elusive. Improved d quantification of the organic compounds present in
Fred L. Eisele (left) holds a joint appointment as principal research scientist at the Georgia Institute of Technology and as senior research associate at the National
surements. It i As air masses mogeneous (chemically), air flows extremely complex, and surface effects far from negligible, the ability to measure OH may add little to our understanding of the photochemistry of this highly polluted environment. Continued improvement, not only of OH measurement technology but alm of the ability to characterize the chemistry, dynamics, and history of an air mass, is necessary to meet our future goals of understanding and predicting the behavior of the natural atmosphere and the effects of an-
Tuck, A. F. Science 1990,250,75. (2)Kolb, C. E. Rev. Geophys. 1991,29Supplement, 25. (3)Lo an,J. A.; Prather, M. J.; McElroy, Geophys. Res. 1981,86,7210 W. Rev. 1986,86, 69. (4)Atkinson, R. (5)Crosley, D. I Conference Re. port MP92- 135”;SRI International: Menlo Park. CA. December 1992. (6)Mount, G: H.J Geophys. Res. 1992,97, 2427. (7) Hubler, G.; Perner, D.; Platt, U.; nissen, A.; Ehhalt, D. H. J. Geophys. 1984,89,1309. ( 8 ) Armerdin W.; Herbert, A.; Spiekermann, M.; &ter, J.; Comes, F.J. Freseniw Z. Anal. Chem. 1991,340,654. (9)Baardsen, E. L.;Terhune, R. W.App1. Bays. Left. 1972,21,209. (10)Davis, L.I.; Guo, C.; James, J . V.; et al.J. Geophys. Res. 1985,90,12835. (11) Rodgers, M. 0.; Bradshaw, J. D.; Sandholm, S. T.; e t al. J. Geophys. Res. 1988,90,12819. (12)Beck, S.M.; Bendura, R. J.; McDougal, D. S.; et al.J. Geophys. Res. 1987,92, 1977. (13)Copeland, R. A.; Jeffries, J. B.; Crosley, D. R. Chem. Phys. Lett. 1987,138, 425. (14)Copeland, R. A.;Wise, M. L.; Crosley, D. R.J. Phys. Chem. 1988,92,5710. (15)Smith, G. P.; Crosley, G
dJ.
95,10155. (26)Eisele, F.L.;Tanner, D. J.J. Geophys. Res. 1991, 96,9295. (27)Mount, G. H.; Eisele, F. L. Science 1992,256,1187. (28)Prinn, R. G.GeoPhYS. Res. Left. 1986, 12,597. (29)Prinn, R.; Cunnold, D.; Rasmussen, R.; et al. Science 1987,238,945. (30)Felton, C. C.; Sheppard, J. C.; Campbell, M. V. Environ. Sei. Technol. 1990, 24,1841. (31) Charlson, R, J,;Schwartz, S. E.; Hales, J. M.;et al. Science 1992,255, 423.
F
ionization MS techniques for measuring trace atmospheric compounds. John D-Byadshaw is a Principal research scientist at the Georgia Institute of Technology. He received his Ph.D. in 1980 from the University of Florida, where he studied analytical applications of LIF methods. His research interests include the development offield-based Seand photofiagmenquential tation LIF sensors for measuring trace levels of compounds in the atmosphere.
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