The Long and...
Quality. Reliability. Reproducibility. When you are deciding which column to use in your new method, you must con sider all of these factors. That's why we offer the Waters NovaPak® column product line. But what good is a column that offers all these features if it isn't the size column you want? Waters knows that a variety of column choices is just as impor tant as the quality of the column you buy. That's why we are
expanding our Nova-Pak product line from 3.9 diameter columns to include NEW Steel Cartridge columns in 4.6 χ 150 mm and 4.6 χ 250 mm dimensions. These steel cartridge columns are avail able in C]s and Cg, and with reusable, finger-tight endfittings.
CIRCLE 95 ON READER SERVICE CARD
measurement techniques. In fact, the broad acceptance that OH concentra tions can be accurately m e a s u r e d will probably not be realized until a successful intercomparison of several different instruments under a vari ety of conditions is achieved. Thus, in the near future several OH instru ments will be tested in informal intercomparisons, and a formal inter comparison (i.e., w i t h " b l i n d " submission of m e a s u r e m e n t s ) will probably be made two or three years down the road. This does not, however, mean that nothing useful will be done until that time. Some OH m e a s u r e m e n t s are already being t a k e n seriously and are beginning to inspire researchers to work on newly identified problem areas. For some time, long-path ab sorption studies conducted near ur ban European sites have suggested t h a t measured OH concentrations are consistently lower t h a n those p r e d i c t e d by fast p h o t o c h e m i c a l model calculations. In 1991 the in tercomparison in the Colorado moun t a i n s of the l o n g - p a t h absorption and the i o n - a s s i s t e d chemical in s t r u m e n t s m e n t i o n e d earlier was also part of a limited photochemistry study. Whereas agreement between the two instruments was reasonably
good, the OH concentrations m e a sured were three to five times lower than calculated values (27). This re sult has led to the belief that not all OH losses were being accounted for a n d t h a t u n m e a s u r e d compounds might be responsible at least in part for the discrepancy. An i n t e r c o m p a r i s o n w a s m a d e during this past summer at the same location and with the same investi gators as in the latter intercompari son, but with the addition of an in situ low-pressure LIF OH measure ment technique. This intercompari son c o n t a i n e d a p h o t o c h e m i s t r y study portion t h a t emphasized the identification and quantification of hydrocarbon species that could be re s p o n s i b l e for OH loss. T h u s , OH m e a s u r e m e n t s are already helping identify areas where additional in vestigation is needed, and OH mea surements will gain more credibility with each successful intercompari son.
What next? Although OH measurements are be coming both more credible and more sensitive, generally they are not be coming any simpler or less expen sive, a n d the applications of such measurements must be well planned.
938 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
REPORT The measurement of OH by itself is of little value; in order for a n OH measurement to provide new insight, it must be imbedded in a reasonably complete photochemistry study. A realistic goal for such studies would be to further our u n d e r s t a n d i n g of the natural photochemical processes that control the chemical and physi cal properties of the E a r t h ' s atmo sphere. In addition, although photochem istry is often considered to be prima rily a mechanism for producing the strong oxidants that help cleanse the atmosphere, these same oxidants also react with atmospheric constitu ents, such as sulfur compounds, to form some very nonvolatile products, such as sulfuric and methane sulfon ic acid. These compounds leave the gas phase at their first opportunity, resulting in aerosol growth and, u n der the proper conditions, probably new particle formation. The result ing aerosol growth/production can then have both direct and indirect ef fects on the scattering of incident so lar radiation and thus affect climate (31). Although sulfur may not play a major role in determining OH con centrations, OH and related a t m o spheric oxidants probably do play major roles in controlling s u l f a t e aerosol formation. As these types of processes in the natural atmosphere are better understood, the effects of natural and anthropogenic emissions must be studied on both a local and a global basis. Eventually, the effects of future emission scenarios will be not only understood but also accu rately predicted. The location, times, and types of future emissions might be chosen in such a way as to mini mize t h e i r d e t r i m e n t a l effects or even optimize their positive effects on the environment. Although OH h a s l a r g e l y eluded m e a s u r e m e n t since its importance was recognized more t h a n two decades ago, contin ued advances in a number of technol ogies over the next decade will make its m e a s u r e m e n t not only possible but also well tested and available us ing a variety of different techniques. To t a k e the next few steps along the road to a better understanding of the natural tropospheric photochem ical process, several other measure ment advances are also required. Im p r o v e m e n t s m u s t be m a d e in our ability to m e a s u r e H 0 2 , which can act as a reservoir for OH and is also a strong atmospheric oxidant spe cies. Less emphasis has been placed on the measurement of H 0 2 than on OH and, as a result (despite its typi-
cally much higher atmospheric con centration), its accurate m e a s u r e ment has also been elusive. Improved identification and quantification of the organic compounds p r e s e n t in the atmosphere (especially oxygen ated species) is required if the loss of OH and the production of organoperoxy compounds (commonly referred to as R0 2 ) are to be properly taken into account. This is not a simple problem, because the speciation of these compounds probably v a r i e s greatly with location. When OH measurements are made at highly polluted sites such as ur ban and i n d u s t r i a l a r e a s or a r e a s heavily affected by biomass burning, it is hard to say how much more dif ficult it will be to take accurate mea surements. It is quite possible, how ever, t h a t the measurement will be easier to perform t h a n to interpret. As air masses become highly inhomogeneous (chemically), air flows extremely complex, and surface ef fects 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 m e a s u r e m e n t technology b u t also of the ability to characterize the chemistry, dynamics, and history of an air mass, is necessary to meet our future goals of u n d e r s t a n d i n g and predicting the behavior of the n a t u ral atmosphere and the effects of an thropogenic perturbations.
phys. Res. 1990, 95, 16427. (16) Hoell, J. M. NASA Conf. Pub. 2332, 1982. (17) Heaps, W. S.; McGee, T. J.J. Geophys. Res. 1985, 90, 7913. (18) Stimpfle, R. M.; Anderson, J. G. Geo phys. Res. Lett. 1988, 15, 1503. (19) Stimpfle, R. M.; Wennberg, P. O.; Lapson, L. B.; Anderson, J. G. Geophys. Res. Lett. 1990, 17, 1909. (20) Hard, T. M.; O'Brien, R. J.; Cook, T. B.; Tsongas, G. A. Appl. Opt. 1979, 18, 3216. (21) Crosley, D. R.; Hoell, J. M. NASA Conf. Pub. 2448, 1986. (22) Chan, C. Y.; Hard, Τ. Μ.; Mehrabzadeh, Α. Α.; et a l . / Geophys. Res. 1990, 95, 18569. (23) Bradshaw, J. D.; Rodgers, M. O.; Davis, D. D. Appl. Opt. 1984, 23, 2134. (24) Bradshaw, J. D.; Van Dijk, C. Mea surement of Atmospheric Gases, SPIE 1433 1991; Vol. 91. (25) Sandholm, S. T.; Bradshaw, J. D.; Dorris, K. S.; et a l . / Geophys. Res. 1990, 95 10155 (26)' Eisele, F. L.; Tanner, D. J. / Geophys. Res. 1991, 96, 9295. (27) Mount, G. H.; Eisele, F. L. Science 1992 256 1187 (28) Prinn,'R. G. Geophys. Res. Lett. 1985, 12, 597. (29) Prinn, R.; Cunnold, D.; Rasmussen, R.; et al. Science 1987, 238, 945. (30) Felton, C. C; Sheppard, J. C; Camp bell, M. V. Environ. Set. Technol. 1990, 24, 1841. (31) Charlson, R. J.; Schwartz, S. E.; Hales, J. M.; et al. Science 1992, 255, 423.
Fred L. Eisele (left) holds a joint appoint ment as principal research scientist at the Georgia Institute of Technology and as se nior research associate at the National Center for Atmospheric Research. He re ceived his B.S. degree in physics from Rensselaer Polytechnic Institute in 1970 and his Ph.D. in physics from the Univer sity of Vermont in 1975. He is currently developing sensitive selected-ion chemical ionization MS techniques for measuring trace atmospheric compounds. John D. Bradshaw is a principal re search scientist at the Georgia Institute of Technology. He received his Ph.D. in 1980 from the University of Florida, where he studied analytical applications ofLIF methods. His research interests in clude the development of field-based se quential multiphoton and photofragmen tation LIF sensors for measuring trace levels of compounds in the atmosphere.
...short of it.
References (1) Albritton, D. L.; Fehsenfeld, F. C ; Tuck, A. F. Science 1990, 250, 75. (2) Kolb, C. E. Rev. Geophys. 1991, 29 Sup plement, 25. (3) Logan, J. Α.; Prather, M. J.; McElroy, W. M. /. Geophys. Res. 1981, 86, 7210. (4) Atkinson, R. Chem. Rev. 1986, 86, 69. (5) Crosley, D. R. "SRI 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.; Piatt, U.; Tonnissen, Α.; Ehhalt, D. H./. Geophys. Res. 1984, 89, 1309. (8) Armerding, W.; Herbert, Α.; Spiekermann, M.; Walter, J.; Comes, F. J. FreseniusZ. Anal. Chem. 1991, 340, 654. (9) Baardsen, E. L.; Terhune, R. W. Appl. Phys. Lett. 1972, 21, 209. (10) Davis, L. I.; Guo, C; James, J. V.; et al./ Geophys. Res. 1985, 90, 12835. (11) Rodgers, M. O.; Bradshaw, J. D.; Sandholm, S. T.; et al./. Geophys. Res. 1985, 90, 12819. (12) Beck, S. M.; Bendura, R. J.; McDougal, D. S.; et a l . / Geophys. Res. 1987, 92, 1977. (13) Copeland, R. Α.; Jeffries, J. B.; Cros ley, D. R. Chem. Phys. Lett. 1987, 138, 425. (14) Copeland, R. Α.; Wise, M. L.; Cros ley, D. R./ Phys. Chem. 1988, 92, 5710. (15) Smith, G. P.; Crosley, D. R.J. Geo
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CIRCLE 95 ON READER SERVICE CARD
