The Elusive Hydroxyl Radical Measuring OH in the ... - ACS Publications

The Elusive Hydroxyl Radical Measuring OH in the Atmosphere Fred L. Eisele1 Georgia Tech Research Institute Georgia Institute of Technology Atlanta, GA 30332

John D. Bradshaw School of Earth and Atmospheric Sciences Georgia Institute of Technology Atlanta, GA 30332

Understanding the fundamental physical and chemical processes occ u r r i n g w i t h i n the E a r t h ' s a t m o sphere is critically important to the development of predictive capabilities t h a t can a c c u r a t e l y describe past, present, and future conditions. 0003-2700/93/0365-927 A/$04.00/0 © 1993 American Chemical Society

Over the past two decades significant progress has been made in elucidating m a n y of t h e s e i m p o r t a n t fundamental processes from a mechanistic standpoint. Indeed, progress in a t m o s p h e r i c c h e m i s t r y h a s r e sulted in an impressive blossoming

REPORT in our understanding of the detailed chemical mechanisms and kinetics of many critical processes. Even so, observational measurements within the Earth's atmosphere continue to uncover new findings t h a t are not

easily reconciled with current theoretical predictions. These findings often indicate that key aspects of the chemical and physical couplings in the atmosphere have yet to be fully elucidated. T h u s , t h e study of the E a r t h ' s a t m o s p h e r e , especially its chemistry, remains highly reliant on observations for critically examining the state of this science (J). The list of reactions known to be i m p o r t a n t in t h e a t m o s p h e r e h a s grown at a seemingly exponential rate. These reactions can be either 1 Mailing address: Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO 80307-3000

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Sulfate aerosol growth

Additional oxidant formation

so2

Hydrocarbons A

0 2 and H 2 0

NO and 0 2

OH

ducing it from the free atmosphere i n t o a n a n a l y s i s device, a n d t h e p r e p a r a t i o n of a stable calibration standard is not possible. Rapid gasphase reactions of OH in the atmo­ sphere are also in p a r t responsible for the fact that its atmospheric con­ centration typically remains in t h e s u b - p p t v r a n g e ; t h u s , it m u s t be measured at concentrations t h a t are o r d e r s of m a g n i t u d e l o w e r t h a n those of most other compounds of at­ mospheric interest. In this REPORT we will describe the various ways of measuring OH in the atmosphere, including both di­ rect and indirect measurement tech­ niques, and discuss the future of OH measurement technology. O H in the atmosphere

r Hydrogenated fluorochlorocarbons

Many other^ natural and anthropogenic compounds

N 0 2 and NH 3

Conversion into more readily removed compounds

Removal or conversion

Figure 1. Processes in which the hydroxyl radical is involved.

homogeneous (occurring in the gas phase) or heterogeneous (occurring w i t h i n or on aerosols). W i t h t h i s growth has come a demand to simul­ taneously measure an ever increas­ ing suite of atmospheric constituents to rigorously test the "completeness" of current understanding. These de­ mands can challenge the analytical capabilities of current instrumenta­ tion, and i n s t r u m e n t a l limitations have often had a serious impact on the ability to accurately quantify nu­ m e r o u s c o m p o u n d s in t h e a t m o ­ sphere—even those t h a t seemingly should be straightforward, such as water vapor. The importance of water vapor to atmospheric chemical and physical processes is indeed unparalleled, yet only recently has i n s t r u m e n t a t i o n become available that can accurately and reliably measure it under atmo­ spheric conditions at molar mixing ratios over the few to h u n d r e d s of parts per million by volume (ppmv) range found in the upper troposphere and stratosphere. No single instru­ ment can as yet unambiguously mea­ sure water vapor throughout the tro­ posphere and the stratosphere, where mixing ratios span 4 orders of magnitude from about 5% by volume at low altitudes over warm tropical

marine environments to a few ppmv in stratospheric air. As another example, the reliable m e a s u r e m e n t of N 0 2 h a s only r e ­ cently been d e m o n s t r a t e d over the few to hundreds of parts per trillion by volume (pptv) range of mixing ra­ tios found in t h e n o n u r b a n tropo­ sphere. However, even t h e s e m e a ­ surements may not be unequivocal, because interferences may exist from concomitant N 0 2 - c o n t a i n i n g gases such as pernitric acid ( H 0 2 N 0 2 ) . In addition, current instrumentation is approaching its limits in meeting the increasing demands of airborne sam­ pling scenarios for which more pre­ cise (< ±10%) m e a s u r e m e n t s over short time intervals (< 10 s) at the 1 0 - 2 0 - p p t v mixing ratio range are desired. This is necessary to increase spatial resolution to the domain over which atmospheric inhomogeneities can occur (< 2 km h o r i z o n t a l a n d < 50 m vertical). Similar challenges are faced when attempting to measure many impor­ t a n t atmospheric compounds (1, 2). This is especially t r u e for t h e hydroxyl radical because its highly re­ active n a t u r e makes it particularly difficult to measure. Because of its rapid destruction on surfaces, great care must be exercised when intro-

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Despite its low concentration, OH plays a preeminent role in control­ ling the oxidation, and thereby t h e subsequent removal, of many natural a n d m a n m a d e compounds e m i t t e d into the E a r t h ' s atmosphere (3, 4). The types of processes in which OH is involved are shown in Figure 1. Primary production of OH begins via t h e p h o t o l y s i s of 0 3 ( 0 3 + hx> —» O x D + 0 2 ) followed by the reaction ΟλΌ + H 2 0 -> 20H. Numerous other coupled secondary r e a c t i o n p a t h ­ ways, such as H 2 0 2 + hx> —> 2 0 H and H 0 2 + NO -> N 0 2 + OH, also con­ t r i b u t e to the production of a t m o ­ spheric OH. Because it is highly re­ active with a variety of compounds, the OH produced via these processes quickly disappears. In cleaner regions of the E a r t h ' s troposphere, where many compounds are removed oxidatively by OH, the chemical loss of OH is controlled pri­ marily by the reactions OH + CO —> C 0 2 + Η and C H 4 + OH -> C H 3 + H 2 0 . In regions affected by the input of either natural or manmade emis­ sions, the loss of OH can be affected by a wide variety of other compounds such as higher molecular weight hy­ drocarbons. The balance between OH produc­ t i o n a n d loss is t h e o r e t i c a l l y d e ­ scribed by photolytic and chemical reaction sequences that are referred to collectively as t h e "fast p h o t o ­ chemical" cycle of the atmosphere. The production and loss p a t h w a y s for OH result in a rapid establish­ ment of pseudo steady-state concen­ t r a t i o n s of OH on the time scale of seconds. It has been estimated t h a t midday p h o t o s t a t i o n a r y - s t a t e OH concentrations range from - 1 χ 10 7 molecules/cm 3 (- 0.5 pptv at 1 atm pressure) to - 1 χ 10 6 molecules/cm 3 or less, depending on ambient levels

of 0 3 , H 2 0 , UV solar radiation, and a host of compounds t h a t participate in photochemical cycles related to OH. To perform robust tests of the com­ pleteness of the fast photochemical theory, the absolute concentration of OH must be measured with an accu­ racy and precision sufficiently better than the known uncertainties of the reaction rates and input parameters, such as the concentration of various compounds ( 0 3 , H 2 0 , CO, NO, N 0 2 , H 2 0 2 , and n o n m e t h a n e h y d r o c a r ­ bons) t h a t are used in p r e d i c t i v e models (5). We will focus primarily on OH measurement techniques that can be used to make so-called local OH c o n c e n t r a t i o n m e a s u r e m e n t s . The localized n a t u r e of these mea­ surements implies t h a t OH and the numerous parameters and other compounds needed to test fast photo­ chemical theory can all be character­ ized over relevant spatial volumes and temporal scales. The aforementioned l i m i t a t i o n s suggest t h a t OH m e a s u r e m e n t in­ s t r u m e n t a t i o n m u s t be capable of measuring atmospheric OH concen­ trations unambiguously within about ± 30% at the 1 χ 10 6 molecules/cm 3 level. This sensitivity requires an in­ strumental limit of detection (LOD) in the low-10 5 molecules/cm 3 range. For ground-based i n s t r u m e n t a t i o n measuring local atmospheric OH, the time period for measurement should be limited to less than - 10 min. This temporal limitation originates from a desire to minimize changes in atmo­ spheric conditions and composition, thus allowing a more robust inter­ pretation of the photochemical sig­ nificance of the measurements. For instrumentation that is deployed on aircraft-sampling platforms, the time scale over which significant spatial inhomogeneity of the atmo­ sphere can be encountered is much shorter. In consideration of t h e s e constraints, it is p e r h a p s not sur­ prising that the measurement of at­ mospheric OH concentration has in­ deed been an elusive and challenging endeavor. To accurately quantify OH or any other trace atmospheric constituent, several measurement criteria must be satisfied. F i r s t , t h e d e t e c t i o n technique used must be specific; it m u s t be capable of d e t e c t i n g t h e compound of interest and only the compound of interest at the desired concentration. If a d d i t i o n a l com­ pounds are detected by t h e t e c h ­ nique, some means for isolating the portion of the signal associated with the compound of i n t e r e s t m u s t be

found. Second, t h e m e a s u r e m e n t technique must provide a reproduc­ ible and well-characterized response to a given preexisting concentration of the compound of interest. In some cases, alterations of the ambient con­ centration of the compound of inter­ est by the m e a s u r e m e n t itself can pose problems. In addition, an accurate means of calibrating the nonabsolute mea­ s u r e m e n t t e c h n i q u e s m u s t be de­ vised. This poses a technical chal­ lenge because stable gas s t a n d a r d s are not available for this highly reac­ tive free radical. If the calibration procedure cannot be carried out un­ der ambient atmospheric sampling conditions, the various p a r a m e t e r s that might affect the transfer of the i n s t r u m e n t c a l i b r a t i o n — s u c h as changes in absorption linewidth with pressure and temperature or changes in excited-state collisional quench­ ing in l a s e r - i n d u c e d fluorescence (LIF)—must be taken into account. Finally, a means for reliable deter­ m i n a t i o n of t h e i n s t r u m e n t ' s t r u e

A

methods are techniques that interro­ gate the air in the vicinity of the in­ strument, usually by flowing ambi­ ent air into some portion of it.

Direct measurement Techniques based on long-path UV absorption and LIF are generally the only two direct OH m e a s u r e m e n t techniques that have been applied to t h e m e a s u r e m e n t of a t m o s p h e r i c OH. Of all the various techniques, these two basic methods also share the distinction of having been under development for the longest time. L o n g - p a t h UV a b s o r p t i o n . Long-path UV absorption spectro­ photometry is the only direct m e a ­ surement technique that, in princi­ ple, is capable of making an absolute d e t e r m i n a t i o n of a t m o s p h e r i c OH c o n c e n t r a t i o n . Conceptually, t h i s method is quite simple and relies only on fundamental parameters to determine OH concentrations. Multi­ line, broadband, or t u n a b l e l a s e r s provide powerful, well-collimated light sources. The light from these

lthough long path absorption techniques can he used to determine OH concentrations

more directly than can other OH measurement techniques, they have several limitations

blank response under ambient atmo­ spheric conditions m u s t be e s t a b ­ lished. In general, atmospheric measure­ m e n t t e c h n i q u e s can be classified into several basic categories. Direct methods encompass techniques that, in principle, attempt to directly mea­ sure a net signal response induced by a t m o s p h e r i c c o n c e n t r a t i o n s of OH. Indirect methods are techniques for which the signal response is di­ rectly induced by some other com­ pound that can be chemically related to atmospheric concentrations of OH. Remote methods are techniques that interrogate volumes of air a t a dis­ tance from the instrument. In situ

lasers is allowed to traverse a large but well-known distance and is typi­ cally reflected back to a detection de­ vice close to t h e l i g h t source, as shown in Figure 2. Beer's law provides the needed re­ l a t i o n b e t w e e n OH c o n c e n t r a t i o n and several measurable parameters In I/I0 = aLn where / and I0 are the a t t e n u a t e d and unattenuated laser light intensi­ ties, respectively, σ is the OH ab­ sorption cross-section at the wave­ l e n g t h ^ ) of interest, L is the total light p a t h l e n g t h , and η is the OH c o n c e n t r a t i o n to be d e t e r m i n e d . Even t h o u g h / and L can be mea-

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Detector

Monochromator

Laser

Telescope

Figure 2. Typical long-path absorption measurement.

sured directly and with good accu­ racy, several problems a r i s e . The very small ambient concentrations of OH require t h a t the m e a s u r e m e n t pathlengths be on the order of kilo­ m e t e r s or even t e n s of kilometers. Thus, these measurements are often made between a laboratory contain­ ing a light source and a detector or a retroreflector on a nearby mountain. Even with 10-km optical paths, the net absorption from atmospheric OH is typically seen only in the third or fourth decimal places. To complicate matters further, the atmosphere also contains m a n y other optically active species (some of which still have not been chemi­ cally identified) a t c o n c e n t r a t i o n s well in excess of ambient OH levels. In a d d i t i o n , s c a t t e r i n g by a t m o ­ spheric gases and aerosols, as well as scintillations induced by refractive index changes, can effectively atten­ uate optical power along the absorp­ tion pathlength. This combination of factors results in optical attenuation t h a t is orders of magnitude larger t h a n t h a t induced by atmospheric concentrations of OH. Thus the char­ acterization of I0 as the optical power entering the path of absorbers can­ not be used for these techniques. Additional complications can arise because the σ t h a t one determines q u i t e a c c u r a t e l y as a function of wavelength for individual molecules

is n o t t h e s a m e a s t h e p r e s s u r e broadened σ' t h a t m u s t be used at atmospheric pressure. The laser used to m e a s u r e OH can also photolytically g e n e r a t e OH along t h e m e a ­ surement path via the UV photolysis of ambient 0 3 (i.e., 0 3 + hu -» 0 1 D + 0 2 and O a D + H z O -> 20H). Over the past two decades, how­ ever, many* of these difficulties have been overcome and apparently mean­ ingful determinations of ambient OH concentrations down to the mid-10 5 molecules/cm3 range have been made. By expanding the laser beam with a telescope and by choosing OH t r a n s i t i o n s at longer wavelengths, where 0 3 absorbs less strongly, un­ wanted OH production can be made negligible. The pressure and wave­ length dependence of σ are becoming better known, and thus the value of σ' at atmospheric pressure often can be determined to within better than ± 10%. The determination of /„ and inter­ ferences from other optically active species, however, still pose major problems. Because t h e effective I0 cannot be quickly determined by re­ moving OH from the optical path, it is often inferred by examining the probe b e a m i n t e n s i t i e s a t w a v e ­ lengths adjacent to OH absorption lines using a technique commonly re­ ferred to as differential optical ab­ sorption spectrophotometry. If the

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other optically active species or opti­ cal attenuation effects are spectrally broad a n d u n s t r u c t u r e d compared with OH, this technique can work quite well. This is often not the case, however, p a r t i c u l a r l y n e a r u r b a n areas, where compounds with highly structured spectra, such as S 0 2 and C H 2 0 , typically alter the absorption spectrum. For this reason, nearly all longpath absorption measurement tech­ niques use several different spectral lines to measure OH concentrations. P u l s e d XeCl excimer l a s e r s (6) or broadband picosecond lasers (7) are used as pseudo-continuum sources in regions of the Χ2Π (ν" = 0) -» Α 2 Σ (ν' = 0) OH r o t a t i o n a l l y r e s o l v e d t r a n s i t i o n s n e a r 308 n m . In r e l a ­ tively clean air masses, additional credibility is lent to this technique when similar OH concentrations can be independently and simultaneously calculated, using a number of differ­ ent OH absorption lines (6). In pol­ luted air masses, multiline absorp­ tion m e a s u r e m e n t s a r e n e e d e d to deconvolute the OH absorption spec­ t r u m from t h a t of other interfering species (7). There is also an absorption tech­ nique in which the optical p a t h has been folded back and forth about 100 t i m e s in a 6-m open s t r u c t u r e d White cell. In this system the air to be s t u d i e d p a s s e s n e a r l y u n o b ­ structed between the end mirrors (8). This application of the absorption technique offers both a direct and an in situ m e a s u r e m e n t capability for OH, but it requires the measurement of even lower absorbances because of t h e r e l a t i v e l y short overall p a t h length. The instrument also depends l a r g e l y on one or two a b s o r p t i o n lines to d e t e r m i n e OH c o n c e n t r a ­ tions, using the rapid scanning of a narrow-band laser, and may be sus­ ceptible to spectral interferences. Although long-path absorption techniques can be used to determine OH c o n c e n t r a t i o n s more d i r e c t l y t h a n can o t h e r OH m e a s u r e m e n t techniques, they have several limita­ tions. They are inherently less mo­ bile than other techniques; often, in­ strumentation located at two widely separated points (many kilometers apart) is required. Even the smallest folded-path apparatus using the ab­ sorption technique is more than 6 m long. Long-path measurements also determine the average OH concen­ tration along the entire path, a r e ­ sult t h a t h a s both a d v a n t a g e s and disadvantages. At present, the disad­ v a n t a g e s are most pronounced be­ cause ancillary measurements of the

many related chemical species needed to model the photochemistry can be obtained only by in situ tech­ niques. L o n g - p a t h absorption m e a s u r e ­ ment techniques are being developed for a number of photochemically im­ portant compounds, and in the fu­ t u r e t h e s e t e c h n i q u e s should i m ­ prove o u r a b i l i t y to m o d e l t h e p h o t o c h e m i s t r y a l o n g t h e optical path. As these new techniques come on line, the benefits offered by longpath absorption—including easy ac­ cess to air m a s s e s r e m o v e d from ground surface effects (such as high over a valley), m e a s u r e m e n t s t h a t are characteristic of a large portion of an air m a s s , a n d t h e direct a n d passive n a t u r e of t h e m e a s u r e ­ ment—can be better exploited. Mea­ surements could also be made along several different paths from one cen­ tral laboratory site. LIF. As first conceived, LIF also appeared to offer a simple, direct technique for the measurement of at­ mospheric OH (9). The electronic structure of OH seemingly provided a straightforward measurement op­ portunity. The wavelength of excita­ tion for t h e Χ 2 Π ( v " = 0) -» Α 2 Σ + (v' = 1) transition near 282 nm was in the region accessible by the newly available high-powered, frequencydoubled dye lasers. In addition, the rapid Α 2 Σ + (ν' = 1) -> Α 2 Σ + (ν' = 0) relaxation rate allowed highly effi­ cient detection of nonresonant fluo­ rescence from the Α 2 Σ + ( ν ' = 0) —» Χ2Π (ν" = 0) transition n e a r 308 m while a v o i d i n g t h e d e t e c t i o n of 282-nm scattered light. In practice, this simplistic approach was severely challenged by the realities of an at­ mosphere t h a t would not so easily relinquish the detection of its most important prize. The first limitation occurred in the form of OH that was artificially pro­ duced from t h e i n t e r a c t i o n of t h e supposedly benign UV laser beam with atmospheric 0 3 . In early cases, the laser used to probe atmospheric OH a c t u a l l y g e n e r a t e d m o r e OH than was present in the atmosphere. This artifact arose from the sequence 0 3 + hv -> 0*D + 0 2 a n d O x D + H 2 0 -» 0 Η Χ 2 Π ( v " = 0) + ΟΗΧ 2 Π ( v " = 0,1,2), which occurred effi­ ciently even on n a n o s e c o n d t i m e scales. Significant reductions in the UV probe laser flux were needed to bring this so-called 0 3 / H 2 0 interfer­ ence under control. Unfortunately, the sensitivity of t h e i n s t r u m e n t s was also a p p r e c i a b l y reduced b e ­ cause the LIF sensitivity is propor­ tional to the UV probe laser flux.

As a m e a n s for dealing with the 0 3 / H 2 0 interference appeared to be in hand, a second significant limita­ tion was uncovered in the form of a temporally varying fluorescence background component arising from the UV excitation of other concomi­ tant gases and aerosols in the atmo­ sphere. This background resulted in a n o n s t a t i o n a r y noise t h a t limited the ability to reach the photon statis­ tics noise limits of background. To overcome these limitations, rapidscanning and dual-wavelength dif­ ferential fluorescence excitation schemes—similar to those for longp a t h UV a b s o r p t i o n — w e r e devel­ oped (10, 11). Sufficient p r o g r e s s a p p e a r e d to have been made in resolving these problems to launch a formal interc o m p a r i s o n p r o g r a m in t h e e a r l y 1980s with the goal of rigorously ex­ a m i n i n g t h e s e m e t h o d s (12). The

appeared

LIF

to offer

a simple

direct technique for measurement of atmospheric OH overall outcome of t h e s e activities was disappointing—the methods used to resolve the 0 3 / H 2 0 interfer­ ence also reduced sensitivity enough that meaningful measurement could not be made because the instrument operated predominately at the LOD ( - 1 - 2 χ 10 6 OH/cm 3 for 3 0 - 6 0 - m i n integration times). Researchers con­ cluded t h a t a n order of m a g n i t u d e improvement in detection limits was needed for future i n s t r u m e n t s , and because the techniques used in this study basically followed square-rootdependent, photon statistics back­ ground-limited performance, a 2-order-of-magnitude improvement in basic sensitivity appeared to be necessary. It seemed impossible to improve the sensitivity to this extent unless new approaches could be de­ veloped to reduce the magnitude of

both the laser-generated 0 3 / H 2 0 in­ terference and the background fluo­ rescence without sacrificing OH sig­ nal strength. The collective efforts of the various groups engaged in t h e s e activities were not, however, entirely wasted. Significant advances were made in understanding the fundamental photodynamics underlying those tech­ niques (13-15) and in creating tech­ nologies t h a t would be needed for future instrumentation. In particu­ lar, reliable internal reference sys­ tems were developed for the monitor­ ing and compensation of changes in l a s e r w a v e l e n g t h a n d energy, a n d methods were developed and imple­ mented for in-field calibration under atmospheric conditions (11). In addition, new methods were ex­ plored for reducing the 0 3 / H 2 0 in­ terference while maintaining sensi­ tivity. Initially t h e 0ΎΌ + H 2 0 -» 2 0 H r e a c t i o n t i m e was used as a m e a n s of segregating the artificial l a s e r - g e n e r a t e d OH from a t m o ­ spheric OH. In one such method an attempt was made to use shorter la­ ser pulses (on the order of 300 ps) to probe atmospheric OH before signifi­ cant amounts of O x D could form OH (16). Although three- to fourfold im­ provements were made in the instru­ m e n t a l LOD, t h i s approach was abandoned because of stability prob­ lems in the laser pulse-slicing tech­ nique. During this time, significant ad­ vances were also being made in the d e v e l o p m e n t of L I F - b a s e d t e c h ­ niques for use in the s t r a t o s p h e r e , where the combination of higher OH concentrations, lower pressure (far less collisional deactivation of elec­ tronically excited OH), lower H 2 0 vapor concentrations (smaller poten­ tial for 0 3 / H 2 0 interference), a n d generally cleaner conditions (signifi­ cantly reduced background fluores­ cence from concomitant gases and aerosols) all contributed to the suc­ cessful development of LIF i n s t r u ­ ments capable of making meaningful remote and in situ measurements of OH in this portion of the atmosphere (17-19). An e l e g a n t l y e n g i n e e r e d version of one of these i n s t r u m e n t s based on recently available solidstate lasers has resulted in compact instruments capable of making unat­ t e n d e d r o u t i n e s t r a t o s p h e r i c OH measurements. In parallel with these activities, other researchers realized that simi­ lar attributes might be obtainable in an instrument for tropospheric mea­ surements using an artificially gen­ erated low-pressure environment

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Ambient air Tunable laser light in multipass cell

Collected scattered light . Fast, gated detector

OH fluorescence to

c g> GO

Vacuum pump Other scattering

Figure 3. Multipass single-photon LIF measurement scheme. Fluorescence is collected by optics that are perpendicular to the airflow and the multipass laser light, shown in cross section as small ovals.

(20). R e d u c e d - p r e s s u r e s a m p l i n g systems offer two advantages. First, the 0 * ϋ + H 2 0 -> 2 0 H reaction rate is significantly slower because of the lower p a r t i a l p r e s s u r e s of 0 3 a n d H 2 0 . Second, a large portion of the laser-generated background fluores­ cence has a significantly shorter ra­ diative lifetime than does the excited Α 2 Σ + (ν'= Ο) state of OH, thereby al­ lowing temporally delayed gated sig­ nal detection techniques to reduce measured background levels without sacrificing signal strength. Even with these advantages, how­ ever, instrument sensitivity was still insufficient (21). In subsequent ver­ sions multipass cells were used to improve sensitivity, but once again 0 3 / H 2 0 interference problems in­ creased to unmanageable levels (15). In the most recent generation of in­ struments the OH excitation scheme was shifted to one in which the Χ 2 Π ( ν " = 0) -> Α 2 Σ + (ν" = 0) transition near 308 nm was used (22, 23). This resulted in substantial reduction of the 0 3 / H 2 0 interference problem be­ cause of a significantly smaller 0 3 absorption cross section at t h i s longer wavelength. The initial prob­ lem with this approach related to ad­ equately rejecting scattered light, be­ cause the observed fluorescence emission wavelength was now very n e a r t h a t of t h e excitation w a v e ­ length. Careful attention to the design

and fabrication of t h e i n s t r u m e n t has resulted in a sensor that may be capable of m a k i n g meaningful OH m e a s u r e m e n t s u n d e r tropospheric conditions (5). Some questions have arisen as to whether sampling of the highly reactive OH radical can be carried out q u a n t i t a t i v e l y u s i n g a restrictive orifice sampling inlet un­ der actual field-sampling conditions. Field test/measurement of an instru-

ment such as that shown in Figure 3 should address this concern. In addi­ tion, the instrument was used in in­ formal i n t e r c o m p a r i s o n a c t i v i t i e s during the summer of 1993. Two o t h e r t e c h n i q u e s e m e r g e d conceptually from the earlier efforts to detect OH. One involved the use of a n a r r o w - b a n d XeCl excimer L I F based remote sensing (LIDAR con­ figuration) approach. Unlike the more controlled conditions obtain­ able in the lower p r e s s u r e s y s t e m j u s t described, t h e LIDAR s y s t e m must contend with intense solar r a ­ diation background and a much larger laser-generated scatter/ background fluorescence. This tech­ nique was never fully developed and h a s for t h e most p a r t been a b a n ­ doned. The other approach involves using a sequential two-photon excitation scheme (23, 24) in which either the Χ 2 Π (v" = 0) -> Χ2Π (v" = 1) or Χ 2 Π ( ν " = 0) -> Χ 2 Π ( ν " = 2) vibrational t r a n s i t i o n is first excited, followed by the excitation from one of t h e s e vibrational levels to the Α 2 Σ s t a t e (Figure 4). The fluorescence emission is t h e n d e t e c t e d at a w a v e l e n g t h shorter than that of either excitation laser. This scheme offers two signifi­ cant advantages. First, the UV laser wavelength used for the Χ 2 Π —» Α 2 Σ excitation step is at a sufficiently long wavelength that the 0 3 / H 2 0 in­ terference is virtually nonexistent. Second, the background fluorescence from concomitant gases and aerosols occurs p r e d o m i n a t e l y a t a w a v e ­ length equal to or longer than t h a t of

Air exhaust

Air inlet

-y

* UV laser ~ 345 - 350 nm

IR laser 1.4 μπι 0Γ2.9μΓη 2 v - — νν1 == ο Α^Σ > \ 3 4345 : nm 308 nm

χ2π -

/

2.! !.9μπη v" = 0

v" = 0

Figure 4. Sequential two-photon LIF measurement scheme.

932 A · ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993

REPORT the laser, t h u s allowing v i r t u a l l y complete discrimination against la­ ser-generated background, as was demonstrated by a similar system for the measurement of nitric oxide (25). Thus far, however, it has not been possible to produce an i n s t r u m e n t capable of demonstrating adequate sensitivity because of the lack of suf­ ficiently energetic lasers in the 1.4or 2.9-μπι region. Recent advances are promising and, based on the per­ formance of newly commercialized ΟΡΟ technologies (e.g., the Spectra Physics ΜΟΡΟ), an LOD in the 1 χ 10 5 m o l e c u l e s / c m 3 r a n g e u s i n g a 1-min integration time is predicted. This technique is currently the only airborne-compatible in situ direct measurement technique t h a t can be used to measure OH under the atmo­ spheric conditions r e l e v a n t to t h e troposphere. It is also compatible with the in-field calibration technol­ ogies developed for u s e w i t h i t s single-photon LIF forerunner. Indirect measurement Several t e c h n i q u e s m a k e u s e of chemical rather than optical proper­ ties to measure ambient OH concen­ trations. These chemical techniques can be divided into two categories, real time and photostationary state, which differ by the time scale associ­ ated with chemically converting OH into another, more readily measur­ able c o m p o u n d . R e a l - t i m e t e c h ­ niques quantitatively convert ambi­ ent OH directly, via a 1:1 reaction scheme, whereas p h o t o s t a t i o n a r y state techniques rely on maintaining steady-state ambient OH concentra­ tions for a significantly longer period of time so that the detected reaction products can build up to sufficient levels for analysis. Both types rely on the addition of an exotic compound t h a t r e a c t s w i t h OH a n d is not present in the natural atmosphere in significant concentrations relative to those being measured. S e l e c t e d - i o n MS. Only one real­ time chemical m e a s u r e m e n t tech­ nique is currently being used. In this technique, OH molecules are titrated into gas-phase H 2 3 4 S 0 4 molecules by the addition of 3 4 S 0 2 and subsequent reactions with ambient 0 2 and H 2 0 on a time scale that is short (10 ms) compared with the OH lifetime ( 0 . 1 1.0 s). The resulting H 2 3 4 S 0 4 con­ centration is then measured by a t ­ mospheric pressure selected-ion chemical ionization MS, as shown in Figure 5. Although several gas-phase chem­ ical reactions are involved, the reac­ tion processes are completed in only

Propane addition Periodic OH .production Ambient air

Ε c

Mass H 2 3 4 S0 4

00

Calibration check

4JO +H 2 SO, è reaction

analysis

NO3 34

S02

addition

Figure 5. Real-time titration/selected-ion chemical ionization MS. 10-20 ms and take place in an effec­ tively wall-less flow tube. This tech­ nique offers an LOD of ~ 1 χ 10 5 mol­ ecules/cm 3 with an integration time of 5 min (26). A test of the i n s t r u m e n t calibra­ tion t h a t can be conducted in t h e field has recently been incorporated into t h i s MS t e c h n i q u e . A m b i e n t H 2 0 is q u a n t i t a t i v e l y photolyzed with 184.9-nm photons to generate a known amount of OH in a sampled airflow before entering the measure­ ment apparatus. The sampling and measurement of this known OH con­ centration then provide a test of the instrument's ability to measure OH under ambient conditions. An infor­ mal intercomparison between this latter technique and a long-path ab­ sorption technique was conducted in 1991, and the level of agreement be­ t w e e n t h e s e two t e c h n i q u e s w a s quite good considering the vast dif­ ferences in spatial scales being inter­ rogated (27). Photostationary-state techniques h a v e been used to d e t e r m i n e OH concentrations over a wide range of times a n d a r e a s from 10-s in situ m e a s u r e m e n t s to yearly averaged global e s t i m a t e s . I n t h e s e t e c h ­ niques, the quantity of the compound to be reacted with OH must be suffi­ ciently small that the photochemical steady state t h a t existed before the a d d i t i o n is not significantly per­ t u r b e d . After a k n o w n t i m e , t h e added compound (or its product) is measured and, in conjunction with its reaction rate constant and reac­ tion time, t h e OH concentration is calculated. In the in situ OH tech­ nique, the concentration of the added reactant compound is controlled by mixing it with air in a flow tube. In a controlled-release experiment used to calculate OH on a time scale of hours and over areas on the order of a kilometer, two compounds—one

that will react with OH and one that will n o t — a r e r e l e a s e d in u n i s o n . These compounds m u s t also h a v e identical and ideally near-zero loss by reaction with other compounds (e.g., 0 3 and N 0 3 ) or by photolysis. The concentration of the nonreactive compound is then used to calculate the decrease in concentration of the reactive compound caused by mixing. Any additional decrease in concen­ tration of the reactive compound is then attributed to its reaction with OH (28). On a still larger scale, globally av­ eraged concentrations of OH are cal­ culated from t h e atmospheric life­ t i m e s of long-lived anthropogenic species such a s methylchloroform (29). Lifetimes for t h e removal of these compounds by OH are based on t h r e e factors: t h e m e a s u r e d a t m o ­ spheric concentrations of t h e com­ pounds, relatively well-known emis­ sion rates of the compounds into the atmosphere, a n d best e s t i m a t e s of other losses (which are thought to be small). Although the latter two techniques provide an important means for de­ t e r m i n i n g OH concentrations over extended periods of time and space, they a r e not directly applicable to fast photochemistry and t h u s have t h e i r own very different m e a s u r e ­ ment and data interpretation prob­ lems, which are beyond the scope of this article. Only one in situ photostationarystate technique currently is used. In this t e c h n i q u e , a small a m o u n t of 14 C 0 is added to air t h a t is pulled into a quartz reaction chamber, and the 1 4 C 0 reacts with the OH in the chamber to form 1 4 C 0 2 . The radioac­ tive 1 4 C 0 2 is then cryogenically en­ riched and measured by using either a p r o p o r t i o n a l or a s c i n t i l l a t i o n counter, as shown in Figure 6. The concentration of 1 4 C0 2 —in conjunc-

ANALYTICAL CHEMISTRY. VOL. 65. NO. 21. NOVEMBER 1, 1993 · 935 A

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Ambien air

^

CO + OH —

OH

14

C0 2

^=%,

c? < w

^

t?

HOc 2