New insights into old problems on chemical transformations in photochemical smog are provided by tunable lasers and long path FT-IR spectrometers James N. Pitts, Jr. University of California Riverside, Calif. 9252 1
Barbara J. Finlayson-Pitts California State University Fullerton, Calif. 92634
Arthur M. Winer State wide Air Pollution Research Center Riverside, Calif. 9252 1 Application of tunable lasers and infrared Fourier transform spectroscopy to atmospherically important systems has led to the identification and quantification of a number of species in smog chambers, in laboratory experiments, and in the polluted troposphere. Many of these are labile molecules or free radicals whose presence had been previously postulated on sound kinetics and mechanistic grounds, but not directly observed. Others had not even been suspected to exist in atmospheric systems. Additionally, the development of tunable lasers has greatly aided elucidation of the mechanisms of certain important atmospheric reactions, including primary photodecomposition processes. The identification and accurate measurement of the primary pollutants, intermediates, and products in photochemical air pollution continues to be a high priority in atmospheric and related laboratory studies. Such an improved data base is not only essential for a better understanding of the complex chemical and physical transformations involved, but is also necessary if future oxidant control strategies are to be made more cost-effective and responsive to the complex trade-offs between energy, environmental, and economic constraints (€SAT, May 1977, p 456). Those familiar with the approach traditionally taken by chemists in unraveling the details of reaction mechanisms in the laboratory will readily understand the need to obtain improved analytical information. Without detailed knowledge of the nature and concentrations of the intermediates and products, the mechanism of even a relatively simple laboratory reaction cannot be considered valid. The extent of the challenge facing atmospheric chemists becomes clear when one recognizes that similar information must be obtained for an incredibly complex heterogeneous reaction system involving hundreds of different species. Some of these are stable while others are highly reactive, with lifetimes of the order of seconds or less. All interact in a diurnal cycle that includes periods of both light and darkand at concentrations in the part per million (ppm) range at most, and more frequently in the part per billion (ppb) range for molecules, and even lower for free radicals. 568
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Thus the validation of complex computer kinetic models of photochemical smog, which are becoming increasingly important in the development and implementation of oxidant control strategies, and which often involve a hundred or more reactions, demands identification and measurement of a large number of species. While traditional wet chemical methods have yielded valuable information in the past about the nature and concentrations of the major species involved in photochemical smog-to an increasing extent during the past 10 years-sensitive and specific physical techniques have been developed and applied. Thus, it is only recently that some of the important short-lived intermediates and labile products, postulated in the past, have been identified and quantitatively measured. The advances in this area are due in large measure to the development of high resolution optical detection techniques such as tunable lasers and Fourier transform infrared spectroscopy (FT-IR). For example, the development of ultraviolet tunable dye lasers was necessary for the direct detection of at least one key intermediate species, the hydroxyl radical, in photochemical smog. Such lasers are also used in the elucidation of the wavelength dependence of the photodecomposition modes of important pollutant molecules. The latter absorb solar ultraviolet radiation in the troposphere (-290 < A < 400 nm) and dissociate to produce radicals or atoms capable of initiating the photooxidation of organics. The use of lasers for the remote monitoring of certain primary pollutants such as CO, NO, NO2, and SO2 has been discussed in the companion.article by Hinkley (this issue), and by others. Thus we shall emphasize, instead, recent studies with tunable lasers and Fourier transform infrared spectroscopy, in which certain free radical and labile molecular species present in ambient and simulated atmospheres were directly identified and measured. These species are significant in oxidant control either because of their mechanistic role in establishing kinetic computer models, or alternatively, because of their potential effects on health and visibility, or both. Discussion of combined gas chromatography-mass spectrometry, a powerful analytical tool that has been successfully applied to the determination of many relatively stable atmospheric species (including particulates), is beyond the scope of this feature. UV tunable dye lasers Several unique properties of lasers make them especially useful in analytical, kinetic, and mechanistic studies of polluted atmospheres. These properties include: the capability of "tuning" the radiation of certain lasers over selected wavelength regions in the infrared, visible, and ultraviolet regions the generation of highly monochromatic radiation with little spatial diffusion of the beam (extremely good collimation) the availability of a wide range of power outputs (photon flux) ranging from relatively weak to extremely intense radiation. A notable achievement made possible by the development of lasers has been the recent detection and measurement in ambient air of low steady-state concentrations of the hydroxyl
radical (OH), the key species responsiblefor initiating the free radical oxidation of hydrocarbons, and indeed most organics, in the natural and polluted troposphere. Sources of ambient OH include: the reaction of electronically excited oxygen atoms, 0 ('D), with water the reaction of NO with the hydroperoxyl radical (H02) the photolyses of nitrous acid (HONO) and hydrogen peroxide (H202)by solar UV radiation. The role of ambient OH was suggested by Leighton almost two decades ago, and confirmed by indirect, though convincing, kinetic evidence acquired in laboratory systems over the last 10 years. Only recently, the first direct determination of OH in ambient air was reported by Wang, Niki, Weinstock, and co-workers at Ford Motor Company. Subsequently, Davis et al., then at the University of Maryland, did the first measurements of OH conVolume 11, Number 6,June 1977 569
centrations in the troposphere as a function of location and altitude by using an aircraft as the instrument platform. While differing somewhat in approach and detail, both of these research teams used tunable UV dye laser systems to monitor OH radicals by their resonance fluorescence. The dye laser was directed into ambient air samples and tuned across a region centered at 282.6 nm, a wavelength that is absorbed by the OH present. A small fraction of the electronically excited OH radicals thus generated return to the ground state by emitting fluorescence, which is detected at 309 nm. The magnitude of this signal, when correctedfor background, is a direct measure of the ambient OH concentration which in the initial experiments ranged from 106-107 radicals/cm3.Such concentrations are consistent with the observed rates of hydration decay (owing to OH attack) in smog chambers (€SAT, July 1976, p 692) and in the atmosphere (ES&T,March 1976, p 256). as well as with levels predicted by computer modeling studies. Laser magnetic resonance Techniques for the detection of radicals such as HOz and R 0 2 under atmospheric conditions have not yet been developed. Nevertheless, several detection methods have been successfully applied in laboratory studies where these radicals are present at much higher concentrations and in simpler systems than the atmosphere. For example, laser magnetic resonance has been applied by Howard and Evensen of the NOAA Laboratories to kinetic studies of OH, HOZ,and other free radicals, and Radford and Russell have observed the methoxy radical (CH,O) by this technique. In these studies, absorption of laser radiation in the far infrared results in transitions between rotational energy levels, which have been brought into resonance by application of a magnetic field. This method affords high sensitivity for free radicals and when combined with a discharge flow system, for example, offers a direct and powerful method for determining absolute reaction rate constants of these radicals with a variety of atmospherically important species. HOz, in particular, is a species whose kinetics are notoriously 570
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difficult to study; laser magnetic resonance is one of the few direct techniques available for such studies. However, while such laboratory detection methods for free radicals may ultimately prove adaptable to ambient conditions, the stringent requirements of high sensitivity and specificity, in the presence of a host of other pollutants, may preclude their successful application in atmospheric systems in the near future. Modes of photodecomposition The high power and monochromaticity of lasers have also made them valuable in studying the modes of photodecomposition of primary and secondary pollutants under solar irradiation. For example, ozone in the troposphere is important not only because of its impact on human health and on agriculture, but also because upon absorption of actinic ultraviolet light (290 X 400 nm) it photodissociates into molecular oxygen plus an oxygen atom. If the light is sufficiently energetic (X 5 318 nm), the oxygen atom can be produced in an electronicallyexcited state, O(’D); a certain fraction of the O(’D) atoms then react with water vapor to form OH radicals. While the wavelength dependence of the quantum yield for O(’D) and, in particular, the precise onset for its production (an important input into chemical models of the stratosphere as well as the troposphere) were in dispute for some years, the use of tunable laser radiation with narrow linewidths has now clarified the wavelength dependence. Tunable UV lasers also have been useful in elucidating the photochemistry of formaldehyde (HCHO), a key photoinitiator of oxidant. Upon absorbing UV radiation, formaldehyde dissociates by two competing paths, to H HCO and to H2 GO. The free radicals Hand HCO react with molecular oxygen to produce HOZ, which is very important in initiating and propagating oxidation chain reactions in smog. In addition, it is speculated that HC03 may be formed and may then abstract hydrogen atoms from organics to form formic acid (HCOOH), which has been observed in both laboratory studies and ambient air. Unfortunately, the results 0f.a number of earlier laboratory studies of the photolysis of formaldehyde were in severe disagreement with respect to the relative, as well as absolute,
