Understanding the photochemical, radiative, and dynamical processes that are at work in the Earth's atmosphere is a critical prerequisite for furthering our knowledge of global scale environmental change. For example, our understanding of the chemistry of the stratosphere has improved significantly during the past two decades and led to international efforts to limit the introduction of long-lived halogenated compounds into the atmosphere. These political actions were taken only when the scientific evidence linking chlorine and ozone destruction became constrained by atmospheric observations. In contrast, the chemistry and dynamics of the troposphere is only now beginning to be quantitatively understood. Precise and accurate measurements, made on short temporal and spatial scales, will be essential for further progress. The delicate balance between chemistry and dynamical processes such as convective transport in the tropics is well known, but a number of issues have arisen which demand new laboratory techniques that can investigate the fundamental physical and chemical processes at work along with novel instrumentation possessing ppt level sensitivity that can cover the tremendous temporal and spatial scales involved.
For example, stable isotopes have long served as an invaluable tool in geochemical studies of the solid and liquid reservoirs of the earth. As outlined more fully below, stable isotopes also have the potential to provide critical new insights into a variety of issues in atmospheric science, yet have been only rarely pursued. In part this arises because the measurements are difficult, requiring precisions and accuracies in the part per ten thousand range, but also because the science that can be pursued with stable isotopic studies of atmospheric trace gases is only now becoming clear. We have begun a new laboratory program that utilizes stable isotopes to examine the fate of important biogenic trace gases such as nitrous oxide, and are developing new in situ measurement approaches for such species using infrared laser induced fluorescence. With Paul Wennberg's group we are also beginning a new effort in Br/BrO detection in the vacuum ultraviolet, as described below.
Stables Isotopes and Nitrous Oxide Photolysis
Nitrous oxide exists in the Earth's atmosphere at a current concentration of approximately 310 ppbv and is increasing at an annual rate of about 0.25-0.31%. Despite being present in only trace amounts, nitrous oxide exerts a large influence on the terrestrial climate in two major ways: (1) It is one of the main greenhouse gases because of its long atmospheric lifetime (~100-150 years) and its large radiative forcing capabilities (nearly 200 times that of carbon dioxide), and (2) It is the principal source of NOx to the stratosphere, which plays a fundamental role in the catalytic depletion of ozone. The importance of nitrous oxide and its increase in the atmosphere make it one of the six gases targeted for regulation at the 1997 Kyoto Climate meeting.
However, the global budget of nitrous oxide is not presently well quantified. NNO, is formed mainly via microbes feeding on organic material as part of the nitrogen cycle. Once it is released from the soils and oceans, it is essentially inert in the troposphere. In the stratosphere, NNO is removed principally via photodissociation with a minor contribution (~10%) from oxidation by excited state oxygen atoms. In an effort to balance the NNO budget, various minor sources, as yet poorly constrained, have been identified. However, the total sources investigated to date can only account for approximately two-thirds of the well-established sinks.
Multi-isotopic analyses have provided useful constrains on the sources and sinks for other atmospheric species, but are only now being successfully applied to nitrous oxide. Researchers have begun to measure delta15N (d15N) and delta18O (d18O) isotopic shifts in NNO from the surface and deep ocean, cultivated and tropical rain forest soils, and the troposphere. There are also nine samples collected cryogenically from lower stratosphere. While the observed fractionation values scatter over a large range, the data do show a general trend: relative to tropospheric NNO, soil flux samples are significantly depleted in both d15N and d18O, surface oceanic samples are slightly depleted while deep ocean samples exhibit enrichment, and the stratospheric data appear to be strongly enriched in the heavy isotopes. It has been speculated that back-fluxing from stratosphere to the troposphere is responsible for balancing the light emission from soils. In the wake of these measurements, a variety of new chemical processes involving NNO have been suggested, and while these speculations have yet to be quantified they do indicate that additional field measurements and laboratory studies of NNO isotopes will be able to provide a new understanding of this pivotal biogenic species.
For example, in recent theoretical analyses of the photochemical destruction of nitrous oxide by Yuk Yung and Chip Miller, it was noted that the UV absorption spectra of the heavy isotopomers of nitrous oxide will be blue-shifted relative to the parent isotopomer due to the differences in their ground state zero point vibrational energies (ZPEs). This coupled with the spectral shape of the solar UV flux window around 205 nm in the lower stratosphere would lead to the preferential photodestruction of 14N14N16O relative to the heavy isotopomers. To test this hypothesis, we have doubled the visible output from a 355 nm Nd:YAG-pumped type II beta barium borate-optical parametric oscillator (BBO-OPO) to obtain intense tunable UV light to wavelengths as short as 205 nm. The UV light was directed into a coolable reaction cell by graduate student Hui Zhang to perform photolysis measurements over the 205-220 nm range. Pure NNO was mixed with a much larger amount of nitrogen quenching gas to ensure that no interference from reactions of NNO with singlet atomic oxygen produced by the photolysis occurs. NNO was then recovered and analyzed for d15N and d18O mass spectrometrically by Dr. Thom Rahn in Martin Whalen's group at Scripps. The figure below presents an overview of the results at 207.4 nm and 193 nm (with an excimer laser) which clearly shows a considerable enrichment in the heavy isotopes. Thom is now a post-doc in the group at Caltech, and continuing this work in a joint project with Professors John Eiler and Yuk Yung.
In Situ Instrumentation for Stable Isotope Measurements
Utilizing the isotopic signatures outlined above to examine global biogeochemistry requires extremely precise measurements, and so to date only mass spectrometers have been able to achieve the requisite dynamic range and precision. As a result, flask sampling is most often employed and the number of measurements is therefore limited both spatially and temporally. We have begun to develop new technologies with support provided by the NASA Planetary Instrument Definition and Development Program (PIDDP) that we believe will make possible the in situ measurements of abundances and stable isotope ratios in important radiatively and biogenically active gases such as carbon dioxide, carbon monoxide, water, methane, nitrous oxide, and hydrogen sulfide to very high precision (0.1 per mil or better for the isotopic ratios, for example). Such measurements, impossible at present, could provide pivotal new constraints on the global (bio)geochemical budgets of these critical species, and could also be used to examine the dynamics of atmospheric transport on the Earth, Mars, Titan, and other solar system bodies. We are combining of solid state light sources with imaging of the IR laser induced fluorescence (IR-LIF) via newly available detector arrays. Even under ambient terrestial conditions, the LIF yield from vibrational excitation of species such as water and carbon dioxide should produce emission measures well in excess of ten billion photons/sec from samples volumes of order 1 c.c. These count rates can, in principle, yield detection limits into the sub-ppt range that are required for the in situ isotopic study of atmospheric trace gases. While promising, such technologies are relatively immature, but developing rapidly, and there are a great many uncertainties regarding their applicability to in situ IR-LIF planetary studies. We have begun a three year program with PIDDP support to combine microchip near-IR lasers with low background detection axes and state-of-the-art Rockwell Science Center HgCdTe detector arrays developed for astronomical spectroscopy to investigate the sensitivity of IR-LIF under realistic planetary conditions, to optimize the optical pumping and filtering schemes for important species, and to apply the spectrometer to the non-destructive measurement of stable isotopes in a variety of test samples. These studies form the necessary precursors to the development of compact, lightweight stable isotope/trace gas sensors for future planetary missions. A schematic of the cryognenic imaging system (with our current InSb detector array), and possible overtone versus fundamental pumping schemes, are illustrated in the figures below. A fuller description of this research can be found by dowloading a recent white paper (Link to PDF File, 7.7 MB).
Tropospheric Bromine Chemistry
The lowest 1 km of the marine atmosphere (called the planetary boundary layer) plays a pivotal role in the chemical and radiative processes that shape the Earth's climate. It is in the boundary layer that much of the ozone present in the lower atmosphere is destroyed. In addition, most of the reduced organic compounds emitted to the atmosphere are oxidized by chemistry occurring in this small region of the atmosphere. Over the last twenty years, there has been considerable speculation that bromine may play an important role in the chemistry of the marine boundary layer. Theoretical models have suggested that reactions involving bromine can lead to destruction of ozone and, possibly, dimethylsulfide. Since the atmosphere is both one of the easiest regions of the planet to affect, it is imperative we understand these processes.
Ozone, for example, is a major source of the hydroxyl radical, the reactive molecular fragment that initiates the degradation of most of the effluent of the biosphere, including man's. The oxidation of dimethylsulfide generates sulfuric acid which in turn leads to the formation of the marine aerosol - the ubiquitous marine haze. Taken together, these species strongly influence the air quality of Southern California, particularly in the Los Angeles Basin.
The oxidative chemistry of bromine is thought to be driven by minute abundances of the bromine monoxide radical (BrO) and bromine atoms (Br). These compounds are extremely reactive; for example, at a concentration of four parts per trillion (4 ppt, or only four molecules for every 1012 molecules of air), BrO can destroy nearly all the ozone present in the boundary layer in only a few days! In fact, BrO present at a concentration even a factor of ten smaller will still play an important role in global marine boundary layer chemistry. But, how much Br and BrO is present in the global marine boundary layer? No one knows. Only in the high arctic springtime has BrO been detected in the troposphere, where the unique chemistry and dynamics produce BrO concentrations greater than 10 ppt. At this abundance, BrO can barely be detected using the technique of differential optical absorption spectroscopy , but only over regions several km in size and with integration periods approaching twenty-four hours!
Clearly, to address the importance of bromine globally and in regional settings an extremely sensitive and selective technique for measuring Br and BrO is required. Together with the Wennberg group we are developing just such a technique, in this case based on vacuum ultraviolet-laser induced fluorescence (VUV-LIF). By coupling newly available light source and sensor technology, we hope to be able to measure BrO and Br atom at concentrations less than 0.5 ppt in situ and thereby answer directly the question of whether or not bromine is an important oxidant in the troposphere. This has never been performed because of the difficulty producing a laser operating in the vacuum ultraviolet - the spectral region where Br can be detected. BrO can be studied with the same apparatus through its conversion to Br atoms by reaction with NO.
"Stable Isotope Fractionation in the Ultraviolet Photolysis of N2O'' Thom Rahn, Hui Zhang, Martin Whalen, & Geoffrey A. Blake 1998, Geophys. Res. Lett. 25, 4489.
"Fractionation of 14N15N16O and 15N14N16O During Photolysis at 213 nm" Hui Zhang, Paul O. Wennberg, Vincent H. Wu, & Geoffrey A. Blake 2000, Geophys. Res. Lett. 27, 2485. (Link to PDF File, 218 kB)
"Positionally Dependent 15N Fractionation Factors in the UV Photolysis of N2O Determined by High Resolution FTIR Spectroscopy" Fred Turatti, David W. Griffith, M.B. Esler, Thom Rahn, Hui Zhang, & Geoffrey A. Blake 2000.Geophys. Res. Lett. 27, 2489. (Link to PDF File, 1.44 MB)
"Photodissociation of Peroxynitric Acid in the Near-IR" Coleen M. Roehl, Sergey A. Nizkorodov, Hui Zhang, Geoffrey A. Blake, & Paul O. Wennberg 2002, J. Phys. Chem. A 106, 3766.
"Photolytic Fractionation of Stratospheric Nitrous Oxide" Geoffrey A. Blake, M.C. Liang, C.G. Morgan, & Yuk L. Yung 2002, Geophys. Res. Lett., submitted.
Other Links of Interest
Kinetics, spectroscopy, & atmospheric chemistry research groups:
Mitchio Okumura, CaltechAtmospheric chemistry field investigators I am friends/work with:
The ISOGEOCHEM web page (job listings, conferences, etc.)
| Molecular Astrophysics | Cluster Spectroscopy | Light Sources |
| G.A. Blake Home Page | GPS Home Page | Chemistry Home Page |
| Astronomy Home Page | ESE Home Page |