Yuk L. Yung

   
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Professor of Planetary Science

Division of Geological and Planetary Sciences
California Institute of Technology
Pasadena, CA 91125

Research CV Bibliography LaGESI Cassini OCO

Atmospheric Chemistry and Global Change

Three themes underline the research program of Professor Yuk Yung and his colleagues: What are the processes controlling the current chemical state of the earth's upper atmosphere? What is the proper description of chemistry and radiative transfer in planetary atmospheres in the current epoch? What are the processes that have governed the evolution of atmospheres in the solar system to what exist at the present time? These various studies require a close interaction between theory, modeling, laboratory spectroscopic and kinetic studies, and observations. JPL groups with whom there is much contact do most of the laboratory work and measurements and the campus effort focuses on modeling and data comparison. A major modeling tool used until now is a highly interactive one-dimensional computer code in which vertical transport and chemistry are coupled. Recently developed for studying the terrestrial atmosphere and other planetary atmospheres is a two-dimensional model description of chemistry interacting with global-scale dynamical fields. Such a model is necessary for analysis of global data sets. These data sets were recently acquired by satellite experiments and will be acquired by upcoming NASA projects monitoring the Earth such EOS (Earth Observing System) and to other planets (Cassini, a mission to Saturn and Titan). We are also in the process of developing a three-dimensional chemical tracer model. A new effort has recently been initiated between Caltech and JPL to utilize global data sets for the study of global environmental change. See http://www.lagesi.caltech.edu for details.

Prof. Yung is the author of two books and more than 100 professional papers. The complete list of papers is referred to in the bibliography. The two books are listed as follows.

Books

Photochemical Studies of Chemistry in the Outer Solar System

The goal of our research program is to gain a quantitative understanding of chemical processes and their coupling with atmospheric dynamics in reducing atmospheres of the outer solar system, with a particular focus on Infrared Space Observatory (ISO) observations and future experiments such as the Cassini Mission to Saturn and Titan.

For over a quarter of a century since Strobel's (1973) pioneering work, the study of hydrocarbon chemistry has become a centerpiece of atmospheric chemistry in the outer solar system. Methane, the parent molecule of hydrocarbons, is ubiquitous in reducing atmospheres. The pathways for the synthesis of more complex hydrocarbons are identified in the laboratory and the products are readily observed in the atmospheres. A fundamental understanding of the chemistry of hydrocarbons has the following implications. 1. The chemistry is unique and rich and the same reactions apply to all atmospheres in the outer solar system. 2. Once the chemistry is understood, the distribution of the chemical species may be used to infer atmospheric transport. 3. The hydrocarbon chemistry inevitably leads to the formation of high molecular weight products, giving rise to aerosols known as the Axel-Danielson dust. The latter exerts a profound influence on atmospheric radiation and transport. 4. On Titan the atmospheric chemistry results in massive, irreversible loss of the atmosphere and provides a case study for atmospheric evolution in the solar system. 5. The hydrocarbon chemistry is the simplest example of organic synthesis and may provide insight into abiotic synthesis and origin of life on early Earth. 6. The hydrocarbon chemistry is expected to apply to extrasolar planets and can provide a basis for comparative planetology between solar system planets and extrasolar planets.

Our work is divided into two related tasks addressing the relevance of recent Infrared Space Observatory (ISO) observations and future experiments such as the Cassini Mission for critically testing our understanding of the hydrocarbon chemistry. The first task is a synthesis model that employs a consistent set of photochemical reactions applied to all the atmospheres of the Jovian planets and Titan. Starting from Gladstone et al., (1996), this effort will allow us to arrive at a quantitative understanding of hydrocarbon chemistry. Comparing the various atmospheres determine in particular the components of the chemistry that are most important as well as insights into transport.

The second task is related to using the model for studying the atmospheric evolution and the history of volatiles on Titan using isotopic fractionation. The simple chemistry of hydrocarbons, together with the high rates of production, provides a unique window into the evolution of an atmosphere.

Publications related to this project

Thesis

The Chemical and Isotopic Constraints for Biology on Ancient and Present Mars

Our modeling effort is naturally divided into two components related to the inhabitability and inhabitance of Mars. The emphasis of the first goal is to evaluate the various loss processes that are important for the evolution of the Martian environment and to relate the evolutionary history to available isotopic measurements. The second goal is the relationship of the planetary environment to the origin and continuation of life on Mars.

Liquid water is the quintessence of all known forms of life on Earth. The surface of Mars is covered by ancient fluvially generated channels, strongly suggesting the existence of a warmer climate that could sustain the flow of water (Carr, 1996) and life. That is very different from the current cold and arid climate. What happened to all that water and how much of it remains on Mars today? This is central to the question of inhabitability on Mars.

The question was answered in a classic paper by McElroy (1972) using a combination of Mariner 9 observations that the planet is surrounded by a corona of H atoms (Barth, et al.,1972) and a model of the aeronomy of Mars. The escape flux of hydrogen was inferred to be 1-2 x 108 cm-2 s-1. According to the theory of McElroy, there is a corresponding escape of O atoms at half the rate of hydrogen escape, so the net result is the loss of H2O from Mars. Over geological time the recent escape rate implies a total loss of water equal to 280 mbar in the atmosphere, or 3 m of water uniformly spread over the Martian surface. This is probably a lower limit because in the past there was probably more water in the atmosphere and the sun could have been more active. Inclusion of this possibility raised the amount of escaped water to 50 m (Kass and Yung, 1995; Kass and Yung, 1999; Johnson and Luhmann, 1998). For comparison, we note that the mean column-integrated H2O in the atmosphere today is 8.8 mm, five orders of magnitude less than the minimum amount of water that have escaped. The atmosphere of Mars today is too cold to contain much water vapor. The bulk of the remains of the once wet planet must be sequestered either at the poles (Zuber, et al., 1998) or in the ground. A crucial question is: "How big is the reservoir?" What new information is available to address the key questions concerning the planetary environment on Mars?

Publications related to this project

Thesis

Chemistry and Transport in the Terrestrial Atmosphere in a Multi-dimensional-model

We have two primary scientific goals: the hydrological cycle of the stratosphere and the modeling of atmospheric chemistry and and biogenic sources and sinks of methyl bromide. Our efforts are aimed at integrating new information obtained by spacecraft, shuttle and oceanic measurements to achieve a better understanding of the chemical and dynamical processes that are needed for realistic evaluations of human impact on the global environment. The focus is the exchange between the stratosphere and the troposphere, and between the troposphere and the biosphere. The major datasets we use for detailed comparison with model results include those obtained by the Stratosphere Aerosol and Gas Experiment (SAGE II), the Halogen Occultation Experiment (HALOE) and the Atmospheric Trace Molecular Spectroscopy (ATMOS).

The second task is to initiate an in-depth examination of the atmospheric and oceanic budget methyl bromide (CH3Br). We will address some essential aspects of the coupling between the marine and terrestrial environments and the atmosphere that are poorly quantified at present for CH3Br, whose regulation has recently been discussed at the highest levels of government. A major goal is to characterize sources, sinks, and distributions of CH3Br, including coupled biological, chemical, and physical interactions. We will use the Caltech/JPL CTM to carry out the following scientific objectives: (a) to characterize the latitudinal and seasonal distribution of CH3Br and (b) to identify (or constrain) the missing CH3Br and (c) to assess the impact of global warming on the oceanic sources of CH3Br.

Publications related to this project

Application of Global Data Sets to Study Global Change

One major reason why the fundamentally important issue of climate variability is a belated subject is the lack of long time series of carefully calibrated global data sets. As poignantly pointed out by Rasool (1999), "It is clear that to build a 10- to 20-year history of planetary-scale changes with the needed accuracy to assess the predicted global climate change requires extreme caution. The reason is obvious. The "weather satellites" [NOAA (National Oceanic and Atmospheric Administration) series] that are used to unravel these long-term and highly subtle changes were not designed for this job. They were made to observe the changes in weather patterns from one day to the next, not to monitor climate changes from one year to another. The latter can only be achieved with great difficulty, if at all." If we date the birth of the space age with the launching of the Sputnik in 1957, that is only four decades ago. Within this short period, the bulk of global data sets were collected for the purpose of meteorology, not climatology. This is the ultimate limit that motivates the collection of the next generation of global data by the National Aeronautics and Space Administration's (NASA) Earth Observing System (EOS).

While very few remote sensing instruments are precisely calibrated enough to reveal interannual variations in the cryosphere, we can nevertheless learn from the study of one well calibrated decadal record of the ultraviolet (UV) reflectivity of the Earth taken by the Nimbus-7 Total Ozone Mapping Spectrometer (TOMS). The TOMS instrument measured the incoming and the backscattered solar radiance, from which reflectivity was derived [Herman et al., 1991; Brest et al., 1997]. It was largely based on the integrity of this data set that the problem of anthropogenic impact of chlorofluorocarbons (CFCs) was solved and the loop involving hypothesis, modeling, global data, enviromental policy and regulation was successfully closed (WMO, 1999). Nothing is unusual about this data set except for its remarkable precision, which is about 1% per decade. This makes the TOMS data set ideal for tracking interannual variability and trends in surface reflectivity. Recently Kuang and Yung (1999) investigated the interannual variability of the Earth's UV albedo using TOMS (Version 7) monthly average data from 1979 to 1990. The results are summarized in Figure 1. The interannual variability is computed as the standard deviation of the interannual anomaly, which is the residual after the seasonal cycle is removed.

There are a number of important conclusions we can draw from Figure 1. First, the largest interannual variability in the tropical Pacific Ocean is due to El Niño, with its characteristic spatial and temporal pattern. Second, the bulk of the oceans has very little variability within the limits of the precision of the data (1%). This is a surprising result in view of the possibly large cloud feedbacks to climate change (IPCC, 1996). Third, the interannual variability on land is dominated by snow/ice. Fourth, in the polar oceans, the interannual variability is mostly due to sea ice. We have checked our results against Nimbus-7 SMMR derived Global Snow Cover and Snow Depth [Chang et al., 1987] and the International Satellite Cloud Climatology Project Cloudiness and Cloud Optical Depth [Rossow and Schiffer,1991]. There is good correlation with the snow/ice data but little correlation with the cloud data. As far as we know, this is the first time a global perspective of the variability of the UV reflectivity of the Earth over decadal time scale has been made. It remains a challenge for the climate models to reproduce this variability.

Perhaps the single most important result from the study of Kuang and Yung (1999) is that the interannual variability of the Earth's albedo (especially in Spring) on land is dominated by snow/ice, and not by clouds. This interannual variability could be the major driver of changes in the atmosphere and the biosphere. It is plausible that the interannual variability of snow/ice, through interactions with the atmosphere and biosphere, is responsible for the interannual variability of atmospheric CO2.

Figure 1. The interannual variability for the Earth’s UV albedo derived from TOMS reflectivity. The interannual variability is computed using the standard deviation of the interannual anomaly data, which is the residue after the seasonal cycle is removed. Data taken from TOMS Version 7 1979-1992.

Publications related to this project

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