Mark H. Thiemens Research Group Stable Isotopes Lab at the University of California San Diego, UCSD Department of Chemistry and Biochemistry Anthony Fry Dean Division Physical Sciences
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The research in the Thiemens group is directed towards resolution of several issues of contemporary interest in global climate. The sampling efforts are global in distribution and also include aircraft, balloon and rocket collections. The projects presently underway include the following:


As a constitutent species of the troposphere, ozone is responsible for billions of dollars loss annually to crops and forests from growth retardation. Health effects are well documented and in San Diego county alone, more than 1400 people are hospitalized annualy due to respiratory and cardiovascular problems associated with ozone inhalation. There are many outstanding issues with regard to ozone, in particular, what are its sources, transformation mecahnisms and exxtent of transport. Following development of the ability to perform high precision ozone isotopic measurements, we have become engaged in a field sampling effort to address these issues. The isotope ratios of atmospheric ozone provide specific information which relates to its chemical and meteorological history.


Tropospheric carbon monoxide is an important species because it serves (via a chemical reaction chain) as a major source of atmospheric ozone. In addition, it ultimately removes the hydroxyl free radical in the troposphere. This radical is also the predominant removal agent for essentially all reduced molecules such as methane, sulfur (e.g. sulfur dioxide and hydrogen sulfide) and volatile organic compounds. As the hydroxyl radical concentrations decrease, the lifetime of methane and its concentration increases and proportionately more leaks into the stratosphere. Once in the stratosphere, methane is a major catalytic ozone destruction agent. In addition, its stratospheric oxidation produces water, which is significant at these altitudes. It's infra-red absorption of planetary radiation alters the thermal structure of the stratosphere and concomitantly, all the atmosphere. The budget of carbon monoxide is poorly characterized and our global isotopic measurements are providing new insight into the atmospheric budgets.


Atmospheric nitrous oxide is a greenhouse agent, more effective on a per molecule basis than even carbon dioxide. Due to it's long (>110 years) lifetime, it enters the stratosphere where it is a dominant catalytic destruction agent of ozone. One of the greatest concerns with nitrous oxide is the inability to adequately describe it's budget; nearly 30% of its sources, presumably anthropogenic, are unrecognized at present. We have a global collection program to perform isotopic measurements on nitrous oxide to address this outstanding issue and detect and define it's sources. Aircraft, ground based and rocket sampling programs are utilized in this effort.


As a result of a novel isotope effect, stratospheric carbon dioxide acquires an anomalous isotopic composition. This isotopic composition provides what is perhaps the most sensitive measure of ozone activity in the stratosphere. As this is a reflection of overall stratospheric chemistry, the measurements are unique and couple powerfully with other chemical measurements and models. The anomalous isotopic composition startospheric carbon dioixde acquires also serves as a unique tracer of stratosphere-troposphere mixing. Our group pursues carbon dioxide collection from a global range of ground based, balloon, aircrfat and rocket samples.


It is well known that atmospheric sulfate aersols play an important role in global climate mediation in their capacity to serve as cloud condensation nuclei (CCN). There are many important unanswered questions confronting the atmospheric commmunity at the present time. How these particles are formed from gas phase precursors, what their atmospheric lifetime is and what proportions derive from homogeneous ( gas phase sulfur dioxide oxidation) or heterogenous (gas-lquid oxidation) processes is inadequately understood. What the importance is of other sulfur bearing molecules requires greater definition. We are measuring all four stable sulfur isotopes as well as all three oxygen isotopes in size fractionated atmospheric aerosol samples to address these issues.

Last updated: 2006

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