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Final Report: The Measurement of the Oxygen Isotopic Composition of Tropospheric Ozone
EPA Grant Number: R823117Title: The Measurement of the Oxygen Isotopic Composition of Tropospheric Ozone
Investigators: Thiemens, Mark
Institution: University of California - San Diego
EPA Project Officer: Shapiro, Paul
Project Period: August 15, 1996 through September 14, 1999
Project Amount: $211,727
RFA: Exploratory Research - Chemistry and Physics of Air (1995)
Research Category: Engineering and Environmental Chemistry
Description:
Summary/Accomplishments (Outputs/Outcomes):Even though tropospheric ozone constitutes only some 10% of the total ozone column, this modest amount exerts a vital influence of the chemical composition and oxidation state of the atmosphere. Ozone, via its photolysis, creates atomic oxygen, which subsequently produces two hydroxyl radicals following its reaction with water. The hydroxyl radical is well known to be the driving oxidant for nearly all reduced species, including CO, CH4 and SOx. In addition, ozone possesses spectral activity in the infrared region and, as a result is an important greenhouse agent.
Ozone concentrations have more than doubled at mid to high latitudes in the northern hemisphere over the past century. This doubling has profound influence upon human activities in the widest sense. First, ozone is well known (more than 100 years) to be a lung irritant. It has been estimated by the EPA that more than 140 million people live in ozone non attainment areas. In such areas, e.g. suburban areas downwind of source regions, will typically have high ozone concentrations which persist for hours. At O3 levels above 120 ppb, respiratory function effects have been clinically demonstrated to occur. Lung function reduction and increases in respiratory symptoms, such as airway reactivity, permeability and inflammation are typical indications. For people with breathing disorders, such as asthmatics, ozone exposure means restricted activities and increased medication.
Along with pernicious health effects, damage to agriculture is well documented. Net photosynthesis, and consequently crop yield, result from interaction with ozone. This has profound effects for areas such as the eastern United States. For example, ozone exposure to fast growing plants in the eastern U.S. resulted in a $2.1 billion cost due to diminished plant productivity. This is a result of a modest 25% increase in ozone. Forests are also known to incur significant damage due to ozone reactions.
The overall chemistry of ozone is complex, with concentrations demonstrated to be highly variable, with >1% yr-1 increases over many areas of the world. The complexity arises as a result of photochemical and transport processes on differential spatial and temporal scales. Present wisdom considers two tropospheric O3 sources, transport from the stratosphere and in situ photochemical production. These processes are balanced by two main loss processes, deposition at the Earth's surface, and in situ chemical destruction. An outstanding issue has been the quantification of these processes. In particular, variation in the inadequately understood sink processes means that there will be a significant variation in the O3 atmospheric lifetime. The situation is exacerbated in the winter, when O3 lifetimes are enhanced.
This grant utilized EPA funds to develop the ability to measure O3 isotopes at sufficiently high precision to address the issue of transport and removal mechanism by a new approach.
TECHNIQUE DEVELOPMENT
The most difficult component of the proposed work was the development of a technique to quantitatively collect O3. We have successfully developed the technique, which was published (Johnston and Thiemens with EPA acknowledgment, reprint attached). Ozone is collected with a cryogenic system. At ambient O3 concentrations, a collection temperature and pressure of 55.0 ? 0.1 K, 5.75 torr, respectively, is required. In this P-T regime, we have carefully established utilizing control experiments, that we attain efficient O3 trapping without O2 contamination. The difficulty with the original design was that O3 and Xe are collected together, and xenon decreases the O3 isotopic measurement precision. The average sample composition is 74% Xe, 21% O2 and 4% N2, with traces of argon. An off-line separation system has now been used where O3 (decomposed to O2) and Xe and transferred to a molecular sieve (13 x 60/80 mesh) at 77K. This sieve trap is used as a separation media by warming to 153 K and allowing the O2 to release ahead of the xenon. This system (reported in Chakraborty, Johnston and Thiemens, 1998) results in nearly 100% purification of the O2. The final composition is ~97% O2, 2% N2 and 1% Ar. At this purification level, our isotopic precision has been enhanced for d18O, d17O by nearly an order of magnitude. This entire apparatus is also portable, thus permitting transport to varying sampling sites. As part of this grant, several sites were utilized for measurement campaigns.
Due to the low level of ozone in the atmosphere, the ability to measure ultra small samples for all three oxygen isotopes is imperative. The average sample size is 0.08?0.03 micromoles of O2. With the funds provided by the EPA, we have been able to modify the stable isotope ratio mass spectrometer to perform the d17O, d18O measurements on these modest amounts of O3.
ATMOSPHERIC MEASUREMENTS
In the measurement programs, several distinct environments were utilized for ozone collection and isotope measurements. Those areas are as follows: (complete details are provided in Johnston and Thiemens, 1997).
(1) La Jolla, CA
This sampling region, part of the greater San
Diego area, has a population in excess of 2.7 million people. Ozone
concentrations occasionally exceed federal standards. The meteorology is
dominated by a semi-permanent Pacific high pressure system in the marine coastal
environment. Westerly Pacific winds predominate the sampling site was located
less than 1 mile from the Pacific Ocean. The isotopic composition of ozone in
the past several years reflects the steady state between ozone formation and
decomposition. The specifics of the data reflect the complexity of O3 chemistry.
The steady state fractionation factors associated with O3 formation are well
known. The observed isotopic results may not be strictly accounted for in terms
of ozone formation. These results require that a component of the La Jolla
isotopic variability must be accounted for by O3 decomposition processes. In
particular, it appears from most recent observations that there is a coupling to
NO concentrations with the NO + O3 reaction establishing the secondary
fractionation factor. Future measurements which determine the isotopic
fractionation factor for this reaction may permit us to utilize the isotope
measurements to determine the relative "age" of the ozone.
(2) White Sands Missile Range, New Mexico
A sampling campaign in
the WSMR was undertaken. This is a remote, high desert region in southern
central New Mexico. The d17O, d18O composition of the WSMR data was distinct
from the La Jolla measurements. The data suggest that known decomposition
processes are likely to have been important. The WSMR suggest transport as being
the main source of O3 to the WSMR. This sampling campaign has been particularly
important in evaluating the effect of long-range transport upon isotopic
composition.
(3) Pasadena, California
This area (California Inst. Technology
site) is urban, with an approximate population of >14 million people in the
greater Los Angeles area. Air quality in this region has been characterized by
the EPA as "extreme". The O3 isotopic data from this sampling campaign is
completely distinct, with the data defining a mass fractionation line. This has
been interpreted by Johnston and Thiemens as reflecting the dominance of sink
reactions, presumably the O3 + NO. These measurements have been important in
verifying the contention that the O3 stable isotope ratio measurements may be
utilized as a measure of O3 heritage and transport.
(4) White Mountain Research Station (12,500 feet, S. Calif.)
During March, 1997, a field sampling campaign to the White Mountain Research
Station was undertaken. This was the first winter occupation of the site in 2
decades. During this time of year, at 12,500 feet, stratospheric incursions are
frequent. The ozone isotopic composition from this time period exhibited the
largest 17O, 18O enrichment of any samples taken to date. The short lived
cosmogenic radionuclide 35S (87 day half life) was also measured for its
activity. It was observed that its activity was more than double ground level,
thus confirming the presence of an enhanced stratospheric component.
Furthermore, it is known that stratospheric O3 isotope possess a greater
isotopic enrichment compared to tropospheric, thus, all evidence supports the
notion that this is stratospheric O3. One of the most ill defined components of
the O3 budget, is the magnitude of the stratospheric O3 injections into the
troposphere. The funded work has demonstrated that stable isotope ratio measurements have the capacity to address this issue.
Journal Articles on this Report: 2 Displayed | Download in RIS Format
| Other project views: | All 2 publications | 2 publications in selected types | All 2 journal articles |
| Type | Citation | ||
|---|---|---|---|
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Chakraborty S, Johnston JC, Thiemens MH. Construction of a portable collection system for the isotopic analysis of tropospheric ozone. Analytical Chemistry. |
R823117 (Final) |
not available |
|
|
Johnston JC, Thiemens MH. The isotopic composition of tropospheric ozone in three environments. Journal of Geophysical Research 1997;102(D21):25395-25404. |
R823117 (Final) |
not available |
Air, Geographic Area, Scientific Discipline, RFA, Physics, Chemistry, Atmospheric Sciences, EPA Region, State, tropospheric ozone, oxidants, California (CA), fate and transport, ambient air, ozone, isotopic measurement technique, field measurements, stratospheric ozone, atmosphere, California
Progress and Final Reports:
Original Abstract