Upcoming Presentations:
Pittcon '99 in Orlando, Florida, Thursday March 11, 1999 (Morning session)
Ultratrace Detection of Sulfur Dioxide in Air, A Novel Technique
Nicks Jr., D. K. and R. L. Benner
Abstract: The Continuous Sulfur Dioxide Detector (CSD) is a sensitive, high time resolution instrument for reliable measurements in the atmosphere. This new technology is based on a successful SO2 measurement technique involving sulfur chemiluminescence detection (SCD) that has been validated in a rigorously blind experiment sponsored by the National Science Foundation (NSF). Due to the sensitivity of the SCD, denuder separation technology and optimized fluid dynamics, the CSD is capable of measuring trace levels (<100 parts-per-trillion by volume (pptv)) of sulfur dioxide on the order of seconds. Novel digital signal processing with phase-locked amplification of the detector signal enhances the precision and temporal resolution of the CSD. By recording the raw detector signal at 10 Hz the data retrieval can be optimized for any particular application by modifying the parameters used in the data processing. The analysis can therefore be tailored to the environmental conditions, enhancing signal to noise when sulfur dioxide levels are low and optimizing for temporal resolution when levels are high. The instrument has advantages over existing technologies such as chromatography in that it provides accurate and reliable measurement of low pptv sulfur dioxide in sub-minute time resolution. The high temporal resolution and sensitivity of the CSD allows for more detailed study of the atmosphere from a moving platform such as an aircraft. The CSD has applications in many fields where sulfur dioxide is found in air, including process stream monitoring, pollution source monitoring and pollution receptor/compliance monitoring as required by the Clean Air Act.
Peer Reviewed Publications:
Click on title for Abstract:
Benner, R. L., J. Wu and D. K. Nicks Jr., Development
and evaluation of the diffusion denuder-sulfur chemiluminescence
detector for atmospheric SO2 measurements,
J. Geophys. Res., 102, 16,287-16,291, 1997.
Presentations:
Click on title for Abstract:
Nicks Jr., Dennis K., W. Lee Bamesberger, Richard L. Benner,
David R. Crosley, Sherry O. Farwell, Paul D. Goldan
and Douglas L. MacTaggart, Summary
of experimental set-up, results and conclusions from The Gas-phase Sulfur Intercomparison
Experiment Phase II (GASIE II), Presented at: The 1997 Fall Meeting
of the American Geophysical Union, San Francisco, CA.
Benner, Richard, Carl S. Benson, Nicole South, Dennis K. Nicks Jr.,
The Dynamics and Fate of Contaminants
in the Arctic Snowpack,
Presented at: The 1997 Fall Meeting of the American Geophysical Union,
San Francisco, CA.
Back to: Dennis' Homepage
Complete Abstracts for the Publications Listed Above:
Title: Development and Evaluation of the Diffusion Denuder-Sulfur Chemiluminescence Detector for Atmospheric SO2 measurements.
Benner, R.L., J. Wu, and D. K. Nicks Jr., J. Geophys. Res., 102, 16287-16291, 1997.
Abstract: The University of Alaska Fairbanks participated
in the GASIE intercomparison to evaluate the sulfur chemiluminescence detector
(SCD). The SCD is a relatively new sulfur selective detection technology
[Benner and Stedman, 1989, 1990, and 1994; Benner, 1991; Stedman and Benner,
1995]. The system used during GASIE is referred to as the Diffusion
Denuder/Sulfur Chemiluminescence Detector (DD/SCD). We chose to use
a diffusion denuder to separate SO2 from other sulfur
species rather than a gas chromatograph because one of our long-term goals
is to develop the DD/SCD into a unique system capable of resolving low
pptv levels of SO2 on time scales of one minute or
less. The principles of detection of The DD/SCD and how it was operated
during GASIE are presented.
Results from the GASIE program are presented in detail by Goldan
et al. and Crosley et al. (this issue). In this paper we briefly
describe what was learned about the Diffusion Denuder/Sulfur Chemiluminescence
Detector (DD/SCD) technique from GASIE. To address the problem, The
DD/SCD has been completely redesigned and rigorously tested in our laboratory.
The modifications made, how these modifications eliminate many of the problems
found during GASIE and results from subsequent laboratory testing are discussed.
Title: Summary of experimental set-up, results and conclusions from The Gas-phase Sulfur Intercomparison Experiment Phase II (GASIE II).
Dennis K. Nicks Jr., W. Lee Bamesberger, Richard L. Benner, David R. Crosley, Sherry O. Farwell, Paul D. Goldan and Douglas L. MacTaggart
Presented at: The 1997 Fall Meeting of the American Geophysical Union, San Francisco, CA.
Abstract: The Gas-Phase Sulfur Intercomparison Experiment phase
II (GASIE II) was conducted in 1997 to determine whether recent modifications
to the Diffusion Denuder Sulfur Chemiluminescence Detector (DD/SCD) eliminated
interferences principally from water vapor observed during phase 1 of GASIE
in 1994. The DD/SCD utilizes a K2CO3
coated denuder that selectively removes SO2 from the
sample gas. The sample gas flow is switched via a solenoid valve
between sample air and SO2-denuded sample air
resulting in a signal modulation proportional to the SO2
mixing ratio.
The sample gas was created in-situ by the Automated Sulfur Gas Dilution
System (ASGDS). The ASGDS consists of a commercial time proportional
dilution system that underwent extensive modifications and was optimized
for sulfur gases. The ASGDS is capable of delivering SO2
at mixing ratios in the part-per-trillion (pptv) range in numerous gas
matrices. Quality assurance and control of the ASGDS sample gas was
maintained by using two commercial SCDs. One was used as a continuous
monitor of total sulfur gas mixing ratios in the sample gas. A second
was interfaced to a gas chromatograph with a cryogenic trap/focusing apparatus.
The DD/SCD technique for field measurement of trace atmospheric (<200pptv)
SO2 was subjected to a range of analyte mixing ratios
and potential interfering gases. The potential interferents included
were H2O vapor, CO2,
CO, CH4, nitrogen oxides (NOx),
CH3SCH3
(DMS) and O3. GASIE II was conducted
in a rigorously blind fashion, so the DD/SCD and ASGDS investigators were
kept unaware of each other's results. Results of the experiments
and conclusions drawn from the data will be presented.
Title: The Dynamics and Fate of Contaminants in the Arctic Snowpack
Richard L. Benner, Carl S. Benson, Nicole South, Dennis K. Nicks
Jr.: Geophysical Institute and Department of Chemistry, University
of Alaska Fairbanks, Fairbanks 99775.
Presented at: The 1997 Fall Meeting of the American Geophysical
Union, December 8-12, 1997.
Abstract: Atmospheric contaminants that are deposited in
the Arctic to the terrestrial and marine environments could have a profound
effect on the ecosystem. It is therefore important to understand
how chemical species are deposited to the surface and what processes influence
them once deposited.
The physical morphology and stratigraphy of Arctic snow have been studied
for several decades. These studies have revealed a dynamic system
that undergoes dramatic changes over the winter season. These changes
are driven by a very strong temperature gradient between the ground, which
is slightly below freezing and the air above that is well below freezing.
The temperature gradient produces a vapor pressure gradient, movement of
water vapor and recrystallization within the snow pack. A few measurements
of the electrical conductivity have hinted that the chemical composition
of the snowpack also changes during the winter. It is not surprising
that the physical changes in the snow are accompanied by chemical changes,
but the direction and magnitude of the changes are rather unexpected.
We have spent the last three winter seasons investigating the stratigraphy
of ion content in snow around Fairbanks. In these studies we are
attempting to determine what ions are changing, quantify the changes and
determine the mechanism causing the changes. These studies have concentrated
on two sampling sites equipped with a simple set-up. Prior to the
first snowfall we prepared a fenced-in area containing three snow packs:
1) snow on bare ground where free exchange of heat flux, ions and water
vapor could occur, 2) snow on a clean plastic sheet which maintained the
thermal contact but isolated the snow from exchange of ions and water vapor,
and 3) snow on a clean table which isolated the snow from exchange of ions
and water vapor as does the plastic sheet, but this case also eliminates
thermal contact with the underlying ground so strong temperature gradients
did not occur in the snow. Results from these studies and how they
have allowed us to eliminate server plausible hypotheses about dynamics
of contamination in the snow will be discussed.