Transcript 48x48 Poster Template - 2015 AGU Fall Meeting

DIRECT DEMONSTRATION OF THE GREENHOUSE EFFECT
Daniel Jaffe, Sonya Malashanka, Kevil Hall, Noah Bernays
v
Department of Science and Technology, University of Washington, Bothell, WA, United States, Department of
Atmospheric Sciences, Univ. of Washington, Seattle, WA, United States.
Abstract
Consider these three “theories:” climate change, evolution, and gravity. Why
are two of them hotly debated by non-scientists, but not gravity? In part, the answer
is that climate change and evolution are more complex processes and not readily
observable over short time scales to most people. In contrast, the “theory of gravity”
is tested every day by billions of people world-wide and is therefore not challenged.
We will describe two experiments. In the first, we use FTIR spectroscopy to
quantify the CO2 content of ambient air and indoor/classroom air. For this
experiment, we use a commercial standard of 350 ppm CO2 to calibrate the
absorption features. Once the CO2 content of ambient air is found, it is useful for
students to compare their observed value to background data (e.g. NOAA site in
Hawaii) and/or the “Keeling Curve”. This leads into a discussion on causes for local
variations and the long-term trends. This experiment is currently used in our general
chemistry class but could be used in many other science classes.
In the second experiment, we use a simple plexiglass tube, approximately 15
cm long, with IR transparent windows. The tube is first filled with dry nitrogen and
exposed to an IR heat lamp. Following this, the tube is filled with pure, dry CO2.
Both tubes warm up, but the tube filled with CO2 ends up about 0.7 degrees C
warmer. It is useful to compare this 15 cm column of CO2 to the column in the
earth’s atmosphere, which is equivalent to approximately 2.7 meters of pure CO2.
Both of the above experiments should lead to a greater understanding of the
scientific basis for the greenhouse effect.
The 15 cm gas cell has two IR
transparent windows on either side,
and it will be placed directly into the
FTIR cavity. See photos to the left.
First the 15 cm cell will be filled with
nitrogen to purge the cell. We found it
important to scrub the nitrogen with
ascarite to eliminate residual CO2.
A background spectrum will be taken
of this purged cell. The cell will then be
filled with 355ppm carbon dioxide. A
sample spectrum will be taken. This is
repeated with inside air and outside air,
much like the previous experiment.
The data will be analyzed the same
way.
In this example, the inside ambient air
concentration was 571 ppm. See
Graph to the left for example of data.
Experiment One Learning Goals
Detection of CO2 by FTIR
This General Chemistry Experiment is used to quantify the amount of carbon dioxide
present in various samples of air.
The 15 cm gas cell is first purged with Nitrogen, which is used as a background.
Then it is filled with 355ppm carbon dioxide. Carbon dioxide has an absorbance peak
at 2360nm. A spectrum is taken of the 355ppm Carbon Dioxide, and the absorbance
value of the peak at 2360 wavenumbers is noted. This is repeated for:
-Inside room air
-Outside room air
These three absorbance spectra and their three absorbance peaks for carbon dioxide
are analyzed using beers law,
A=E*L*C
To calculate the amount of carbon dioxide in each sample.
Typically there will be more carbon dioxide outside/inside than the 355ppm standard.
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Temp probe with
digital readout
IR in
IR out
Heat lamp
Gas in
Gas out
Graph of 3 spectra overlay for 15 cm gas cell
We first tried using a 10 meter gas cell, instead of the 15 cm gas cell, but found
that most of the absorption values were too large. So we built a smaller 15 cm
cell at relatively low cost. The 15 cm gas cell is recommended compared to
the 10 meter gas cell, because the 15 cm gas cell has absorbance values that
are small enough to be used to calculate the amount of carbon dioxide
present.
Experiment One
IR transparent
windows
The amount of carbon dioxide will vary with classroom ventilation and what is
happening outside the classroom. Students will have to use their knowledge of
greenhouse gasses and IR spectroscopy to explain possible reasons why there is
more carbon dioxide present outside or inside.
Experiment two
This experiment shows that IR light is absorbed by some gases but not others. The
heat lamp is the source of IR light and as it shines through the 15 cm gas cell, and
some of the light is absorbed by the gas inside. To get reliable data, this experiment
needs to be done in a controlled laboratory environment, where the room temperature
variations can be carefully controlled.
Experiment Two Results and Summary
After each gas is introduced into the cell, it takes time to reach temperature
equilibration. Initially, addition of both gases tend to cool the cell. However, after a
few minutes the data show that having carbon dioxide (CO2) in the cell increased
the temperature by about 1 degree C when compared to having only nitrogen (N2)
in the cell. This is due to the IR absorption of CO2. The temperature difference is
relatively small due to the small size of our tube. To more closely mimic the entire
column of CO2 in Earth’s atmosphere we would need to use a tube of pure CO2
that is roughly 4 times longer. This experiment shows that CO2 is a greenhouse
gas and plays an important role in warming the Earth.
Why are these experiments
important to education?
It is important for students of all ages to understand the
greenhouse effect. These experiments provide a
straightforward way to observe and understand the
greenhouse effect.
Understanding the greenhouse effect through
observation will lead to a greater acceptance of the
occurrence of climate change.