TEMPLATE DESIGN © v DIRECT DEMONSTRATION OF THE GREENHOUSE EFFECT 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. Experiment One Detection of CO 2 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. 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. 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 (CO 2 ) in the cell increased the temperature by about 1 degree C when compared to having only nitrogen (N 2 ) in the cell. This is due to the IR absorption of CO 2. The temperature difference is relatively small due to the small size of our tube. To more closely mimic the entire column of CO 2 in Earth’s atmosphere we would need to use a tube of pure CO 2 that is roughly 4 times longer. This experiment shows that CO 2 is a greenhouse gas and plays an important role in warming the Earth. 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. Daniel Jaffe, Sonya Malashanka, Kevil Hall, Noah Bernays Department of Science and Technology, University of Washington, Bothell, WA, United States, Department of Atmospheric Sciences, Univ. of Washington, Seattle, WA, United States. Graph of 3 spectra overlay for 15 cm gas cell Gas out Gas in IR in Heat lamp IR transparent windows Temp probe with digital readout IR out Experiment One Learning Goals Experiment Two Results and Summary 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. 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.