Case Study: Greenhouse Effect
Electromagnetic (EM) Spectrum Worksheet
- Go to the webpage - http://lectureonline.cl.msu.edu/~mmp/applist/Spectrum/s.htm This should provide access to the applet.
- The applet displays a scale with wavelength and frequencies listed along it. Note that the wavelength unit here is meters (1 m = 106 μm = 109 nm).
- Click anywhere on the wavelength scale (black region) or drag the blue bar across the scale. The exact wavelength corresponding to the location of the blue bar is listed below the scale (in meters and nm). Also listed below the scale (in blue color) is the name of the spectrum associated with that wavelength.
Using the EM spectrum applet, answer the following questions:
- Explore the Electromagnetic spectrum applet and fill out the table below relating the wavelength ranges associated with ultraviolet (UV), visible, infrared, and microwave spectra. Don’t forget to include units (units conversion: 1 m = 106 mm = 109 nm; 1000 nm = 1 μm).
- What wavelength spectrum is associated with heat radiation? Do you know any devices that rely on detecting these wavelengths?
Heat radiation is in infrared wavelengths. Infrared detection is used in thermal imaging, night vision goggles, certain non-contact thermometers, etc. For examples see http://coolcosmos.ipac.caltech.edu/cosmic_kids/learn_ir/index.html
- Arrange the spectrum ranges – infrared, UV, microwave, and visible – in the order of increasing wavelengths.
In the order of increasing wavelengths: UV, visible, infrared, microwave
Blackbody Spectrum Worksheet
- Open the blackbody radiation applet (http://phet.colorado.edu/en/simulation/blackbody-spectrum).
- Select a temperature by moving the slider (start with "Sun")
- Use the x-axis and y-axis zoom in and zoom out buttons until the blackbody emission spectrum becomes visible. Typical choices for axis limits are: (a) Sun: x-axis: 0 to 3 and y-axis: 0 to 100; (b) Earth: x-axis: 0 to 48 and y-axis: 0 to 0.0001.
Using the applet answer the following questions:
- Use the blackbody spectrum applet to fill out the table below:
Note that the peak wavelength is the x-axis value corresponding to the maximum of the blackbody intensity curve (red curve).
Note: Temperature units conversion: Fahrenheit (F) to Celsius (C): C = (F - 32)*5/9; Celsius (C) to Kelvin (K): K = 273 +C
- Using Excel, plot the relationship between temperature and peak wavelength. Remember to label the axes.
- Which of the above bodies mostly radiate light (i.e., emit EM radiation in the visible wavelengths) and which bodies will mostly radiate heat (i.e., emit EM radiation in the infrared regime). Recall the wavelengths associated with the visible and infrared spectrum.
Mostly light: Sun
Mostly heat: Light Bulb, Oven, Earth, Humans
- The area under the curve (i.e. the size of the space under the red curve) represents the net energy radiated out of a blackbody at the selected temperature. Explore how the area under the curve changes with temperature.
- Assuming that the curve is a triangle, calculate the area of the triangle as the product of the wavelength range and peak intensity.
Peak intensity: The maximum y-axis value of the blackbody intensity curve.
Wavelength range: A blackbody emits radiation over all wavelengths, and thus there is no finite wavelength range associated with a blackbody radiation. Here, lets define wavelength range as the width of the blackbody intensity curve at half the peak intensity level.
- Using Excel, plot the Area under the curve as a function of body temperature. Remember to label the axes.
- Assuming that the curve is a triangle, calculate the area of the triangle as the product of the wavelength range and peak intensity.
- Do incandescent light bulbs (3000 K) radiate more light or more heat?
The net light from a blackbody is proportional to the area of the emission curve in the visible wavelengths (400-750 nm). The net heat from a blackbody is proportional to the area of the emission curve in the infrared regime (~ 1 – 100 μm). For an incandescent light bulb, the area of the emission curve in the infrared regime is greater than the area in the visible regime, thus an incandescent light bulb generates more heat than light.
Atmospheric Gases and EM Radiation Worksheet
- Open the greenhouse gas and light applet or go to the link http://phet.colorado.edu/en/simulation/molecules-and-light
- Select a molecule (start with CO2).
- Choose an EM radiation spectrum (i.e., select Microwave, infrared, visible, or ultraviolet option as desired).
- Move the slider on the lamp to start the flow of photons with energy (and wavelength) corresponding to the chosen spectrum.
Using the Atmospheric gases and EM radiation applet, answer the following questions:
- Fill out the table below. Write “yes” or “no” to indicate whether a molecule interacts with photons of the selected EM radiation spectrumA molecule is considered to have interacted with photons of the selected EM radiation, if it absorbs the incident radiation and gets excited. Note that, as the molecule returns to its original state, it re-emits radiation in all directions.
- Recall the wavelength range corresponding to Earth’s radiative emission. Which of the above molecules interacts with Earth’s EM radiation? What do we call this group of gases?
The Earth's radiative emission is largely in the infrared regime. The gases that interact with infrared emission include: CO. CO2, H2O, NO2, and O3. These gases are often referred to as greenhouse gases.
- In the applet, select CO2 molecule and infrared wavelength spectrum. As CO2 absorbs infrared radiation and remits radiation, can you see that some of the radiation is directed back towards the lamp? Describe how this is similar to what happens between Earth’s surface and its atmosphere with CO2 molecules.
The Earth’s temperature is such that it emits primarily in the infrared regime. As the infrared emission from Earth’s surface is absorbed by carbon dioxide molecules (and other greenhouse gas molecules) in the atmosphere, they get excited, and then re-radiate the absorbed energy as infrared emission. Some of this re-radiated energy is directed back to Earth’s surface, thus increasing the net energy received at the surface of Earth.
- Which of the above molecules interacts most significantly with microwaves? How is this knowledge used in everyday cooking?
Water molecules effectively absorb microwave radiation, making them a necessary ingredient in microwave cooking. Water (externally added or already existing in vegetables) gets heated by absorption of microwaves and the neighboring molecules get heated on collision with the hot water molecules.
- Does ozone interact with solar radiation? In what wavelength range? How is this interaction important for life on Earth?
Yes, ozone interacts with solar radiation, by absorbing UV light (wavelengths smaller than ~ 350 nm). This action of ozone protects us from the harmful effect of exposure to UV radiation.
Greenhouse Gases and EM Radiation Worksheet
- Open the greenhouse gas applet link http://www.kcvs.ca/site/projects/chemistry_files/CO2/co2new.swf
- If the website homepage opens up, click on the picture under "Collisional Heating CO2 in the Atmoshpere".
- Click on "Start"
- In the bottom of the screen, click on choices of: IR spectrum and wavelength (nm).
- Waves/particles with energies corresponding to the selected wavelength will pass near or through the selected molecule.
Using the Greehouse Gases and EM Radiation applet, answer the following questions:
- Under the “Gas” option in the menu bar, select Carbon Dioxide. Move the vertical bar (with a diamond symbol at the center) to different wavelengths and notice the behavior of the molecule at these wavelengths. What do you think the red line represents? How does the molecule behave when the value of red line is ~ 1 (for example at a wavelength of 4300 nm)? And when it’s ~ 0 (for example at a wavelength of 3333nm).
The red line is the infrared (IR) absorption spectrum of the selected molecule. A value of ~ 1 indicates no absorption, i.e., wavelengths which pass through the molecule unaffected. A value of ~ 0 indicates strong or complete absorption of energy at those wavelengths, i.e., these wavelengths are not allowed to pass through the molecule.
- Set the diamond bar to a wavelength of 4300 nm. Click on the “Atmosphere” button in the bottom of the screen to visualize the interaction between the excited molecule and the neighboring molecules in the atmosphere. How does an excited molecule return to its original state?
The heat from the excited carbon dioxide molecule is transferred to its neighboring N2 and O2 molecules in the atmosphere via molecular collisions. The CO2 molecule eventually reaches the same temperature as the background atmosphere. The warm CO2 and neighboring molecules radiate EM energy in all directions, some of which reach Earth’s surface.
- Write the important absorption wavelengths associated with the different molecules in the table below.
- Which of the gases in the chart above have absorption wavelengths corresponding to Earth’s blackbody radiation? What is this common terminology used to refer to this group of gases?
CO2, H2O, and N2O. These gases are commonly referred to as greenhouse gases.
- In the above list of gases, which ones are not greenhouse gases?
O2 and N2
- The different greenhouse gases in the above table (CO2, H2O, and N2O) absorb EM radiation in different wavelengths. Select CO2 molecule and display Earth’s blackbody radiation curve. Considering Earth’s blackbody radiation curve, which of the CO2 absorption modes may be more important for determining Earth’s temperature?
The absorption mode closer to the peak of the Earth’s radiation curve (i.e., the one at 15,000 nm) may be the more important absorption peak from the perspective of Earth’s atmospheric temperature.
- In this applet, the interaction of packets of EM radiation with an individual molecule is demonstrated. Would the net absorbed energy change, if the number of molecules is increased to two?
EM radiation packets will be absorbed if the path of the molecule coincided with that of the energy packet. If the number of molecules is increased, more energy packets would be intercepted and thus more energy would be absorbed.
- Considering the current concentrations of major greenhouse gases, (http://cdiac.ornl.gov/pns/current_ghg.html): Note that the units of concentration are parts per million (ppm), parts per billion (ppb), and parts per trillion; 1 ppm = 1000 ppb = 106 ppt), which greenhouse gas may be the most important in determining Earth’s temperature?
The current concentrations of major greenhouse gases are:
Carbon dioxide(CO2): 389 ppm
Methane(CH4): 700 ppb
Nitrous oxide(N2O): 270 ppb
Tropospheric ozone(O3): 25 ppb
As the concentration of a greenhouse gas determines the net energy absorbed, CO2 is the most important greenhouse gas in Earth’s atmosphere.
Greenhouse Gases and Earth's Temperature Worksheet
- Open the greenhouse effect applet or click on http://phet.colorado.edu/en/simulation/greenhouse
- Slide the “Greenhouse Gas Concentration” option to “none”, i.e., select a CO2 concentration of 0.
- Note the temperature on the thermometer.
- Vary the greenhouse gas concentrations by selecting different time period options (by selecting options under “Atmosphere during”).
- Note the temperatures and CO2 concentrations for the different settings.
- Lastly, slide the “Greenhouse Gas Concentration” option to “Lots”, i.e., select a “very high” CO2 concentration.
Using the greenhouse gases and Earth's temperature applet, answer the following questions:
- What do the yellow stars represent? What primary wavelengths do the yellow stars correspond to?
The yellow stars represent the photons (energy particles) from Sun that reach Earth. These photons are primary in the visible wavelengths.
- What do the red stars represent? What wavelength do the red stars correspond to?
The red stars represent photons emitted by Earth. As Earth’s electromagnetic radiation is primarily in wavelengths longer than 4 m, these stars have wavelengths in the infrared regime.
- Under “Atmosphere during …” click on “Ice age”. Observe the motion of yellow photons. In what direction(s) (up or down) do they travel?
The yellow photons travel downwards from Sun towards Earth and are absorbed by the Earth’s surface in the “brown regions” and reflected back towards the Sun in “white regions” where ice is present.
- Under “Atmosphere during …” click on “Today”. Observe the motion of red photons. In what direction(s) (up or down) do they travel?
The red photons, in general, travel from Earth to space. When greenhouse gases are present, Earth’s EM radiation is absorbed by these gases and some of the infrared radiation from the warm atmosphere is re-directed back towards the Earth.
- Complete the table below (for the “Future” scenario (600 pm) move the greenhouse gas slider bar to half-way between “Today” and “Lots”):
- Using Excel, plot temperature (y axis) as a function of CO2 concentrations (x-axis). Remember to label the axes. (Use a value of 600 ppm to represent a “very high” concentration.)
- How are the numbers of infrared photons coming back to Earth related to the concentration of greenhouse gases? Explain why.
As the concentration of greenhouse gases are increased, more of Earth’s electromagnetic infrared radiation is absorbed by the atmosphere, making the atmosphere warmer. A warmer atmosphere radiates more effectively, resulting in an increased re-radiation of infrared photons from the atmosphere to Earth’s surface.
- What does the addition of clouds do to the Earth’s average temperature? (Increase the “number of clouds” on the right corner of the applet, from 0 to 3.) How do the cloud layers interact with the solar and infrared photons?
The temperature of Earth decreases slightly because of the reflection of sunlight by the clouds.
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