Frequently Asked Questions About Ozone (1)
Scientific Assessment of Ozone Depletion: 1998
World Meteorological Organization Global Ozone Research and Monitoring Project - Report No. 44
National Oceanic and Atmospheric Administration
National Aeronautics and Space Administration
World Meterological Organization
United Nations Environment Programme
How Can Chlorofluorocarbons (CFCs) Get to the Stratosphere If They're Heavier than Air?
CFCs reach the stratosphere because the Earth's atmosphere is always in motion and mixes the chemicals added into it.
CFC molecules are indeed several times heavier than air. Nevertheless, thousands of measurements from balloons, aircraft, and satellites demonstrate that the CFCs are actually present in the stratosphere. This is because winds and other air motions mix the atmosphere to altitudes far above the top of the stratosphere much faster than molecules can settle according to their weight. Gases such as CFCs that do not dissolve in water and that are relatively unreactive in the lower atmosphere are mixed relatively quickly and therefore reach the stratosphere regardless of their weight.
Measured changes in the concentration of constituents versus altitude teach us more about the fate of compounds in the atmosphere. For example, the two gases carbon tetrafluoride (CF4, produced mainly as a by-product of the manufacture of aluminum) and CFC-11 (CCl3F, used in a variety of human activities) are both heavier than air.
Carbon tetrafluoride is completely unreactive at altitudes up to at least 50 kilometers in the atmosphere. Measurements show it to be nearly uniformly distributed throughout the atmosphere (as illustrated in the figure below, the abundance of CF4 is nearly the same at all altitudes where measurements have been made). There have been measurements over the past two decades of several other completely unreactive gases, both lighter than air (neon) and heavier than air (argon and krypton), that show that they also mix upward through the stratosphere regardless of their weight.
CFC-11 is unreactive in the lower atmosphere (below about 15 kilometers) and is similarly uniformly mixed there, as shown in the figure. However, the abundance of CFC-11 decreases as the gas reaches higher altitudes, because it is broken down by high-energy solar ultraviolet radiation. Chlorine released from this breakdown of CFC-11 and other CFCs remains in the stratosphere for several years, where every chlorine atom destroys many thousands of molecules of ozone.
Atmospheric Measurements of CFC-11 and CF4
Frequently Asked Questions about Ozone
- [How can chlorofluorocarbons (CFCs) get to the stratosphere if they're heavier than air?]
- What is the evidence that stratospheric ozone is destroyed by chlorine and bromine?
- Does most of the chlorine in the stratosphere come from human or natural sources?
- Can natural changes such as the Sun's output and volcanic erruptions be responsible for the observed changes in ozone?
- When did the Antarctic ozone hole first appear?
- Why has an ozone hole appeared over Antarctica when CFCs and halons are released mainly in the Northern Hemisphere?
- Is there an ozone hole over the Arctic?
- Is the depletion of the ozone layer leading to an increase in ground-level ultraviolet radiation?
- Does ozone depletion cause climate change?
- How severe is the ozone depletion now?
- Is the ozone layer expected to recover? If so, when?