Vertical Profiling of Atmospheric Ozone
Keith Nunn
Department of Chemistry, Western State College,Gunnison,Colorado, 81231
Abstract:
Oxygen-containing molecules in the atmosphere, specifically diatomic oxygen and ozone, play a vital role in protecting life on the surface of the earth from harmful ultraviolet radiation. The intensity of different types of ultraviolet light at different altitudes and the concentrations of various oxygen-containing molecules give the ozone concentration in the stratosphere a distinct profile. Since the ozone layer varies depending on latitude and time of year, it is important to develop simple and inexpensive techniques for the vertical profiling of ozone concentrations. Atmospheric measurements are often taken from balloons or airplanes where weight and size are issues, so it is important that the instrument be compact and light weight. The ultraviolet photometer ozonesonde is the preferred method of the Environmental Protection Agency due to the photometers resistance to interference effects from other trace gases found in the atmosphere. Traditionally the ultraviolet photometer has been too heavy due to battery weight for many balloon flights. This project will develop and test a light weight single wavelength photometer to record a vertical profile of atmospheric ozone. By employing a highly efficient light source and other efficient instrumentation, and by powering the device with photovoltaic cells, the weight of the instrument can be kept below 1.5kg.
Introduction:
Oxygen-containing molecules in the atmosphere, specifically diatomic oxygen and ozone, play a vital role in protecting life on the surface of the earth fromharmful ultraviolet radiation. While not all ultra violet radiation is filtered out by ozone and diatomic oxygen in the atmosphere, the most biologically harmful portion of the ultra-violet spectrum, the UV-C and most of the UV-B radiation is. The intensity of ultra-violet light at different altitudes and the concentrations of various oxygen containing molecules give the ozone concentration in the stratosphere a distinct profile. The region of the stratosphere where the ozone concentration is greatest, known as the ozonelayer, is often found between 15-35km depending on latitude and time of year.
Absorption spectra of diatomic oxygen and ozone show how these oxygen molecules effectively filter out the sun’s ultraviolet radiation. Diatomic oxygen absorbs light with wavelengths between 70-250nm. At wavelengths greater than 250nm, a miniscule
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amount of light is absorbed by diatomic oxygen in a decreasing manner. Ozone absorbs light in the ultraviolet region between 220-320nm, with a peak absorption at 254nm.
The combination of diatomic oxygen and ozone effectively filter out the ultraviolet radiation from the sun up to wavelengths of about 290nm. The remaining ultraviolet light not filtered by the atmosphere strikes the surface of the earth. Thus the UV-B light striking the earth has wavelengths between 290-320nm and all of the UV-A light passes to the earth’s surface. Fortunately, the longer wavelength light in the UV-B and UV-A regions is less harmful than the more energetic shorter wavelength light.
Altitude significantly affects the concentrations of oxygen-containing molecules and is responsible for the distinct profile of the ozone layer. At altitudes above 30km, high UV-C intensity promotes the production of ozone. However, at these altitudes most of the oxygen present is in the form of atomic oxygen. When two oxygen atoms collide, they combine to form molecular oxygen. The molecular oxygen is short lived in the presence of high intensity UV-C light, as photodecomposition occurs rapidly, producing once again atomic oxygen. At altitudes 15-35km, the density of air is greater resulting in higher concentrations of molecular oxygen. An abundance of molecular oxygen at these altitudes also arises from molecular oxygen above filtering out much of the UV-C radiation through photochemical decomposition. However, enough UV-C radiation penetrates this layer to cause the photolysis of molecular oxygen. The atomic oxygen then collides with the abundant molecular oxygen to form ozone. At altitudes below
15km, the amount of UV-C radiation remains insufficient to produce significant amounts of ozone (1).
Since the ozone layer varies depending on latitude and time of year, it is important to develop simple and inexpensive techniques for the vertical profiling of ozone concentrations. Atmospheric measurements are often taken from balloons or airplanes where weight and size are issues, so it is important that the instrument be compact and
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light weight. Several types of instruments are available that conform to the above characteristics. Most commonly used are ozonesondes utilizing an electrochemical
concentration cell. The electrochemical concentration cell is the lightest and least expensive method for measuring atmospheric ozone. In the electrochemical cell iodine is
oxidized to I3. However, the electrochemical concentration cell experiences interferences from NO2 and SO2 in polluted air samples.
An alternative to the electrochemical concentration cell is an ultraviolet photometer. The ultraviolet photometer is the preferred method of the Environmental Protection Agency due to the photometers resistance to interference effects from other trace gases found in the atmosphere. Traditionally the ultraviolet photometer has been too heavy due to battery weight for many balloon flights. By employing a highly efficient light source and other instrumentation, and powering the device with photovoltaic cells, the weight of the instrument can be kept below 1.5kg. The ultra-violet photometer allows for a light weight compact instrument with minimal interferences and high sensitivity (2).
Method
A single wavelength ultraviolet photometer will be constructed for the purpose of vertically profiling ozone concentrations above the eastern plains of Colorado in August of 2004. A balloon will transport the photometer to an altitude of 100,000 feet at which point the photometer will be released from the balloon and allowed to drift down to the ground via a parachute. NASA provides the balloon through funding of the Demosat Project. Ozone concentrations will be measured continuously as a function of altitude during the balloon ascent.
The photometer will utilize a low pressure mercury lamp whose emission spectrum includes an intense line at 253.7nm. Since the lamp is of low power, shorter wavelength mercury emissions will not result in the photodecomposition of diatomic oxygen and the
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formation of ozone. An interference filter centered at a wavelength of 253.7nm will shield the photodiode detector from ambient light (2). Small fans will be placed at the
inlet and outlet of the instrument to ensure adequate sampling during flight. To ensure constant power the electronic systems will be powered by a six-volt battery wired in
parallel with photovoltaic cells. Temperature and pressure measurements will also be recorded during flight. All data will be stored on-board using a HOBOdata acquisition system and retrieved post flight.
The photometer will be calibrated in the laboratory using a calibration system based on the vapor pressure of ozone. Gaseous ozone will be generated using acorona discharge system. The ozone will be frozen in a test tube submerged in liquid nitrogen. The temperature of the liquid nitrogen will be varied by varying the pressure above the surface of the liquid nitrogen. As the temperature of the solid ozone varies, so will its vapor pressure. By knowing the temperature of the liquid nitrogen, the vapor pressure of the ozone can be calculated. The gaseous ozone produced from sublimation will be carried out of the test tube and into a sample chamber by nitrogen gas. The gas from the sample chamber will then be passed through the photometer. By knowing the response of the instrument and the concentration of ozone in the gas sample, the instrument can be calibrated (3).
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Literature Cited
1)Baird C., Environmental Chemistry 17-74 (W.H. Freeman and Company, New York, 1998).
2)Bognar J.A., Birks J.W., Analytical Chemistry 68, 3059-3062 (1996).
3)Mauersberger K., Hanson D., Morton J., Rev. Sci. Instrum. 58, 1063-1066 (1987).