A

Atmosphere

1. The atmosphere is the column of air that lies above the Earth's surface. The density of this air decreases as you proceed up from the surface. The air in the atmosphere consists of 78% Nitrogen, 21% Oxygen, and 0.9% Argon. The remaining 0.1% of the atmosphere consists of ozone, water vapor, carbon dioxide, methane, helium, and neon. The atmosphere is divided into different regions. The lowest two layers are the troposphere and the stratosphere respectively. These two layers contain more than 99% of the atmospheric molecules. 2. A unit of pressure. One atmosphere (atm.) is equal to 760 mmHg (millimeters mercury) or 101.325 kPa (kilopascals).

Atom

All matter is made up of atoms. Atoms are made up of protons, neutrons, and electrons. The number of protons an atom has, determines that atoms chemical properties. Ozone is made up of three oxygen atoms.

B

Brewer Spectrophotometer

A ground-based instrument that measures the amount of ozone in the atmosphere. The Brewer spectrophotometer measures the amount of UV light from the sun at 5 different wavelengths between 306 and 320 nm.

C

Chlorofluorocarbons (CFC's)

CFC's were used as a refrigerator coolant and as propellants in aerosol cans. CFC's were thought of as a great replacement for ammonia because ammonia is very flammable. CFC's are safe, nonreactive (inert) molecules in the lower atmosphere (troposphere), but in the stratosphere, where the UV radiation is intense, CFC's are broken down. UV light breaks the bond between the carbon and chlorine on the CFC molecule. With the highly reactive chlorine atom, now free, it reacts with ozone and destroys it.

D

Direct Sun

Ozone measurements are often taken with spectrophotometers using direct sunlight as the light source. Direct Sun measurements require moderately bright sunlight. During days that are overcast, spectrophotometers, such as the Dobson, can still take ozone measurements using the zenith sky (light from directly overhead).

Dobson Spectrophotometer

A ground-based instrument that measures the amount of ozone present in the atmosphere. The Dobson spectrophotometer was designed by Gordon Dobson in the 1930's. The Dobson spectrophotometer measures UV light from the sun at two to six different wavelengths of between 305 and 345 nm.

Dobson Unit

The standard unit of measure for ozone. If all of the ozone from the Earth's surface to outer space were compressed to standard temperature and pressure (STP), the ozone layer or its thickness would be about 3 mm thick. At STP, 3mm of ozone is 300 Dobson Units (DU). 1mm = 100DU.

E

Element

An element is characterized by a particular type of atom. The number of protons an atom contains determines its chemical properties.

F

G

H

I

J

K

Kilometer (km)

Unit of length equal to 10³ or 1,000 meters. Often referred to when measuring large distances.

L

M

Millimeter (mm)

Unit of length equal to 10^-3 or 1/1000 meter. Often referred to when measuring small distances.

Molecule

Atoms combine in various ways to form molecules. Three oxygen atoms bound together form one ozone molecule.

N

Nanometer (nm)

Unit of length equal to 10^-9 or 1/1,000,000,000 meter. Often referred to when measuring light waves.

O

Ozone

Ozone is a molecule made of three oxygen atoms bound together. Ozone is written by chemists as O₃. High energy ultraviolet light from the sun is damaging to living cells. Ozone absorbs most high energy ultraviolet light before it reaches the Earth's surface. Because ozone is very reactive, ozone in the air we breathe is harmful. Ozone reacts with tissues in our sinuses and lungs. Therefore, we are lucky that most ozone lies in the stratosphere at elevations greater than 8 km.

Ozone Hole

Since the 1970's, the amount of ozone over Antarctica has dropped to about one third of normal levels during the spring. This depletion of ozone, believed to be caused by chlorine put into the atmosphere by man, is referred to as the ozone hole.

Ozone Layer

Most of the ozone in the atmosphere is in the stratosphere (about 10 - 50 km above earth's surface). This is what is called the ozone layer or the stratospheric ozone layer. The layer refers to the thickness of the ozone.

P

Photochemical Reaction

The dissociation or excitation of a molecule caused by the absorption of a photon. How a molecule will react to absorbing a photon depends on the photon's energy and the molecule absorbing the photon. Excitation of a molecule can cause the molecule to react with other atoms or molecules more readily. Dissociation of a molecule can cause the dissociated parts of the molecule to react more readily with other atoms or molecules. A CFC molecule is dissociated by high energy ultraviolet light in the stratosphere.

Photolysis

1. (Noun) The dissociation of a molecule caused by the absorption of a photon. 2. photolyze (Verb) The action of a molecule breaking apart after absorbing a photon. Ozone is photolyzed by ultraviolet light.

Photon

Particles of light energy.

Q

R

S

Spectrophotometer

Light can be separated into its individual wavelengths by passing the light through a prism. When light passes through matter, it may be reflected, emitted, transmitted or absorbed. By knowing the intensity of the light before passing it through matter and measuring the intensity of the light after it is passed through the matter, the amount of matter in the sample can be measured.

Standard Temperature and Pressure (STP)

0° Celcius and 1 atmosphere.

Stratosphere

The section of the atmosphere between 8 and 16 km to 50 km above the surface of the Earth. In the stratosphere, the temperature generally increases with increasing height, as opposed to the troposphere, where temperature decreases with increasing height.

T

Total Ozone Mapping Spectrometer (TOMS Satellite)

The latest spectrophotometer aboard the Earth Probe satellite launched by NASA in 1996. TOMS is a spectrophotometer that measures the amount of ultraviolet light from the sun that is reflected back from the Earth. The TOMS spectrophotometer measures ultraviolet light at wavelengths between 312 and 360 nm.

Total Column Ozone

The amount of ozone contained in a column of air. A column of air consists of the area from the Earth's surface to outer space.

Tropopause

The tropopause is a transition region of the atmosphere between the troposphere and the stratosphere. The troposphere and the stratosphere have markedly different properties. The tropopause is located just above the troposphere and just below the stratosphere. The height of the tropopause changes during the seasons. The tropopause is located at higher altitudes in the summer and fall and lower altitudes in the winter and spring. This change in altitude, changes with the abundance of ozone overhead.

Troposphere

Region of the Earth's atmosphere that extends from the Earth's surface to between 8 and 16 km. In the troposphere, the temperature decreases with increasing height, as opposed to the stratosphere, where the temperature generally increases with increasing height. The troposphere is convective, meaning that it mixes vertically. Nearly all clouds are in the troposphere.

U

Ultraviolet (UV) Radiation

Light that is not visible to the human eye. Ultraviolet light is higher in energy than visible light. Ultraviolet light has a wavelength in the 200-400 nm region. Visible light has a wavelength in the 400-800 nm region. The high energy ultraviolet light is harmful to living cells in plants and animals.

UV-A

Lower energy ultraviolet light from the Sun with a wavelength between 320-400 nm. UV-A light is not absorbed by ozone, thus all of the UV-A light is allowed to reach the surface of the Earth.

UV-B

Ultraviolet light from the Sun with a wavelength between 290-320 nm. Ozone absorbs most UV-B light before it reaches the surface of the Earth.

UV-C

High energy ultraviolet light from the Sun with a wavelength between 200-290 nm. Ozone and oxygen molecules absorb all UV-C light before it can reach the Earth's surface.

V

Visible Light

Light that is visible to the human eye. Visible light is lower in energy than ultraviolet light. Visible light has a wavelength in the 400-800 nm region.

W

Wavelength

The distance between the peaks on a wave. A high frequency (higher energy) wave has a shorter distance between its peaks. A low frequency (lower energy) wave has a longer distance between its peaks.

X

Y

Z

Zenith Sky

The sky directly overhead. Ozone measurements are often taken with spectrophotometers using the amount of light directly overhead, rather than the light directly from the sun. Zenith sky measurements can be taken on days that are overcast.