Ozone (\(\text{O}_3\)) is a naturally occurring gas found throughout the Earth’s atmosphere. It exists in two distinct layers with different effects on health and the environment. Stratospheric ozone forms a protective shield that absorbs harmful solar ultraviolet radiation. Conversely, ground-level ozone (tropospheric ozone) is a pollutant detrimental to respiratory health and ecosystems. Measuring ozone concentration is necessary for monitoring atmospheric health and ensuring local air quality standards are met, requiring specialized techniques and specific units.
The Standard Measures of Ozone
Ozone concentration units depend on the atmospheric region being measured. For ground-level ozone related to air quality, concentration is expressed as a ratio of ozone volume to total air volume. This is typically communicated in parts per billion (ppb) or parts per million (ppm). These units are practical for measuring the low concentrations of ozone pollutant found in the troposphere.
For assessing the total amount of ozone in the entire vertical column, essential for stratospheric monitoring, the Dobson Unit (DU) is used. The DU represents the total thickness the ozone layer would occupy if compressed to standard temperature and pressure (STP). One Dobson Unit is equivalent to a layer of pure ozone 0.01 millimeters thick at STP. This measure quantifies the total number of ozone molecules overhead, integrating concentrations from the surface to the top of the atmosphere.
Monitoring Ground-Level Ozone
Ground-level ozone, a significant component of urban smog, is primarily measured using continuous monitoring methods deployed at stationary air quality stations. The standard reference method is Ultraviolet (UV) photometry. This technique exploits the property that ozone molecules strongly absorb UV light at approximately 254 nanometers.
The UV photometer draws a sample of ambient air into an absorption cell and exposes it to a UV light beam. The instrument first measures the light intensity after passing it through an ozone-free sample, created by scrubbing the ozone out using a catalytic converter. It then measures the light intensity after passing it through the untreated ambient air sample containing ozone.
Ozone molecules in the untreated air absorb UV light, causing the measured intensity to decrease. The difference in UV light intensity between the ozone-free sample and the ambient air sample is directly proportional to the ozone concentration. This relationship is quantified using the Beer-Lambert Law, allowing the instrument to determine ozone concentration with high precision, typically in the parts per billion range.
Chemiluminescence
Another technique used for ground-level measurement is chemiluminescence, which relies on a chemical reaction that produces light. Air is mixed with a reactant gas, often nitric oxide (NO). Ozone reacts with the nitric oxide to form nitrogen dioxide (\(\text{NO}_2\)) in an electronically excited state. As the excited molecules return to their ground state, they emit a photon of light, a phenomenon known as chemiluminescence. The intensity of this emitted light is measured by a sensitive photodetector and is directly proportional to the original ozone concentration. Both UV photometry and chemiluminescence provide the real-time data required for air quality forecasting and regulatory compliance.
Assessing Total Column Ozone
Measuring the total amount of ozone in the stratosphere involves quantifying the entire vertical column of gas. Ground-based remote sensing instruments, such as the Dobson and Brewer Spectrophotometers, are used for this process. Both instruments measure the solar ultraviolet (UV) radiation that reaches the Earth’s surface at specific wavelengths.
These spectrophotometers compare the intensity of a UV wavelength that is strongly absorbed by ozone with the intensity of a nearby UV wavelength that is only weakly absorbed. By analyzing the ratio of these intensities, scientists calculate the total ozone present in the vertical path between the instrument and the sun. The result is expressed as total column ozone in Dobson Units. The Brewer Spectrophotometer, a more modern successor, uses multiple wavelength pairs and is often automated for continuous operation.
Ozonesondes
To obtain a precise vertical profile of ozone concentration, especially in the stratosphere, balloon-borne instruments called ozonesondes are utilized. These lightweight instruments are attached to weather balloons and ascend to altitudes of up to 35 kilometers. The ozonesonde contains an electrochemical concentration cell (ECC) that draws in ambient air. Inside the ECC, ozone reacts with a potassium iodide solution, generating an electrical current. The strength of this electric current is directly proportional to the amount of ozone present at that specific altitude. As the balloon rises, the ozonesonde transmits this ozone data, along with temperature and pressure readings, back to a ground station, providing a high-resolution vertical map of ozone concentration.
Satellite Monitoring
For global mapping and long-term monitoring, satellite instruments provide a comprehensive view of the ozone layer. Instruments such as the Total Ozone Mapping Spectrometer (TOMS) and the Ozone Monitoring Instrument (OMI) use the backscatter ultraviolet (BUV) technique. These satellites measure the intensity of solar UV radiation that is backscattered from the Earth’s atmosphere and surface. By comparing the amount of UV light emitted by the sun with the amount that returns to the satellite at different wavelengths, the total column ozone is calculated. OMI, which continues the long-standing TOMS record, provides daily global coverage, allowing researchers to track the recovery of the ozone layer following international agreements.