The ozone layer
Earth's atmosphere
The Earth's atmosphere is composed of several layers. We live in the 'troposphere', where most of the weather occurs, such as rain, snow, and clouds. Above the troposphere is the 'stratosphere', an important region where phenomena such as the ozone hole and global warming originate. The supersonic jet airliner, Concorde, flew in the lower stratosphere, whereas subsonic commercial airliners usually fly in the troposphere. The narrow region between these two parts of the atmosphere is called the 'tropopause'.
In the upper stratosphere, ozone is generated when ultraviolet radiation (sunlight) strikes the stratosphere, dissociating (or 'splitting') oxygen molecules (O2) to atomic oxygen (O). The atomic oxygen quickly combines with further oxygen molecules to form ozone.
O2 + hv -> O + O (1)
O + O2 -> O3 (2)
(1/v = wavelength < ~ 240 nm)
It is ironic that at ground level, ozone is a health hazard - it is a major constituent of photochemical smog. However, ozone in the stratosphere is important for our survival. In the stratosphere, ozone absorbs some of the potentially harmful ultraviolet (UV) radiation from the Sun, at wavelengths between 240 and 320 nm, which could otherwise lead to an increase in the incidence of skin cancer and damage the Earth's ecosystem in a variety of ways. Although UV radiation splits ozone molecules, ozone can reform through the following reactions, resulting in no net loss of ozone:
O3 + hv -> O2 + O (3)
O + O2 -> O3 (2)
Ozone is also destroyed by the following reaction:
O + O3 -> O2 + O2 (4)
Reaction (2) becomes slower with increasing altitude, while reaction (3) becomes faster. The concentration of ozone is a balance between these competing reactions.
In the upper atmosphere, atomic oxygen dominates where UV levels are high. Moving down through the stratosphere, the air gets denser, UV absorption increases, and ozone levels peak at roughly 20 km. As we move closer to the ground, UV and ozone levels decrease.
The missing reactions
At the ground level, ozone is a health hazard
A problem was found in the 1960s when scientists realised that the loss of ozone given by reaction (4) was too slow. It could not remove enough ozone to give the values seen in the real atmosphere. There had to be other, faster reactions that were controlling the ozone concentrations in the stratosphere.
What is the ozone hole?
Over the past 15 years, stratospheric ozone has been depleted at certain times of the year over Antarctica, and recently, over the Arctic as well. This is mainly due to the release of human-made chemicals containing chlorine such as ChloroFluoroCarbons (CFCs), but also compounds containing bromine, other related halogen compounds, and nitrogen oxides (NOx). CFCs are a common industrial product used in refrigeration and air conditioning systems, as propellants in aerosol products and solvents, and in the production of some types of packaging. Nitrogen oxides are a by-product of combustion processes, including aircraft emissions.
The current levels of depletion have served to highlight a surprising degree of instability in the atmosphere, and the amount of ozone loss is still increasing.
During the winter polar night, sunlight does not reach the South Pole. A strong circumpolar wind develops in the middle to lower stratosphere. These strong winds are known as the 'polar vortex'.
This has the effect of isolating the air over the polar region.
Since there is no sunlight, the air within the polar vortex can get very cold. In fact, special clouds can form once the air temperature drops to about -80C°. These clouds are called Polar Stratospheric Clouds (PSCs), but they are not the clouds that you are used to seeing in the sky, which are composed of water droplets. PSCs first form as nitric acid trihydrate. As the temperature gets colder, however, larger droplets of water-ice with nitric acid dissolved in them can form. However, their exact composition is still the subject of intense scientific scrutiny. These PSCs are crucial for ozone loss to occur.
It's important to appreciate that these reactions can only take place on the surface of polar stratospheric clouds, and that they are very fast. This is why the ozone hole was such a surprise. Heterogeneous reactions, that is those that occur on surfaces like the stratosphere, were neglected in atmospheric chemistry before the ozone hole was discovered. Another ingredient then, are these heterogeneous reactions which allow reservoir species of chlorine and bromine to be rapidly converted into more active forms.
This dramatic fall in ozone was caused by the use of human-made chemicals known as 'halogens' which include the well-known CFCs commonly used in fridges and as propellants in aerosols. These CFCs have made their way into the upper atmosphere, where the much stronger UV radiation from the Sun has broken them down into their component molecules, releasing the potentially damaging chlorine and bromine atoms, which, given the right conditions, can destroy ozone.
To recap then, the requirements for ozone loss are:
- The polar winter leading to the formation of the polar vortex that isolates the air within it
- Cold temperatures forming inside the vortex, cold enough for the formation of PSCs. As the vortex air is isolated, the cold temperatures and the PSCs persist.
- Once the PSCs form, heterogeneous reactions take place and convert the inactive chlorine and bromine reservoirs to more active forms of chlorine and bromine.
- No ozone loss occurs until sunlight returns to the air inside the polar vortex and allows the production of active chlorine and initiates the catalytic ozone destruction cycles.
Ozone loss is rapid. The ozone hole currently covers a geographic region a little bigger than Antarctica, and extends nearly 10 km in altitude in the lower stratosphere.
The amount of ozone above a point on the Earth's surface is measured in Dobson units (DU) - typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations. If all the ozone in this column were to be compressed to standard temperature and pressure (STP) (0 deg C and 1 atmosphere pressure) and spread out evenly over the area, it would form a slab approximately 3 mm thick. 1 DU is defined to be 0.01 mm in thickness at STP; the ozone layer over Labrador then is 300 DU.
Global Ozone Monitoring Experiment
The Global Ozone Monitoring Experiment (GOME) is an instrument that was added to the ERS-2 satellite's payload when it was launched on 20 April 1995. GOME is a nadir-viewing spectrometer which observes solar radiation transmitted through or scattered from the Earth's atmosphere or from its surface. The recorded spectra is used to derive a detailed picture of the atmosphere's content of ozone, nitrogen dioxide, water vapour, oxygen/oxygen dimmer, bromine oxide, and other trace gases. The ERS-2 orbit provides global Earth coverage every three days. On 1 March 2002, Envisat, ERS-2's successor, was launched. Onboard, its instrument SCIAMACHY continues this worldwide measurement campaign.
Ozone measurements and the GOME instrument characteristics
Atmospheric studies from space began more than 20 years ago. Missions were dedicated to the measurement of ozone and several other atmospheric constituents (CH4, HNO3, HO2, NO2). Plans for continuing such measurements cover the next decade.
ESA's long-term programme for stratospheric chemistry and dynamics studies was approved in 1987. Various instruments are being flown on Envisat.
The NASA and NOAA US organisations have been providing ozone maps for about 20 years. In the 1990s, Russian and Japanese programmes commenced. Prior to the launch of ERS-2, several European initiatives were taken at national level, focusing on instruments dedicated to atmospheric studies to be flown on space stations, such as Spacelab-1 and MIR-2.
The most common ozone measuring instruments that have been operated so far include the Total Ozone Mapping System (TOMS) and the Solar Backscattered Ultra-Violet (SBUV/2). TOMS was operated on the NASA satellite Nimbus-7 from 1978 to 1994, and on the Earth Probe Satellite from 1996 until today. It's also been operated on the Russian METEOR-3 and the Japanese ADEOS mission. The SBUV/2 was operated on the NOAA operational satellite series. Both instruments are mapping spectrometers in the ultraviolet/near ultraviolet spectral region. TOMS has 6 channels, whereas SBUV/2 has 12. In addition to measuring the ozone total column, the SBUV/2 instrument can measure the stratospheric ozone profile, which is the ozone concentration by altitude.
Another instrument that has been operating on the NOAA TIROS-N satellites is the TIROS Operational Vertical Sounder (TOVS). On this instrument, three infrared radiometers are used to derive the ozone total column.
It is worth pointing out that the SBUV/2 spectrometer has 12 measuring bands in the 255-340 nm region, and a spectral resolution of 1 nm. This same spectral resolution is covered by 850 of GOME's 4096 detectors at a spectral resolution of 0.1 nm.
The TOMS has 6 measuring bands in the 311-380 nm region, and a spectral resolution of 1 nm. It appears to give a quicker global coverage than GOME, due to its shorter measurement time. Moreover, both SBUV/2's and TOMS's accuracy are hampered by the degradation of the instrument diffuser plate.
Additional space missions for atmospheric chemistry studies are NASA's Earth Radiation Budget Experiment (ERBE), in operation since 1984, and the Upper Atmospheric Research Satellite (UARS), in operation since 1991. The ERBE carries the Stratospheric Aerosol and Gas Experiment II (SAGE II) sensor that measures the attenuated solar radiation in seven bands in the spectral interval: 0.385 - 1.02 micro-m. The UARS carries various limb-sounding instruments as well as the SOLar STellar Comparison Experiment (SOLSTICE) for Sun irradiance measurements. The SOLSTICE's spectral interval is of 120 - 420 nm. The sounding instruments include the Microwave Limb Sounder (MLS), the Cryogenic Limb Array Etalon Spectrometer (CLAES) and the HALogen Occultation Experiment (HALOE). The last two instruments measure the solar radiation in the mid-infrared spectral region. The vertical resolution of the measured profiles ranges from 2 to 5 km.
GOME instrument characteristics
GOME spectral bands and spectral resolution
| Band |
1A |
1B |
2 |
3 |
4 |
| Spectral Interval (nm) |
237-307 |
307-315 |
312-406 |
397-609 |
576-794 |
| Spectral Resolution (nm) |
0.2 |
0.2 |
0.17 |
0.29 |
0.33 |
GOME Polarisation Measuring Devices (PMD)
| Channel |
1 |
2 |
3 |
| Band (nm) |
300-400 |
400-600 |
600-800 |
The GOME instrument measures ozone vertical column density and vertical profile measurements in the troposphere and the stratosphere with moderately high vertical resolution. Moreover, additional trace gas vertical columns can be detected either on a routine basis, like for example NO2, or on occasional events, like HCHO and SO2, particularly during ozone hole conditions (ClO, BrO, etc.).
New and improved data have been acquired due to the increased GOME spectral range and spectral resolution. Ozone retrieval from GOME data can be carried out either by the traditional Backscattering Ultra Violet (BUV) technique, or by means of the more recent Differential Optical Absorption Spectroscopy (DOAS) technique. This enables the comparison with time series of ozone data, acquired with one of the two techniques, and a check on the consistency of the two estimates based on the same GOME data. The integration of data time series from different sources is crucial for ozone trend analysis.
Please visit the SCIAMACHY page to know more about its specifications.
Ozone, trace gas and aerosol retrieval
With the DOAS technique, a total slant column amount of a given atmospheric absorbing gas can be derived from the combination of the Earth radiance spectra and the Sun irradiance, through the analysis of the features which are specific to the absorbing gas under investigation. This total amount can be worked out regardless of knowledge of the atmosphere or of the ground surface properties.
The gas vertical column can be obtained by working out the best fit between the simulated and the observed Earth spectra at given spectral bands. The wavelengths have to be carefully selected in order to minimise the interference with other atmospheric processes.
Some atmospheric constituents and related issues are worth considering further. Some of the following conclusions stem from sensitivity studies carried out during the GOME instrument implementation phase, so they should be compared to recent results obtained by GOME and SCIAMACHY.
It should be noted that all of the above-mentioned compounds, with the exception of chlorine monoxide and nitrogen trioxide, have been successfully retrieved from GOME and SCIAMACHY data.
Ozone O3
Ozone bands: Hartley < 310 nm; Huggins about 310-350; Chappius around 500 nm
Hartley and Huggins bands have been used mainly by means of the SBUV technique for stratospheric ozone vertical column density and profile retrieval, whereas Chappius bands have been used for ozone vertical column (including the troposphere) retrieval.
Huggins bands can be used for tropospheric and stratospheric ozone profile determination because they are temperature-dependent, and the temperatures in the troposphere and in the stratosphere are different. The GOME instrument is particularly suitable for the detection of the absorption feature variations, due to temperature changes.
The retrieval of the ozone profile in the stratosphere or in the troposphere is made possible by the different penetration depth of the solar radiation at different wavelengths. The penetration depth depends also on the solar radiation path length, which varies with the solar zenith angle (SZA). It is therefore season and latitude dependent.
Favourable bands for ozone measurements are as follows.
Stratospheric ozone
| Summer |
at all Latitudes |
spectral bands < 310 nm |
| Winter |
below 35 deg. Lat. |
spectral bands < 310 nm |
Winter |
above 50-55 deg. Lat. |
spectral bands about 500 nm |
Tropospheric ozone
| Summer |
at all Latitudes |
spectral bands 325-340 nm |
| Winter |
below 35 deg. Lat. |
spectral bands 325-340 nm |
One of the goals of the GOME instrument is the retrieval of tropospheric ozone.
From sensitivity studies carried out prior to the launch, it was estimated that GOME would retrieve tropospheric ozone vertical profiles with a height resolution of about 5 km and vertical column density with an accuracy of a few %. This is consistent with the obtained initial results. Other sensitivity studies have focused on the detectability of the polar ozone hole conditions, by considering a 20% ozone column decrease at latitudes of about 80 degrees.
Oxygen O2/O4
O2 bands: about 690 and 760 nm O4 bands: about 360, 476 and 576 nm
The O2/O4 bands can be used for retrieving cloud top height. Their absorption is sensitive to the presence of solar radiation backscattered by the clouds. O2 absorption near 760 nm is considered for deriving the cloud cover fraction. The O4 576 nm band is sensitive to Polar Stratospheric Clouds (PSC).
The O2 concentration is proportional to the O4 concentration. This represents an additional condition when retrieving the penetration depth.
Nitrogen Dioxide NO2
Bands: range 300-600 nm
Typical contents are of the order of 10-40 volume parts per trillion (pptv) in clean air, and 10-100 pptv in polluted air. Tropospheric NO2 is highly variable in time and space. Its variability is lower in the afternoon.
The retrieval accuracy, estimated from sensitivity studies, is of the order of magnitude of 1 pptv. NO2 is another trace gas, together with ozone, that is detectable on a routine basis.
The temperature dependence, at present under investigation, might allow for the separation of the stratospheric and tropospheric amounts.
Formaldehyde HCHO
Bands: range 250-360 nm
The window of 300 -350 nm is most suitable for total column measurements, although it can be obscured by tropospheric ozone. Moderate pollution events (1015 molec.x cm-2 total column), related to biomass burning or smog episodes, should be detectable in the troposphere at low latitudes.
Sulphur Dioxide SO2
Bands: range 290-315 nm
It should be detectable under moderate pollution conditions in the boundary layer (typical value of 20 ppbv) and low ozone concentrations with an accuracy of a few %. Sulphur dioxide in the stratosphere is climatologically important, as it forms sulphate aerosols that affect both the radiation balance and the ozone chemistry by heterogeneous reactions.
Chlorine Monoxide ClO
Bands: range 220-310 nm
It should be detectable during ozone hole conditions with an accuracy of 10%. Typical threshold values for detection are between 1013 and 1015 cm-2. Together with BrO, it is part of the ozone catalytic destruction cycles.
Chlorine Dioxide OClO
Bands: range 280-440 nm
Should be detectable during ozone hole conditions with an accuracy of a few % with typical values of 1014 cm-2. Typical daytime column amounts of 2 x 1013 cm-2 should be detectable with an accuracy of 10%. It is an indicator of chlorine activation. Relatively high values are observed during ozone hole conditions (perturbed conditions). In general it is higher at night because of a lack of photochemical reactions.
Bromine Monoxide BrO
Bands: around 349 nm and 355 nm
Should be detectable during ozone hole conditions with an accuracy of 10%, with typical values of 2.5 x 1013 molec. cm-2 and 10-25 ppbv in the lower stratosphere. During normal conditions the expected accuracy is of about 15%. Unlike ClO, BrO is always present; along with ClO, it is part of the ozone catalytic destruction cycles.
Nitrogen Trioxide NO3
Band: 662 nm
It is more abundant during nighttime, as it is photolysed during the day. One of the ways it is produced is by its reaction with ozone.
Glossary of Terms
Ozone
A molecule consisting of three oxygen atoms. Ozone strongly absorbs ultraviolet light with wavelengths in the range of around 290 to 300 nanometres (0.000000001 metres), which makes it useful in the stratosphere because it protects life on Earth from the damaging effects of this radiation. It is also a very reactive compound, which makes it harmful here, because it can damage the lung tissue of those who breathe it.
Ozone layer
The level of the atmosphere which has a peak in ozone concentration. As electromagnetic radiation comes in from the Sun and hits the Earth's atmosphere, certain wavelengths in the ultraviolet range (180-240 nanometres) are absorbed by and break apart oxygen molecules, which are made of two oxygen atoms. Some of the resulting unattached pairs of oxygen atoms then recombine into triplets to form ozone. Different ranges of wavelengths of ultraviolet (290-300 nm) are strongly absorbed by ozone, which breaks down as a result and reforms into molecular oxygen again. The higher up in the atmosphere you go, the thinner the air is and the less oxygen there is to absorb the 180-240 nm ultraviolet to form ozone. This means that ozone concentrations tend to decrease as you go higher up into the atmosphere. The lower down in the atmosphere you go, the more oxygen the ultraviolet has to pass through to get there, and the greater the chances are that it has already been absorbed to create ozone somewhere higher up. This means that towards the bottom, the ozone amounts tend to be significantly lower. From roughly 12 to 30 km, the two tendencies balance out, and the highest ozone concentrations are found there, in what is called the 'ozone layer'.
Total ozone
The total amount of ozone in a column of air stretching from the Earth's surface to space.
Dobson Unit
The unit of measure for total ozone. If you were to take all the ozone in a column of air stretching from the surface of the Earth to space, and bring all that ozone to standard temperature (0 Celsius) and pressure (1013.25 millibars, or one atmosphere, or 'atm'), the column would be about 0.3 centimetres thick. Thus the total ozone would be 0.3 atm-cm. To make the units easier to work with, the 'Dobson Unit' (DU) is defined to be 0.001 atm-cm. Our 0.3 atm-cm would be 300 Dobson Units.
ChloroFluroCarbons (CFCs)
Formerly a common industrial product used in refrigeration systems and air conditioners, as well as propellant in aerosol products and solvents, and in the production of some types of packaging. Although chemically inert in the lower atmosphere (troposphere), they are taken to very high altitudes where they are broken down into their constituents by the stronger sunlight (UV) at these altitudes. It is the chlorine formed in this process that causes the damage to ozone. The manufacture and use of CFCs in industry has been severely curtailed following the Montreal Protocol and subsequent amendments. See also halogens.
Halogens
A class of halide (containing Chlorine, Bromine or Iodine) compounds, including CFC. These can break down to form various ozone-depleting radicals.
Polar Vortex
A distinct column of cold air contained over the pole, especially the South Pole, by meteorological effects. Develops during the polar winter when the Polar Regions are in polar night, when sunlight does not reach the poles. Wind speed around the vortex may reach 100 metres per second. The vortex establishes itself in the middle to lower stratosphere. It is important because it isolates the very cold air within it.
Rayleigh scattering
It is typical of the atmospheric gas molecules scattering the solar radiation. The scattered energy is inversely proportional to the power 4 of the wavelength of the incident radiation. It also depends on the refractive index of the scatterers. The blue colour of the sky results from such a phenomenon.
Mie scattering
It is typical of atmospheric aerosols, scattering the solar radiation. The scattered energy has an oscillating behaviour, depending on the particle size parameter. Forward scattering prevails over backward scattering. It results in the reddish/bluish colour of the particles.
Solar Backscattering Ultra Violet (SBUV) technique
The SBUV technique, for stratospheric ozone measurements, is based on the observations (nadir) of the solar backscattered light at different wavelengths (at least 2) for which there is significantly different ozone absorption. Therefore absolutely calibrated radiance measurements are needed. The accuracy of the measurements, from the presently available radiometers, depends on the diffuser plate stability and on its Bi-directional Reflectance Distribution Function (BRDF) knowledge.
Differential Optical Absorption Spectroscopy (DOAS) technique
The DOAS technique, for atmospheric trace gas retrieval, is based on a filtering procedure, where the measured atmospheric spectra (divided by a reference spectrum), aims at eliminating the broad spectral features while retaining the narrow spectral features. The former is due to scattering by air molecules and aerosols, as well as to the absorption by aerosols and clouds. The latter are due to trace gas absorption. The filtered spectra are then correlated to the gas species absorption cross section, as derived by laboratory measurements. The advantage of this technique is that it does not rely on absolute radiance measurements.
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