|Volume 57(1) — January 2008
Global atmospheric ozone monitoring
by Johannes Staehelin*
Ozone molecules are concentrated in the stratosphere mainly between altitudes of 10 and 40 km. They determine the temperature structure of the stratosphere and, by absorbing harmful ultraviolet radiation, safeguard life on this planet.
Isolated ozone observations were made in the 1920s, but systematic measurements began only about 50 years ago. At present, more than 70 agencies in some 50 WMO Member countries are contributing ozone observations to WMO’s Global Atmosphere Watch (GAW), providing data essential for understanding the state of, and changes to, the ozone layer. These data started to be analysed carefully when, in the early 1970s, scientific findings highlighted the potential of chlorofluorocarbons (CFCs) and halons to destroy ozone with serious environmental implications.
It was not until the mid-1980s, however, that we obtained convincing evidence of ozone destruction as demonstrated by the dramatic ozone decline in the Antarctic spring. This discovery was made in 1985 by British scientists performing ground-based measurements from the Halley station at 76°S, where ozone observations had been carried out since 1956.
During the last 20 years, WMO, in collaboration with the United Nations Environment Programme (UNEP), has coordinated the preparation of a series of scientific assessments, the most recent of which was published in early 2007. They are based on the achievements of hundreds of scientists, from both developed and developing countries and on contributions from many national agencies. Ground-based measurements play an essential role, both for trend studies in various regions of the world and also for ground-truthing of satellite measurements. These assessments provided the basis for the UNEP-led negotiations for conclusion of the Vienna Convention on the Protection of the Ozone Layer (1985) and its Montreal Protocol (1987). Today, these assessments form the basis for amendments and adjustments to the Montreal Protocol, the most recent of which was agreed upon in Montreal in September 2007. The current Montreal Protocol requires drastic reductions in the use of CFCs and halons and an accelerated phase-out of hydrochloroflurocarbons.
Long-term recovery of the ozone layer from the effects of ozone-depleting substances is, according to the most recent WMO/UNEP Scientific Assessment of Ozone Depletion, expected to span much of the 21st century. It is estimated to happen around 2050 in middle latitudes and around 2065 in the Antarctic. This is five and 15 years later, respectively, than projected in the previous (2002) Assessment.
Failure to comply with the Montreal Protocol would delay, or could even prevent, recovery of the ozone layer. It is, therefore, of importance to continue systematic, high-quality observations of the ozone layer in all regions of the world. It is also becoming more and more apparent that there are important linkages between ozone depletion and climate change. Ozone itself, ozone-depleting substances and many of their substitutes are also greenhouse gases; changes in ozone affect the climate and changes in climate affect ozone.
Because of its importance as a greenhouse gas, ozone is one of the essential climate variables (ECVs) targeted by the Global Climate Observing System (GCOS). In 2007, the GAW Dobson and Brewer spectrophotometer networks and the GAW ozone balloon-sonde network were adopted as baseline networks of GCOS. In this article, a description is given of the GAW ozone observing system and the contributing Southern Hemisphere Additional Ozonesondes (SHADOZ) network and the Network for the Detection of Atmospheric Composition Change (NDACC).
The GAW Programme was established in 1989, merging the Global Ozone Observing System (GO3OS) and the Background Air Pollution Monitoring Network, with increased emphasis on quality assurance and global partnership and a focus on six measurement groups: ozone, UV radiation, greenhouse gases, aerosols, major reactive gases and precipitation chemistry. Through the GO3OS system and now GAW, there is a history of global monitoring of atmospheric ozone (total ozone as well as profile ozone) going back to the early 1970s.
GO3OS was built in response to the threat of anthropogenic destruction of the ozone layer. In 1974, ozone depletion by chlorine was first discussed by Stolarski and Cicerone and Molina and Rowland. The latter two scientists first described the source of reactive chlorine species by the release of chlorine from anthropogenically emitted chlorofluorocarbons. The full extent of anthropogenic ozone depletion was found in 1985, when Farman et al. discovered the ozone hole over the Antarctic.
The Montreal Protocol of 1987 has been successful. The amount of ozone-depleting substances is now slowly going down (about 1 per cent per year) after reaching a peak in the late 1990s. Nonetheless, the Antarctic ozone hole of 2006 was the largest on record. This was due to the unusually cold and stable south polar vortex in the austral spring of 2006. This demonstrates that the degree of ozone loss depends not only on the atmospheric burden of ozone-depleting halogens but also on the meteorological conditions. It also shows the close linkage between ozone depletion and climate change.
The total ozone measurements operated under the umbrella of GAW are based on spectrophotometer measurements, using either the Sun or the zenith sky as the light source. Today, measurements from about 80 Dobson and about 50 Brewer instruments are regularly reported to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) in Toronto, which was established in 1960.
Ozone profile information can be obtained from several types of instruments operated from the ground. Reliable and detailed information up to an altitude of approximately 35 km can be obtained by measurements with an electrochemical cell from small balloons which typically burst at 25-35 km. Ozone profiles are also measured by lidar and microwave instruments, having their particular strengths in the stratosphere and mesosphere.
Consisting today of a global partnership of managers, scientists and technical experts in 112 countries, GAW is coordinated by the Joint Scientific Steering Committee of the Open Programme Area Group on Environmental Pollution and Atmospheric Chemistry (OPAG-EPAC) of the WMO Commission for Atmospheric Sciences (CAS) with the assistance of the WMO Atmospheric Environnment Research Division of the Research Department.
The GAW office and its Ozone SAG have been actively involved in supporting the Vienna Convention and its Montreal Protocol, as well as the United Nations Framework Convention on Climate Change (UNFCCC) through contributions to the Implementation Plan and the Second Report on the Adequacy of the Global Observing Systems for Climate by the Global Climate Observing System (GCOS). Essential climate variables that need to be systematically measured globally in order to address major issues are officially recognized by the UNFCCC. These include total ozone and profile ozone and GAW has been designated by WMO and GCOS as the lead international programme in furthering the observational requirements for ozone. Since October 2007, the Dobson, Brewer and ozonesonde networks constitute baseline networks of GCOS.
A further noteworthy development that has taken place in the last three years is the establishment under the Integrated Global Observing Strategy partnership of a strategy related to atmospheric composition issues. The Integrated Global Atmospheric Chemistry Observations (IGACO) strategy provides a framework for global observations under the Global Earth Observations System of Systems. IGACO involves 13 classes of atmospheric chemistry variables, total ozone and profile ozone being key variables. The IGACO strategy is the basis for the next generation GAW programme of 2008-2015 and will be implemented by the research community under the leadership of GAW with the assistance of GCOS, its Atmospheric Observation Panel for Climate and the Global Earth Observations System of Systems. The IGACO-Ozone/UV office has been established at the Finnish Meteorological Institute and the implementation plan of IGACO-Ozone/UV is currently in the final stages of review.
The ozone component of GAW is a mature, organized system. Details can be found on the WMO Website (http://www.wmo.int/pages/prog/arep/gaw/gaw_home_en.html). The Ozone SAG is responsible with the JSSC of OPAG-EPAC for implementing the GAW Strategic Plan. It reviews and writes measurement guidelines and data-quality objectives and guides the GAW office at WMO in the maintenance, development and communication of products and services of the network.
To assist in the latter, numerous quality assurance/science activity centres conduct intercomparisons and provide audits and training at the national and international levels. The GAW Station Information System (GAWSIS) (http://www.empa.ch/gaw/gawsis/) is an on-line query and mapping facility supported by the MeteoSwiss GAW Programme and the Swiss Federal Laboratories for Materials Testing and Research that documents the stations, their measurements and points of contact, as well as up-to-date information on data submission to the World Ozone and UV Data Centre operated by Environment Canada. The WMO/GAW World Ozone and UV Data Centre collects, documents and archives data and quality-assurance information and makes them freely available to the scientific community for analysis and assessments.
Total ozone observations
Within the GAW Global Atmospheric Ozone Monitoring Network, total ozone is measured and reported regularly at some 80 stations by Dobson instruments. The longest continuous data series is from the Swiss Alpine site Arosa going back to 1926. The station instruments of the present network need to be regularly compared (at least once every four years) with regional standard instruments by intercomparisons, which is done by side-by-side comparison. The regional standard instruments are compared at least at the same intervals with the Dobson world primary instrument.
The Brewer instrument is based on the same basic measuring principle as the Dobson spectrophotometer but using more modern technology. It has been commercially produced since the middle of the 1980s and is designed for automatic measurements. The number of Brewer instruments has been increasing during the last decades. The GAW World Reference Standard is based on the triad of instruments operated by Environment Canada. An additional Brewer triad is located at the Brewer calibration centre at Izaña (Tenerife) operated by the National Institute of Meteorology of Spain. The Langley plot measurements performed at Izaña provide important redundancy of the calibration scale of the Brewer triad. Brewer instruments are supposed to be compared with standard instruments at least every two years.
UV-visible differential optical absorption spectroscopy
UV-visible spectrometers using differential optical absorption spectroscopy technique are deployed at several NDACC stations. Some of the stations use the standardized SAOZ (Système d’analyse par observations zénithales) instrument, developed by the French National Scientific Research Centre, and others use “home-built” spectrometers. The quality of the measurements is checked regularly through intercomparisons, where a large number of instruments are compared side by side. New instruments are not accepted in the network until they have proven their performance through an intercomparison campaign.
Ozone profile measurements
Ozone-profile measurements from small balloons (1 200-2 000 g) have been regularly performed by electrochemical sensors since the early 1970s and are what most stations use today.
Additional ozonesonde stations in the tropics have been established within the SHADOZ project, supported by the US National Aeronautics and Space Administration (NASA). The 13 SHADOZ stations are jointly operated with GAW.
Electrochemical sensors are produced by two manufacturers and two different solute concentrations are currently used in the network. Both makes of sensor and solute concentrations affect the measurements of the ozone profiles. Sonde intercomparisons in the JOSIE environmental simulation chamber in Jülich, intercomparisons with many combinations of sensor brand and solute concentration conducted on the same balloon carrying a reference UV-photometer (e.g. BESOS campaign) and measurements of two sondes from the same balloon (dual flights) are used to study the influence of these factors on profile measurements from electrochemical sondes. The vertical resolution, taking into account the response time of the sensor is about 200-300 m.
There are 25 ozonesonde stations affiliated with NDACC. The electrochemical sonde was accepted as an NDACC measurement in 1995. Most of these stations are also GAW stations. NDACC has served to promote additional stations, filling gaps in remote areas that are not otherwise accessed by GAW members.
Ozone lidars and microwave radiometers
In addition to electrochemical sondes, lidars and microwave radiometers are used to determine upper-air ozone profiles. Lidars typically cover the altitude range 10-50 km and microwave radiometers cover the range 20-70 km. The vertical resolution of a lidar ozone profile is typically around 100-200 m. A microwave profile has a vertical resolution of approximately 5-10 km. In the same way as with UV-visible spectrometers, lidars and microwave radiometers in the NDACC network undergo regular intercomparisons and new instruments must prove the quality of the measurements before being accepted as part of NDACC.
Most NDACC ozone lidar stations and NDACC stations with microwave spectrometers are also GAW stations.
The GAW Global Atmospheric Ozone Monitoring Network consists of global, regional and contributing partner stations (e.g. SHADOZ and NDACC). GAW stations are operated by WMO Member States, while contributing partner stations are operated by other independent networks. Total ozone (Dobson and Brewer spectrophotometers) and profile ozone (ozonesondes) measurements are carried out according to standard operating procedures and data- quality objectives developed by the WMO Ozone SAG across GAW and contributing networks. Dobson, Brewer and sonde data are submitted to the World Ozone and Ultraviolet Radiation Data Centre, either directly from the stations or via the NDACC and SHADOZ data centres.
Though quite extensive in its global coverage, the network continues to grow to match the needs of the climate research community as defined periodically in the scientific assessments of WMO/UNEP and the Intergovernmental Panel on Climate Change. Currently, there are gaps in Asia, Africa and South America. Although satellite data are not a formal part of these networks, GAW, through its Strategic Plan 2008-2015, is developing the capability to link and integrate surface-based, aircraft and satellite data following the requirements of the IGOS-IGACO strategy.
Contributors to GAW for total ozone
Fifty-two countries are registered at WOUDC as actively contributing total ozone data from Dobson and/or Brewer instruments. GAW contributors also develop and provide network operational products, such as global and regional calibration centres. They contribute on a number of levels, from collecting and analysing data, to providing quality control, to producing documents and delivering products such as those by WOUDC. For instance, the World Calibration Centre for Dobson supported by NOAA/ESRL/Global Monitoring Division has a long history of on-site calibrations, audits, training and maintenance at stations in developing countries and occasionally in developed countries. In addition, there are regional Dobson calibration centres hosted by the National Meteorological Service of Argentina (Buenos Aires), the Japan Meteorological Agency (Tokyo), the Australian Bureau of Meteorology (Melbourne) and the Deutsche Wetterdienst (Hohenpeissenberg).
The World Reference for Brewer measurements is held by Environment Canada and a Regional Calibration Centre is hosted by the National Institute of Meteorology at Izaña, Tenerife, Spain.
A Website with information on Dobson calibration is maintained by the Dobson ad hoc Committee of WMO and hosted by the Czech Hydrometeorological Institute.
Because of the contributions of its many partners dedicated to generating the highest-quality data, GAW is able to adapt to pressing global needs driven by new scientific questions, to provide exemplary quality assurance mechanisms, to remain up to date with developments in instrumentation and data management and to develop useful and informational products and services for the global community.
Contributors to GAW for ozonesonde profile data
Thirty countries are registered in WOUDC as actively contributing ozone profile data from ozonesondes. The WMO World Calibration Centre for Ozonesondes is hosted by Forschungszentrum Jülich, Germany. Several Jülich ozonesonde intercomparison experiments have been carried out during the last decade in order to understand the characteristics and differences between different types.
The Balloon Experiment on Standards for Ozone Sondes (BESOS) was hosted by the University of Wyoming, USA, in April 2004 and a payload of 12 ozonesondes was compared with a UV ozone photometer. Several workshops have been held to discuss the results of the campaign and these results will lead to official WMO standard operating procedures for ozonesondes. Most of the stations that earlier used the Brewer-Mast sondes have switched to the more modern electrochemical sondes. Dual flights have been carried out at these stations over a period of several years in order to be able to splice the times-series. Dual flights have also been carried out at a number of stations with the two main brands of electrochemical ozonesondes to obtain a better understanding of the differences between them.
Contributors to SHADOZ for ozonesonde profile data
A number of stations were operating in the southern hemisphere tropics and subtropics, but with differing frequency and reporting procedures. SHADOZ was designed to remedy this data discrepancy by coordinating launches, supplying additional sondes in some cases and by providing a central archive location. Data are collected in a timely manner and made openly available through the SHADOZ Website to the scientific community as a whole. Several institutions contribute to the SHADOZ network, with station facilities and purchase of ozonesondes. Among the contributors are NASA, NOAA and MeteoSwiss.
Contributors to NDACC
NDACC changed its name from Network for the Detection of Stratospheric Change in 2005 in order to reflect the increased breadth of measurement techniques and related research. When the network was planned during the latter half of the 1980s, the focus was stratospheric ozone depletion. It is evident, however, that the instruments deployed in NDACC can address several issues other than ozone depletion, such as temperature change, changes in water vapour and atmospheric aerosols.
The regions of the atmosphere that are observed range from the free troposphere to the mesosphere. The network consists of more than 70 stations distributed in the two polar regions, middle latitudes in both hemispheres and in the tropics. These stations are operated by institutions in 21 countries. The observations and related research are funded nationally funding and by the European Commission through the Framework Research Programmes. NDACC operations started in 1991 and the number of stations is still increasing. The main goals of the network are to provide consistent, standardized, long-term measurements of atmospheric temperature, trace gases, particles, ultraviolet radiation and physical parameters centred around the following priorities:
The primary objective of the GAW quality assurance system is to ensure that the data deposited in WOUDC are consistent, meet GAW data- quality objectives and are supported by a comprehensive description of methodology. The system involves quality assurance/science activity centres and calibration centres that ensure the quality of observations through adherence to measurement guidelines established by the Scientific Advisory Groups and through calibrations that are traceable to World Calibration Standards. The SHADOZ and NDACC ozonesonde stations, which largely overlap with the GAW network, follow the same quality assurance routines as in GAW.
Long-term education, training, workshops, calibration station audits/visits and twinning are also provided to build capacities in atmospheric sciences in the GAW network. These capacity-building activities are of increased importance as many GAW stations in developing countries have become operational.
GAW procedures address the quality of an observation through maintenance of components of the entire measurement process, from operational procedures at stations to submission of properly quality-controlled data to the World Data Centre. The recommended GAW principles are as follows (GAW Strategy of Implementation, 2001, Report No.142):
In addition, certain measurement principles are parameter-specific:
Data archiving and processing centres
The six GAW WDCs are each responsible for archiving one or more GAW measurement parameter(s) or measurement type(s). The data centres are operated and maintained by their individual host institutions. They collect, document and archive atmospheric measurements and the associated metadata from measurement stations worldwide and make these data freely available to the scientific community. In some cases, WDCs also provide additional products, including data analyses, maps of data distribution and data summaries. The most recent addition to the GAW family of World Data Centres (WDCs) has been established at the German Aerospace Centre. This centre will give one-stop access to satellite data on atmospheric composition.
Data analysis, distribution and application
GAW Station Information System (GAWSIS)
The Swiss Federal Laboratories for Materials Testing and Research hosts the GAWSIS Website (http://www.empa.ch/gaw/gawsis). This is a useful tool for obtaining an overview of the GAW system. It contains detailed information on measurement programmes, contact persons, exact location of stations and a direct link to the ozone data stored at WOUDC.
World Ozone and Ultraviolet Radiation Data Centre
From this Website (http://www.woudc.org), anyone can access measurement data that have been contributed by ozone stations throughout the world. Users are requested to note that they should properly reference the data if they use or publish them by citing the contributors and the source of the data. Some examples of data distribution products are:
SHADOZ data archive
The SHADOZ Website (http://croc.gsfc.nasa.gov/shadoz/) gives easy and open access to the all the data collected from the SHADOZ network. A clickable map of the stations leads to the database where one can download data or look at ready-made plots. The data are stored in a simple self-explanatory ASCII format.
Data archive of NDACC
The NDACC data protocol states that investigators should submit data within one year of the measurement. The data will then reside for one year in a limited-access part of the data centre. After that, it is copied to an anonymous ftp site for public access. Investigators can, of course, tell the data centre manager to make data available earlier than required in the data protocol. This arrangement is put in place so that data providers have time to calibrate and validate their data. The data are stored in ASCII format and can be accessed through the NDACC Website (http://www.ndacc.org).
Other data centres
The British Atmospheric Data Centre (http://badc.nerc.ac.uk/home/index.html) contains a copy of the NDACC data centre.
The Norwegian Institute for Air Research collects ozonesonde data from European ozonesonde stations in near-real-time (same day), converts them to the CREX code form and passes them on to the European Centre for Medium-Range Weather Forecasts.
At the request of CAS, GAW has produced bi-weekly Antarctic ozone bulletins during the ozone hole season from August to December every year since the late 1980s. The Arctic ozone bulletin has been issued annually since 2006. These bulletins make extensive use of both near-real-time data and long-term climatological data from a number of GAW and NDACC stations. Satellite data are also used.
GAW partners all aim to improve applications of the database. These improvements are related to evolving scientific questions. The standard operating procedures of Dobson instruments are currently being updated and those for Brewer instruments and electrochemical sondes are nearing completion. In addition, the collaboration between the Ozone SAG and the NDACC Steering Committee is planned to be improved.
As noted earlier, the GAW network continues to expand with scientific needs, in collaboration with SHADOZ and NDACC. An important research topic concerns documenting the effect of the reduction of ozone-depleting substances on the ozone layer. While the decrease in anthropogenic ozone-depleting substances, enforced by the Montreal Protocol, is well documented through their decreasing concentrations in the troposphere, their influence on the ozone layer is much more difficult to quantify. Further activities aiming at integrating ground-based, satellite and regular aircraft measurements are planned for IGACO-Ozone/UV, for which the implementation plan is currently under review.
In order to obtain high-quality global ozone datasets from satellite instruments suitable for long-term trend analysis, the validation of satellite by high-quality ground-based measurements has high priority. Historically, GAW was more involved in the quality assurance/quality control of ground-based data. By implementing IGACO, WMO becomes more involved in the comparison of satellite and ground-based ozone measurements.
NDACC is now putting more emphasis on water vapour in the free troposphere, stratosphere and mesosphere. There are large uncertainties around the trend of stratospheric water vapour, with satellite data and frost-point hygrometer data giving opposite results for the last few years. Water vapour is already measured by some microwave instruments and some lidar systems. However, there is a need for balloon soundings to collect data in the upper troposphere and lower and middle stratosphere.
Significant research results
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Deuber, B., N. Kämpfer and D.G. Feist, 2004: A new 22-GHz radiometer for middle atmospheric water vapor profile measurements, IEEE Transactions on Geoscience and Remote Sensing, Vol. 42, No. 5, (doi:10.1109/TGRS.2004.825581)
Farman, J.C., B.G. Gardiner and J.D. Shanklin, 1985: Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction, Nature, 315, 207-212, .
Fioletov, V.E, J.B. Kerr, C.T. McElroy, D.I. Wardle, V. Savastiouk and T.S. Grajnar, 2005: The Brewer reference triad, Geophys. Res. Lett., 32, 20,805,.
Froidevaux, L., W.G. Read, T.A. Lungu, R.E. Cofield, E.F. Fishbein, D.A. Flower, R.F. Jarnot, B.P. Ridenoure, Z. Shippony, J.W. Waters, J.J. Margitan, I.S. McDermid, R.A. Stachnik, G.E. Peckham, G. Braathen, T. Deshler, J. Fishman, D.J. Hofmann and S.J. Oltmans, 1996: Validation of UARS microwave limb sounder ozone measurement, J. Geophys. Res.-Atmos., 101, 10 017–10 060.
Johnson, B.J., S.J. Oltmans, H. Vömel, H.G.J. Smit, T. Deshler and C. Kroger, 2002: Electrochemical concentration cell (ECC) ozonesonde pump efficiency measurements and tests on the sensitivity to ozone of buffered and unbuffered ECC sensor cathode solutions, J. Geophys.Res., 107(D19), 4393, doi:10.1029/2001JD000557.
Johnston, H., 1971: Reductions of stratospheric ozone by nitrogen oxide catalysts from supersonic transport exhaust, Science, 173, 517-522.
Keckhut P.S. McDermid and D. Swart et al., 2004: Review of ozone and temperature lidar validations performed within the framework of the Network for the Detection of Stratospheric Change, Journal of Environmental Monitoring, 6 (9), 721-733.
Molina, M.J. and F.S. Rowland, 1974: Stratospheric sink for chlororfluoromethanes, chlorine atom catalyzed destruction of ozone, Nature, 249, 810-812.
Pommereau, J.-P., and F. Goutail, 1988: O3 and NO2 ground-based measurements by visible spectrometry during Arctic winter and spring 1988, Geophys. Res. Lett., 15, 891-894.
Staehelin, J., A. Renaud, J. Bader, R. McPeters, P. Viatte, B. Högger, V. Bugnion, M. Giroud and H. Schill, 1998: Total ozone series of Arosa (Switzerland). Homogenization and data comparison, J. Geophys. Res., 103, 5827-5841.
Staehelin, J., N.R.P. Harris, C. Appenzeller and J. Eberhard, 2001: Ozone trends: a review, Rev. Geophys., 39, 231-290,.
Steinbrecht, W., H. Claude, F. Schönenborn, I. S. McDermid, T. Leblanc, S. Godin et al., 2006: Long-term evolution of upper stratospheric ozone at selected stations of the Network for the Detection of Stratospheric Change (NDSC), J. Geophys. Res., 111, D10308, doi:10.1029/2005JD006454.
Stolarski, R.S. and R.J. Cicerone, 1974: Stratospheric chlorine: a possible sink for ozone, Can. J. Chem., 52, 1610-1615.
Thompson, A., J.C. Witte, H.G.J. Smit, S.J. Oltmans, B.J. Johnson, V.W.J.H. Kirchhoff and F.J. Schmidlin, 2007: Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998-2004 tropical ozone climatology: 3. Instrumentation, station-to-station variability, and evaluation with simulated flight profiles, J. Geophys Res., 112, D03304, doi:10.1029/2005JD007042.
Vandaele, A.C., C. Fayt, F. Hendrick, et al., 2005: An intercomparison campaign of ground-based UV-visible measurements of NO2, BrO, and OClO slant columns: Methods of analysis and results for NO2, J. Geophys. Res., 110 (D8), Art. No. D08305 APR 26.
WMO, 1985: Atmospheric Ozone, Chapter 14 (Ozone and temperature trends), Global Ozone Res. and Monit. Proj., Report No. 16, Vol. III, WMO, Geneva, Switzerland.
WMO, 1989: Report of the International Ozone Trends Panel 1988, Global Ozone Res. and Monit. Proj., Report No. 18, Geneva.
WMO, 2007: 2006 Scientific Assessment of Ozone Depletion: Global Ozone Research and Monitoring Project, Report No. 50, Geneva.