Event K085
1997/98
Event Personnel:
Ian Boyd
Brian Connor
Sylvia Nichol
John Robinson
Stephen Wood (Event Leader)
March 1997-February 1998
Popular Science Summary
The Antarctic ozone hole, a rapid loss of more than half of the ozone of Antarctica each springtime, remains a striking example of how easily mankind can change the atmosphere. The unusual conditions which lead to this loss; a combination of sunlight, cold temperatures inside a stable polar vortex, and anthropogenic chemicals, provide a unique opportunity to study the photochemical and dynamic processes that control the amount of ozone. Knowledge gained in its study can be applied to the task of predicting how such processes affect ozone in other parts of the world as well as their direct impacts in the Antarctic and southern hemisphere.
At Arrival Heights the lowest amount of ozone measured in 1997 was 136 Dobson units (DU), in October as expected and comparable to the minimum amounts in recent years (eg. 130 DU in 1995, 141 DU in 1996). Ozone amounts varied greatly from day to day, and amounts typical of unperturbed regions, as high as 325 DU, were also measured in October, reflecting the large dynamically induced changes in ozone. This occurs where motion of the polar vortex can bring air from outside the vortex, less depleted in ozone, above Arrival Heights.
Chlorine monoxide, ClO, is arguably the most important indicator of perturbed ozone chemistry, and has been measured intermittently in the Antarctic since 1986. The dedicated ground-based microwave receiver installed at Scott Base in February 1996, in collaboration with the State University of New York, now provides year round measurements of ClO, showing enhancements prior to and during the period of ozone loss.
Measurements of a range of chlorine, nitrogen and bromine trace gases that interact with ozone were made at Arrival Heights and Scott Base, using a variety of ground-based spectrometers. These measurements can be used to test models of the chemistry of ozone depletion and predictions of how that chemistry changes as chlorine levels respond to controls on source gases under the Montreal Protocol. The ability to measure trace gases at Arrival Heights has been enhanced by the replacement of the existing infrared instrument by one with higher resolution and wavelength range, in collaboration with the University of Denver. Gases important because of their radiative effects will be investigated, in addition to those important in controlling ozone. Analysis of these measurements is continuing. Modelling will be an increasing feature of future work.
Proposed Programme
METHODOLOGY
Despite the age of the design, the Dobson spectrophotometer is still regarded by the World Meteorological Organisation (WMO) as the standard instrument for ground-based ozone measurement. It measures ozone indirectly by measuring the attenuation of solar radiation at ground level in a few wavebands in the ultraviolet region of the solar spectrum. The measurement obtained is of the total amount of ozone in the vertical column. Even though satellite measurements of ozone are now made routinely, their calibration depends on accurate ground based measurements.
The spectrometers that work at near ultraviolet and visible wavelengths (typically 350-500 nm) use a diffraction grating to separate the wavelengths and either a photomultiplier or a linear array of photodiodes as the detector to measure light intensity. In the former case the grating is scanned and the detector signal recorded as a function of time to yield a spectrum. In the latter case the grating is set to determine the wavelength range to sample and the photodiode responses read to provide the spectrum. They generally work with scattered zenith skylight but operation with direct sunlight or moonlight is possible. Analysis of the spectra involves taking a ratio of each measured spectrum with a control spectrum to remove solar features and then comparison of the measured ratio spectrum to a calculated model spectrum. Parameters of the model spectrum, including the amounts of the trace gases of interest, are adjusted in a least squares fitting procedure to yield the best agreement with the measured spectrum. The interfacing of commercial spectrometers to computers to control them and log their data is based on similar systems developed at NIWA Lauder and run in many parts of the world. The analysis software has been developed at Lauder and is continually being improved.
The measurements made in the infrared employ a different method of separating wavelengths known as Fourier transform spectroscopy. The instruments are scanning Michelson interferometers that split the light into two paths and then recombine them. The intensity seen at the detector is a function of the path length difference and the wavelength. By scanning the path length difference through a range of values a signal called an interferogram is built up that can be converted to a spectrum by a Fourier transform. There is a direct relationship between the maximum path difference and the resolution of the calculated spectrum. The Bomem interferometer had a maximum path difference of 50 cm and so a resolution of 0.02 cm-1 while it’s replacement, the Bruker 120M has a maximum path difference of 257 cm and can achieve resolutions of 0.0035 cm-1. Both instruments have two detectors to cover a range of wavelengths from 2 m m to 10 m m but the Bruker has provision for taking spectra in six filter passbands, rather than the two of the Bomem, and is considerably more automated so that all routine operations are controlled by computer. The detectors are cooled with liquid nitrogen to improve the ratio of signal to noise. The spectra recorded use direct sunlight fed to the instrument with sun tracking mirrors and contain absorption lines of many gases. Quantities of the those gases, and in some cases their vertical profiles, are determined by fitting the measured spectra to synthetic spectra with least squares or optimal estimation techniques. The software has been developed at the University of Denver, NASA Langley and NIWA Lauder. Identical techniques are used by these groups at other sites.
Microwave emission by atmospheric ClO near 1.1 mm wavelength is measured by a heterodyne spectrometer. The heterodyne technique allows very high spectral resolution revealing the details of the pressure-broadened line shape. Because the line is pressure broadened, its shape is sensitive to the vertical distribution of ClO and that distribution can be retrieved with vertical resolution of about 10 km. The range of sensitivity of the current instrument is about 17-50 km. Although the technique has been used for over a decade, the ClO instrument is one of a new generation that underwent validation at Mauna Kea, Hawaii, before it was moved to Scott Base. A very similar instrument has been measuring ozone at Lauder since 1992. NIWA is responsible for the operation of the instrument and data reduction will be carried out at the State University of New York. The need to keep the external reflector of this instrument free of snow is unique to polar measurements.
The Dasibi instrument monitors surface ozone with an ultraviolet absorption measurement. Ambient air is pumped through a cell and the intensity of ultraviolet light shining through this cell from a mercury lamp is measured. Periodically the airflow is diverted to a chemical filter that removes ozone from the air and the lamp’s intensity measured as a control. Corrections are required to allow for the ambient pressure and the instrument temperature, so these parameters are logged by a computer along with the intensity measurements. The unit is a commercially available one; the logging software has been developed by the Climate Monitoring and Diagnostic Laboratory group of NOAA in Boulder and both are in use elsewhere.
An increasing feature of this programme is to use the satellite data link to transfer data back to New Zealand soon after it is acquired. Checks of the data quality can be made much more effectively, and potential problems identified and rectified.
ACHIEVEMENTS:
This season has been one of consolidating the improvements and changes made in the previous season. The single most important change from 1996/97 was the installation of the high-resolution Bruker Fourier transform spectrometer. Initial mechanical and computer problems were frustratingly difficult to track down and rectify. In addition, we needed to learn the best way to make measurements and to make most effective use of the instrument. While the focus of this report is on activities in Antarctica, this is just a part of the entire project. Data analysis and interpretation are equally important. The personnel listed on the title page of this report are only those that travelled to the ice in 1997/98. Several more, both in NIWA and with collaborating groups, are involved. The new FTS brings the requirement for new analysis methods and the handling of much larger volumes of data. Considerably more information is contained in the high resolution FTS measurements, both in the number of gases that can be investigated and the possibility of getting height information. Progress is this area had been made, but there is much work yet to do.
The measurement programme of ozone with the Dobson Spectrophotometer was maintained. There was a noisy bearing in it which was traced and lubricated. The total ozone data from the past year have been submitted to the World Ozone Data Centre. The springtime data were also included in the "real-time" Antarctic Ozone Bulletins that were issued by WMO.
The nitrogen dioxide measurements have continued successfully. Because of the length of the data record for Arrival Heights, it has been the practice over recent years to have duplicate systems making this measurement. Last season, the duplicate instrument was set up in a specially prepared attic in the Hatherton Lab at Scott Base. This season the computer controlling that instrument was upgraded to allow a computer network connection, allowing people at Lauder to check it’s performance and retrieve data via the satellite link. On the atmospheric scale the two sites are effectively identical, but increased reliability comes from having them physically separated and on independent mains supplies. Data from these instruments are analysed at Lauder and added to the database there to allow study of the dynamics and chemistry in the Antarctic stratosphere.
The Bruker FTS did not perform particularly well when it was first installed. There were some problems identified that affected data qualify. The biggest of these was that some intermittent fault appeared to be causing the automatic gain to be set too high and the detectors to saturate, rendering the measurements unusable. A few spectra that did not have this fault were obtained. Considerable time was spent in tracing the fault, and it was eventually (in August 1997) found to be a loose limit switch. Other problems, such as faults in the internal He-Ne reference laser, and problems of "etaloning" affecting one wavelength channel, have been rectified at various times throughout the season.
The diode-array spectrometer used for measurements of BrO and OClO performed well, a relief after trouble in earlier years. Results from BrO retrievals were published. Analysis of 1997 data is proceeding, as is the intercomparison of OClO measurements from a NOAA instrument also operated at Arrival Heights in spring 1996.
After giving sufficient trouble last season that parts had to be sent to the US for fault-diagnosis, performance of the microwave experiment at Scott Base throughout 1997 was good, except for a minor problem with the phase locking of the local oscillator. Early spring measurements show a surprising level of ClO in the upper stratosphere prior to the return of sunlight in spring and, as expected, strong enhancements in ClO at lower altitudes associated with the rapid depletion of ozone around September. During the spring period another ClO instrument, one that has been used for spring campaigns there for a number of years, was operated by the US programme at Little House, between McMurdo and Scott Base. A comparison of the two instrument’s measurements over this period will allow measurements of the Scott Base instrument to be related to earlier measurements. Improvements to the system that keeps the instrument’s external mirror clear of snow with bursts of compressed air have been made and these appear to be extremely worthwhile.
Many of the measurements made form part of the Network for the Detection of Stratospheric Chance (NDSC) and the data are submitted to the NDSC database held in the US. The Lauder group also has strong links with the Japanese Improved Limb Atmospheric Spectrometer (ILAS) campaign. The satellite instrumentation for this was launched on August 17, 1996 but failed prematurely in April 1997. Two scientists from Lauder worked in Japan during 1997 to assist with the validation of data from the instrument, but have now returned. Preparations for a second ILAS experiment are proceeding.
Publications
For earlier publications see previous season’s reports
PUBLICATIONS:
Connor, B. J., N. B. Jones, S. W. Wood, J. G. Keys, C.
P. Rinsland, and F. J. Murcray, Retrieval of HCl and HNO3
profiles from ground-based FTIR data using SFIT2, in Proceedings of
the XVIII Quadrennial Ozone Symposium, L'Aquila, Italy, L'Aquila,
Italy, accepted 1997.
Keys, J. G., S. W. Wood, X. Liu, F. J. Murcray, and R. L. de Zafra, Partitioning of stratospheric chlorine during Antarctic spring as seen from ground-based infrared solar absorption and microwave observations, in Proceedings of the XVIII Quadrennial Ozone Symposium, L'Aquila, Italy, L'Aquila, Italy, accepted 1997.
Kreher, K., P. V. Johnston, S. W. Wood, and J. G. Keys, Ground-based observations of OClO, BrO and NO2 during 1995 at Arrival Heights (77.8°S) Antarctica, in Proceedings of the XVIII Quadrennial Ozone Symposium, L'Aquila, Italy, L'Aquila, Italy, accepted 1997.
Kreher, K., P. V. Johnston, S. W. Wood, B. Nardi, and U. Platt, Ground-based measurements of tropospheric and stratospheric BrO at Arrival Heights (78°S), Antarctica, Geophys. Res. Lett., 24, 3021-3024, 1997.
Kreher, K., T. Wagner, U. Friess, U. Platt, S. W. Wood, P. V. Johnston, and B. Nardi, Observation of enhanced tropospheric bromine oxide in the Antarctic, in International Symposium on Atmospheric Chemistry and Future Global Environment, Nagoya, Japan, pp. 78-81, 1997.
PRESENTATIONS
Connor, B.J., Aspects of error analysis for FTIR column
retrievals, presented at Infrared Working Group Meeting, Network for Detection
of Stratospheric Change, Pasadena, California, 2 June 1997.
Connor, B. J. An overview of Antarctic atmospheric research at NIWA Lauder. Presented at the Department of Physics and Astronomy, University of Canterbury, 15 August 1997.
Keys, J. G., The Antarctic ozone hole, paper presented at UV Radiation and its Effects: an update, National Science Committee for Climate Change, Christchurch, 4-5 December, 1997.
Kreher, K., P. V. Johnston, J. G. Keys, S. W. Wood, G. E. Bodeker, W. A. Matthews, and U. Platt, Spectroscopic measurements of halogen and nitrogen trace gases in the Antarctic stratosphere and troposphere, paper presented at STELAB, Toyokawa, Japan, March 5, 1997.
Matthews, W. A., Ozone changes, causes and expected future changes, paper presented at UV Radiation and its Effects: an update, National Science Committee for Climate Change, Christchurch, 4-5 December, 1997.
Matthews, W. A., S. W. Wood, G. E. Bodeker, B. J. Connor, P. V. Johnston, N. B. Jones, and K. Kreher, ILAS corelative measurements of stratospheric constituents from Arrival Heights, Antarctica, paper presented at 2nd ADEOS workshop, Yokohama, Japan, 10-14 March, 1997.
Matthews, W. A., and G. J. Fraser, Antarctic knowledge base profile: physical sciences. Invited review paper, paper presented at Antarctic Science beyond 2000, Christchurch, 16-18 April, 1997.
Matthews, W. A., The Antarctic stratosphere - the laboratory in the sky, presented at October meeting, Royal Society of New Zealand, Wellington, 28 October, 1997.
Nardi, B., G. D. Donfrancesco, and S. W. Wood, Sounding the skies: Balloons, lidars, satellites, presented at Sunday Science Lecture Series, National Science Foundation, McMurdo Station, 21 September, 1997.
Nichol, S. E., and S. W. Wood, The 1997 Antarctic ozone hole, paper presented at Australasian Atmospheres and Oceans '98, Australian Meteorological and Oceanographic Society and New Zealand Meteorological Society, Wellington, 8-12 February, 1998.
Wood, S. W., S. E. Nichol, and I. S. Boyd, Seasonal Variations in NO2 at 45º, 54º, and 78º South, paper presented at Australasian Atmospheres and Oceans '98, Australian Meteorological and Oceanographic Society and New Zealand Meteorological Society, Wellington, 8-12 February, 1998.
Wood, S. W., B. J. Connor, J. G. Keys, K. Kreher, S. E. Nichol, P. V. Johnston, and N. B. Jones, Antarctic Atmospheric Research, poster presented at Antarctic Science Beyond 2000, Christchurch, 16-18 April, 1997.
Wood, S. W., T. S. Stephen, N. B. Jones, B. J. Connor, F. J. Murcray, and J. G. Keys, FTIR observations of chlorine and nitrogen trace gases as 78 degrees South during the 1997 austral spring, paper presented at AGU Fall Meeting, American Geophysical Union, San Francisco, 8-12 December, 1997.
OTHER ARTICLES
Connor, B.J., and T. S. Clarkson, Severe 1997 Ozone Hole
Reaches Maximum. Released 17 October 1997. Story carried by newspapers
throughout New Zealand. Interviews with Connor or Clarkson appeared on
IRN, Radio NZ, TVNZ, and TV3. Also carried by Reuters, AFP, the
Malaysia Straits-Times, and Antarctic Magazine.
PLANNED PUBLICATIONS
Work is progressing in a number of areas, including the utilisation
of the new measurements available and in investigating the use of other
data to enhance the ground based data recorded in the Antarctic. Results
from this work will be submitted to refereed journals.
Acknowledgements
This work was funded by the Foundation of Research Science and Technology.
It is closely linked to similar work carried out under other FRST contracts
at Lauder in Central Otago. It relies on the commitment of collaborative
partners. It is also dependent on the excellent logistic and technical
support from the New Zealand Antarctic Institute in Christchurch and at
Scott Base.
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