THOMAS M. STEPHEN, JOHN R. OLSON, RENATE VAN ALLEN, and FRANK J. MURCRAY, Department of Physics and Astronomy, University of Denver, Denver, Colorado 80208-2238
The University of Denver operates two Fourier transform infrared (FTIR) spectrometers on the continent of Antarctica. These instruments have been upgraded in the last 2 years. The Amundsen-Scott South Pole Station instrument, installed in 1995, is an emission spectrometer capable of atmospheric observations during both the austral winter and summer. The instrument at McMurdo Station, installed in 1996, is a solar absorption instrument in use during the austral summer.
The University of Denver Atmospheric Emission Radiometric Interferometer-Extended (AERI-X) was installed in the annex of Antarctic Submillimeter Telescope and Remote Observatory (AST/RO) at South Pole Station in December 1995 by John Olson and Renate Van Allen. In December 1996, John Olson and Linda Wigoda (a high school teacher from St. Charles, Louisiana, participating in the National Science Foundation Research Opportunity Awards program) traveled to South Pole Station to perform routine maintenance and calibration tasks. As of this writing, the instrument has collected data over two austral winters and parts of two austral summers.
AERI-X is a Michelson-type interferometer that is described in detail in Olson et al. (1996). This instrument detects atmospheric emission in the range of 650-1,250 inverse centimeter (cm-1) with a spectral resolution of 0.1 cm-1. This region includes an atmospheric transmission window, at 12 microns, allowing for the quantification of stratospheric constituents, including nitric acid (HNO3) and water vapor (H2O). This analysis uses an atmospheric line-by-line model. A comparison between calculated emission spectra and observed spectra, is presented in figure 1.
A consistent series of observations using the AERI-X allows for the study of seasonal variability of important stratospheric constituents. For example, figure 2 shows the denitrification of the stratosphere in the austral fall and the subsequent slow recovery of HNO3 during the austral spring in 1996 ( see also Van Allen, Liu, and Murcray 1995). A paper on the observed denitrification in connection with the dehydration of the atmosphere is in preparation.
The spectral range of the AERI-X instrument also encompasses two of the carbon dioxide (CO2) emission bands, at 11.1 and 15 microns. These bands, important in radiative transfer calculations, may also lead to methods for atmospheric sounding. By careful analysis, these bands may be used to retrieve the temperature profile of the atmosphere as a function of altitude.
The AERI-X represents an improvement over previously deployed instruments. This improvement is rather broad based. Earlier instruments of this type had a resolution of 1 cm-1 (Van Allen, Murcray, and Liu 1996), and although these instruments performed well, higher spectral resolution benefits both studies involving stratospheric chemistry and radiative transfer measurements. Data are now collected continuously rather than twice a day. This improved time resolution allows for a complete monitoring of changes developing over the course of hours.
An FTIR spectrometer has been operated on Arrival Heights since 1989 by the University of Denver. This instrument is operated in the Antarctic New Zealand observatory by their science technicians. Until December of 1996, a Bomem DA-2 was in operation there. This instrument was replaced at that time by a Bruker 120m FTIR spectrometer. The new instrument is capable of solar absorption measurements in the spectral region 800-5,000 cm-1 at a spectral resolution of 0.0025 cm-1, a factor of four better than the older instrument. The Bruker instrument uses two detectors and five optical filters to span this spectral region. This replacement was motivated by the desire to install a more versatile instrument capable of obtaining a higher resolution data set. The Bruker instrument was installed at the Antarctic New Zealand observatory at Arrival Heights in December 1997 by Thomas M. Stephen and Shaima Nasiri (an undergraduate National Science Foundation Research Experience for Undergraduates participant) from the University of Denver and Steve Wood and John Robinson of the National Institute of Water and Atmosphere, Crown Research Institute, New Zealand.
The solar absorption instrument at Arrival Heights differs from the previously discussed AERI-X instrument at a very fundamental level. The AERI-X measures the emission of radiation from molecules present in the atmosphere; therefore, the atmosphere itself is the source. The Bruker instrument is configured to measure the absorption of radiation by molecules in the atmosphere. It generally uses the Sun as a spectral source, although the Moon can also be used after some modification to the instrument. A solar absorption spectrometer is much more sensitive to trace gas constituents, can be used to profile a wide variety of atmospheric species (Nakajima et al. in press), and is generally better able to provide column amounts for a variety of atmospheric constituents. It is the accurate determination of total column amounts of stratospheric constituents that is a driving force for this research for the Network for the Determination of Stratospheric Change (for general information on this program see the Web page at http://climon.wwb.noaa.gov ).
The new spectrometer has been in operation for less than a year now, 3 months of sunlight. It is still undergoing a "shake-down" period. Preliminary results of observations of chlorine nitrate (ClONO2) during the austral spring will be presented at the 1997 Winter American Geophysical Union (AGU) meeting.
New instruments have been installed at South Pole Station and at McMurdo Station. These instruments will allow for a higher temporal density of data taken at higher resolution. The high resolution obtained by these instruments will allow for more intensive modeling efforts in both the fields of atmospheric chemistry and radiative transfer.
We would like to thank Simon Balm, Jeanne Kelley, and Paul Sullivan for taking excellent care of our experiment at South Pole Station. In addition, we thank the Antarctic New Zealand Program and the National Institute of Water and Atmosphere, Crown Research Institute for their hospitality and help. In particular, we are grateful to their winter-over science technicians, Michael Mahon and Hermene Binnie.
This work was supported by National Science Foundation grant OPP 95-26913 and National Aeronautics and Space Administration grant NAG2-351.
Nakajima, H., X. Liu, I. Murata, Y. Kondo, F.J. Murcray, M. Koike, Y. Zhao, and H. Nakajima. In press. Retrieval of vertical profiles of ozone from high-resolution infrared solar spectra at Rikubetsu, Japan. Journal of Geophysical Research .
Olson, J.R., J. Van Allen, P.F. Fogal, F.J. Murcray, and A. Goldman. 1996. Calibrated 0.1-cm-1 IR emission spectra from 80N. Applied Optics , 35, 2797.
Van Allen, R., X. Liu, and F.J. Murcray. 1995. Seasonal variation of the atmospheric nitric acid over the South Pole in 1992. Geophysical Research Letters , 22, 49-52.
Van Allen, R., F.J. Murcray, and X. Liu. 1996. Mid-infrared measurements of the atmospheric emission over the South Pole using a radiometrically calibrated Fourier transform spectrometer. Applied Optics , 35, 1523-1530.