RAMAS: MICROWAVE RADIOMETER FOR STRATOSPHERIC
TRACE GASES AT GREENLAND
N. Buschmann1, A. Kleindienst1, M. Hoock1, K. Künzi1, J. de La Noë2, N. Schneider2, H.
Jønch-Sørensen3, A. Gross3, M. Chipperfield4 and G. Nedoluha5
1
2
Institute of Environmental Physics, University of Bremen, Germany,
Observatoire de Bordeaux, University Bordeaux 1, France, 3Danish Meteorological Institute, Denmark,
4
University of Leeds, UK, 5Naval Research Lab, USA
Abstract
Microwave Radiometers, key sensors at NDSC stations, are adversely affected by high tropospheric
water vapour content. Presently available stations in the Arctic are located between sea level and
600m, where poor meteorological conditions still severely limit the temporal coverage.
RAMAS (Radiometer for Atmospheric Measurements at SUMMIT) takes advantage of the unique
SUMMIT station (72° N, 38° W) in the interior of Greenland at an altitude of 3200 m. Presently a
microwave sensor is being developed for deployment at the SUMMIT research station in spring
2003. The observing band for the radiometer will cover the frequency range from 265 to 281 GHz.
RAMAS will allow to measure essentially O3, ClO, N2O and HNO3. This comprises a set of
primary species involved in polar ozone chemistry, and an important dynamical tracer (N2O), which
will permit the separation of chemical and dynamical effects on O3.
It is planned to have RAMAS operational at SUMMIT in early summer 2003. After an appropriate
validation and test phase we expect RAMAS to become eventually part of the primary Arctic
NDSC station.
Introduction
The stratospheric ozone layer protects life on earth from harmful ultra-violet radiation of the sun.
Since the discovery of the Antarctic ozone hole large parts of the world population have become
aware that human impact greatly affects the composition of the earth's atmosphere. During the last
decade similar effects have been observed over the Arctic and over northern Europe.
The maximum loading of the stratosphere with chlorine was found to happen around the year 2000,
and consequently also the maximum ozone depletion. How long the recovery of the ozone layer will
take is unclear. State-of-the-art models predict 10 to 30 years, however these models are
incomplete, e.g. they can not reproduce the quantitative details of observed ozone declines.
Therefore it is an absolute requirement to continuously observe the ozone layer and key constituents
of ozone destruction. Polar regions, where the largest ozone decline has been observed, are good
indicators to detect recovery of the ozone layer.
Project outline
Within the project RAMAS (Radiometer for Atmospheric Measurements at Summit) a new
microwave sensor is being developed for deployment at the Summit research station (72° N, 38° W)
in the interior of Greenland at an altitude of 3200 m. Presently available stations in the Arctic are
located between sea level and 600 m, where the observing conditions are still severely limited due
to high tropospheric attenuation and poor weather conditions. Summit station provides the only high
altitude site available in the Arctic, which allows year-round millimetre-wave measurements. Figure
1 shows the improvement obtained at Summit compared to the Arctic station Ny-Ålesund
(Spitzbergen) at sea level. Compared to the similar radiometer situated at sea-level in Ny-Ålesund a
factor of 5 increase in signal strength is expected for Summit.
Fig. 1: Signal strength of ClO at Summit and Ny Ålesund. Solid lines: Intensity of the ClO emission line for
activated chlorine (Cl) after transmission through the troposphere (monthly climatological mean water vapour
profiles). Dashed lines: Same for unactivated chlorine. Dots: Same for inactivated Cl but calculated for water vapour
profiles derived from radio sondes launched at Summit. Grey shaded area: Range of expected signal intensity.
Instrument details
Microwave radiometers measure the thermal radiation induced by rotational transition lines of
atmospheric gas molecules. The spectrally well resolved line shapes contain altitude information
through pressure broadening of the particular emission line. This effect is used to retrieve altitude
profiles of the volume mixing ratio of the respective atmospheric constituents. The observing band
for RAMAS will cover the frequency range from 265 to 281 GHz, which contains a large number of
interesting species and is best suited for a site with low tropospheric water vapour content. Figure 2
shows emission lines of different trace gases within the observing range of RAMAS.
The principal objective of this new microwave sensor is to measure ozone (O3), chlorine monoxide
(ClO), nitrous oxide (N2O) and nitric acid (HNO3) over an altitude range of approximately 12 to 55
km and an accuracy of 0.2 ppmv for O3, 0.3 ppbv for ClO and HNO3 and 10-30 ppbv for N2O.
The performance of this radiometer is mainly characterised by three components: First the mixer in
the front-end will be a Superconductor-Insulator-Superconductor (SIS) junction cooled down to 4
K, permitting to achieve a single side band system noise temperature of less than 400 K. Secondly
the first local oscillator and quasi optical elements are tuned automatically by the measurement
control unit to cover seven channels each covering a band of 1.9 GHz at selected frequencies in the
265 – 281 GHz range. This allows to operate the instrument automatically. Thirdly for the real time
spectrum analysis two acousto optical spectrometers (AOS) are used each with an instantaneous
band width of 1 GHz and a frequency resolution of 1.1 MHz. This allows to retrieve constituent
profiles down to 12 km and up to 5 km minimum with an expected vertical resolution of 6 – 8 km in
the lower and 8 – 12 km in the upper stratosphere. A schematic view of the instrument is presented
in figure 3 where the three main components characterizing the performance of the radiometer are
described. The new developed system uses a minimum of optical elements to avoid standing waves,
based on an idea of De Zafra et al. (1994) and is optimised in space to account for the limited
conditions within the instrument container.
Fig. 2: Results of forward model calculation for RAMAS at Summit in the frequency range 265-282 GHz. The top
panel shows all lines stronger than 10 mK in the selected frequency range, the bottom panels present 5 of 7 observing
channels.
Fig. 3: Schematic of the RAMAS instrument.
Outlook
After successful assembling and installation of the instrument in Bremen, Germany, RAMAS will
be transported to Greenland and installed at Summit during the spring 2003. During the first winter
season (winter 2003/2004) there will be one scientist staying at Summit to take care of the
instrument and control the measurements. Summer 2004 gives time to perform modifications to the
instrument if needed and to prepare the controlling unit for continuous measurements. During the
following winter, routine operation and maintenance can be done by regular station personnel. After
an appropriate validation and test phase, it is planned to make the RAMAS instrument part of the
primary Arctic NDSC stations and to perform year-round observations.
The project is supported by the European Commission (Fifth framework programme). The Summit
station is operated by the U.S. National Science Foundation (NSF) and the operation of RAMAS at
Summit depends on the logistical and financial support of NSF.
References
De Zafra, R. L., C. Trimble, J.M. Reeves, D. Cheng, D. Shindell: MM-Wave spectroscopy of
stratospheric trace gases at the South Pole over an 11-Month cycle: O3, N2O, HNO3, NO2 ClO.
IEEE International Geoscience and Remote Sensing Symposium, Proceedings, Vol III, 1684-1686,
1994