Clare Lee

Temperature effects on the 15–85 μm spectra of olivines and pyroxenes

Far-infrared spectra of laboratory silicates are normally obtained at room temperature even though the grains responsible for astronomical silicate emission bands seen at wavelengths >20 μm are likely to be at temperatures below ∼150 K. In order to investigate the effect of temperature on silicate spectra, we have obtained absorption spectra of powdered forsterite and olivine, along with two orthoenstatites and diopside clinopyroxene, at 3.5±0.5 K and at room temperature (295±2 K). To determine the changes in the spectra the resolution must be increased from ∼1 to 0.25 cm−1 at both temperatures, because a reduction in temperature reduces the phonon density, thereby reducing the width of the infrared peaks. Several bands observed at 295 K split at 3.5 K. At 3.5 K the widths of isolated single bands in olivine, enstatites and diopside are ∼90 per cent of their 295-K widths. However, in forsterite the 3.5-K widths of the 31-, 49- and 69-μm bands are, respectively, 90, 45 and 31 per cent of their 295-K widths. Owing to an increase in phonon energy as the lattice contracts, 3.5-K singlet peaks occur at shorter wavelengths than do the corresponding 295-K peaks; the magnitude of the wavelength shift increases from ∼0–0.2 μm at 25 μm to ∼0.9 μm at 80 μm. In olivines and enstatites the wavelength shifts can be approximated by polynomials of the form ax+bx2 where x=λpk(295 K) and the coefficients a and b differ between minerals; for diopside this formula gives a lower limit to the shift. Changes in the relative absorbances of spectral peaks are also observed. The temperature dependence of λpk and bandwidth shows promise as a means to deduce characteristic temperatures of mineralogically distinct grain populations. In addition, the observed changes in band strength with temperature will affect estimates of grain masses and relative mineral abundances inferred using room-temperature laboratory data. Spectral measurements of a variety of minerals at a range of temperatures are required to quantify these effects fully.

SAFIRE-A: Spectroscopy of the Atmosphere Using Far-Infrared Emission/Airborne

A new instrument named SAFIRE-A (Spectroscopy of the Atmosphere using Far-Infrared Emission/Airborne), which can operate on high-altitude platforms, has been developed for the study of the atmospheric composition through limb-scanning emission measurements. The instrument is a polarizing Fourier transform spectrometer that operates in the far infrared with a resolution of 0.004 cm(−1). SAFIRE-A uses efficient photon noise limited detectors and a novel optical configuration, which provide a cold pupil and field stop as well as cold narrow bandpass filters to enhance its sensitivity. The instrument was successfully operated on an M-55 stratospheric research aircraft in the polar regions during the winter 1996–97 Airborne Polar Experiment. The instrument design, aircraft integration, and performances attained in the field campaign are described and discussed. The atmospheric emission spectrum is measured with an rms noise accuracy of 0.5 K (measured in brightness temperature) in each spectral element near 20 cm(−1) with a 30-s measurement time.

Stratospheric HBr concentration profile obtained from far-infrared emission spectroscopy

Hydrogen bromide (HBr) is the principal bromine sink species for the ozone loss chemistry induced by bromine‐containing gases in the stratosphere. We report a 1994 balloon‐based measurement of the daytime stratospheric HBr profile between 20 and 36.5 km altitude. The average concentration result of 1.31±0.39 parts per trillion in volume (pptv) and an analysis for the concentration versus altitude profile are consistent with previously reported measurements. These results strengthen the evidence for a significantly higher HBr concentration than that predicted by current photochemical models which, on the basis of recent kinetics results, do not include significant HBr production by the reaction branch, BrO + HO2 → HBr + O3.

Minor constituent concentrations measured from a high altitude aircraft using high resolution far-infrared Fourier transform spectroscopy.

Far-infrared emission spectroscopy has beendemonstrated to be a valuable method for remotesensing of trace species in the stratosphere, with theability to simultaneously detect a number of keychemical species. SAFIRE-A is a new far-infraredFourier Transform (FT) spectrometer which has beenspecifically designed to operate on board of a highaltitude aircraft in the lower stratosphere and uppertroposphere regions where relatively few remotesensing measurements have been made. Using newtechnology, the sensitivity of the FT spectrometermethod has been substantially improved for the longwavelength region. Results are reported formeasurements of O3, HNO3 and N2O at 17and 19 km using a detection window near 23 cm-1.Geographical and altitude variability of the volumemixing ratio of these constituents and their relativecorrelation are discussed. Ozone measurements agreewell with in situ measurements, except in regions ofstrong stirring and mixing associated with deformationof the northern vortex edge. Whilst SAFIREmeasurements of trace gases do not capture all of thelocal variability seen by rapid in-situ techniques,they can indicate horizontal variability close to, butnot intercepted by, the aircrafts flight path. Apossible detection of ClO at the low background levelsexpected outside the polar vortex is also reported.

Cosmic-ray flux detected by an IR bolometer operated on board of a stratospheric aircraft

Abstract A bolometer was flown as an infrared detector on a stratospheric aircraft at altitudes of approximately 19 km during the APE campaign, which took place in Rovaniemi (Finland) in the period December 1996–January 1997. The signal impulses produced in the bolometer by the thermalisation of the ionisation induced by cosmic charged particles in the sensitive layer of the detector have been analysed. A statistics of approximately 4000 cosmic-ray events was recorded during four flights. In the period of the observations the solar activity was near its minimum. The observations from a stratospheric aircraft has made possible the collection of a first sampling of cosmic-ray flux variation with latitude and altitude. A comparison is made with previous balloon observations at the same altitudes. Model calculations are compared to the observed cosmic-ray energy loss distribution in the bolometer. The results are useful for the understanding of cosmic-ray background effects occurring in applications of high sensitivity bolometers in aircraft-borne measurements.

An introduction to ISMAR

ISMAR (International SubMillimetre Airborne Radiometer) is currently under development by the Met Office. This instrument can carry a range of receivers in a scanning instrument platform designed and certified for airborne use. It will be used for scientific atmopsheric research, as a satellite demonstrator and for validation/calibration of ICI (Ice Cloud Imager).

Development of a submillimeter/far-infrared radiometer for cirrus measurements

We introduce a low cost, lightweight and compact polarisation sensitive radiometer for the measurement of Cirrus clouds in the submilimeter and far-infrared region (10-50 cm-1). It is widely recognised that enhanced global measurements of cirrus properties are essential to the development of General Circulation and Climate Prediction Models (GCMs) since cirrus clouds have a strong effect on the Earths Global Radiation Budget. We introduce a project currently under development in Cardiff, to design and build a novel instrument suitable for aircraft deployment in order to measure Ice Water Path (IWP) along with cirrus particle size and shape. The radiometer will capitalise on the on going measurements of the NASA led, Fourier Transform interferometer based, Far-Infrared Sensor for Cirrus (FIRSC) instrument for which Cardiff has been closely associated. Data from FIRSC campaigns is being used to select optimum radiometer channels that exhibit good sensitivity to specific cirrus. This new multi-channel radiometer will however have some key advantages over similar spectroscopic instruments for example: portability, increased optical efficiency, a multi-angle field of view and a reduced integration period leading to an improved spatial resolution. The radiometer will benefit from the application of state-of-the-art submm/FIR polariser and solid filter technology currently being developed in Cardiff.

Far-infrared remote sensing measurements of cirrus clouds during AFWEX

Remote sensing measurements of cirrus clouds are crucial for improving global climate models. Spectral measurements of the far-infrared region provide especially useful information to retrieve cirrus ice water path and particle size properties. Earth radiance spectra of this region have been obtained for a range of cloud conditions using the airborne instrument FIRSC (Far-InfraRed Sensor for Cirrus). The instrument flew on board the high altitude Proteus aircraft in the ARM-FIRE Water Vapor Experiment (AFWEX) during the Nov - Dec 2000 Intensive Operations Period. FIRSC is a Martin-Puplett type Fourier transform spectrometer with two channels covering the ranges 10 - 33 cm-1 and 80 -140 cm-1 at a resolution of 0.1 cm-1. It has achieved a noise equivalent temperature of approximately 1K at 30 cm-1 using a scan duration of 4 seconds. For the first time this far-infrared data has been compared to data from the thermal IR NAST-I (NPOESS Airborne Sounder Testbed - Interferometer), which was part of the same payload during the AFWEX flights. Retrievals of cirrus ice water path and particle size from the FIRSC data are presented.

ICEMuSIC — A new instrument concept for mm-wave observations of ice clouds, and temperature and humidity sounding from space

We present a new mm-wave instrument concept for the study of the role of ice clouds in the global climate system, and for atmospheric temperature and humidity sounding. Ice cloud parameters in particular are poorly understood, and represent the biggest uncertainty in current global climate models.

The cloud and precipitation airborne radiometer - Populating the international sub-millimetre airborne radiometer

An upgrade funded by the European Space Agency to the Met Office ISMAR instrument as a risk reduction for potential future spaceborne radiometers is described. This adds four channels between 118GHz and 664GHz, to measure cloud, precipitation, and cirrus cloud. The receivers are all heterodyne based, with multiplied local oscillators and sub-harmonic mixers. The manufacturing of the receivers is underway and testing of the overall channels is starting. This will be followed by integration of all the channels onto ISMAR. The integrated instrument is to have its test flight on the FAAM aircraft in November 2012 and the project is to conclude in early 2013.