Acousto-optic measurements of tropospheric nitrogen dioxide column density by solar spectroscopy

Measurements of column densities of atmospheric gases can be achieved by a solar spectroscopic method that uses differential optical absorption spectroscopy. Because of the scintillation of sunlight intensity in the atmosphere, the variation in intensity will introduce a low-frequency modification of the measured solar spectrum. A spectral capturing-type CCD spectrometer takes milliseconds to capture a solar spectrum, and the effect of sunlight scintillation is negligible. In contrast, without scintillation correction, a scanning-type spectrometer spending minutes to obtain a complete solar spectrum will introduce some amount of errors. We demonstrate an intensity-compensation technique in a scanning-type spectrometer, based on a solid-state acousto-optic tunable filter, for solar spectroscopic measurements.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. III. Presence of aerosols in the middle stratosphere

The aerosol extinction measurements in the ultraviolet and visible wavelengths by the balloonborne spectrometer Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) show that aerosols are present in the middle stratosphere, above 25-km altitude. These observations are confirmed by the extinction measurements performed by a solar occultation radiometer. The balloonborne Laboratoire de Météorologie Dynamique (LMD) counter instrument also confirms the presence of aerosol around 30-km altitude, with an unrealistic excess of micronic particles assuming that only liquid sulfate aerosols are present. An unexpected spectral structure around 640-nm observed by SALOMON is also detectable in extinction measurements by the satellite instrument Stratospheric Aerosols and Gas Experiment III. This set of measurements could indicate that solid aerosols were detected at these altitude ranges. The amount of soot detected up to now in the lower stratosphere is too low to explain these measurements. Thus, the presence of interplanetary dust grains and micrometeorites may need to be invoked. Moreover, it seems that these grains fill the stratosphere in stratified layers.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. 1. Analysis of aerosol extinction spectra from the AMON and SALOMON balloonborne spectrometers

Aerosol extinction coefficients have been derived in the 375-700-nm spectral domain from measurement in the stratosphere since 1992, at night, at mid- and high latitudes from 15 to 40 km, by two balloonborne spectrometers, Absorption par les Minoritaires Ozone et NO(chi) (AMON) and Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NO(chi) (SALOMON). Log-normal size distributions associated with the Mie-computed extinction spectra that best fit the measurements permit calculation of integrated properties of the distributions. Although measured extinction spectra that correspond to background aerosols can be reproduced by the Mie scattering model by use of monomodal log-normal size distributions, each flight reveals some large discrepancies between measurement and theory at several altitudes. The agreement between measured and Mie-calculated extinction spectra is significantly improved by use of bimodal log-normal distributions. Nevertheless, neither monomodal nor bimodal distributions permit correct reproduction of some of the measured extinction shapes, especially for the 26 February 1997 AMON flight, which exhibited spectral behavior attributed to particles from a polar stratospheric cloud event.

Optical and physical properties of stratospheric aerosols from balloon measurements in the visible and near-infrared domains. II. Comparison of extinction, reflectance, polarization, and counting measurements

The physical properties of stratospheric aerosols can be retrieved from optical measurements involving extinction, radiance, polarization, and counting. We present here the results of measurements from the balloonborne instruments AMON, SALOMON, and RADIBAL, and from the French Laboratoire de Météorologie Dynamique and the University of Wyoming balloonborne particle counters. A cross comparison of the measurements was made for observations of background aerosols conducted during the polar winters of February 1997 and January-February 2000 for various altitudes from 13 to 19 km. On the one band, the effective radius and the total amount of background aerosols derived from the various sets of data are similar and are in agreement with pre-Pinatubo values. On the other hand, strong discrepancies occur in the shapes of the bimodal size distributions obtained from analysis of the raw measurement of the various instruments. It seems then that the log-normal assumption cannot fully reproduce the size distribution of background aerosols. The effect ofthe presence of particular aerosols on the measurements is discussed, and a new strategy for observations is proposed.

Ultraviolet-visible bulk optical properties of randomly distributed soot

The presence of soot in the lower stratosphere was recently established by in situ measurements. To isolate their contribution to optical measurements from that of background aerosol, the soot's bulk optical properties must be determined. Laboratory measurements of extinction and polarization of randomly distributed soot were conducted. For all soot, measurements show a slight reddening extinction between 400 and 700 nm and exhibit a maximum of 100% polarization at a scattering angle of 75 +/- 5 degrees. Such results cannot be reproduced by use of Mie theory assumptions. The different optical properties of soot and background stratospheric aerosol could allow isolation of soot in future analyses of stratospheric measurements.

Measurement of Stratospheric Chromatic Scintillation with the AMON-RA Balloonborne Spectrometer

The balloonborne instrument AMON (which is a French acronym for Absorption par les Minoritaires Ozone et NO(x)) has been modified to record chromatic scintillation during stellar occultation by the Earth's atmosphere. A 14-channel spectrophotometer with a sampling rate of 10 Hz was added, and the modified instrument, AMON-RA, performed successful measurements of the setting star Alnilam during the third European Stratospheric Experiment on Ozone (THESEO) project. Unambiguous records of the chromatic scintillation were obtained, to our knowledge for the first time from above the atmosphere, and some of its basic properties are reported. The properties of atmospheric structures that are responsible for this chromatic scintillation were found to be consistent with those of previous monochromatic measurements performed from space. A maximum chromatic delay of 2.5 s was observed for widely different wavelengths.

Differential optical absorption spectroscopy instrument for stratospheric balloonborne trace-gas studies

A newly developed UV-visible instrument for differential optical absorption spectroscopic measurements of atmospheric trace gases from balloon platforms is described. Direct solar light at daytime in the near-ultraviolet (320.6-422.6-nm) and the visible (417.6-670.7-nm) spectral ranges can be simultaneously analyzed for the atmospheric column abundances or profiles of O(3), NO(2), NO(3), BrO, OClO, O(4), H(2)O, and possibly other species (HNO(2), IO, CH(2)O). Compared with previously used balloonborne UV-visible spectrometers, the instrument has the superior properties of low mass (42 kg), low power consumption (30 W), decreased spectral drift that is caused by temperature and pressure changes, low detector dark current, and low spectrometer stray light. The three last-named characteristics are achieved by enclosure of the entire spectrometer in a pressurized and thermostated container and by inclusion of separately thermostated photodiode array detectors. The optical setup is simplified to reduce its weight. The spectral stray light is reduced by suppression of the higher-order and zero-order grating reflections by use of light traps and in the UV by addition of a dispersive prism preanalyzer. The major instrumental design characteristics and the instrumental performance as tested in the laboratory and during several stratospheric balloon flights are reported.

SALOMON: a new, light balloonborne UV-visible spectrometer for nighttime observations of stratospheric trace-gas species

A new, light balloonborne UV-visible spectrometer, called SALOMON, is designed to perform nighttime measurements of stratospheric trace-gas species by using the Moon as a light source. The first flight, performed on 31 October 1998 at mid-latitude with a float altitude of 26.7 km, allowed the performance of the pointing system to be checked and vertical profiles of ozone, NO(2), NO(3), and possibly OBrO to be obtained. First the instrument and then the performance of the pointing system and the detector are described. Finally the vertical profiles are compared with other profiles obtained at the same location five years before with the heavier balloonborne spectrometer AMON, which uses a star as the light source.