Edward J. Llewellyn

Retrieval of stratospheric O3 and NO2 profiles from Odin Optical Spectrograph and Infrared Imager System (OSIRIS) limb-scattered sunlight measurements

Scientific studies of the major environmental questions of global warming and ozone depletion require global data sets of atmospheric constituents with relevant temporal and spatial resolution. In this paper global number density profiles of O3 and NO2 are retrieved from Odin/OSIRIS limb-scattered sunlight measurements, using the Maximum A Posteriori estimator. Differential Optical Absorption Spectroscopy is applied to OSIRIS radiances as an intermediate step, using the wavelength windows 571-617 nm for O3 and 435-451 nm for NO2. The method is computationally efficient for processing OSIRIS data on an operational basis. Results show that a 2-3 km height resolution is generally achievable between about 12 km and 45 km for O3 with an estimated accuracy of 13\% at the peak and between about 15 km and 40 km for NO2 with an estimated accuracy of 10\% at the peak. First validations of the retrieved data indicate a good agreement both with other retrieval techniques applied to OSIRIS measurements and with the results of other instruments. Once the validation has reached a confident level, the retrieved data will be used to study important stratospheric processes relevant to global environmental problems. The unique NO2 data set will be of particular interest for studies of nitrogen chemistry in the middle atmosphere.

Volume emission rate tomography from a satellite platform.

The possibility of retrieving horizontal atmospheric structure from a series of limb images taken aboard a satellite is discussed and a maximum likelihood expectation maximization algorithm is developed. Examples of the retrieval of horizontal structure with this algorithm, for different S/N (signal-to-noise) ratios and different structures, are presented. It is shown that with this algorithm and even in the presence of substantial observational noise, a S/N equal to 10 for a single observation, it is possible to retrieve both horizontal and vertical atmospheric structure.

Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite

[1] Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A ‘‘snapshot’’ meridional cross section between 40� N and 70� S is shown from observations during a fraction of an orbit. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: optical, Sun-synchronous, polar-orbiting, Fraunhofer, Ring effect, iterative onion peel

The concentration of atomic oxygen in the mesosphere and thermosphere

Abstract The determination of atomic oxygen concentrations in the mesosphere and thermosphere has long been of interest: a knowledge of these concentrations has important implications for many processes in the terrestrial atmosphere, e.g. vertical transport and auroral excitation mechanisms. In recent years there has been an intense on-going debate as to whether the concentration in the vicinity of the atomic oxygen peak, near 100 km, is low (1 × 1011 cm−3) or high (1 × 1012 cm−3). Since different investigators have advanced various views there has been extreme confusion among many researchers. In an attempt to explain, and understand, the reasons for the apparently wide range in assumed atomic oxygen concentrations a special session was included in the San Francisco Fall Meeting of the American Geophysical Union. A number of these papers are collected together in this issue of Planetary and Space Science. While these papers do not answer the question of high or low concentrations they do provide a convenient focus for much of the available data and interpretation.

Singlet molecular oxygen in planetary atmospheres

Abstract Recent observations of the oxygen airglow emissions in the different planetary atmospheres are reviewed and the vibrational development of the various emissions is discussed. A model which can describe this vibrational development is also presented. The excitation mechanisms for each of the emissions are considered and from an examination of the volume emission profiles in the Earths atmosphere it is shonw that the emissions from the low-lying states Are probably excited by energy transfer from some precursor. From a comparison of the nightglows of Earth and Venus it is suggested that this precursor is the c 1 Σ u − state and that this is preferentially excited in the three-body recombination of atomic oxygen. It is also suggested tht vibrational excitation may be extremely important in this energy transfer.

Rocket Measurements of the O2 Infrared Atmospheric (0-0) Band in the Nightglow: The Vestigial Dayglow Components

The contribution made by the undecayed component of the dayglow O2 (a1∆g) population to the nightglow O2(a1∆g-X3∑ g - ) IR atmospheric band emission is assessed. The time dependence of the evening twilight O2(a1∆g) production rates and the delayed response of the resulting IR atmospheric band emission is considered in some detail. The vestigial dayglow IR atmospheric band emission profiles expected under the conditions of a number of recent rocket-borne radiometer experiments are presented. These vestigial dayglow emission profiles are compared with those that have been previously estimated by Lopez-Moreno et al. (1986) and McDade et al. (1987) who employed much simpler kinetic models. The possible impact of differences between the present and the previously estimated vestigial emission rates upon our understanding of the nightglow IR atmospheric band excitation mechanisms is discussed.

The photodissociation of vibrationally excited ozone in the upper atmosphere

Abstract The photodissociation of vibrationally excited O3 in the sunlit mesosphere is investigated. Dissociation rate coefficients for specific vibrationally excited states of O3 are calculated for the conditions of an overhead sun. Possible vibrational enhancements of the O(1D) and O2(a1Δg) production rates are assessed. It is shown that such enhancements should make only minor contributions to the daytime production of these species in the mesosphere and lower thermosphere.

Impact of ambient O2(a1Δg) on satellite‐based laser remote sensing of O2 columns using absorption lines in the 1.27 µm region

Determination of CO2 mixing ratio columns from space using Laser Absorption Spectroscopy (LAS) requires simultaneous measurements of CO2 number density columns and knowledge of the dry atmospheric surface pressure. One approach to determining the surface pressure is to make an LAS column measurement of O2 number density in the 7857.3–7921.7 cm−1 (1.27 µm) region of the O2(1Δ) state. A complicating factor in the LAS O2 measurement is the presence of a permanent but spatially variable natural source of airglow from the O2(1Δ) state. In addition, the laser radiation can induce stimulated emission from the ambient O2(1Δ) state and also cause stimulated absorption and emission from the ground state O2 molecules as the laser beam passes through the atmosphere. Finally, the upwelling surface-reflected solar radiation is an additional source of background radiation. The effects of these additional radiation sources on the LAS measurement of O2 are examined. The surface-reflected solar radiation produces the largest background at 3 orders of magnitude more intense than the laser backscatter signal, while the airglow is of the same order of magnitude as the laser backscatter. The stimulated emission from ambient O2(a1Δg) is found to be about the same order of magnitude as the laser radiation. These effects are evaluated under noon, twilight, and midnight conditions at midlatitudes, the equator, and the pole. The stimulated emission is in the same direction and in phase with the laser signal, its contamination of the LAS O2 measurement prevents a full sunlight determination of surface pressure.

Near-ultraviolet limb imaging spectrograph for sounding rockets

As part of an ongoing investigation of airglow emissions of the upper atmosphere, an intensified CCD imaging spectrograph has been developed for a sounding rocket project called GEMINI (general excitation mechanisms in nightglow). The instrument, known as LISA (limb-imaging spectrograph for airglow), will be used to measure the limb profiles of some important nighttime airglow emission features. The observed limb profiles will be analyzed to provide atmospheric temperatures and density profiles of excited atomic and molecular species of interest to specific modeling problems in the mesopause and lower thermosphere. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in late 1993 or early 1994. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. The authors describe the design capabilities of the LISA instrument, which include a spectral range of 310 to 390 nm, a wavelength resolution of [approximately]0.3 nm, a height resolution of 1 km, and a theoretical count rate of 0.04 count R[sub [minus]1] s[sub [minus]1], where R represents rayleighs. The imager design is discussed and they present the results of some laboratory test performed by means of an artificial source of the oxygen nightglow emission.

Visible airglow limb imaging spectrograph for sounding rockets

As part of an ongoing investigation of airglow emissions from space, we have developed an intensified CCD imaging spectrograph for a sounding rocket project called General Excitation Mechanisms In Nightglow (GEMINI). The instrument, known as Limb Imaging Spectrograph for Airglow (LISA) will be used to measure the limb profiles of some important nighttime airglow emission features. The GEMINI rocket is to be launched from White Sands Missile Range, New Mexico, in early 1993. The payload will be three-axis stabilized and absolute pointing will be derived from a star video camera. In this paper the imager design is discussed and we present the results of some laboratory tests performed using an artificial source of the oxygen nightglow emission.