N. D. Lloyd

The OSIRIS Instrument on the Odin Spacecraft

The optical spectrograph and infrared imager system (OSIRIS) on board the Odin spacecraft is designed to retrieve altitude profiles of terrestrial atmospheric minor species by observing limb-radiance profiles. The grating optical spectrograph (OS) obtains spectra of scattered sunlight over the range 280-800 nm with a spectral resolution of approximately 1 nm. The Odin spacecraft performs a repetitive vertical limb scan to sweep the OS 1 km vertical field of view over selected altitude ranges from approximately 10 to 100 km. The terrestrial absorption features that are superimposed on the scattered solar spectrum are monitored to derive the minor species altitude profiles. The spectrograph also detects the airglow, which can be used to study the mesosphere and lower thermosphere. The other part of OSIRIS is a three-channel infrared imager (IRI) that uses linear array detectors to image the vertical limb radiance over an altitude range of approximately 100 km. The IRI observes both scattered sunlight and the airglow emissions from the oxygen infrared atmospheric band at 1.27 mum and the OH (3-1) Meinel band at 1.53 mum. A tomographic inversion technique is used with a series of these vertical images to derive the two-dimensional distribution of the emissions within the orbit plane.

Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport.

Indirect Injection Aerosols in the stratosphere, especially submicron-hydrated sulfuric acid droplets, are an important factor influencing climate variability. Stratospheric sulfate aerosols can form from sulfur dioxide that has been transported from the underlying troposphere. Large volcanic eruptions can inject sulfur dioxide and other material into the stratosphere, but smaller volcanoes have been thought not to be energetic enough to do so. Bourassa et al. (p. 78) used satellite data to show that sulfur dioxide from the 2011 eruption of the Nabro stratovolcano in Eritrea was lofted into the stratosphere by deep convection associated with the Asian summer monsoon. Even moderate volcanic eruptions can inject sulfur dioxide into the stratosphere with the help of the Asian monsoon. The Nabro stratovolcano in Eritrea, northeastern Africa, erupted on 13 June 2011, injecting approximately 1.3 teragrams of sulfur dioxide (SO2) to altitudes of 9 to 14 kilometers in the upper troposphere, which resulted in a large aerosol enhancement in the stratosphere. The SO2 was lofted into the lower stratosphere by deep convection and the circulation associated with the Asian summer monsoon while gradually converting to sulfate aerosol. This demonstrates that to affect climate, volcanic eruptions need not be strong enough to inject sulfur directly to the stratosphere.

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

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

A rocket tomography measurement of the N+2 3914 Å emission rates within an auroral arc

Abstract A rocket tomography experiment designed to measure the two-dimensional distribution of the N + 2 3914 A volume emission rates within an auroral arc is described. A simple filter photometer on board a sounding rocket, which was launched during the ARIES auroral campaign, was used to measure the 3914 A auroral brightnesses at elevation angles ranging from 0° to 360° in the plane of the rocket trajectory. The measured auroral brightnesses have been tomographically inverted to recover the local 3914 A volume emission rates as a function of both altitude and latitude within the arc. The tomographic inversion procedure, which is based upon a maximum probability algebraic reconstruction approach, is described and the implications of the results for studies of auroral excitation processes are briefly discussed.

Response to comments on "Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport".

Fromm et al. and Vernier et al. suggest that their analyses of satellite measurements indicate that the main part of the Nabro volcanic plume from the eruption on 13 June 2011 was directly injected into the stratosphere. We address these analyses and, in addition, show that both wind trajectories and height-resolved profiles of sulfur dioxide indicate that although the eruption column may have extended higher than the Smithsonian report we highlighted, it was overwhelmingly tropospheric. Additionally, the height-resolved sulfur dioxide profiles provide further convincing evidence for convective transport of volcanic gas to the stratosphere from deep convection associated with the Asian monsoon.

Comparison of the Odin/OSIRIS stratospheric ozone profiles with coincident POAM III and ozonesonde measurements

We present first statistical comparison results for stratospheric ozone density profiles retrieved from Odin/OSIRIS limb scattered radiance with 1220 coincident POAM III and 205 coincident ozonesonde measurements. Profiles are compared on a monthly basis from November 2001 to October 2002. Most of the time, differences between OSIRIS mean profiles and those measured by POAM III and ozonesondes were 5-7% between 15 km and 32 km, and within 15% above 32 km. In April-July 2002, OSIRIS mean profiles appear shifted downward by ∼1 km, introducing a difference of about 10% with POAM III and about 25% with ozonesonde profiles between 15 km and 32 km. This study demonstrates that outside the April-July 2002 period, the OSIRIS ozone profiles agree well with coincident ozonesonde and POAM III ozone profiles and make a valuable addition to the international ozone database available for research into global ozone change.

Odin/OSIRIS observations of stratospheric BrO: Retrieval methodology, climatology, and inferred Bry

A 7+ year (2001–2008) data set of stratospheric BrO profiles measured by the Optical Spectrograph and Infra-Red Imager System (OSIRIS) instrument, a UV-visible spectrometer measuring limb-scattered sunlight from the Odin satellite, is presented. Zonal mean radiance spectra are computed for each day and inverted to yield effective daily zonal mean BrO profiles from 16 to 36 km. A detailed description of the retrieval methodology and error analysis is presented. Single-profile precision and effective resolution are found to be about 30% and 3–5 km, respectively, throughout much of the retrieval range. Individual profile and monthly mean comparisons with ground-based, balloon, and satellite instruments are found to agree to about 30%. A BrO climatology is presented, and its morphology and correlation with NO2 is consistent with our current understanding of bromine chemistry. Monthly mean Bry maps are derived. Two methods of calculating total Bry in the stratosphere are used and suggest (21.0 ± 5.0) pptv with a contribution from very short lived substances of (5.0 ± 5.0) pptv, consistent with other recent estimates.

OSIRIS A Decade of Scattered Light

Into year 11 of a 2-yr mission, OSIRIS is redefining how limb-scattered sunlight can be used to probe the atmosphere, even into the upper troposphere.