Robert E. Veiga

Validation of SAGE II ozone measurements

Stratospheric aerosol and gas experiment (SAGE) II satellite-borne measurements of the stratospheric profiles of NO2 at sunset have been made since October 1984. The measurements are made by solar occultation and are derived from the difference between the absorptions in narrow bandwidth channels centered at 0.448 and 0.453 μm. The precision of the profiles is approximately 5% between an upper altitude of 36 km and a latitude-dependent lower altitude at which the mixing ratio is 4 ppbv (for example, approximately 25 km at mid-latitudes and 29 km in the tropics). At lower altitudes the precision is approximately 0.2 ppbv. The profiles are nominally smoothed over 1 km except at altitudes where the extinction is less than 2×10−5/km. (approximately 38 km altitude), where 5 km smoothing is employed. The profile measurement noise has an autocorrelation distance of 3–5 km for 1 km smoothing and more than 10 km for 5 km smoothing. The absolute accuracy of the measurements is estimated to be 15% based on uncertainties in the absorption cross-sections and their temperature dependence. Comparisons against two sets of balloon profiles and atmospheric trace molecules spectroscopy experiment (ATMOS) measurements show agreement within approximately 10% over the altitude range of 23 to 37 km at mid-latitudes. SAGE II NO2 measurements are calculated to be approximately 20% smaller at the mixing ratio peak than average limb infrared monitor of the stratosphere (LIMS) measurements in the tropics in 1979. They show acceptable agreement with SAGE I sunset NO2 measurements in the tropics in 1979–1981 when the limited resolution and precision of the SAGE I measurements and the differences between the two measurement techniques are considered.

Observations of reduced ozone concentrations in the tropical stratosphere after the eruption of Mt. Pinatubo

The eruption of Mt. Pinatubo (15oN, 122oE) on June 15 and 16, 1991, placed a large amount of SO2 and crustal material in the stratosphere. Based on measurements of decreases of stratospheric ozone after previous volcanic eruptions, it was expected that the aerosols deposited into the stratosphere (both directly and as a result of SO2 conversion into particulate sulfate) by this eruption would give rise to significant ozone depletions. To check for such an effect, ozone profiles obtained from ECC sondes before and after the eruption at Brazzaville, Congo (4oS, 15oE), and Ascen- sion Island (8oS, 14oW), are examined. Aerosol profiles determined from a lidar system in the western Pacific (4 o- 6o1,,1, 125oE) show that most of the material injected into the stratosphere is located between 18 and 28 km with highest mounts at 24-25 km. For the period 3-6 months after the eruption, decreases in ozone are found at 16 to 29 km, with peak decreases as large as 20% found at 24 km. Integrated between 16 and 28 km, a decrease of 13-20 Dobson units is observed when the ozonesonde data after the Pinatubo eruption are compared with those prior to the eruption. The altitude at which the most pronounced ozone decrease is found strongly correlates with peak aerosol loading deter- mined by the lidar. In addition, a small increase in ozone density is found above about 28 kin. Mechanisms that might explain the results such as heterogeneous chemistry, radiative effects, and dynamics are discussed.

Stratospheric aerosol and gas experiments I and II comparisons with ozonesondes

Ozone profiles measured by the Stratospheric Aerosol and Gas Experiments (SAGE) I and II are compared with ozonesonde profiles at 24 stations over the period extending from 1979 through 1991. Ozonesonde/satellite differences at 21 stations with SAGE II overpasses were computed down to 11.5 km in the midlatitudes, to 15.5 km in the lower latitudes, and for nine stations with SAGE I overpasses down to 15.5 kin. The set of individual satellite and ozonesonde profile comparisons most closely colocated in time and space shows mean absolute differences relative to the satellite measurement of 6 -- 2% for SAGE II and 8 +- 3% for SAGE I. The ensemble of ozonesonde/satellite differences, when averaged over all altitudes, shows that for SAGE II, 70% were less than 5%, whereas for SAGE I, 50% were less than 5%. The best agreement occurred in the altitude region near the ozone density maximum where almost all the relative differences were less than 5%. Most of the statistically significant differences occurred below the ozone maximum down to the tropopause in the region of steepest ozone gradients and typically ranged between 0 and -20%. Correlations between ozone and aerosol extinction in the northern midlatitudes indicate that aerosols had no discernible impact on the ozonesonde/satellite differences and on the SAGE II ozone retrieval for the levels of extinction encountered in the lower stratosphere during 1984 to mid-1991.

SAGE III measurements

SAGE III is a NASA EOS instrument designed to provide long term measurements of ozone, aerosol, water vapor, and other gases in the atmosphere. The instrument was launched on a Russian Spacecraft Meteor 3M on December 10, 2001. This paper will provide a brief discussion of the SAGE III data that will be made available to the science community to perform study on problem related to global climate change issues. The SAGE III measurement strategy, data retreival technique, and the expected quality of the data products will be discussed. Preliminary data obtained from the instrument will be presented.

Preliminary assessment of possible aerosol contamination effects on SAGE ozone trends in the lower stratoshere

Abstract An investigation of the validity of long term ozone trends in the lower stratosphere derived from SAGE I and II measurements is described. At altitudes below approximately 20 km, it is important to separate the ozone and aerosol contributions to SAGE extinction at 600 nm. The correlation between SAGE II measurements of ozone and aerosols indicates that most of the variability in these parameters is associated with physically induced variations resulting from quasi-horizontal motions of air parcels. The SAGE ozone measurements are however found to be as much as 20% larger than coincident ozonesonde measurements between 15 and 20 km altitude. A sudden change in the difference at approximately 14.5 km altitude for which there is a change in the SAGE aerosol retrieval procedure suggests that SAGE ozone trends below 20 km altitude may be more sensitive to aerosol variations. Between 20 and 25 km altitude, however, both SAGE and the ozonesondes indicate a reduction in ozone of approximately 0.5%/year over the period 1979 to 1989 at mid-latitudes of the Northern Hemisphere.

SAGE II observations of a previously unreported stratospheric volcanic aerosol cloud in the northern polar summer of 1990

Analysis of aerosol extinction profiles obtained by the spaceborne SAGE II sensor reveals that there was an anomalous increase of aerosol extinction below 18.5 km at latitudes poleward of 50[degrees]N from July 28 to September 9, 1990. This widespread increase of aerosol extinction in the lower stratosphere was apparently due to a remote high-latitude volcanic eruption that has not been reported to date. The increase in stratospheric optical depth in the northern polar region was about 50% in August and had diminished by October 1990. This eruption caused an increase in stratospheric aerosol mass of about 0.33 [times] 10[sup 5] tons, assuming the aerosol was composed of sulfuric acid and water. 13 refs., 2 figs., 1 tab.

Estimated SAGE II ozone mixing ratios in early 1993 and comparisons with stratospheric photochemistry, aerosols and dynamics expedition measurements

An empirical time-series model for estimating ozone mixing ratios based on Stratospheric Aerosol and Gas Experiment II monthly mean ozone data for the period October 1984 through June 1991 has been developed. The modeling results for ozone mixing ratios in the 10- to 30- km region in early months of 1993 are presented. In situ ozone profiles obtained by a dual-beam UV-absorption ozone photometer during the Stratospheric Photochemistry, Aerosols and Dynamics Expedition campaign, May 1–14, 1993, are compared with the model results. With the exception of two profiles at altitudes below 16 km, ozone mixing ratios derived by the model and measured by the ozone photometer are in relatively good agreement within their individual uncertainties. The identified discrepancies in the two profiles are discussed.

SAGE III meteor mission: pre-launch preparation, post-launch operation, and initial on-orbit data

The Stratospheric Aerosol and Gas Experiment III (SAGE III) Meteor mission was originally scheduled for launch in the early summer of 2001. This paper will discuss the overall SAGE III/Meteor mission and provide a description of the instrument performance based on different pre-launch tests that have been performed over the last two years. Pre- launch tests include instrument radiometric throughput and calibration; wavelength calibration; polarization response; and in-atmospheric testing including sun and moon viewing. The resulting data demonstrate the capability of the instrument to provide high spectral resolution atmospheric spectral measurement in the visible to the near IR wavelength region with a high SNR. The instrument has been integrated onto the Meteor spacecraft at the NIIEM facility in Russia. Since the launch of the Meteor SAGE III has been delayed until the end of 2001, this paper will only provide a description of the planned initial operation of the SAGE III instrument after launch.

SAGE III test model: ground-based coincident measurements with the SAGE III flight instruments and field characterization measurements

Five functional UV-VIS-NIR spectrometer/telescopes were built for the Stratospheric Aerosol and Gas Experiment III (SAGE III) satellite instrument project. Three will be on satellite platforms in the early part of the decade, and a fourth, the SAGE III Test Model (TM) is functioning as a ground-based instrument. The fifth is used as a software test-bed to support Mission Operations for the space instruments. The SAGE III instrument is spatially scanning UV-VIS-NIR holographic grating spectrometer using a cooled CCD detector. This paper presents performance results from the TM instrument. The TM instrument has been used in cross calibration studies during which one of the SAGE III flight instruments directly sampled the Sun, Moon, and twilight sky from the ground. Comparisons of SNRs, and relative responsivities are presented. TM zenith twilight spectra and measurement sensitivities are presented including a comparison of its twilight radiance measurements with the MODTRAN 3.7 atmospheric radiance/transmittance model. In addition the TM results of sampling planetary and stellar spectra are presented.

Direct solar and lunar viewing ground testing to simulate Earth orbit scenarios with the Stratospheric Aerosol and Gas Experiment III (SAGE III) space instrument

The instrument description and ground test simulations of on- orbit scenarios for the Stratospheric Aerosol and Gas Experiment III (SAGE-III) are presented. SAGE-III is a spectrographic instrument that has been developed in the U.S. and will orbit aboard a Russian Meteor-3M spacecraft beginning Fall of 1999. It will orbit at a nominal altitude of 1020 km and inclination of 99.6 degrees for global coverage. The instrument will measure the attenuated solar and lunar radiation from 290 nm to 1550 nm wavelength range through the stratosphere. The radiant data are normalized to the non- attenuated radiation measured above the atmosphere during each occultation event. The data are used to calculate the vertical distribution of stratospheric aerosols, ozone and other species that are critical in studying trends and global change. After on-orbit operations being, the autonomy of the instrument will not need up-link commands to acquire science data or to transmit the data back to the United States and Russia.