Mission Studies the Composition
of Earth's Middle Atmosphere
Dirk Offermann and Robert R. Conway

Dirk Offermann, Physics Dept., University of
Wuppertal, 42097 Wuppertal, Germany; and
Robert R. Conway, Naval Research Labora-
tory, Washington, DC 20375

Remote sensing of the middle atmosphere from Earth orbit has provided a wealth of two- and three-dimensional images over the last 15 years. Although these images have been powerful tools for testing chemical-dynamical models, they lacked high-horizontal spatial resolution. In contrast, high-altitude aircraft experiments provided high spatial resolution along the aircraft flight path at the expense of limited area coverage.
The aircraft observations made during the Antarctic and Arctic ozone campaigns proved invaluable in unraveling the respective roles of dynamics and chemistry in the formation of the ozone hole during the austral spring. The importance of high-resolution measurements is further suggested by the predictions of increasingly detailed three-dimensional models which show that trace gases are distributed with spatial structures on many scales and that these structures can form clear signatures of specific dynamical activity.
A recent space shuttle flight deployed a joint German-U.S. experiment, called CRISTA-SPAS (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere-Shuttle Pallet Satellite), to map with unprecedented horizontal and vertical resolution the trace gas inventory of the middle atmosphere on a global scale. The experiment also acquired global maps of the distribution of hydroxyl in the upper stratosphere and mesosphere, a measurement that had been missing in middle atmosphere research.
CRISTA-SPAS completed 8 days of space flight during the ATLAS-3/SSBUV/CRISTA-SPAS space shuttle mission [Miller, 1995] .The CRISTA-SPAS scientific payload consisted of two new instruments developed to acquire global maps of the important trace gases in the altitude region from 15 to 150 km: CRISTA and the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI).
The infrared limb scanning emission measurements of CRISTA can resolve dynamical structures with horizontal scales in the range from 300 to 600 km with a vertical resolution of 2 km. MAHRSI measures resonance fluorescence of ultraviolet sunlight by OH and NO at very high spectral resolution to derive global maps of these two important trace gases.


The CRISTA Experiment


CRISTA uses three telescopes and four spectrometers. Its objective is to measure variations in the concentrations of atmospheric trace gas with high spatial resolution in horizontal and vertical scales. This allows CRISTA to resolve small structures formed by the motions of air caused by winds and atmospheric waves [Offermann, 1993] . To obtain high spatial resolution, fast measurement speed is needed. This was achieved through the experiment's design:


* Trace gases were selected that possess
the potential for significant gradients in their
mixing ratio and are strong infrared emitters
(see Table 1).
* The resolving power of the spectrome-
ters was intentionally made low to obtain a
high signal for the expected light intensities.
* The spectral range covered was divided
into several slices, which were measured si-
multaneously. In total, 26 spectral channels
are operated in parallel.
* The whole instrument was cooled to liq-
uid helium temperatures. This allowed a high
measurement speed. The whole spectral
range was measured in 1 s.


This design gives CRISTA a horizontal spatial resolution of 300 to 500 km along the orbital ground track, depending on the altitude range to be covered. To further increase the horizontal resolution of the measurements, the lines ot sight of CRISTA's three telescopes are separated by 18° in azimuth. Therefore their tangent points in the middle atmosphere-that is, the minimum height above the Earth from which rays enter the instrument at a particular instant-are 650 km apart along the horizon, defining the crosstrack resolution of the measurements.

Table 1. Trace Gases Measured by CRISTA


Species

Wavelength, µm

NO

5.3

NO2

6.2

H2O

6.3

CH4

7.7

N2O

7.8

N2O5

8.0

O3

9.6

F12

10.8

HNO3

11.3

F11

11.8

Aerosol

12.0

CO2, T

12.6

CO2, #, T

15.0

N2O

17.0

H2O

58.0

HF

61.0

O (³P)

63.0

HCl

69.0
Gases in the lower six lines
are measured by the center
telescope only.

CRISTA performed almost flawlessly during flight and accumulated more than 50,000 vertical density profiles for each of the trace gases in the upper section of Table 1. A number of measurement modes were used, including scans of different altitude regions of the atmosphere, scans of restricted altitude regions to oversample with the highest spatial resolution, and tests of detector behavior.
By the time of the CRISTA flight the south polar vortex, which controls the meterological conditions involved in the formation of the ozone hole, was no longer centered around the pole but had moved northward toward South America. There it took on an egg-shaped form with its tip located west of Cape Horn. Despite the relatively low inclination (57°) of its orbit, CRISTA-SPAS passed above this northern extension of the vortex acquiring the high-resolution horizontal profiles shown in Figure 1 . The figure shows a sequence of raw radiance (that is, apparent total brightness) spectra, for the energy region from 780 through 860 wave numbers (that is, the inverse of the wavel telescopes in the most southern part of one orbit). Spectra in Figure la are from the southern track and those in Figure 1b are from the northern track. The emission features in the spectra are mostly due to CO2 and CFCl3 as indicated. The data are all from an altitude of about 15 km, and spectra with corresponding sequence numbers were taken simultaneously. The spectra are separated in time by 53 s. Movement of the spacecraft in this part of the orbit is almost entirely eastward. The measurements begin over the Pacific ocean and travel toward the southern tip of South America.
Fig. 1. The horizontal extent of the south polar vortex mapped using a sequence of infrared spectra observed by CRISTA. These plots show a measurement series recorded as the spacecraft passed above the vortex. Each profile shows the raw intensities of CO2 and CFCl3 at a tangent altitude of about 15 km progressing from west to east (front to back). Figure 1a shows the southerly track and 1b shows the northerly track; the tracks are separated horizontally by 1300 km. Spectra with corresponding numbers are taken simultaneously, and sequential spectra are separated by 400 km. The profiles with low CFCl3 intensities are inside the vortex (see text).



The south polar vortex is characterized by strong downward flow of higher altitude air. Chlorofluorocarbons are particularly good indicators of its horizontal extent because outside the vortex, their mixing ratios decrease rapidly with altitude above 15 km. When high-altitude air descends to 15 km inside the vortex, the observed mixing ratios at that altitude become small. This signature is clearly seen in Figure 1a: the sequence starts outside the vortex with spectra numbers 1 and 2 showing strong emission in the 840-855 wave number region (shown in red in Figure 1). The intensity decreases considerably in the third spectrum and continues to fall to fairly low levels in spectrum number 6. Spectra 6 through 9 are essentially identical, i.e., well inside the vortex. Spectra 10 and 11 show the eastern slope toward the edge, while 12 and above show high and almost constant intensities, indicating that the measurements are outside the vortex once again and thus clearly mark its eastern edge. The northern track (Figure 1b) shows a somewhat different structure: spectra 1 through 6 show a gradual decrease. The subsequent measurements, however, do not remain constant but rise abruptly, indicating that the vortex was muchnarrower in this region. Thus the measurements indicate that the southern track (Figure la) cuts through the body of the vortex, while the northern track touches only its northern tip. This figure demonstrates the capability of CRISTA to study spatial structures in the atmosphere. The signatures seen in this example are also seen in the spectra of other gases and at higher altitudes. When taken together, they will form a detailed picture which can help to provide new insight into the effects of dynamics on the composition of the atmosphere.


The MAHRSI experiment

MAHRSI is a middle ultraviolet (190-320 nm) spectrograph experiment designed to measure the abundance of OH and NO in the middle atmosphere. OH is the most important radical in the photochemistry of ozone and plays a powerful role as an oxidizer throughout the atmosphere. Despite its prevasive importance in atmospheric chemistry, there are few direct measurements of its abundance in the middle atmosphere with which to validate photochemical models. Although much has been learned form recent aircraft in situ observations in the lower stratosphere [Wennberg et al. , 1994] , until the CRISTA-SPAS mission, there were no global maps of its abundance distribution available.
MAHRSI measures the OH density profile by observing the bright airglow produced by the resonance fluorescence of sunlight in the OH A-X (0,0) band around 310 nm. The difficulty with this measurement arises from sunlight Rayleigh-scattered by the ambient atmosphere, which generates an equally bright emission that contains the complex Fraunhofer spectrum formed when gases in the solar atmosphere absorb solar radiation. Separation of these two signals can best be achieved using very high spectral resolution. NO is also important photochemically, and is a tracer of dynamical transport from the lower thermosphere into the mesosphere and stratosphere. It produces a bright resonance fluorescence emission in the band system around 215 nm which has been used previously by many rocket and satellite experiments to measure its global distribution.The MAHRSI measurement of NO above 100 km complements the CRISTA IR measurement at 5.3 microns. It also will provide temperature measurements of the region around the mesopause.
As mentioned above, the success of the OH measurement depended in part on the ability to accurately estimate and remove the Rayleigh-scattered background signal. Observation of the Moon during the flight provided a convenient measure of the precise shape of the solar spectrum uncontaminated with atmospheric emissions or absorptions, including OH and O3.
The data in Figure 2 illustrate how the OH signal is retrieved. The upper panel compares the observed spectrum at 62 km (black curve) with a background spectrum (blue dashed curve), scaled in magnitude to fit the wavelength regions of the atmospheric spectrum known to be free of OH lines. The lower panel shows the difference between the two spectra, that is, the observed intensity minus the background (black curve). Plotted over the remaining residual is the normalized theoretical prediction of the OH emission lines (red dashed curve), smoothed to the MAHRSI spectral resolution measured in the laboratory. The agreement of the observed OH spectrum with the theory provides convincing evidence of a clear OH detection.
The MAHRSl experiment generated a sequence of spectra taken at discrete altitudes to form a series of limb scans, each acquired at a different local time and extending above a different geographic location. The altitude region scanned depended on the emission.These limb scans are reduced to total in-band radiances profiles by background subtraction and calibration. A constrained inversion algorithm computes from these radiances the number density profiles. Each profile then represents a "snapshot" of the vertical distribution at a specific local time of day, day of year, and geographic location.
Fig. 2. MAHRSI observation of OH. This figure shows how the OH emission is recovered from the dayglow spectrum from an altitude of 62 km by subtracting the bright Rayleigh-scattered background. In the upper panel, the black curve is the average of 23 2.2-s integrations at a tangent height of 62 km, and is compared to the blue dashed curve, which shows the background spectrum scaled to fit the observation. The black curve in the lower panel shows the difference between the upper curves. The red dashed curve shows the theoretical prediction of the OH fluorescence spectrum, smoothed to the MAHRSI resolution and normalized to the data at 309.1 nm. The agreement demonstrates a clear detection of the OH emission. The OH abundance profile is inferred from altitude scans of these spectra.

When combined with the ozone and water vapor measurements of CRISTA and other instruments on the combined mission, the OH data from MAHRSI provide the first opportunity to test our theoretical understanding of ozone loss in the upper stratosphere and mesosphere (that is, altitudes from 40 to 90 km). Model data comparisons will be performed by using a two-dimensional chemical-dynamical model to predict the basic distribution fields for the important trace gases. These fields will be validated using a variety of data. For gases with important diurnal variations (like OH), a one-dimensional chemical-diffusion model will be used to compute the local time variation at selected locations.

The CRISTA/MAHRSl Campaign

More than 30 scientific groups from 10 countries participated in a 4-week campaign,called the CRlSTA/MAHRSl campaign, that began 1 week before the Space Shuttle launch. The scientific objectives of the campaign were to validate the CRISTA and MAHRSI measurements; determine the dynamical conditions of the middle atmosphere before, during, and after the Shuttle mission; and measure data not accessible to CRISTA or MAHRSI. The main validation objectives for CRISTA were temperatures and ozone densities. For temperature comparisons, meteorological rockets with Falling Spheres or Datasondes were launched from Wallops Island, Va., during each CRISTA overpass and were accompanied by radiosonde releases. Three rockets and three balloons were launched during the campaign. Temperature measurements during the CRISTA overpasses were also taken by a number of lidar stations and other ground based experiments. Ozone validation was mainly performed by flights from Hohenpeissenberg, Germany, during each CRISTA overpass. These experiments were complemented by Dobson and ozone lidar measurements. Further ozonesonde releases were performed at Wallops Island during the CRISTA overpasses. A number of other ground stations took additional ozone measurements during the campaign.
MAHRSI OH measurements will be compared to the total column measurements of the Pepsios interferometer at Fritz Peak Observatory, Colorado. Coordinated microwave measurements of HO2 and NO from the Kitt Peak Observatory were also performed.
A detailed description of the CRISTA/MAHRSI campaign is given in the Campaign Handbook [Bittner and Offermann, 1994]. CRISTA and MAHRSI experiment teams worked closely with colleagues from the ATLAS-3 and SSBUV experiments and will continue to work together as data analysis proceeds.

Data Evaluation


The CRISTA and MAHRSI experiments produced a data rate of 125 kbit/s, which was stored on board by tape recorders. Once the data have been reduced, they will be distributed to scientific working groups that were formed for data utilization. Each working group is focused on a specific scientific issue to be addressed by the CRISTA, MAHRSI, and the Campaign data. A total of 35 working groups have been formed so far.

Acknowledgments


CRISTA-SPAS is part of the ASTRO-SPAS program, which is based on a Memorandum of Understanding between NASA and the German Space Agency DARA. DARA provided the spacecraft and was responsible for its management while NASA provided launch services and overall science integration with the ATLAS-3/SSBUV/CRISTA-SPAS mission. CRISTA was built and operated by the University of Wuppertal with the support of DARA, Bonn, and MWF, Düsseldorf. MAHRSI was built and operated by the Naval Research Laboratory and was partially supported by the Office of Naval Research, the U.S. Air Force Space Test Program, and the Strategic Environmental Research and Development Program.

References

Bittner, M., and D. Offermann, CR/STA/MAHRSl
Campaign Handbook, Tech. Rep. , University
of Wuppertal, Wuppertal, Germanny, 1994.
Miller, T. L., and S. A. Smith, Mission studies
Earth's atmosphere and solar input, Eos
Trans. AGU, in press, 1995.


Offermann, D., CRISTA: A Space Shuttle experi-
ment for middle atmosphere small scale struc-
tures, in Coupling Processes in the Lower
and Middle Atmosphere, edited by E. V.
Thrane et al., pp. 389-401, Kluwer Academic,
Hingham, Mass., 1993.


Wennberg, P.O., et al., Removal of stratospheric
O2 by radicals In situ measurements of OH,
HO2, NO, NO2, ClO, and BrO, Science, 266,
398, 1994.