Atmosphere High Resolution Spectrograph Investigation Research Articles

First UV satellite observations of mesospheric water vapor

We report the first UV satellite observations of mesospheric water vapor. The measurements are of nonthermal OH prompt emission between 300–330 nm produced directly from the photodissociation of water vapor by H Lyman‐α. This technique is most sensitive to water vapor concentrations between 70–90 km altitude. We present OH data from two limb scanning experiments: the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) and the Optical Spectrograph and Infra‐Red Imager System (OSIRIS). Interpretation of the lower resolution (∼1 nm) OSIRIS spectra requires the rotational emission rate factors for OH(1,1) solar fluorescence between 313–318 nm, which we present for the first time herein. Comparison of water vapor concentration profiles with the most coincident profiles from the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite shows agreement to within 30% between 75–80 km for both MAHRSI and OSIRIS. We discuss the benefits of this promising new approach to measuring upper mesospheric water vapor and the need for new laboratory measurements to improve the analysis.

OH observations of space shuttle exhaust

We report the unexpected observation of a large hydroxyl (OH) cloud north and east of the United States a day after a space shuttle launch in November, 1994. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) observed OH(0,0) solar fluorescence near 309 nm while staring toward a tangent altitude of 87 km, where OH can be produced from water vapor photodissociation. The OH(0,0) band has a rotational temperature of 252 ± 23 K corresponding to an altitude of 110 ± 3 km, where nearly half of the shuttle's main engine water vapor exhaust is released on ascent. The location of the cloud one day after injection into the atmosphere implies that its average velocity is between 26–40 m/s northward. We also report strong evidence of water ice measured simultaneously along the same line of sight, suggesting that water vapor exhaust is redistributed by condensation and sedimentation.

PMCs and the water frost point in the Arctic summer mesosphere

In August, 1997 the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) obtained vertical profiles of OH number density and polar mesospheric cloud (PMC) brightness by scanning the limb up to 71° N while the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) obtained co‐located vertical profiles of temperature. MAHRSI OH densities are converted to water vapor using a one‐dimensional model that assumes photochemical equilibrium. By combining water vapor profiles with CRISTA temperatures we map the frost point both vertically and horizontally in the Arctic summer mesosphere. Our data show that supersaturation can exist between 80‐87 km suggesting that growth of ice particles is limited to these altitudes. Additionally, we find that not only is supersaturation an insufficient condition for a PMC but also that PMCs can exist in apparently unsaturated air.

Discovery of a water vapor layer in the Arctic summer mesosphere: Implications for polar mesospheric clouds

We report the discovery of a layer of enhanced water vapor in the Arctic summer mesosphere that was made utilizing two new techniques for remotely determining water vapor abundances. The first utilizes Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) OH measurements as a proxy for water vapor. The second is a reanalysis of Halogen Occultation Experiment (HALOE) water vapor data with a technique to simultaneously determine polar mesospheric cloud (PMC) ice particle extinction along with the water vapor abundance. These results reveal a narrow layer of enhanced water vapor centered between 82–84 km altitude and coincident with PMCs, that exhibits water vapor mixing ratios of 10–15 ppmv. This indicates that a higher degree of supersaturation is present in the PMC region, and that PMCs are thus more efficient at sequestering total water (both ice particles and vapor) within the layer, than previously believed.

Satellite observations of upper stratospheric and mesospheric OH: The HOxdilemma

We report the first observations of the vertical distribution of hydroxyl (OH) from the upper stratosphere to the mesopause. The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) made these measurements in August 1997. The data confirm the results from the earlier November 1994 MAHRSI mission that were confined to altitudes above 50 km, namely that mesospheric OH densities are 25 to 35% lower than predicted by standard photochemical theory. However, the new observations show that below 50 km the OH density increases rapidly and at 43 km altitude it is larger than that expected from standard theory. This represents a serious dilemma for our understanding of odd‐hydrogen chemistry because the same key reactions are thought to dominate OH/HO2 partitioning in both regions. We show that neither standard photochemical theory nor any previously proposed changes are adequate to explain the OH observations in both the upper stratosphere and mesosphere.

The 2.5 THz heterodyne spectrometer THOMAS: Measurement of OH in the middle atmosphere and comparison with photochemical model results

The interpretation of recent odd hydrogen measurements in the stratosphere from balloons and in the mesosphere from space indicates a serious lack of understanding in atmospheric HOx chemistry. In order to resolve these persisting problems, coincident measurements of HOx molecules and/or measurements that cover both altitude regions are desirable. In this work, the airborne 2.5THz heterodyne spectrometer Terahertz OH Measurement Airborne Sounder (THOMAS) is introduced. Since the first THOMAS measurements in 1994/1995, the spectrometer was significantly improved by modification or replacement of individual components. The THOMAS instrumental setup and properties are presented together with a retrieval algorithm for atmospheric parameters based on a Phillips‐Tikhonov regularization scheme. Furthermore, the results of a complete error assessment are given. In August 1997, during the second CRISTA/MAHRSI campaign (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere / Middle Atmosphere High Resolution Spectrograph Investigation), OH observations were performed by THOMAS covering altitudes between about 30 and 90 km over a full diurnal cycle. Hydroxyl column densities derived from THOMAS measurements are presented and compared to photochemical model results. The model calculations using the standard HOx chemistry systematically show higher values by about 15% for the 40–90 km and 50–90 km OH columns. Moreover, a recently proposed change of an HOx chemistry reaction rate is included into the comparison which, for the same altitude intervals, yields OH column densities that are about 10% lower than the THOMAS measurements. A detailed comparison of the THOMAS and MAHRSI measurements is presented in a seperate publication [Englert et al., 2000].

Middle Atmosphere High Resolution Spectrograph Investigation

The Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) was developed specifically to measure the vertical density profiles of hydroxyl (OH) and nitric oxide (NO) in the middle atmosphere from space. MAHRSI was launched on its first flight in November 1994 on the CRISTA‐SPAS satellite that was deployed and retrieved by the space shuttle. The instrument measured the radiance profiles of ultraviolet solar resonance fluorescence on the Earth's limb with a spectral resolving power of 15,600 at a wavelength of 308 nm and 7200 at 215 nm. The instantaneous height of the field of view projected to the tangent point was about 300 m. OH limb radiance measurements were made between altitudes of 90 and 30 km, and each limb scan extended over a horizontal distance of 1200 km. For NO a limb scan extended between altitudes of 140 and 76 km and over a horizontal distance 700 km. Observations were made from 52°S latitude to 62°N latitude. The OH measurements have been inverted to provide the first global maps of the vertical distribution of OH between 90 and 50 km. The data show a detailed history of the morning formation of a strongly peaked layer of OH at an altitude of 68 km. This layer was produced by solar photodissociation of a thin layer of water vapor peaked at 65 km extending between 30°S and 35°N observed contemporaneously by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite. MAHRSI was successfully flown for a second time in August 1997 under conditions that extended the geographical coverage to 72°N latitude and local solar time coverage through the afternoon hours. This paper provides a detailed description of the experiment and instrumentation, of the algorithms used to reduce the spectral data and perform the inversions, and presents examples of key results from the 1994 flight.

MAHRSI observations of nitric oxide in the mesosphere and lower thermosphere

In November, 1994, the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) observed the distribution of NO between the altitudes of 76 and 140 km by measuring limb intensity profiles of solar resonance fluorescence in the NO A ²Σ → X ²II (1,0) γ band near 215 nm. The observations were made from the ASTRO‐SPAS (Shuttle Pallet Satellite) spacecraft which was deployed and retrieved by the space shuttle. The data provided a seven hour snapshot of lower thermospheric and mesospheric NO from sunrise near 48°S to sunset near 61° N latitude following a period of low solar and high geomagnetic activity. Inferred peak lower thermospheric NO densities ranged from 3 × 107 cm−3 near the equator to 2 × 108 cm−3 at high northern latitudes, roughly consistent with previous observations for the same conditions. Individual vertical density profiles showed substantial structure and large orbit to orbit variations, suggesting that the distribution of mesospheric and lower thermospheric NO is dynamically influenced.

Implications of Satellite OH Observations for Middle Atmospheric H 2 O and Ozone

Satellite observations by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) have produced global measurements of hydroxyl (OH) in the atmosphere. These observations reveal a sharp peak in OH density near an altitude of 65 to 70 km and are thus consistent with observations from the Halogen Occultation Experiment (HALOE) on the NASA Upper Atmosphere Research Satellite (UARS), which showed an unexplained H 2 O layer at the same level. Analysis of stratopause (about 50 kilometers) OH measurements and coincident ozone observations from the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment reveals that the catalytic loss of ozone attributable to odd-hydrogen chemistry is less than that predicted with standard chemistry. Thus, the dominant portion of the ozone deficit problem in standard models is a consequence of overestimation of the OH density in the upper stratosphere and lower mesosphere.

Mesospheric HOx photochemistry: Constraints from recent satellite measurements of OH and H2O

We use recent measurements of OH made with the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) along with satellite observations of water vapor abundances, to study odd‐hydrogen photochemistry in the mesosphere. The MAHRSI data sampled primarily the morning part of the diurnal variation of OH, and here we focus on one orbit of data that is representative of the MAHRSI observations during the mission. Our approach is to use a photochemical model, with input H2O abundances fixed to observed values, to simulate the diurnal variation of mesospheric HOx species. Models of OH using standard recommended HOx chemical rate coefficients are found to substantially overpredict OH between 5065 km. Proposed modifications to HOx chemistry that produce lower OH and higher HO2 and O3, yield better agreement in the lower mesophere but worsen the agreement at the observed OH peak near∼70 km. We find that neither standard HOx chemistry, nor models incorporating proposed HOx modifications, adequately reproduce the observed OH density profile over the entire 50–75 km altitude range for any of the observed early to mid morning local times.

Satellite measurements of hydroxyl in the mesosphere

The global distribution of hydroxyl (OH) in the middle atmosphere was recently measured by the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) on a satellite deployed and retrieved by the space shuttle. During 75 orbits, MAHRSI acquired 1800 daytime limb scans of the OH ultraviolet solar resonance fluorescence intensity. Each limb scan extends over the altitude region from 30 to 90 km and across 10° of latitude between 53°S and 63°N. OH number densities were retrieved using a Twomey regularization scheme constrained by the smoothness of the retrieved profile. Results provide a detailed description of the diurnal variation of mesospheric OH. Midmorning OH densities had a well defined peak of about 6 ×106 cm³ near 70 km, a broad minimum centered near 64 km, and rose to about 1 × 107 cm³ at 50 km. This profile is in substantial disagreement with photochemical model predictions [Summers et al., this issue]. The observations are compared with the two previous measurements.