William S. Heaps

Raman dial measurements of stratospheric ozone in the presence of volcanic aerosols

Since the eruption of Mt. Pinatubo in June, 1991, measurements of atmospheric species which depend on Rayleigh scattering of radiation, have been severely compromised where the volcanic aerosol cloud exists. For the GSFC stratospheric ozone lidar, this has meant that ozone determination has been impossible below approximately 30 km. The GSFC lidar has been modified to detect Raman scattering from nitrogen molecules from transmitted laser wavelengths. The instrument transmits two laser wavelengths at 308 nm and 351 nm, and detects returns at four wavelengths; 308 nm, 332 nm, 351 nm, and 382 nm. Using this technique in conjunction with the Rayleigh DIAL measurement, ozone profiles have been measured between 15 and 50 km.

Collisional deactivation rates for A 2Σ+ (ν′ = 1) state of OH

Abstract Rates for rotational transfer, vibrational crossover, and electronic quenching have been determined for specific rotational levels, N ′, of A 2 Σ + , ν′ = 1, OH radicals in the presence of N 2 and O 2 quenching gases. Measurements of total rates of depletion of specific rotational levels, rates of energy transfer out of the ν′ = 1 state, and fluorescence spectra were made at room temperature utilizing a low-pressure OH flow system. Total rates of depletion and rotational transfer rates for N 2 and O 2 quenching gases show a weak dependence upon N ′. The N 2 quenching rates for these processes and for vibrational crossover were faster than the O 2 rates. Conversely, the O 2 rates for electronic quenching were greater than the N 2 rates.

Stratospheric ozone and hydroxyl radical measurements by balloon-borne lidar

A balloon-borne lidar system for the measurement of ozone and hydroxyl radical in the stratosphere has been constructed and flown by Goddard Space Flight Center. On this flight ozone concentration at altitudes between 20 and 37 km were determined with vertical resolution of approximately 0.5 km. In addition, horizontally resolved ozone measurements with 0.15-km resolution were obtained over a 2-km range. The temporal variation of the hydroxyl radical concentration was measured in the 34-37-km altitude region, ranging from 40 parts/trillion shortly after noon to approximately 5 parts/trillion 2 h after sunset. Improvements in the system are proposed to increase the sensitivity of the instrument below the parts/trillion level, permitting hydroxyl determinations in the 20-30-km altitude range.

Development of a Fabry?Perot interferometer for ultra-precise measurements of column CO2

Progress on a passive Fabry?Perot-based instrument for detecting column CO2 through absorption measurements at 1.58 ?m is described. In this design, solar flux reaches the instrument platform and is directed through two channels. In the first channel, transmittance fringes from a Fabry?Perot interferometer are aligned with CO2 absorption lines so that absorption due to CO2 is primarily detected. The second channel encompasses the same frequency region as the first, but is comparatively more sensitive to changes in the solar flux than absorption due to CO2. The ratio of these channels is sensitive to changes in the total CO2 column, but not to changes in solar flux. This inexpensive instrument will offer high precision measurements (error <1%) in a compact package. Design of this instrument and preliminary ground-based measurements of column CO2 are presented here as well as strategies for deployment on aircraft and satellite platforms.

Quenching and rotational transfer rates in the ν′ = 0 manifold of OH (A 2Σ+)

Abstract We report quenching and rotational transfer rates out of the N ′ = 1–5 levels of the A 2 Σ + (ν′ = 0) state of hydroxyl radical (OH) in the presence of oxygen and nitrogen quenching gases. Total depletion rates out of individually excited rotational levels and quenching rates out of the ν′ = 0 manifold were measured at room temperature. Quenching rates out of initially excited states show a weak dependence upon the rotational quantum number of the initially populated state. Electronic quenching by oxygen was significantly faster than by nitrogen while rotational transfer rates measured with nitrogen were faster than those using oxygen as the collider.

Lidar temperature measurements during the Tropical Ozone Transport Experiment (TOTE)/Vortex Ozone Transport Experiment (VOTE) mission

Temperature profiles were acquired on the Tropical Ozone Transport Experiment (TOTE) and Vortex Ozone Transport Experiment (VOTE) flights by the Goddard Airborne Raman Lidar. The objective was to compare the temperature product from this new instrument with those generated by the Jet Propulsion Laboratorys Microwave Temperature Profiler and Goddard Space Flight Centers ground-based Stratospheric Ozone Lidar. Simultaneous measurements were made between the Airborne Raman Lidar and O3 lidar on three dates; average temperatures differences varied from ∼0.5 K-5 K over the 16–24 km altitude region. Comparisons between the Microwave Temperature Profiler and Airborne Raman Lidar for all flights (17) during TOTE/VOTE were performed over the same altitude region. The average temperature difference, Airborne Raman Lidar minus Microwave Temperature Profiler, was +2.3 K. In comparing temperatures between the Airborne Raman Lidar and Goddards Data Assimilation Model, the ARL showed small scale structures not evident in those of the model for altitudes between ∼16 and 43 km.

UV Raman Cross Sections in Nitrogen

In an effort to resolve ambiguities concerning nitrogens vibrational (Q-branch) Raman cross section at wavelengths near ∼300 nm, a series of measurements were carried out in the region between 282 and 355 nm. Members of our group have employed nitrogen Raman as a means of calibrating the detection efficiency of a ballon-borne lidar system used to measure stratospheric OH. The only known measurement in the region of interest (at 300 nm) displayed a factor-of-two enhancement over that expected theoretically, thereby introducing a significant uncertainty into the measurement. Nitrogen cross sections were measured at 282 and 306 nm, where uncertainty existed between theory and experiment, and at 355 nm, where good agreement was known to exist. As a check upon the technique, vibrational (Q-branch) Raman cross sections in methane and oxygen were also measured.

Lidar technique for remote measurement of temperature by use of vibrational-rotational Raman spectroscopy

Atmospheric temperature can be measured remotely by a lidar system that measures the ratio of backscattered signals from vibrational-rotational Raman scattering by N(2) to pure vibrational Raman scattering. We present simulations of the performance of an airborne lidar system (based on the Goddard Airborne Raman lidar) that employs this technique.

Total column oxygen detection using a Fabry-Perot interferometer

A passive instrument based on a Fabry-Perot interferometer was designed and used for oxygen atmospheric column absorption measurements. The instrument operates in the oxygen A-band spectral region from 759 to 771 nm. Surface solar irradiation reflected off the Earth is detected using two channels at two wavelengths—one for pressure sensing and the other for temperature sensing. Each channel of the O2 column measurement system consists of two subchannels—Fabry-Perot and reference. Solid Fabry-Perot etalons were designed and used to confine the response to the O2 absorption bands. The etalons have free spectral ranges of 0.575, 0.802, and 2.212 nm. Two narrow bandpass filters (760 to 764 and 767 to 771 nm) were also used. The instrument is sensitive to changes in oxygen column as small as 0.88 mbar for ground-based experiments and 5 mbar for airborne measurements. The major advantages of the optical setup are its compactness, high sensitivity, high signal-to-noise ratio, and stability for both ground and airborne experiments.

Differential Radiometers Using Fabry–Perot Interferometric Technique for Remote Sensing of Greenhouse Gases

A new type of remote-sensing radiometer based upon the Fabry-Perot (FP) interferometric technique has been developed at NASAs Goddard Space Flight Center and tested from both ground and aircraft platforms. The sensor uses direct or reflected sunlight and has channels for measuring the column concentration of carbon dioxide at 1570 nm, oxygen lines sensitive to pressure and temperature at 762 and 768 nm, and water vapor (940 nm). A solid FP etalon is used as a tunable narrow bandpass filter to restrict the measurement to the gas of interests absorption bands. By adjusting the temperature of the etalon, which changes the index of refraction of its material, the transmission fringes can be brought into nearly exact correspondence with the absorption lines of the particular species. With this alignment between absorption lines and fringes, changes in the amount of a species in the atmosphere strongly affect the amount of light transmitted by the etalon and can be related to gas concentration. The technique is applicable to different chemical species. We have performed simulations and instrument design studies for CH4, 13CO2 isotope, and CO detection.