W. A. Traub

Validation of measurements of water vapor from the Halogen Occultation Experiment (HALOE)

The Halogen Occultation Experiment (HALOE) experiment is a solar occultation limb sounder which operates between 2.45 and 10.0 μm to measure the composition of the mesosphere, stratosphere, and upper troposphere. It flies onboard the Upper Atmosphere Research Satellite (UARS) which was launched in September 1991. Measurements are made of the transmittance of the atmosphere in a number of spectral channels as the Sun rises or sets behind the limb of the atmosphere. One of the channels, at 6.60 μm, is a broadband filter channel tuned to detect absorption in the ν2 band of water vapor. This paper describes efforts to validate the absolute and relative uncertainties (accuracy and precision) of the measurements from this channel. The HALOE data have been compared with independent measurements, using a variety of observational techniques, from balloons, from the ground, and from other space missions, and with the results of a two-dimensional model. The results show that HALOE is providing global measurements throughout the stratosphere and mesosphere with an accuracy within ±10% over most of this height range, and to within ±30% at the boundaries, and to a precision in the lower stratosphere of a few percent. The H2O data are combined with HALOE measurements of CH4 in order to test the data in terms of conservation of total hydrogen, with most encouraging results. The observed systematic behavior and internal consistency of the HALOE data, coupled with these estimates of their accuracy, indicate that the data may be used for quantitative tests of our understanding of the physical and chemical processes which control the concentration of H2O in the middle atmosphere.

Smithsonian stratospheric far‐infrared spectrometer and data reduction system

The Smithsonian far-infrared spectrometer is a remote sensing Fourier transform spectrometer that measures the mid- and far-infrared thermal emission spectrum of the stratosphere from balloon and aircraft platforms. The spectrometer has had nine successful balloon flights from 1987 to 1994, flying at float altitudes of 36–39 km and collecting 131 hours of midlatitude stratospheric limb spectra. The spectrometer also flew on a NASA DC-8 aircraft, as part of the second Airborne Arctic Stratospheric Expedition (AASE-II), collecting 140 hours of overhead spectra at latitudes ranging from the equator to the north pole. We present here a brief description of the instrument, a discussion of data reduction procedures, an estimation of both random and systematic errors, an outline of the procedure for retrieving mixing ratio profiles, and an explanation of the method of deriving temperature and pressure from the far- and mid-infrared spectra.

Simultaneous measurements of stratospheric HOx, NOx, and Clx: Comparison with a photochemical model

We report simultaneous measurements of the stratospheric concentration profiles of OH, HO2, H2O2, H2O, O3, HNO3, NO2, N2O, HCl, HOCl, and ClO during a midlatitude balloon flight in 1989. Measurements were made over almost an entire diurnal cycle by the Smithsonian Astrophysical Observatory Far-Infrared Spectrometer (FIRS-2) and the Jet Propulsion Laboratory Balloon Microwave Limb Sounder (BMLS). We analyze these measurements using a photochemical model constrained by observations of long-lived gases. Measured HOx species (OH and HO2) and H2O2 show fair agreement with theory throughout the diurnal cycle. Measurements of HNO3 are higher than theory near the concentration peak, while the levels of NO2 are consistent with the model at most altitudes. Measurements of ClO and HOCl are less than predicted concentrations, suggesting a source of HCl in addition to the reaction of Cl with CH4. Possibilities for such a source include a minor HCl + O2 product channel for the reaction of ClO with OH and a minor HCl + O3 channel for the reaction of ClO with HO2.

Detection of HBr and upper limit for HOBr: Bromine partitioning in the stratosphere

We determine mixing ratio profiles for HBr and upper limits for HOBr in the stratosphere with precisions up to 1.7 and 4.8 parts per trillion, respectively, using the combined data from 7 flights of our far-infrared spectrometer. The measurements suggest that in the range 22–34 km the average mixing ratio of HBr is 2.0±0.8, and that the average mixing ratio of HOBr is less than 2.8 ppt. Our measurements of HBr are in reasonable agreement with a photochemical model which includes 0 or 2% production of HBr through the reaction of BrO with HO2, but in strong disagreement with a model including 5 or 10% HBr production.

Nitric acid in the middle stratosphere as a function of altitude and aerosol loading

We present remote-sensing measurements of the abundance of nitric acid (HNO3) in the lower and middle stratosphere (between 16 and 40 km) covering the period 1989–1997. The measurements were made with the Smithsonian Astrophysical Observatory Far-Infrared Spectrometer (FIRS-2) under a wide range of aerosol surface area density. We compare our measurements with the results of a photochemical steady state model to test our understanding of the chemistry of HNO3 under a variety of conditions. We find that HNO3 is significantly overestimated by the model at altitudes above 22 km, with the difference increasing with increasing altitude and decreasing aerosol surface area density. The agreement between modeled and measured HNO3 can be improved by either decreasing the rate of OH + NO2 by 35% or by using newly measured rate constants for the reactions OH + NO2 and OH + HNO3, but significant differences remain. We discuss these observations in the context of possible uncertainties in the calculated photolysis rate of HNO3 at wavelengths near 200 nm, uncertainties in the observations, errors caused by the use of constrained steady state models, and possible missing sink reactions for HNO3.

Measurement of chlorine nitrate in the stratosphere using the ν4 and ν5 bands

We present laboratory measurements of the absorption cross sections of the ν5 band (540–580 cm−1) of chlorine nitrate made under stratospheric conditions of temperature and pressure. We use the measured cross sections and observed mid-infrared stratospheric emission spectra to retrieve mid-latitude vertical mixing ratio profiles for both day and night in the altitude range 20–34 km, and compare the results with a sunset profile obtained simultaneously from solar absorption measurements of the ν4 Q branch at 780 cm−1. The profiles obtained using these two methods are shown to be in excellent agreement, validating retrievals using the ν4 band. We also find good agreement in this altitude range between the observed day/night ratio and the results of a 1-D photochemical model

Estimating the abundance of ClO from simultaneous remote sensing measurements of HO2, OH, and HOCl

Using a simple photochemical model we derive the mixing ratio profile of ClO in the altitude range 20–38 km from simultaneous measurements of HO2, OH, HOCl, temperature, pressure, and ozone. The measurements were made with the FIRS-2 far-infrared spectrometer during a balloon flight on September 29, 1992. We compare the derived ClO with the ClO profile obtained by the SLS instrument while flying on the same gondola. The good agreement between the two profiles validates our simple model and confirms the relevant rate constants and photolysis cross sections.

The far-infrared spectroscopy of the troposphere (FIRST) project

The far-infrared spectroscopy of the troposphere (FIRST) project is under development by NASA through its Instrument Incubator Program (IIP) administered by the Earth Science Technology Office. The objective of the FIRST project is to develop and demonstrate the technology needed to routinely observe from space the far-infrared spectrum between 15 and 100 micrometers in wavelength. This spectral region contains about half of the outgoing longwave radiation from the Earth and its atmosphere and is responsible for about half of the natural greenhouse effect. Radiative cooling of the free troposphere occurs almost exclusively in the far-infrared. The far-infrared emission is modulated almost entirely by water vapor, the main greenhouse gas. Cirrus clouds exhibit significant climate forcing in the far-infrared. Despite this fundamental science, the far-infrared has remained almost unobserved directly, primarily due to technological limitations. The FIRST project is advancing technology in the areas of high throughput interferometers, broad bandpass beamsplitters, and detector focal planes to enable routine measurement of the far-infrared from space. FIRST will conduct a technology demonstration on a high altitude balloon platform in Spring 2005.

Response of lower stratospheric HCl/Cl y to volcanic aerosol: Observations from aircraft, balloon, space shuttle, and satellite instruments

Vertical profiles of HCl/Cly measured by space shuttle (Atmospheric Trace Molecule Spectroscopy (ATMOS)) and balloon (MkIV) instruments from 1985 through 1994 show that the increased volcanic aerosol from Mount Pinatubo had little effect on chlorine partitioning above 21 km. Below 21 km, however, large changes in HCl/Cly were observed. Aircraft (Aircraft Laser Infrared Absorption Spectrometer), balloon (MkIV, Far Infrared Spectrometer 2), space shuttle (ATMOS), and satellite (Halogen Occultation Experiment) measurements below 21 km reveal an increase in HCl/Cly between early 1993 and late 1997 of 31±9%. This growth rate is 2 to 3 times larger than model estimates of the effect of the change in aerosol surface area and volume over this time period. For 1993 the five independent direct measurements of HCl/Cly produce a mean value of 0.57±0.05, whereas by 1997 the mean value was 0.75±0.07, in excellent agreement with the pre-Pinatubo mean of 0.74±0.11. Measurements made in 1992 at the peak of the post-Pinatubo midlatitude aerosol loading demonstrate large variability, which is qualitatively consistent with model predictions. This variability appears to be attributable to the temperature sensitivity of the Cly partitioning when aerosol abundances are high.

The Far-Infrared Spectroscopy of the Troposphere (FIRST) project

The radiative balance of the Earth is influenced strongly by radiative cooling associated with emission of radiation by water vapor at far-infrared (far-IR) wavelengths greater than 15 /spl mu/m and extending out beyond 60 /spl mu/m. the distribution of water vapor and cirrus cloud associated far-IR radiative forcings and feedbacks are well-recognized as major uncertainties in understanding and predicting future climate. Despite this fundamental importance, far-IR emission (spectra or band-integrated) has rarely been directly measured from space, airborne, or ground-based platforms. Current and planned operational and research satellites typically observe the mid-infrared only to about 15.4 /spl mu/m. The Far-Infrared Spectroscopy of the Troposphere (FIRST) project is an investment by NASA through the Instrument Incubator Program (IIP) to develop a space-based capability to measure the infrared spectrum to 100 /spl mu/m.