A. F. Tuck

The Halogen Occultation Experiment

The Halogen Occultation Experiment (HALOE) was launched on the Upper Atmosphere Research Satellite (UARS) spacecraft September 12, 1991, and after a period of outgassing, it began science observations October 11. The experiment uses solar occultation to measure vertical profiles of O3, HCl, HF, CH4, H2O, NO, NO2, aerosol extinction, and temperature versus pressure with an instantaneous vertical field of view of 1.6 km at the Earth limb. Latitudinal coverage is from 80°S to 80°N over the course of 1 year and includes extensive observations of the Antarctic region during spring. The altitude range of the measurements extends from about 15 km to ≈ 60–130 km, depending on channel. Experiment operations have been essentially flawless, and all performance criteria either meet or exceed specifications. Internal data consistency checks, comparisons with correlative measurements, and qualitative comparisons with 1985 atmospheric trace molecule spectroscopy (ATMOS) results are in good agreement. Examples of pressure versus latitude cross sections and a global orthographic projection for the September 21 to October 15, 1992, period show the utility of CH4, HF, and H2O as tracers, the occurrence of dehydration in the Antarctic lower stratosphere, the presence of the water vapor hygropause in the tropics, evidence of Antarctic air in the tropics, the influence of Hadley tropical upwelling, and the first global distribution of HCl, HF, and NO throughout the stratosphere. Nitric oxide measurements extend through the lower thermosphere.

Hemispheric asymmetries in water vapor and inferences about transport in the lower stratosphere

Both satellite water vapor measurements and in situ aircraft measurements indicate that the southern hemisphere lower stratosphere is drier than that of the northern hemisphere in an annual average sense. This is the result of a combination of factors. At latitudes poleward of ∼50°S, dehydration in the Antarctic polar vortex lowers water vapor mixing ratios relative to those in the north during late winter and spring. Equatorward of ∼50°S, water vapor in the lower stratosphere is largely controlled by the tropical seasonal cycle in water vapor coupled with the seasonal cycle in extratropical descent. During the tropical moist period (June, July, and August), air ascending in the Indian monsoon region influences the northern hemisphere more than the southern hemisphere, resulting in a moister northern hemisphere lower stratosphere. This tropical influence is confined to levels beneath 60 mbar at low latitudes, and beneath 90 mbar at high latitudes. During the tropical dry period (December, January, and February), dry air spreads initially into both hemispheres. However, the stronger northern hemisphere wintertime descent that exists relative to that of southern hemisphere summer transports the dry air out of the northern hemisphere lower stratosphere more quickly than in the south. This same hemispheric asymmetry in winter descent (greater descent rates during northern hemisphere winter than during southern hemisphere winter) brings down a greater quantity of “older” higher water vapor content air in the north, which also acts to moisten the northern hemisphere lower stratosphere relative to the southern hemisphere. These factors all act together to produce a drier southern hemisphere lower stratosphere as compared to that in the north. The overall picture that comes from this study in regards to transport characteristics is that the stratosphere can be divided into three regions. These are (1) the “overworld” where mass transport is controlled by nonlocal dynamical processes, (2) the “tropically controlled transition region” made up of relatively young air that has passed through (and been dehydrated by) the cold tropical tropopause, and (3) the stratospheric part of the “middleworld” or “lowermost stratosphere”, where troposphere-stratosphere exchange can occur adiabatically. Satellite water vapor measurements indicate that the base of the “overworld” is near 60 mbar in the tropics, or near the 450 K isentropic surface.

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.

On the evaluation of ozone depletion potentials

Observations of methane, CFC-11, and ozone losses are used along with insights from models and observations regarding interrelationships between tracers to develop a semi-empirical framework for evaluating global ozone depletion potentials. Direct measurements of some hydrochlorofluorocarbons including HCFC-22 in the Arctic lower stratosphere are also used to evaluate the local ozone depletion potentials there. This approach assumes that all of the observed ozone destruction in the contemporary atmosphere is due to chlorine and that the depletion is proportional to the local relative chlorine release. It is shown that the global ozone depletion potentials for compounds with relatively long stratospheric lifetimes such as HCFC-22 and HCFC-142b are likely to be larger than those generally predicted by gas phase chemical models, due largely to the importance of lower stratospheric ozone losses that are not simulated in gas phase studies. The analysis presented suggests that the globally averaged efficiency for ozone depletion by HCFC-22 is as much as a factor of 2 larger than some gas phase model estimates. For compounds with short stratospheric lifetimes such as (CCl4). and (CH3CCl3), on the other hand, gas phase models likely overestimate the ozone depletion potentials for the present-day stratosphere. Observations of polar ozone loss and reactive halogen radical abundances also imply that the globally averaged ozone depletion potentials for brominated species for the contemporary stratosphere could be as much as 1.5–3 times greater than some gas phase model predictions, depending upon lower stratospheric loss processes.

Polar stratospheric cloud processed air and potential voracity in the northern hemisphere lower stratosphere at mid‐latitudes during winter

Small-scale (<1000 km) features in ER-2 measurements of ClO, O3, H2O, N2O, and NOy, outside the lower stratospheric Arctic vortex of 1988–1989 are compared with features on potential vorticity maps from the European Centre for Medium-range Weather Forecasts (ECMWF). The potential vorticity maps are obtained from Tl06 analyses and forecasts. Some of the plots have been truncated to lower resolution (T63 or T42) which smooths out the finer-scale structure. Comparison of these lower resolution plots shows how much detail is lost by excessive smoothing. It is also evident that the forecast plots lose fine-scale structure due to dissipation in the model resulting mainly from horizontal diffusion. We conclude that blobs of air on the maps at latitudes between the vortex edge and 25°N having potential vorticities characteristic of the vortex, did indeed originate from the vortex, but that the real atmosphere is more sharply differentiated (inhomogeneous) than the meteorological analyses, implying that the potential vorticity maps underestimate the amount of peeled-off material. Areal budgets of the ex-vortex air are considered for ER-2 flight days, and are performed for 24-hour forecasts at T63, and analyses at T42, T63, and T106 resolution at θ = 475 K. Finally, it is concluded that the lower stratospheric Arctic vortex of 1988–1989 spread considerable amounts of air to mid-latitudes which had been processed by polar stratospheric clouds, and that this mechanism is a realistic explanation for the wintertime loss of ozone observed over northern mid-latitudes during the last decade.

Chlorine activation and ozone depletion in the Arctic vortex: Observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite

Chlorine-catalyzed ozone destruction is clearly observed during austral spring in the Antarctic lower stratosphere. While high concentrations of ozone-destroying ClO radicals have likewise been measured during winter in the Arctic stratosphere, the chemical ozone depletion there is more difficult to quantify. Here we present observations of the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the vortex region of the Arctic lower stratosphere during the winter and spring months of 1991/1992, 1992/1993, 1993/1994, and 1994/1995. All February measurements indicate an almost complete conversion of the otherwise main chlorine reservoir species HCl to chemically more reactive forms. Using CH4 as a chemically conserved tracer, we show that significant chemical ozone loss occurred in the Arctic vortex region during all four winters. The deficit in column ozone was about 60 and 50 Dobson units (DU) in the winters 1991/1992 and 1993/1994, respectively. During the two winters of 1992/1993 and 1994/1995 a severe chemical loss in lower-stratospheric ozone took place, with local reductions of the mixing ratios by over 50% and a loss in the column ozone of the order of 100 DU.

HALOE Antarctic observations in the spring of 1991

HALOE observations of O3, CH4, HF, H2O, NO, NO2, and HCl collected during the October 1991 Antarctic spring period are reported. The data show a constant CH4 mixing ratio of about 0.25 ppmv for the altitude range from 65 km down to about 25 km at the position of minimum wind speed in the vortex: i.e., the vortex center, and depressions in pressure versus longitude contours of NO, NO2, HF, and HCl in this same region. Water vapor, HF, and HCl enhancement are also observed in the vortex center region above ∼25 km. Between 10 and 20 km, the expected mixing ratio signatures exist within the vortex, i.e., low ozone and dehydration. The water vapor increased by 50%, and the ozone level doubled inside the vortex between October 11 and 24 in the 15 to 20 km layer. These changes imply a time constant for recovery from ozone hole conditions or 19 and 30 days for O3 and H2O, respectively. The data further show the presence of air inside the vortex between 3 and 30 mb which has mixing ratios characteristic of mid latitudes.

Validation of hydrogen fluoride measurements made by the Halogen Occultation Experiment from the UARS platform

The Halogen Occultation Experiment (HALOE) on UARS uses the method of solar occultation limb sounding to measure the composition and structure of the stratosphere and mesosphere. One of the HALOE channels is spectrally centered at 3.4 μm to measure the vertical profile and global distribution of hydrogen chloride. The mean difference between HALOE and 14 balloon correlative underflight measurements ranges from 8% to 19% throughout most of the stratosphere. This difference is within the limits of error bar overlap for the two data sets. The mean differences between HALOE and HCl data from ATMOS flights on the space shuttle is of the order of 15 to 20% for the 1992 flight and 10% for the 1993 flight. Generally, HALOE results tend to be low in these comparisons. Also, comparisons with two-dimensional model calculations and HALOE data are in good qualitative agreement regarding vertical profile shapes and features in a pressure versus latitude cross section. HCl values increase from ∼0.3 parts per billion by volume (ppbv) to 1 ppbv in the lower stratosphere to 2.6 ppbv to 3.3 ppbv just above the stratopause which is the upper limit of HALOE single-profile measurements. There is a dependence of HCl results on the angle between the orbit plane and the Earth-Sun vector with HCl varying by ±9% in the upper stratosphere. This variation appears to be altitude dependent and it is not discernible in the data below about 10 mbar.

Atmospheric radical production by excitation of vibrational overtones via absorption of visible light

We present calculations using a radiative transfer model which predict that in the lower stratosphere at high zenith angles, significant enhancements to the photodissociation rates of HNO3 and HNO4 can result from visible wavelength excitation of OH overtone vibrations containing sufficient energy to cleave the O-O and N-O bonds. The results indicate that atmospheric chromophores such as HONO2, HO2NO2 and H2O2, could make a potentially significant contribution to the production of HOx and NOx. Calculating the relative importance of their effect requires better knowledge of the absolute absorption cross sections, both for vibrational overtones and in the near UV. Stratospheric air masses in which this process could be important are those that experience lengthy exposure at high solar zenith angles: the outer regions of the polar winter vortex and the polar summer anticyclone. We note that the general mechanism may have application elsewhere, such as in the atmospheres of other planets and in generating the diffuse interstellar bands associated with molecular clouds.

HALOE observations of the vertical structure of chemical ozone depletion in the Arctic vortex during winter and early spring 1996-1997

We discuss observations by the Halogen Occultation Experiment on the Upper Atmosphere Research Satellite in the lower stratosphere in the Arctic vortex during winter and spring 1996-1997. Using HF as a chemically conserved tracer, we identify chemical ozone depletion and chlorine activation, despite variations caused by dynamical processes. For the Arctic vortex region, significant chemical ozone loss (up to two thirds around 475 K potential temperature) due to extensive activation of the inorganic chlorine reservoir is deduced, as observed similarly for previous winters. Chemical reductions in column ozone of up to 70-80 Dobson units (DU) in the lower stratosphere are calculated. Both chlorine activation and ozone loss inside the vortex, however, are more variable than observed in previous years.