John E. Harries

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.

Trends in stratospheric humidity and the sensitivity of ozone to these trends

Measurements of stratospheric water vapor and methane from the Halogen Occultation Experiment (HALOE) mounted on the Upper Atmosphere Research Satellite (UARS) are used to investigate changes in stratospheric water vapor over the period 1992–1996 inclusive. An increase in water vapor mixing ratio is found at levels between 30 km and 65 km across the globe which fit, to first order, a linear trend varying with altitude from 40 parts per billion by volume per year (ppbv yr−1) to a maximum of 90 ppbv yr−1 at 45 km. These trends appear to be greater than that expected due to the growth in tropospheric methane over the past several decades, and possible mechanisms accounting for this are discussed. The trend of the combined budget of 2 × CH4 + H2O is approximately constant with altitude with a global mean value of 61±4 ppbv yr−1. On the basis of these estimates, sensitivity studies have been performed using a two-dimensional (2-D) radiative-chemical-dynamical model to assess the impact on concentrations of stratospheric ozone of this degree of change in stratospheric water vapor over timescales consistent with doubling CO2 scenarios. We find that the impact of increased stratospheric water vapor is to enhance the ozone increase in the midstratosphere by ∼1–2% compared to the response due to a doubling of CO2 itself of ∼5–10%. In the upper stratosphere the destruction of ozone is enhanced and the changeover from production to loss is lowered to ∼50 km (from ∼70 km). A chemical mechanism for these processes involving enhanced OH and NO2 is identified.

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.

Stratospheric dryness: Antiphased desiccation over Micronesia and Antarctica

HALOE observations of water vapor and methane during the period 21 September–15 October 1992 are used to examine the role of Antarctic drying in the lower stratosphere. Zonal mean cross-sections of [2 CH4+H2O] show the probability of transport of Antarctic type dryness to latitudes as distant as 20°N, with major water vapor deficits evident between 10 and 100 mb to 10°S. Examination of monthly mean tropical 100 mb temperatures and of Antarctic temperatures suggests that the observations are consistent with stratospheric dryness being achieved by the combined effects of tropopause freeze-drying over the Micronesia region during northern winter and drying through the influence of the very low temperatures over Antarctica during southern winter. This paper presents these intriguing new results, and offers a possible explanation.

On the distribution of mesospheric molecular hydrogen inferred from HALOE measurements of H2O and CH4

The Halogen Occultation Experiment (HALOE) is in orbit on NASAs Upper Atmosphere Research Satellite (UARS), and has been used to make measurements of a number of stratospheric and mesospheric constituents since October 1991. These include, amongst others, water vapour, H2O, and methane, CH4, two principal components of the total hydrogen budget of the middle atmosphere. The third main component is molecular hydrogen, H2, which is not measurable by HALOE or any other UARS sensor. By making the assumption that the total hydrogen content of the middle atmosphere is a conserved quantity, and that these three constituents dominate the budget, it is possible to infer the H2 fields in the mesosphere from the HALOE H2O and CH4 measurements.

Total ozone measured during EASOE by a UV-visible spectrometer which observes stars

Total ozone was measured from Abisko, Sweden (68.4°N, 18.8°E) from January to early March 1992, by a new instrument which uses stars and the Moon as sources of UV-visible light for absorption spectroscopy. In addition, some zenith-sky observations were made. Ozone measurements obtained using both techniques are presented and compared with those from other instruments. Good agreement with simultaneous ozonesonde measurements is observed, but the stellar measurements appear systematically higher than total ozone measured by both SAOZ and TOMS.

Derivation of OH Concentrations from LIMS Measurements

Stratospheric concentrations of OH have been derived from LIMS measurements of minor constituents. Two methods have been used. Assuming that HNO3 and NO2 are in photochemical steady state, LIMS measurements of these species, with knowledge of appropriate rate constants and a calculation of the HNO3 photolysis rate, allow nearly global fields of OH to be derived. The derived profiles show satisfactory agreement with model calculations and the limited number of in situ observations. As a check on our method, OH has also been derived by calculations of its sources and sinks using the LIMS measurements of H2O. The two methods agree extremely well in low latitudes. At higher latitudes the agreement is less satisfactory. This is discussed in terms of the diurnal behaviour of the species and the time constant of the HNO3/NO2 equilibrium.