H. Oelhaf

Remote sensing of vertical profiles of atmospheric trace constituentswith MIPAS limb-emission spectrometers

A deeper understanding of long-term ozone trends and periods of significant ozone depletion as well as of the anthropogenic greenhouse effect requires the concerted actions of experimenters and modelers. With respect to observations, atmospheric constituents need to be measured simultaneously and on a global basis. Fourier-transform infrared spectrometers are especially suited for this measurement task. Apromising and challenging branch of Fourier-transform infrared spectroscopy is its application to limb-emission sounding by the use of cryogenic instrumentation. This method allows the measurements to be made independently of the time of the day. The MIPAS (Michelson interferometer for passive atmospheric sounding) balloon-borne (MIPAS-B) and space-based (MIPAS-S) experiments apply this technique. While the MIPAS-B instrument has already been used for several years for stratospheric process studies, the MIPAS-S instrument is in development for the European Space Agencys ENVISAT mission. Instrumental aspects of these MIPAS experiments are highlighted, the most important results in ozone research achieved with MIPAS-B are reviewed, and a brief overview of the scientific capabilities of the MIPAS space experiment is given.

Design and characterization of the balloon-borne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B2)

MIPAS-B2 is a balloon-borne limb-emission sounder for atmospheric research. The heart of the instrument is a Fourier spectrometer that covers the mid-infrared spectral range (4-14 microns) and operates at cryogenic temperatures. Essential for this application is the sophisticated line-of-sight stabilization system, which is based on an inertial navigation system and is supplemented with an additional star reference system. The major scientific benefit of the instrument is the simultaneous detection of complete trace gas families in the stratosphere without restrictions concerning the time of day and viewing directions. The specifications, the design considerations, the actual realization of the instrument, and the results of characterization measurements that have been performed are described.

Chlorine chemistry and the potential for ozone depletion in the Arctic stratosphere in the winter of 1991/92

We present an analysis of chlorine chemistry in the Arctic stratosphere during the winter of 1991/92 and assess its potential implications for ozone depletion. In accordance with observations of total organic chlorine, ClONO2 and HCl, box model results indicate the following: (1) An almost complete activation of chlorine during the cold winter period. (2) A possible contribution from the heterogeneous reaction HOCl + HCl and the gas-phase reaction CH3O2 + ClO to the complete conversion of HCl to active chlorine. (3) A strong buildup of ClONO2 following PSC disappearance which remains the main chlorine reservoir for about a month, after which HCl becomes dominant. (4) Appreciable chemical ozone loss in the lower stratosphere inside the polar vortex is conceivable for the winter of 1991/92.

Stratospheric ClONO2 and HNO3 profiles inside the Arctic vortex from MIPAS‐B limb emission spectra obtained during EASOE

Vertical profiles of ClONO2 and HNO3 inside the Arctic vortex have been retrieved from infrared limb emission spectra recorded during balloon flights on January 13 and in the night of March 14/15, 1992 from Esrange, Sweden (68°N) as part of the European Arctic Stratospheric Ozone Experiment (EASOE). The instrumentation used was the cryogenic Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B). Low ClONO2 abundances in mid-January indicate that a significant portion of ClONO2 had already been converted at that time. An unexpectedly high ClONO2 amount (1.8 to 3.1 ppbv between 16.1 and 21.5 km altitude) has been inferred from the March flight data. This implies that obviously most of the total available chlorine (ClOy) in the lower stratosphere was then in this reservoir molecule. The measured HNO3 profiles give no hint of any significant layered removal of gaseous HNO3 by condensation on particles or/and sedimentation.

Retrieval of stratospheric O3, HNO3 and ClONO2 profiles from 1992 MIPAS-B limb emission spectra: Method, results, and error analysis

Within the framework of the European Arctic Stratospheric Ozone Experiment, two flights of the balloon-borne MIPAS-B limb emission spectrometer were performed in the Arctic stratosphere from Kiruna, northern Sweden. During the early hours of January 13 and the night from March 14 to March 15, 1992, several limb sequences of infrared spectra were recorded which have permitted the retrieval of vertical profiles of many trace gases relevant for ozone chemistry. In the present work, the retrieval strategy, error estimation strategy, and resulting profiles of O3, HNO3, and ClONO2 are reported. The data analysis procedure, consisting of data preprocessing including calibration, analysis of auxiliary data including temperature profiles and line of sight determination, and retrieval of trace gas profiles, is described in detail. The last is carried out by means of multiparameter nonlinear least squares fitting in combination with onion peeling. An astonishingly high ClONO2 amount of 2.6 ppb by volume at about 19-km altitude was inferred for the March flight. A rigorous error analysis, which takes into account systematic and random errors and their often nonlinear impact on the results, proves the significance of the retrieved trace gas profiles.

Denitrification observed inside the Arctic vortex in February 1995

Balloon-borne in situ measurements of total reactive nitrogen (NO,,) and nitrous oxide (N 2 O) were made from Kiruna (68°N, 21°E), Sweden on February 11, 1995. Ten hours later, N 2 O was again measured by an infrared spectrometer flown on another balloon launched from Kiruna. Both observations were made inside the polar vortex between 380 K (∼14 km) and at least 675 K (∼26 km). In the winter of 1994-1995, temperatures at 475 K (∼19 km) inside the vortex were extremely low, sometimes lower than ice frost point, especially from mid-December to mid-January. The NO y profiles obtained during both the ascent and descent revealed layered structures between 15 and 20 km with mixing ratios ranging from 2.7 to 9.3 parts per billion by volume (ppbv). The observed N 2 O profiles indicate significant downward transport of air due to diabatic cooling in the winter. To quantify the degree of irreversible removal of NO y (denitrification) between 12 and 28 km, the unperturbed values of NO y (i.e., NO * y ) were estimated from the observed N 2 O values using the NO y - N 2 O relationship obtained at midlatitudes by the atmospheric trace molecule spectroscopy ATLAS mission and in situ aircraft and balloon-borne measurements. The largest denitrification was observed at 19± 0.5 km, where the NO y values were lower than the NO * y values by ∼10 ppbv, corresponding to a 70% removal of NO y . In spite of the large uncertainty in NO * y the NO y values generally agreed well with the NO * y values at ∼14 km as well as between 3 and 28 km. The relationship between NO,, and N 2 O measured between 23 and 28 km agreed with that measured above 40 km at northern midlatitudes in fall, indicating that the air masses sampled at 23-28 km over Kiruna were transported from the midlatitude upper stratosphere followed by the descent inside the vortex.

First results of MIPAS/ENVISAT with operational Level 2 code

Abstract Michelson interferometer for passive atmospheric sounding (MIPAS) is operating on board of the ENVISAT satellite and is acquiring for the first time high spectral resolution middle infrared emission limb sounding spectra of the Earth atmosphere from space. An optimized code was developed for the Level 2 near real time analysis of MIPAS data. The code is designed to provide, in an automated and continuous way, atmospheric vertical profiles of temperature, pressure and concentrations of O3, H2O, CH4, HNO3, N2O and NO2, in the altitude range from 12 to 68 km. The performances of the code are herewith derived from the analysis of the first measurements acquired with this instrument. The assumptions made for the development of the optimized code are verified with the real data. The diagnostics of the instrument performances provide indications that there is good agreements with the results obtained by the Level 1 analysis. Consistent geophysical data are retrieved which is a first step towards a more complete assessment of retrieval accuracy. The tests have identified the possibility of measurement improvements by way of some secondary operations such as a correction of the frequency scale and the use of cloud filtering. However, no change in the algorithm baseline appears to be necessary.

Ozone loss rates in the Arctic stratosphere in the winter 1994/1995: Model simulations underestimate results of the Match analysis

We present box model simulations of ozone loss rates in the Arctic lower stratosphere for the winter 1994/1995. The ozone loss was simulated along each of the trajectories of the Match data set for 1994/1995 to conduct a quantitative comparison with the Match results. The simulated ozone loss rates reach their maximum value of ≈ 4 ppb per sunlit hour at the end of January. For this period and for potential temperatures below 475 K the model results are in good agreement with the Match results, but for potential temperatures above 475 K the model underestimates the ozone loss rate by up to a factor of 2. This difference cannot be explained by known uncertainties of the model. Enhanced ozone loss has also been observed in March 1995 for potential temperatures below 475 K. These loss rates are also substantially underestimated by the model, but are within the range of the model uncertainties. In particular, the ozone loss rates simulated for March 1995 strongly depend on the extent of denitrification.

NOy partitioning and budget and its correlation with N2O in the Arctic vortex and in summer midlatitudes in 1997

Vertical profiles of the most important species of nocturnal total reactive nitrogen (NO y = NO 2 + HNO 3 + CIONO 2 + 2 N 2 O 5 + HO 2 NO 2 ) together with its source gas N 2 O were retrieved from infrared limb emission spectra measured by the Michelson Interferometer for Passive Atmospheric Sounding, Balloon-borne version (MIPAS-B) instrument inside the late winter arctic vortex from Kiruna (Sweden, 68°N) on 24 March 1997 and in summer midlatitudes from Gap (France, 44°N) on 2 July 1997. The measured data were compared to calculations performed with the three-dimensional chemistry transport model (CTM) Karlsruhe Simulation model of the Middle Atmosphere (KASIMA). The results show that in the late winter arctic vortex most of the available nitrogen and chlorine is in the form of HNO 3 and CIONO 2 , respectively. An anomalous N 2 O-NO y correlation observed in March 1997 appears to be caused to a large extent by quasi-horizontal mixing of air masses across the vortex edge. However, near 20 km some denitrification of ∼1.5 to 2 ppbv NO y could be observed. The N 2 O profile measured in July 1997 indicates remnants of polar vortex air and is not reproduced by the CTM at the same location. However, the profile shapes of the individual compounds of the NO y family as well as the NO x /NO y ratio are reproduced fairly well by the model.

Optical and microphysical parameters of the Mt. Pinatubo aerosol as determined from MIPAS-B mid-IR limb emission spectra

High-resolution mid-IR limb emission spectra were recorded during a flight of the Michelson interferometer for passive atmospheric sounding, balloon-borne version (MIPAS-B) from Kiruna, northern Sweden (68°N) on March 14/15, 1992. These spectra are affected by the Mt. Pinatubo stratospheric aerosol, which caused an enhanced continuum emission, especially in spectra of low tangent altitudes. Aerosol extinction coefficients were retrieved from MIPAS-B spectra at approximately 60 spectral positions in the 750–980 cm−1 and 1180–1380 cm−1 spectral ranges. Retrieved aerosol extinction coefficients range from 6×10−4 km−1 to 3×10−3 km−1 in tangent altitudes 11.3 km and 14.5 km and from 5×10−5 km−1 to 1×10−3 km−1 in 16.1 km. Their distinct spectral shape indicates the presence of H2SO4-H2O droplets. Compositions and size distribution parameters were retrieved by least squares fitting of Mie-generated spectral extinction coefficients to the ones derived from the spectra. Estimated spectral single-scattering albedos between 0.08 and 0.3 indicate the significance of thermal multiple scattering. Multiple-scattering corrections led to an increase of spectral extinction coefficients by 5–50% with highest changes at lowest tangent altitudes. Accordingly, estimated volume densities have increased by 4–20% to values of 3.66, 2.85, and 0.93 μm3 cm−3 for tangent altitudes 11.3, 14.5, and 16.1 km, respectively. Retrieved H2SO4 weights of 66–70% are in good agreement with values derived from stratospheric temperatures and water vapor partial pressures. Estimated surface densities are systematically low in comparison with in situ size distribution measurements. This finding is explained by the underestimation of small particles by the use of a monomodal size distribution in the analysis. Retrieved effective radii of up to 0.8 μm were found to be consistent with the temporal evolution of the Mt. Pinatubo aerosol.