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The MARSCHALS (Millimetre-wave Airborne Receiver for Spectroscopic CHaracterisation of Atmospheric Limb-Sounding) project has the general objectives of demonstrating the measurement capabilities of a limb viewing instrument working in the millimetre and sub-millimetre spectral regions (from 294 to 349 GHz) for the study of the Upper Troposphere – Lower Stratosphere (UTLS). MARSCHALS has flown on board the M-55 stratospheric aircraft (Geophysica) in two measurements campaigns. Here we report the results of the analysis of MARSCHALS measurements during the SCOUT-O3 campaign held in Darwin (Australia) in December 2005 obtained with MARC (Millimetre-wave Atmospheric-Retrieval Code). MARSCHALS measured vertical distributions of temperature, water vapour, ozone and nitric acid in the altitude range from 10 to 20 km in presence of clouds that obscure measurements in the middle infrared spectroscopic region. The minimum altitude at which the retrieval has been possible is determined by the high water concentration typical of the tropical region rather than the extensive cloud coverage experienced during the flight. Water has been measured from 10 km to flight altitude (~18 km) with a 10% accuracy, ozone from 14 km to flight altitude with accuracy ranging from 10% to 60%, while the retrieval of nitric acid has been possible with an accuracy not better than 40% only from 16 km to flight altitude due to the low signal to noise ratio of its emission in the analysed spectral region. The results have been validated using measurement made in a less cloudy region by MIPAS-STR, an infrared limb-viewing instrument on board the M-55, during the same flight.

The role of ozone in absorbing ultraviolet solar radiation is well known. Ozone also makes a significant contribution to the radiative balance of the upper troposphere and lower stratosphere, such that changes in the distribution of ozone in these atmospheric regions will affect the radiative forcing of climate1,2. Furthermore, tropospheric ozone is the source of the hydroxyl radical which controls the abundance and distribution of many atmospheric constituents, including greenhouse gases such as methane and hydrochlorofluorocarbons. Tropospheric ozone is produced photochemically in situ and is also transported down from the stratosphere, but the relative importance of these two sources to its global budget is poorly understood. High-quality tropospheric and lower-stratospheric ozone profile measurements are available from sondes and lidar techniques, but their geographical sampling is very limited. Complementary satellite measurements of the global ozone distribution in this height region are therefore required to quantify ozone's tropospheric budget and its participation in climate-forcing and tropospheric chemistry. Here we present direct measurements of tropospheric ozone concentrations from space, made by the European Space Agency's Global Ozone Monitoring Experiment. These results demonstrate the potential of satellite measurements to provide self-consistent tropospheric and stratospheric ozone distributions on a global scale.

Atmospheric temperature measurements from the improved stratospheric and mesospheric sounder (ISAMS) are evaluated. Flown on the Upper Atmosphere Research Satellite (UARS), ISAMS obtained 180 days of science data between September 26, 1991 and July 29, 1992. Typically, over 2600 temperature profiles/day were retrieved, spaced every 200 km along the limb-viewing track and nominally extending from 100 to 0.01 mbar (15-80 km). The latitude coverage ranged from 80°S to 80°N, depending on the particular ISAMS/UARS viewing geometry on any day. UARS is in a near-Sun-synchronous orbit, so that while the 15 orbits/d are spaced approximately every 24° longitude around the equator, the sampled local solar time actually changes by 20 min/d. The ISAMS temperature retrieval process is outlined and the various products are described. A detailed error budget for the retrieval is presented and comparisons are made with temperature measurements from other sources. Finally, a table is provided summarizing the best estimates of ISAMS temperature bias and precision. The results suggest a general cold bias of around 1 K in the stratospheric temperatures, with a superimposed 2-3 K warm bias associated with the densest part of the Pinatubo aerosol cloud. The precision of individual profiles is ±2 K throughout the stratosphere but falls off in the mesosphere to about ±10 K at 80 km. The error bars produced by the retrieval appear to be reasonable (although slightly pessimistic) estimates of the precision. Copyright 1996 by the American Geophysical Union.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer measuring the atmospheric emission spectrum in limb sounding geometry. The instrument is capable to retrieve the vertical distribution of temperature and trace gases, aiming at the study of climate and atmospheric chemistry and dynamics, and at applications to data assimilation and weather forecasting. MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004, with scans performed at nominal spectral resolution of 0.025 cm−1 and covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). Only reduced spectral resolution measurements have been performed subsequently. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) and provided a complete set of level-2 operational products (geo-located vertical profiles of temperature and volume mixing ratio of H2O, O3, HNO3, CH4, N2O and NO2) with quasi continuous and global coverage in the period of MIPAS full spectral resolution mission. In this paper, we report a detailed description of the validation of MIPAS-ENVISAT operational ozone data, that was based on the comparison between MIPAS v4.61 (and, to a lesser extent, v4.62) O3 VMR profiles and a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by the European Center for Medium-range Weather Forecasting. A coordinated effort was carried out, using common criteria for the selection of individual validation data sets, and similar methods for the comparisons. This enabled merging the individual results from a variety of independent reference measurements of proven quality (i.e. well characterized error budget) into an overall evaluation of MIPAS O3 data quality, having both statistical strength and the widest spatial and temporal coverage. Collocated measurements from ozone sondes and ground-based lidar and microwave radiometers of the Network for the Detection Atmospheric Composition Change (NDACC) were selected to carry out comparisons with time series of MIPAS O3 partial columns and to identify groups of stations and time periods with a uniform pattern of ozone differences, that were subsequently used for a vertically resolved statistical analysis. The results of the comparison are classified according to synoptic and regional systems and to altitude intervals, showing a generally good agreement within the comparison error bars in the upper and middle stratosphere. Significant differences emerge in the lower stratosphere and are only partly explained by the larger contributions of horizontal and vertical smoothing differences and of collocation errors to the total uncertainty. Further results obtained from a purely statistical analysis of the same data set from NDACC ground-based lidar stations, as well as from additional ozone soundings at middle latitudes and from NDACC ground-based FTIR measurements, confirm the validity of MIPAS O3 profiles down to the lower stratosphere, with evidence of larger discrepancies at the lowest altitudes. The validation against O3 VMR profiles using collocated observations performed by other satellite sensors (SAGE II, POAM III, ODIN-SMR, ACE-FTS, HALOE, GOME) and ECMWF assimilated ozone fields leads to consistent results, that are to a great extent compatible with those obtained from the comparison with ground-based measurements. Excellent agreement in the full vertical range of the comparison is shown with respect to collocated ozone data from stratospheric aircraft and balloon instruments, that was mostly obtained in very good spatial and temporal coincidence with MIPAS scans. This might suggest that the larger differences observed in the upper troposphere and lowermost stratosphere with respect to collocated ground-based and satellite O3 data are only partly due to a degradation of MIPAS data quality. They should be rather largely ascribed to the natural variability of these altitude regions and to other components of the comparison errors. By combining the results of this large number of validation data sets we derived a general assessment of MIPAS v4.61 and v4.62 ozone data quality. A clear indication of the validity of MIPAS O3 vertical profiles is obtained for most of the stratosphere, where the mean relative difference with the individual correlative data sets is always lower than ±10%. Furthermore, these differences always fall within the combined systematic error (from 1 hPa to 50 hPa) and the standard deviation is fully consistent with the random error of the comparison (from 1 hPa to ~30–40 hPa). A degradation in the quality of the agreement is generally observed in the lower stratosphere and upper troposphere, with biases up to 25% at 100 hPa and standard deviation of the global mean differences up to three times larger than the combined random error in the range 50–100 hPa. The larger differences observed at the bottom end of MIPAS retrieved profiles can be associated, as already noticed, to the effects of stronger atmospheric gradients in the UTLS that are perceived differently by the various measurement techniques. However, further components that may degrade the results of the comparison at lower altitudes can be identified as potentially including cloud contamination, which is likely not to have been fully filtered using the current settings of the MIPAS cloud detection algorithm, and in the linear approximation of the forward model that was used for the a priori estimate of systematic error components. The latter, when affecting systematic contributions with a random variability over the spatial and temporal scales of global averages, might result in an underestimation of the random error of the comparison and add up to other error sources, such as the possible underestimates of the p and T error propagation based on the assumption of a 1 K and 2% uncertainties, respectively, on MIPAS temperature and pressure retrievals. At pressure lower than 1 hPa, only a small fraction of the selected validation data set provides correlative ozone data of adequate quality and it is difficult to derive quantitative conclusions about the performance of MIPAS O3 retrieval for the topmost layers.

MARSCHALS is the airborne simulator of a proposed future satellite instrument to measure millimetre-wave limb emission from O 3 , H 2 O, CO and other trace gases in the upper troposphere and lower stratosphere. To achieve comparatively high vertical resolution and pointing stability, MARSCHALS scans the atmospheric limb in 1km vertical steps using a 235mm diameter antenna controlled by a dedicated inertial measurement unit. A quasi-optical network directs radiation from the antenna or an ambient (~300K) or cold (~90K) calibration target into three front-end receivers and suppresses each unwanted side-band by >30dB using multi-layer frequency selective surfaces. Each receiver comprises a waveguide mixer pumped subharmonically by a phase-locked LO and a wideband IF preamplifier. The IF outputs are directed to channeliser spectrometers of 200MHz resolution which instantaneously and contiguously cover 12GHz wide (RF) frequency bands centred near 300, 325 and 345GHz. To identify clouds, images of near-IR sunlight scattered into the limb direction are recorded concurrently by an 850nm wavelength camera. MARSCHALS has been built under ESA contract by a consortium led by Rutherford Appleton Laboratory in the UK, and had its first flights on the Russian Geophysica (M55) aircraft during 2005, culminating in a deployment during the SCOUT-O3 campaign based in Darwin, Australia. This paper describes the MARSCHALS instrument and an initial assessment of its performance, determined on ground and during flight. Keywords: MARSCHALS, limb sounding, millimetre wave, millimeter wave, Geophysica, airborne, MASTER

The Millimetre-wave Acquisitions for Stratosphere-Troposphere Exchange Research (MASTER) instrument is intended to sound the gaseous composition of the upper troposphere and lower stratosphere (UTLS) in a future ESA space mission. A significant and inherent advantage of operation at millimetre and sub-millimetre wavelengths in comparison to limb-sounders operating at infra-red and shorter wavelengths is low sensitivity to cirrus clouds. MASTER will employ relatively small vertical and horizontal spacings between limb views, in order to over-sample the atmosphere in the orbit plane. By viewing each air mass from different directions, and including this information in the retrieval, horizontal as well as vertical structure of atmospheric fields may be captured. In order to examine this tomographic limb-sounding approach for MASTER, a state-of-the-art 2-D radiative transfer model and retrieval model have been developed and used in simulation experiments. A linear analysis has been performed to establish achievable horizontal and vertical retrieval resolution for target species and to identify additional parameters to include in the state vector in order to reduce error sensitivities. A realistic mid-latitude scenario and appropriate instrument and model errors have been considered. By accurately modelling radiative transfer in two dimensions within the orbit plane, and using multiple limb-sequences simultaneously in a 2-D retrieval, a horizontal resolution better than 200 km can be achieved, together with ~2 km vertical resolution for retrievals of water vapour, ozone and other trace gases in the UTLS.

Measurements of nitrogen dioxide (NO2) from the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmosphere Research Satellite (UARS) are assessed. Channel 5 of the instrument was dedicated to observations of nitrogen dioxide and employed pressure-modulation and wideband radiometry to make measurements at 6.2 μm. This dual technique allows simultaneous determination of nitrogen dioxide mixing ratio and the aerosol extinction coefficient at this wavelength and therefore provides nitrogen dioxide data even in the presence of heavy aerosol loading. Approximately 180 days of data, in the period from September 1991 to July 1992, were obtained with, typically, over 2600 profiles per day for each retrieved species, covering an altitude range of 100–0.01 mbar. In this paper the version 10 data are assessed and a full error analysis is described. Comparisons with the Cryogenic Limb Array Etalon Spectrometer (CLAES) on UARS and the Limb Infrared Monitor of the Stratosphere (LIMS) on Nimbus 7 are also presented. It is concluded that the morphology of the retrieved ISAMS fields is robust and consistent with concurrent as well as previous infrared satellite measurements. Random errors are estimated to be of the order of 10% for nighttime and 15% for daytime NO2 near the maxima of the distributions, and systematic errors are estimated to be of a similar size. However, there remains an unresolved systematic difference of about a factor of 2 between ISAMS and CLAES. Both random and systematic errors are likely to be reduced in future versions of the processing.

Limb sounding measurements of infra-red emission at 6.2 μm from the Earth's atmosphere have been made by the Improved Stratospheric and Mesospheric Sounder (ISAMS). This provides a pressure modulated gas-correlation signal and a wideband signal from which NO2 and aerosol extinction at 6.2 μm are jointly retrieved between 100 mb and 0.3 mb. The retrieval scheme is discussed with reference to NO2 and a qualitative comparison made with LIMS data. The ISAMS Northern hemisphere NO2 data for January 9th 1992 are then examined and the importance of dynamical factors highlighted.