The Infrared Atmospheric Sounding Interferometer (IASI) forms the main infrared sounding component of the European Organisation for the Exploitation of Meteorological Satellites's (EUMETSAT's) Meteorological Operation (MetOp)-A satellite (Klaes et al. 2007), which was launched in October 2006. This article presents the results of the first 4 yr of the operational IASI mission. The performance of the instrument is shown to be exceptional in terms of calibration and stability. The quality of the data has allowed the rapid use of the observations in operational numerical weather prediction (NWP) and the development of new products for atmospheric chemistry and climate studies, some of which were unexpected before launch. The assimilation of IASI observations in NWP models provides a significant forecast impact; in most cases the impact has been shown to be at least as large as for any previous instrument. In atmospheric chemistry, global distributions of gases, such as ozone and carbon monoxide, can be produced in near–real time, and short-lived species, such as ammonia or methanol, can be mapped, allowing the identification of new sources. The data have also shown the ability to track the location and chemistry of gaseous plumes and particles associated with volcanic eruptions and fires, providing valuable data for air quality monitoring and aircraft safety. IASI also contributes to the establishment of robust long-term data records of several essential climate variables. The suite of products being developed from IASI continues to expand as the data are investigated, and further impacts are expected from increased use of the data in NWP and climate studies in the coming years. The instrument has set a high standard for future operational hyperspectral infrared sounders and has demonstrated that such instruments have a vital role in the global observing system.

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[1] Lidar observations of volcanic ash are reported, that have been obtained during six flights of the Facility for Airborne Atmospheric Measurements BAe-146 research aircraft over the United Kingdom and the surrounding seas in May 2010, after the eruption of Eyjafjallajökull. Due to safety restrictions, sampling has only been done in areas where forecasted concentrations were smaller than 2000 mg/m 3 . Aircraft in situ measurements of size-distribution permitted evaluation of a coarse extinction fraction (ranging 0.5-1) and a coarse mode specific extinction (0.6-0.9 m 2 /g) for each flight. These quantities were then used to convert the lidar-derived aerosol extinction to ash concentration (with an estimated uncertainty of a factor of two). The data highlight the very variable nature of the ash plume in both time and space, with layers 0.5-3 km deep observed between 2 and 8 km above sea level, and featuring an along-track horizontal extent of 85-550 km. Flights on 14-17 May showed typical concentrations of 300-650 mg/m 3 , and maxima of 800-1900 mg/m 3 in relatively small high density patches. Column loads for these flights were typically 0.25-0.5 g/m 2 (maxima 0.8-1.3 g/m 2 ). Relatively small regions characterized by a larger ash content have been selected, and the distribution of ash concentrations and column loadings within them proved rather broad, showing how fractal and patchy the observed ash layers are. A visual comparison of our data set with the "dust RGB" maps from SEVIRI showed a good spatial correlation for the larger ash content days. Moreover, ash prediction maps output from the NAME dispersion model show reasonable agreement with the overall magnitude of the observed concentrations; in some cases, however, there are positional errors in the predicted plume location, due to uncertainties in the eruption source details, driving meteorology, and in the model itself.

[1] In situ measurements of terrestrial radiation from the C-130 aircraft during the Saharan Dust Experiment (SHADE) are used to quantify the effect of a strong dust outbreak on radiance and brightness temperatures. The dust gives a distinct spectral signature in upwelling and downwelling terrestrial radiation when high spectral resolution data for a dusty day is compared to data from a clear day. A radiative transfer model is used together with a size distribution retrieved from Sun photometers and atmospheric profiles from dropsondes to simulate the radiance data and provide a constraint on the refractive indices of Saharan dust in the terrestrial part of the spectrum. The degree of agreement between observed and simulated brightness temperatures is dominated by the choice of refractive index, the mass loading, and the altitude of the dust layer. The uncertainties in size distribution appear to have less of an effect so long as large particles (radius greater than 1 mm) are included. In the terrestrial spectrum the dust produced a relative warming rate of up to 0.5 K/day below the dust and a relative cooling of up to 0.5 K/day within the dust layer itself. The effect on irradiance due to this dust outbreak was a decrease in upwelling terrestrial radiation at the top of the atmosphere of 6.5 Wm À2and an increase in downwelling terrestrial radiation at the surface of 11.5 Wm À2. The dust led to decreases in brightness temperature of 2-4 K in the window region, consistent with apparent features in the sea surface temperature retrieved from the advanced very high resolution radiometer.

The two experimental techniques of cavity ringdown spectroscopy and high-resolution, long-path Fourier transform spectroscopy have been used to measure quantitative absorption spectra and determine the integrated absorption intensity (Sint,B) for the O2 a 1Δg–X 3Σg− (0,0) band. Einstein A-factors and radiative lifetimes for the O2 a 1Δg v=0 state have been derived from the Sint,B values. The two methods give values for the integrated absorption intensity that agree to within 2%. The value recommended from the results of this study is Sint,B=3.10±0.10×10−24 cm molecule−1, corresponding to an Einstein-A coefficient of A=2.19±0.07×10−4 s−1 and a radiative lifetime of τrad=76 min. The measurements are in excellent agreement with the recent absorption study of Lafferty et al. [Appl. Opt. 37, 2264 (1998)] and greatly reduce the uncertainty in these parameters, for which accurate values are required for determination of upper stratospheric and mesospheric ozone concentrations.