Abstract
Abstract. Altitude-resolved aerosol detection in the upper troposphere and lower stratosphere (UTLS) is a challenging task for remote sensing instruments. Infrared limb emission measurements provide vertically resolved global measurements at day- and nighttime in the UTLS. For high-spectral-resolution infrared limb instruments we present here a new method to detect aerosol and separate between ice and non-ice particles. The method is based on an improved aerosol–cloud index that identifies infrared limb emission spectra affected by non-ice aerosol or ice clouds. For the discrimination between non-ice aerosol and ice clouds we employed brightness temperature difference correlations. The discrimination thresholds for this method were derived from radiative transfer simulations (including scattering) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS)/Envisat measurements obtained in 2011. We demonstrate the value of this approach for observations of volcanic ash and sulfate aerosol originating from the Grímsvötn (Iceland, 64° N), Puyehue–Cordón Caulle (Chile, 40° S), and Nabro (Eritrea, 13° N) eruptions in May and June 2011 by comparing the MIPAS volcanic aerosol detections with Atmospheric Infrared Sounder (AIRS) volcanic ash and SO2 measurements.
Highlights
Aerosol is omnipresent and highly variable in the atmosphere
For the selection of an appropriate window, which cannot be directly adopted from IR nadir measurements due to the strong trace gas emission lines measured in the IR limb geometry, we considered Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) clear air radiance profiles between about 7 and 25 km altitude
In order to demonstrate the benefits of the aerosol cloud index (ACI) and to derive a fixed threshold value that is applicable to MIPAS measurements, we investigated the behaviour of simulated and measured ACI profiles and compared them with the corresponding CI profiles
Summary
Aerosol is omnipresent and highly variable in the atmosphere. In the upper troposphere and lower stratosphere (UTLS) a large variety of aerosol particles, comprising sulfate droplets, volcanic ash, mineral dust, wild fire aerosol, organic material, and meteoritic dust, has been found Studies using high-resolution IR spectra of the spaceborne IR limb instruments Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) (Offermann et al, 1999; Riese et al, 1999) and MIPAS (Fischer et al, 2008) present methods to detect PSCs and distinguish between the three PSC types (ice, supercooled ternary solutions, and nitric acid trihydrate) (Spang and Remedios, 2003; Spang et al, 2004, 2005; Höpfner et al, 2009; Spang et al, 2012) These studies are restricted to the stratosphere and do not tackle the discrimination between aerosol and ice clouds in the troposphere. The spectral sampling is 0.0625 cm−1 and the vertical sampling is 0.5 km
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