Abstract
Abstract. Infrared (IR) retrievals of aerosol optical thickness (AOT) are challenging because of the low reflectance of aerosol layer at longer wavelengths. In this paper we present a closer analysis of this problem, performed with radiative transfer (RT) simulations for coarse and accumulation mode of four main aerosol components. It shows the strong angular dependence of aerosol IR reflectance at low solar elevations resulting from the significant asymmetry of aerosol phase function at these wavelengths. This results in detectable values of aerosol IR reflectance at certain non-nadir observation angles providing the advantage of multiangle remote sensing instruments for a retrieval of AOT at longer wavelengths. Such retrievals can be of importance e.g. in case of a very strong effect of the surface on the top of atmosphere (TOA) reflectance in the visible spectral range. In the current work, a new method to retrieve AOT of the coarse and accumulation mode particles over snow has been developed using the measurements of Advanced Along Track Scanning Radiometer (AATSR) on board the ENVISAT satellite. The algorithm uses AATSR channel at 3.7 μm and utilizes its dual-viewing observation technique, implying the forward view with an observation zenith angle of around 55 degrees and the nadir view. It includes cloud/snow discrimination, extraction of the atmospheric reflectance out of measured brightness temperature (BT) at 3.7 μm, and interpolation of look-up tables (LUTs) for a given aerosol reflectance. The algorithm uses LUTs, separately simulated with RT forward calculations. The resulting AOT at 500 nm is estimated from the value at 3.7 μm using a fixed Angström parameter. The presented method has been validated against ground-based Aerosol Robotic Network (AERONET) data for 4 high Arctic stations and shows good agreement. A case study has been performed at W-Greenland on 5 July 2008. The day before was characterized by a noticeable dust event. The retrieved AOT maps of the region show a clear increase of AOT in the Kangerlussuaq area. The area of increased AOT was detected on 5 July on the ice sheet east of Kangelussuaq, opposite to the observed north easterly wind at ground level. This position can be explained by a small scale atmospheric circulation transporting the mobilized mineral dust upslope, after its intrusion into the upper branch of the circulation. The performed study of atmospheric reflectance at 3.7 μm also shows possibilities for the detection and retrievals of cloud properties over snow surfaces.
Highlights
The large surface spectral reflectance poses significant challenges for the retrieval of aerosol optical thickness (AOT) above snow from observations of the upwelling back scattered radiation at the top of the atmosphere in the visible spectral range
It is known that Advanced Along Track Scanning Radiometer (AATSR) brightness temperature (BT) product measured at 3.7 μm is contaminated by atmospheric reflectance
The simulation shows that the sensitivity of retrieved AOT to the AATSR BT calibration error of ±0.1 K is around several percent
Summary
The large surface spectral reflectance poses significant challenges for the retrieval of AOT above snow from observations of the upwelling back scattered radiation at the top of the atmosphere in the visible spectral range. In order to resolve the uncertainty of aerosol amount, transport and influence on climate, significant temporal and spatial coverage is needed This can only be achieved using active or passive measurements made from orbiting platforms. In addition a wide variety of snow types and temporal evolution of snow properties cause variation of snow bidirectional reflection function (BRDF) Attempts to remove this large unknown surface effect from the TOA reflectance by using an assumed snow model, lead to strong surface contamination of resulting AOT (Istomina et al, 2009), because the assumed surface model cannot account for the existing BRDF variability. Spangenberg et al (2001) discussed the influence of infrared atmospheric reflectance onto AATSR brightness temperature (BT) at 3.7 μm with respect to the problem of cloud detection over snow This idea has been successfully applied for cloud screening over snow using AATSR channel at 3.7 μm in combination with other AATSR channels (Istomina et al, 2010b).
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