Validation of satellite thermal infrared dust aerosol optical depth : challenges and results

Mineral dust is a major component of tropospheric aerosols, and plays a significant role in the Earth’s energy balance (both directly and indirectly through impact on clouds), air quality, health, transportation, and solar energy. Although being natural particles, their emission mechanisms are linked to the ground surface state (vegetation cover, soil type and moisture) and the surface winds, which are all impacted by human activities and climate change. The radiative impact of dust depends on many parameters, such as total amount, vertical distribution, particle size and composition.

The Mineral Aerosol Profiling from Infrared Radiances (MAPIR) algorithm focuses on retrieving dust total amount (in terms of Aerosol Optical Depth or AOD) and vertical distribution from satellite observations in the thermal infrared (TIR), by the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument. As far as aerosols are concerned, the TIR spectral range has the advantage of being sensitive almost exclusively to mineral aerosols (with specific absorption around 10µm), making it intrinsically specific to mineral dust and volcanic ash.

We have recently developed a new version of the MAPIR algorithm (version 5.1), requiring validation. As there is currently no validation data obtained in the thermal infrared, we can only use aerosol data obtained in the solar spectral range. Comparing data obtained with totally different sensors, using a different spectral range (therefore sensitive to different types of particles) and different types of algorithms includes significant challenges. In this contribution, we will discuss those challenges. We will show comparisons of MAPIR dust AOD with AErosol RObotic NETwork (AERONET) Standard Deconvolution Algorithm (SDA) coarse mode AOD, and focus on some specific events where we observe a large difference. For those events, additional satellite data (e.g. Cloud-Aerosol Lidar with Orthogonal Polarization - CALIOP, MODerate resolution Imaging Spectroradiometer - MODIS) will be used for in-depth analysis of the event and investigating the plausible causes for discrepancy.