The mineral aerosol profiling from infrared radiances version 5.1 algorithm and its evaluation

Mineral (desert) dust aerosols are small sand/dust particles entrained by winds from bare areas and possibly transported over long distances. These aerosols are climate forcers and affect human health and many socio-economic sectors. They are therefore important to monitor both in near-real time and on the long term. In this work, the Infrared Atmospheric Sounding Interferometer (IASI) instrument is used to retrieve vertical profiles of mineral dust aerosols concentration, from which a 10 µm aerosol optical depth (AOD) and a mean aerosol altitude are obtained. In addition, the dust AOD is converted to 550 nm using the same aerosol optical properties as in the retrieval. The theoretically calculated conversion factor is 1.78 (the 550 nm AOD being larger than the 10 µm AOD). This conversion step adds significant uncertainties due to different sensitivity at the different wavelengths. Specifically, we present here the new version 5.1 of the Mineral Aerosol Profiling from Infrared Radiances (MAPIR) algorithm and its changes with respect to previous versions. MAPIR v5.1 was used to produce a consistent time series of dust profiles since the start of the IASI observations in 2007 and until now, using data from IASI onboard Metop-A and Metop-C. The capabilities of the instrument and retrieval are illustrated, showing good event detection, expected AOD seasonal cycles, good profiling capabilities and reasonable mean aerosol altitude, good time and cross-platform consistency. A true validation exercise is not possible as there exist no reference aerosols data from thermal infrared measurements (around 10 µm). Therefore, the absolute value of the obtained AOD can not be validated, although the best possible evaluation is provided using data obtained in the visible spectral range. The evaluation shows a strong correlation of respectively about 0.8 and 0.7 between MAPIR dust AOD and AERONET 550 nm coarse mode AOD solar and lunar observations. The MAPIR converted 550 nm AOD shows a small median bias with respect to AERONET, but this bias varies with the AOD, suggesting an underestimation of the AOD conversion factor.