Infrared dust aerosol optical depth retrieved daily from IASI and comparison with AERONET over the period 2007–2016

Abstract

Aerosols represent one of the dominant uncertainties in radiative forcing, partly because of their very high spatiotemporal variability, a still insufficient knowledge of their microphysical and optical properties, or of their vertical distribution. A better understanding and forecasting of their impact on climate therefore requires precise observations of dust emission and transport. Observations from space offer a good opportunity to follow, day by day and at high spatial resolution, dust evolution at global scale and over long time series. Infrared observations allow retrieving dust aerosol optical depth (AOD) as well as the mean dust layer altitude, daytime and nighttime, over oceans and over continents, in particular over desert. Moreover, coarse mode particles are preferentially observed in the infrared, when, in the visible, both larger and finer particles are observed making the distinction between the two modes difficult. Therefore, they appear complementary to observations in the visible. In this study, a decade of the Infrared Atmospheric Sounder Interferometer (IASI) on board European Satellite Metop-A observations, from July 2007 to December 2016, has been processed pixel by pixel, using a “Look-Up-Table” (LUT) physical approach. Important improvements have been brought to our former approach in order to extend it to: 1) daytime retrieval, 2) mid-latitude retrieval, 3) retrieval at the IASI pixel resolution, 4) near real time retrieval (day-1). Moreover, over continents, surface characteristics (pressure, temperature, as well as emissivity spectrum) are now better accounted for. Here, a particular attention is given to the validation of the IASI-retrieved AOD through comparisons with the Spectral Deconvolution Algorithm (SDA) 500nm coarse mode AOD observed at 70 ground-based Aerosol RObotic NETwork (AERONET) sites during the 114months processed. Even if such a comparison requires converting AOD from infrared to visible, inherently leading to significant uncertainties, the two AOD datasets compare well, with an overall correlation of 0.8. For a large majority of sites, correlation ranges from 0.7 to 0.9. Sites with highest correlation are well distributed within the “dust belt” (Sahara, Arabian Peninsula, Mediterranean basin, India and also the Caribbean). Correlations obtained for East-Asia are in general smaller, which might be due to a more complex dust structure (i.e., impact of pollution) and partly due to an increase of the AERONET coarse-mode AOD uncertainty. More generally, the good overall agreement between our restitutions and AERONET AOD demonstrates the ability of infrared sounders to infer dust properties, which opens interesting perspective for a synergy with visible observations.