Retrieving Aerosol Optical Thickness and Altitude of Asian Dust from AIRS Measurements

Abstract

Aerosol optical thickness (AOT) and effective height (zdust) of Asian dust were retrieved from infrared (IR) hyperspectral measurements. In order to examine how accurately dust properties can be retrieved from satellite-borne IR hyperspectral measurements, we first employed a statistical artificial neural network (ANN) method to retrieve the pixel-level AOT and zdust for Asian dust. This was done by relating the Atmospheric Infrared Sounder (AIRS) brightness temperatures for 234 channels, surface elevation, and relative air mass to target parameters (here, AOT and zdust) through an ANN model. Target data collocated with AIRS brightness temperature (TBs) were from the Moderate Resolution Imaging Spectroradiometer (MODIS)-derived AOT and the Cloud Aerosol Lidar Infrared Pathfinder Satellite Observation (CALIPSO)-derived zdust. Results show a correlation coefficient of 0.84 and mean bias of 0.03 for AOT, and a correlation coefficient of 0.79 and mean bias of -0.01 km for zdust, suggesting that dust retrievals from IR hyperspectral measurements are comparable to MODIS-derived AOTs and CALIPSO-estimated dust heights. The potential applicability of IR hyperspectral measurements to dust monitoring, demonstrated and substantiated by the ANN results, brings in a possibility of developing a physical retrieval algorithm for Asian dust. In doing so, the fast Radiative Transfer for Television Infrared Observation Satellite (TIROS) Operational Vertical Sounder (RTTOV) model (version 9.3) was implemented with an updated dust-size distribution and surface emissivity for better simulating the dust effect on top of atmosphere radiances. Also included were the refractive indices giving better Asian-dust simulation results, in comparison to AIRS-measured TBs. Adopting a one-dimensional variational (1DVAR) approach embedded with the implemented RTTOV model, a physical model has been developed for retrievals of Asian dust AOT and effective height. Results show that AIRS-derived IR AOTs are well correlated with the MODIS visible (VIS) AOTs, with correlation coefficients of 0.77–0.96. Effective dust heights also agree well with dust heights derived from CALIPSO backscattered reflectivity profiles. In conclusion, AOT and the effective height of Asian dust can be retrieved from IR hyperspectral measurements with accuracies comparable to MODIS AOT and CALIPSO dust height. Since those parameters can be retrieved on a two-dimensional basis in both day and night, IR hyperspectral sounder measurements can significantly improve dust forecasting through dust monitoring and data assimilation.