Remote sensing of aerosols over land surfaces from POLDER-ADEOS-1 polarized measurements

The polarization measurements achieved by the POLDER instrument on ADEOS-1 are used for the remote sensing of aerosols over land surfaces. The key adv antage of using polarized observations is their ability to systematically c orrect for the ground contribution, whereas the classical approach using na tural light fails. The estimation of land surface polarizing properties fro m POLDER has been examined in a previous paper. Here we consider how the op tical thickness delta (0) and Angstrom exponent alpha of aerosols are deriv ed from the polarized light backscattered by the particles. The inversion s cheme is detailed, and illustrative results are presented. Maps of the retr ieved optical thickness allow for detection of large aerosol features, and in the case of small aerosols, the delta (0) and alpha retrievals are consi stent with correlative ground-based measurements. However, because polarize d light stems mainly from small particles, the results are biased for aeros ol distributions containing coarser modes of particles. To overcome this li mitation, an aerosol index defined as the product AI = delta (0)alpha is pr oposed. Theoretical analysis and comparison with ground-based measurements suggest that AI is approximately the same when using delta (0), and alpha i s related to the entire aerosol size distribution or derived from the polar ized light originating from the small polarizing particles alone. This inva riance is specially assessed by testing the continuity of AI across coastli nes, given the unbiased properties of aerosol retrieval over ocean. Althoug h reducing the information concerning the aerosols, this single parameter a llows a link between the POLDER aerosol surveys over land and ocean. POLDER aerosol index global maps enable the monitoring of major aerosol sources o ver continental areas.