High temporal resolution estimates of columnar aerosol microphysical parameters from spectrum of aerosol optical depth by linear estimation: application to long-term AERONET and star-photometry measurements
2015
This work deals with the applicability of the linear estimation technique (LE) to invert spectral measurements of aerosol optical depth (AOD) provided by AERONET CIMEL sun photometers. The inversion of particle proper- ties using only direct-sun AODs allows the evaluation of pa- rameters such as effective radius (reff/ and columnar volume aerosol content (V/ with significantly better temporal reso- lution than the operational AERONET algorithm which re- quires both direct sun and sky radiance measurements. Sen- sitivity studies performed demonstrate that the constraints on the range of the inversion are very important to minimize the uncertainties, and therefore estimates ofreff can be obtained with uncertainties less than 30 % and ofV with uncertainties below 40 %. The LE technique is applied to data acquired at five AERONET sites influenced by different aerosol types and the retrievals are compared with the results of the opera- tional AERONET code. Good agreement between the two techniques is obtained when the fine mode predominates, while for coarse mode cases the LE results systematically underestimate both reff and V. The highest differences are found for cases where no mode predominates. To minimize these biases, correction functions are developed using the multi-year database of observations at selected sites, where the AERONET retrieval is used as the reference. The derived corrections are tested using data from 18 other AERONET stations offering a range of aerosol types. After correction, the LE retrievals provide better agreement with AERONET for all the sites considered. Finally, the LE approach de- veloped here is applied to AERONET and star-photometry measurements in the city of Granada (Spain) to obtain day- to-night time evolution of columnar aerosol microphysical properties.
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