Spectral discrimination of coarse and fine mode optical depth

[1] The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths (tf and tc) from the spectral shape of the total aerosol optical depth (ta = tf + tc). This purely optical technique avoids intermediate computations of the PSD and yields a direct optical output that is commensurate in complexity with the spectral information content of ta. The separation into tf and tc is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of ta. Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that tc variation is coherent with photographic evidence of thin cloud events and that tf variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of tf and tc are wellcorrelated, if weakly biased, relative to formal inversions of combined solar extinction and sky radiance data. The spectral inversion technique permitted a closer scrutiny of a standard (temporally based) cloud-screening algorithm. Perturbations of monthly or longer-term statistics associated with passive or active shortcomings of operational cloud screening were inferred to be small to occasionally moderate over a sampling of cases. Diurnal illustrations were given where it was clear that such shortcomings can have a significant impact on the interpretation of specific events; (1) commission errors in tf due to the exclusion of excessively high-frequency fine mode events and (2) omission errors in tc due to the inclusion of insufficiently high-frequency thin homogeneous cloud events. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; 4801 Oceanography: Biological and Chemical: Aerosols (0305); KEYWORDS: Sun photometry, aerosol, optical depth, fine mode, coarse mode, cloud screening

[1]  M. McCormick,et al.  Validation of Stratospheric Aerosol and Gas Experiments I and II satellite aerosol optical depth measurements using surface radiometer data , 1994 .

[2]  Andrew J. Heymsfield,et al.  Cirrus Cloud Radiative and Microphysical Properties from Ground Observations and In Situ Measurements During FIRE 1991 and Their Application to Exhibit Problems in Cirrus Solar Radiative Transfer Modeling , 1997 .

[3]  A. Smirnov,et al.  AERONET-a federated instrument network and data archive for aerosol Characterization , 1998 .

[4]  D. Tanré,et al.  Retrieval of aerosol optical thickness and size distribution over ocean from the MODIS airborne simulator during TARFOX , 1999 .

[5]  T. Eck,et al.  Wavelength dependence of the optical depth of biomass burning, urban, and desert dust aerosols , 1999 .

[6]  Alexander Smirnov,et al.  Cloud-Screening and Quality Control Algorithms for the AERONET Database , 2000 .

[7]  Michael D. King,et al.  A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements , 2000 .

[8]  T. Eck,et al.  Modified angström exponent for the characterization of submicrometer aerosols. , 2001, Applied optics.

[9]  T. Eck,et al.  Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space , 2001 .

[10]  T. Eck,et al.  Variability of Absorption and Optical Properties of Key Aerosol Types Observed in Worldwide Locations , 2002 .

[11]  T. Eck,et al.  Optical Properties of Atmospheric Aerosol in Maritime Environments , 2002 .