Spectral discrimination of coarse and fine mode optical depth

Abstract

[1] The recognition that the aerosol particle size distribution (PSD) is effectively bimodal permits the extraction of the fine and coarse mode optical depths (t f and t c ) from the spectral shape of the total aerosol optical depth (t a = t f + t c ). 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 t a . The separation into t f and t c is a robust process and yields aerosol optical statistics, which are more intrinsic than those, obtained from a generic analysis of t a . Partial (optical) validation is provided by (1) demonstrating the physical coherence of the simple model employed, (2) demonstrating that t c variation is coherent with photographic evidence of thin cloud events and that t f variation is coherent with photographic evidence of clear sky and haze events, and (3) showing that the retrieved values of t f and t c 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 t f due to the exclusion of excessively high-frequency fine mode events and (2) omission errors in t c due to the inclusion of insufficiently high-frequency thin homogeneous cloud events.

Origin and pathways of the mineral dust transport to two Spanish EARLINET sites: Effect on the observed columnar and range-resolved dust optical properties

“…However, AE alone does not provide unambiguous information on the relative weight of coarse and fine modes on the AOD. Several authors have discussed how the spectral variation of the Ångström exponent can provide further information about the aerosol size distribution (Eck et al, 1999;O'Neill et al, 2001aO'Neill et al, , b, 2003Fernández-Gálvez et al, 2013). Kaufman (1993) shows that negative values of the difference =AE 440-613 -AE indicate the dominance of fine mode aerosols while positive values of this difference indicate the effect of two separate particle modes.…”

Role of Coarse and Fine Mode Aerosols in MODIS AOD Retrieval: a case study over southern India

“…The size distribution should be ideally split into two parts from centre of the dip. However, we have used a fixed radius of 0.6 µm to separate fine mode and coarse mode AOD as 0.6 µm is the closest to the dip for most seasons and also it is the convention followed for AERONET data (O'Neill et al, 2003). AODs are recalculated for each part separately using the Mie algorithm (Bohren and Huffman, 1983), and the refractive indices and size-distribution retrieved by SKYRAD.…”

“…Location of Gadanki is shown with a white star on the map. Air-masses coming from the northeastern direction via the Bay of Bengal are expected to be rich in anthropogenic aerosols whereas the air-masses coming from the Indian Ocean are expected to be rich in natural aerosols (Corrigan et al, 2006;Jayaraman et al, 2006;Suresh Babu et al, 2007;Rajeev et al, 2010).…”

“…Methods exist that can derive these two parameters from spectral observation of AOD and sky-radiances from ground-based sky-radiometer (Nakajima et al, 1996;Dubovik and King, 2000;O'Neill et al, 2003). However, both the size distribution and the refractive index of aerosol are highly variable in space and time.…”

Using spectral methods to obtain particle size information from optical data: applications to measurements from CARES 2010

“…For example, an additional level of refinement of wavelength-dependent measurements of aerosol optical depth (path-integrated extinction) was introduced by to aid in the interpretation of the data obtained by the ground-based sunphotometer networks AERONET and AEROCAN. Specifically, O'Neill et al (2003 showed that the fine-mode fraction (FMF) of extinction and the fine-mode effective radius (R eff,f ) could be extracted directly from the multi-wavelength optical depth or extinction measurements available from remote sensing. The FMF provides for an approximate discrimination between what are typically naturally produced coarse-mode particles (dust or sea spray) and what are often anthropogenically associated fine-mode particles.…”