Modified Ångström exponent for the characterization of submicrometer aerosols

Abstract

The classical Angström exponent is an operationally robust optical parameter that contains size information on all optically active aerosols in the field of view of a sunphotometer. Assuming that the optical effects of a typical (radius) size distribution can be approximated by separate submicrometer and supermicrometer components, we show that one can exploit the spectral curvature information in the measured optical depth to permit a direct estimation of a fine-mode (submicrometer) Angström exponent (alpha(f)) as well as the optical fraction of fine-mode particles (eta). Simple expressions that enable the estimation of these parameters are presented and tested by use of simulations and measurements.

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

“…This method infers about the AOD increase, caused by fine particle humidification or by the increase of the presence of coarse particles. The use of the spectral dependence of the Ångström exponent curvature give more insight on separating fine and coarse mode contributions to AOD (O'Neill et al 2001a (Valenzuela et al, 2012). Meanwhile days with the same conditions, but with  <-0.3 are dominated by the fine mode.…”

“…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.…”

Contrasting aerosol characteristics and radiative forcing over Hyderabad, India due to seasonal mesoscale and synoptic‐scale processes

“…The positive curvature is attributed to the increased contribution of the coarse aerosols in the near-infrared rather than that of the fine aerosols at shorter wavelengths (Schuster et al, 2006). Thus, despite the fact that the spectral AOD variation as well as the α values alone cannot give sufficient information about the particle size and CSD (O'Neill et al, 2001), the inversion method of King with the combination of the a 2 estimates are valuable tools in revealing the aerosol size distribution and the dominant types, as also revealed over Trivandrum, southernmost India (Beegum et al, 2009b).…”

Variations in the physicochemical and optical properties of natural aerosols in Puerto Rico – Implications for climate

“…The total aerosol optical depth δae is the integral in the vertical of σext,λ, δae is a non-dimensional extensive parameter. AERONET finds the fine fraction with the Spectral Deconvolution Algorithm (SDA; O'Neill, 2001O'Neill, , 2003. Also, AERONET uses the number size distributions to derive the volume size distribution through an inversion algorithm by Dubovik and King (2000).…”