Remote sensing of volcanic ash and sulfur dioxide - Volcanism and Global Environmental Change - page 104

optical properties of volcanic ash. These data feed directly into Mie scattering
models that provide the relevant optical parameters viz. the asymmetry parameter,
the single-scattering albedo and extinction ef ciencies that are needed as input to

the radiative transfer codes.

7.3.4 Satellite retrievals
Infrared remote sensing of volcanic ash was

first developed by Prata ( 1989 ) who
recognised that two channels within the wavelength region 8 m could be used

to discriminate volcanic ash from meteorological water/ice clouds. Subsequent
work by Wen and Rose ( 1994 ) showed how to determine mass loadings and
effective particle size by using microphysical models and the optical properties
of idealised (spherical) ash particles. Further re

- 13 μ

nements and improvements have
been made by Prata and Grant ( 2001 ), Pavolonis et al .( 2006 ), Pavolonis ( 2010 ),
Clarisse et al .( 2010 ) and Prata and Prata ( 2012 ). The determination of mass
loading and effective particle radius relies essentially on two pieces of independent
information or observations: the brightness temperatures at two wavelengths

(most often at 11 a microphysical

model and a radiative transfer model. Different schemes have been proposed to
invert the brightness temperature measurements into the required geophysical
parameters and here we give a graphical explanation of the retrieval of ash from
IR measurements.

μ m and 12 μ m); and two a priori constraints -

Methods
The panels of Figure 7.3 show the brightness temperatures 2 at 11 μ m( T 11 ) and 12
μ ΔT¼T 11 - T 12 , for a MODIS (MODerate

resolution Imaging Spectrometer) Terra image acquired on 8 May 2010 during the
eruption of Eyjafjallajökull. There is a plume emanating from Eyjafjallajökull
dispersing southwards that can be seen in the T 11 and T 12 images and more clearly
in the difference image ( Figure 7.3c ). Figure 7.3d shows a retrieval of mass loading
based on these data, a microphysical model of the ash and a radiative transfer
model. Note that the retrieval is shown for just three levels: 0.2, 2 and 4 g m 2 .
The mass loading (and concentration for a 1-km-thick cloud) can be determined
from the following simpli

m( T 12 ) and the temperature difference,

ed expression:

4

3 ρ τðλÞ

r e
Q ext ðλÞ ,

m l ¼ ð 7 : 3 Þ

2 Brightness temperature is de

ned as the temperature obtained when using the monochromatic measured
radiance in the inverse Planck function.