Re: measuring the mass of the volcanic dust in the atmosphere

Joe,

Clearly this is a question which has gotten a lot of attention this summer!

   The lasers are used in Light Detection and Ranging (LIDAR), which is the
same idea as Radiofrequency Detection and Ranging (RADAR): bounce
electromagnetic waves off of stuff in the atmosphere (cloud and rain
droplets for RADAR, aerosol particles and cloud droplets for LIDAR), and
measured how much is bounced back. For particles with identical properties,
the light scattered back to the detector (the 'backscatter') will be
proportional to concentration.

   However, converting from backscatter to concentration requires several
pieces of additional information about the particles. You need to know:

Absorption: when light strikes the particles, it will be scattered,
absorbed, or it will just pass through. Soot and ash contain a lot of
highly light-absorbing material, and will scatter proportionally less than,
for instance sulfate particles from burning coal.

Particle size: visible light has wavelengths of roughly half a micron
(400-700 nano-meters). Dust and ash particles are generally in this size
range, or somewhat larger, up to about 10 microns. Because the scattering
varies with the relationship between the wavelength of light and the
particle size, multiple wavelengths are often used for particle size
determination.

Particle shape: particles that have a water component will be spherical,
but dry particles will have variable non-spherical shapes. One method to
analyze particle shape is to analyze the polarization of the light
scattered off the particles.

Refractive index: this is a property of the material in the particles that
affects how they interact with light. It is difficult to measure outside of
the laboratory.

   Attempting to get all of this information from a single instrument is a
daunting task. This is why direct measurements use a suite of instruments
on board an aircraft, and why satellite measurements use models to provide
at least a 'first guess' estimate of the aerosol particle properties that
can then be refined using the observations. And of course it is possible to
apply information on aerosol properties from aircraft to improve
measurements using satellite data.

   Getting precise numbers is a daunting task, but for aircraft operations
the most important information is the location and extent of the core part
of the plume, which can then be avoided.

Thanks for the great question!

--Edward Hyer.

Resources: Much of the best information on this topic is in dense academic
journals, but here are two Wikipedia pages covering the basic theory: http://en.wikipedia.org/wiki/Rayleigh_scattering
http://en.wikipedia.org/wiki/Mie_theory