In-Situ Measurement of Particle Extinction


Anthony W. Strawa

Aerosol optical properties are extremely important in assessing climate change. Our lack of sufficient knowledge of aerosol optical properties and their variability in the atmosphere have led the Intergovernmental Panel on Climate Change (IPPC) to rate the effect of aerosol as the most uncertain of all parameters considered important to climate change. Currently, these aerosol properties are obtained from filter samples that measure absorption of black carbon aerosols on a time scale of tens of minutes to hours. Aerosol variability causes significant changes in optical properties on the order of seconds, especially when sampled from aircraft. Thus, the research community is very interested in an instrument that can measure the optical properties of all aerosols, not just black carbon, on a time scale of seconds.

The Cloud and Microphsyics Group at Ames is working with Informed Diagnostics, Inc. (ID) to develop an innovative instrument using cavity ring-down spectroscopy (CRDS) to measure the extinction and scattering coefficients of aerosol and consequently the single-scatter albedo. Our prototype instrument measures extinction and scattering coefficient at 690 nm and extinction coefficient at 1550 nm. The instrument itself is small (24 x 19 inches) and relatively insensitive to vibrations. The prototype instrument has been tested in our lab and used in the field. While improvements in performance are needed, the prototype has been shown to make accurate and sensitive measurements of extinction and scattering coefficients. Combining these two parameters, one can obtain the single-scattering albedo and absorption coefficient, both important aerosol properties. The use of two wavelengths also allows us to obtain a quantitative idea of the size of the aerosol through the Ångstrom exponent. Minimum sensitivity of the prototype instrument is 1.5x10-6 m-1 (1.5 mm-1). The prototype instrument demonstrates 1) fast and accurate measurement of aerosol extinction, 2) measurement of aerosol scattering in a CRDS system, and 3) simultaneous measurement at two laser wavelengths. The instrument proved capable of measuring the extinction of ammonium sulfate aerosol typical of the mid- to high-troposphere background aerosol. At the same time the scattering of the aerosol was measured, and an estimate of its single-scatter albedo could be made. We expect continued development of this technology will lead to a flight-ready instrument within the next two years.


Collaborators: T. Owano, BlueLeaf Networks, Inc.

Point of Contact: Anthony Strawa, 650/604-3437,


[prototype instrument]


[aerosol extinction coefficient plot]