Inventing an Instrument
ESRL scientists need extremely precise instruments to answer critical questions about the atmosphere. Much of the time, there is no catalogue to order from, so ESRL researchers and engineers do the instrument design and construction themselves. Below, we describe one pilot instrument in ESRL’s Chemical Sciences Division: Aerosol Scattering To Extinction Ratio (ASTER). Aerosol comprises tiny particles in the atmosphere with both natural and anthropogenic sources and are known to have climate impacts. Overall, atmospheric aerosol cools the atmosphere, partially offsetting warming by greenhouse gases. However, certain classes of aerosol—such as black carbon or soot—can warm the surrounding atmosphere and are of growing interest to climate scientists. A particle’s “radiative effect” depends, in part, on how it scatters (a cooling effect) and absorbs (warming) sunlight.
Better understanding the behavior of atmospheric aerosol is critical for improving our predictions of how climate change will affect the Earth system. “In climate models, the uncertainty around aerosol effects is usually very large,” said Todd Sanford, who, with Daniel Murphy, is leading ASTER’s development. “The idea of this instrument is to help reduce that uncertainty.”
Today, scientists use a variety of instruments to measure how aerosol scatters and absorbs light. Each instrument has its own set of potential small errors, and there may also be differences between aerosol samples measured in each. That means multiple sets of errors are possible in measurements.
ASTER measures a single aerosol particle’s light scattering and extinction (extinction = scattering + absorption) simultaneously. From this, a scientist can calculate the climaticallyimportant metric of single-scattering albedo. A diode laser sends light into a three-mirror, high reflectivity cavity. Single aerosol particles are introduced into the cavity laser beam, from which they scatter and absorb light. A scattering cell captures and measures scattered light, and extinction is measured from the “leakage” of light through a cavity mirror. The instrument can also provide a measure of a particle’s size.
Waiting for a flight after a field mission in Costa Rica three years ago, ESRL’s Dan Murphy asked Sanford about an instrument Sanford was using to measure single-scattering albedos of single particles. Murphy asked if Sanford ever observed “dips” in the light transmitted through the cavity mirrors. “Actually, we do,” Sanford remembers saying. “And that was it. Until then, we didn’t appreciate that this cavity could measure extinction in a more direct, simpler way.” This development simplified the operation of ASTER, and will make field-deployment more feasible.
ASTER’s diode laser may be rugged, but it’s signal is noisy. Sanford stabilizes the laser by using the cavity as a stable, frequency reference, in a technique called Pound-Drever-Hall frequency stabilization.
In a test of air pulled from outside ESRL, where a nearby construction project produces dust and engine exhaust, Sanford found that most of the sampled particles were purely scattering (cooling), but a small fraction were dark and more absorbing. A more traditional “bulk” measurement could have concluded only that the aerosols were primarily scattering and therefore cooling. “The bulk measurements are very useful, but important information is lost in the averaging process,” Sanford said. “We would have missed these highly absorbing particles, which are very important for climate.”
Sanford hopes to test ASTER in the field sometime this year, in the foothills behind the David Skaggs Research Center in Boulder, Colo. He also wants to compare the instrument’s measurements with existing aerosol optical and composition instruments. With luck, the instrument could be deployed in a ground-based setting during CalNex, a 2009-2010 mission to study climate change and air quality in California. Ultimately, ASTER could fly on research aircraft.