New technique to track aerosol spread more accurately

March 13th, 2009 - 2:51 pm ICT by IANS  

Washington, March 13 (IANS) Aerosols, those tiny, ubiquitous particles in the air, may profoundly affect global climate. But scientists have long struggled to measure their composition, size and global distribution accurately.
A new detection technique and a new satellite instrument (Glory) developed by NASA scientists, the Aerosol Polarimetry Sensor (APS), should help ease the struggle.

Some types of small aerosols - such as black carbon from motor vehicle exhaust and biomass burning - promote atmospheric warming by absorbing sunlight.

Others, like sulphates from coal-fired power plants, exert a cooling effect by reflecting incoming solar radiation back into space. Overall, aerosols present one of the greatest areas of uncertainty in understanding what drives climate change.

But quantifying the influence of aerosols on the atmosphere and climate has been hampered by difficulties in measuring the aerosols themselves.

The problem is especially acute over land, where the glare from sunlight reflecting off Earth’s surface overwhelms the passive imaging instruments scientists typically use to detect aerosols.

In recent years, however, researchers from NASA Goddard’s Institute for Space Studies (GISS) in New York City have developed new remote-sensing techniques to more accurately measure aerosols over land.

Research Scanning Polarimeter (RSP), an aircraft-based version of APS, is the first such instrument to measure polarized light at one particularly important wavelength (2.2 micrometres).

“The 2.2 micrometre channel is critical because it provides the only passive method we have to retrieve accurate and detailed aerosol properties over land surfaces,” said Michael Mishchenko, Glory’s project scientist.

RSP uses crystal prisms to effectively filter out the bright glare from earth’s surface, functioning somewhat like polarised sunglasses by only allowing light waves oriented in specific directions to pass.

According to Brian Cairns, aerosol climatologist at GISS who has pioneered the technique, polarised images have a dull tone that make the subtle hues of aerosols easier to detect amidst the shades of grey land.

“The blueish tint of small aerosols in the atmosphere is more clearly distinguishable with polarized light,” Cairns said, according to a GISS release.

The results were presented in the Journal of Geophysical Research.

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