New aerosol observing technique turns gray skies blue

NASA: Tiny, ubiquitous particles in the atmosphere may play a profound role in regulating global climate. But the scientists who study these particles -- called aerosols -- have long struggled to accurately measure their composition, size, and global distribution. A new detection technique and a new satellite instrument 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, such as sulfates 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. The 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 micrometers). "The 2.2 micrometer 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 polarized sunglasses by only allowing light waves oriented in specific directions to pass. According to Brian Cairns, an aerosol climatologist at GISS who has pioneered the technique, polarized images have a dull tone that make the subtle hues of aerosols easier to detect amidst the shades of gray land….

Roughly 30 percent of the radiation from the sun is reflected back into space, some of it by tiny airborne particles called aerosols. This artist concept depicts the process. Credit: UMBC/Megan Willy