University of Maryland Visibility Database. Visibility in the UM study was the distance a meteorological observer could see clearly from the measurement source. The more aerosols present in the air, the shorter the visibility distance. Using the new database,the researchers show that clear sky visibility over land has decreased globally over the past 30 years, indicative of an increase in aerosols, or airborne particulates, over the world’s continents during that time.

Creation of this database is a big step forward for researching long-term changes in air pollution and correlating these with climate change. And it is the first time we have gotten global long-term aerosol information over land to go with information already available on aerosol measurements over the world’s oceans.

—Professor Kaicun Wang, University of Maryland and lead author of the paper

The researchers compared the visibility data to available satellite data (2000-2007), and found it to be comparable as an indicator of aerosol concentration in the air. Thus, they conclude, the visibility data provide a valid source from which scientists can study correlations between air pollution and climate change.

Aerosol particles—including pollutants such as soot, dust and sulfur dioxide particles—affect the Earth’s surface temperature by either reflecting light back into space, thus reducing solar radiation at Earth’s surface,or absorbing solar radiation,thus heating the atmosphere. The variable cooling and heating effects of aerosols also modify properties of cloud cover and rainfall.

Unlike aerosol particles, carbon dioxide and other greenhouse gases are clear and have no effect on visibility. Sunlight passes right through them, just it does through the oxygen and nitrogen, which are the main constituents of our atmosphere. While the climate warming impacts of increased greenhouse gases are clear, the effects of increased aerosols are not. Studies of the long-term effects of aerosols on climate change have been largely inconclusive up to now due to limited over-land aerosol measurements, according to Wang and his team. However, with this database researchers now can compare temperature, rainfall and cloud cover data from the past 35 years with the aerosol measurements in the new database.

According to the authors, a preliminary analysis of the database measurements shows a steady increase in aerosols over the period from 1973 to 2007. Increased aerosols in the atmosphere block solar radiation, and have thus caused a net “global dimming.” The only region that does not show an increase in aerosols is Europe, which has actually experienced a global brightening, the authors say.

The largest known source of increased aerosols is increased burning of fossil fuels. And a major product of fossil fuel combustion is sulfur dioxide. Thus, the team notes, that their finding of a steady increase in aerosols in recent decades, also suggests an increase in sulfate aerosols. This differs from studies recently cited by the Intergovernmental Panel Climate Change showing global emissions of sulfate aerosol decreased between 1980 and 2000.

NASA technique. Overall, aerosols present one of the greatest areas of uncertainty in understanding what drives climate change. 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.

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, according to Michael Mishchenko, Glory’s project scientist. (Earlier post.)

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.

The new retrieval technique also integrates more information about short-wave polarized light into models used to determine aerosol properties. Past approaches have left short-wave polarized light—which is crucial for calculating how high aerosols reside in the atmosphere and how much radiation the particles are absorbing&mash;largely out of the equation.

RSP is not the first instrument to use polarimetry to measure aerosols. In 1996, a series of three French satellite instruments started measuring aerosols with polarized light. Cairns’ technique measures polarization more accurately, integrates more longwave and shortwave polarization information into the mathematical models, and is the first to provide accurate estimates of aerosol size and composition over land.

The RSP instrument views a point in the atmosphere from more than 100 angles, enabling more complete characterization of aerosols by robust mathematical models. RSP and the soon-to-be-launched APS use parallel optical paths to collect measurements from all wavelengths and polarizations simultaneously. This approach to monitoring the continuously changing scene offers greater accuracy compared to previous polarimeters, which used a rotating filter wheel to measure wavelengths and polarizations sequentially.

To test their technique, Cairns and colleagues from Columbia University and the US Department of Energy conducted a series of field campaigns. In 2003, researchers mounted an RSP instrument on a Cessna 310 airplane and flew the instrument above smoke plumes from wildfires in California’s Simi Valley. In 2005, a J31 research aircraft flew RSP over dust plumes in Oklahoma.

In both cases, the airborne data corresponded well with observations from ground-based photometers known to provide accurate aerosol measurements. By integrating the additional polarization wavelengths into their computer models, Cairns estimated that the technique is several times more accurate than previous measurements. The results were presented in January in the Journal of Geophysical Research.

The techniques developed with the RSP instrument have contributed to the creation of the APS, a spaceborne version that will fly on NASA’s Glory satellite. Functionally identical to RSP, the APS uses slightly different wavelengths of light to improve estimates of ocean color, water vapor, and cirrus clouds.

According to Cairns, APS will be especially useful for identifying and quantifying smaller aerosols, which come mainly from human sources. Smaller aerosols are thought to affect climate more than larger aerosols such as dust and salt. Once Glory launches, scientists expect APS to offer a wealth of new data that will help them peel away long-standing uncertainties about aerosols.

Resources

• Kaicun Wang, Robert E. Dickinson, and Shunlin Liang (2009) Clear Sky Visibility Has Decreased over Land Globally from 1973 to 2007. Science 323, 1468 doi: 10.1126/science.1167549