Shipboard Aerosol Measurements Enhance Climate Models

For 40 days, Rood’s colleague, postdoctoral research associate Christian Carrico, measured the light-scattering properties of aerosols at three wavelengths of light, two upper particle sizes, and under controlled relative humidity conditions. Sailing first between Hawaii and China, the researchers quantified the optical properties of atmospheric aerosols for clean marine conditions. Then they measured the same properties under polluted conditions along the coast of China.

“Along the coast, we also saw very large concentrations of particulate material, including mineral dust,” Rood said. “We were able to quantify their optical properties as a function of increasing and decreasing controlled relative humidity, which had not been done before.” Aerosol particles have the ability to behave differently under increasing compared to decreasing humidity, Carrico said. “While a dry particle may change to a droplet at 75 to 80 percent relative humidity, it may not change back to a dry particle until the humidity drops to 40 percent. That means there can be a range in relative humidity where you could have either a dry particle or a wet droplet, depending on the history of the aerosol.”

Because a particle’s physical state can significantly change its affect on the atmosphere, it is important to know what that state is. “With the results from these shipboard measurements, we will be able to determine whether particles are wet or dry,” said Rood, who will present the results that have been obtained and interpreted by Carrico and doctoral student Pinar Kus at the American Geophysical Union meeting in San Francisco, Dec. 10-14.

The recent field campaign is part of an extensive set of aerosol characterization experiments that began in 1995. International and interdisciplinary in scope, the program has been supported by the National Science Foundation, National Oceanic and Atmospheric Administration, and National Aeronautics and Space Administration. Rood’s group has taken similar aerosol measurements during previous field campaigns at Cape Grim, Tasmania; Sagres, Portugal; and Bondville, Ill.

The shipboard measurements will be combined with ground-, aircraft-, and satellite-based measurements made by other researchers. “These results can then be used to reduce the uncertainties in global climate models and develop better policies related to air pollution and climate change,” Rood said.