Air study looks at the Southeast's cooling trend

Part of the largest U.S. air-quality field study in decades is examining whether human-caused pollution and natural chemical emissions explain a cooling trend in the leafy Southeast.

The Southern Oxidant and Aerosol Study began last month and runs through July 15. It's part of a five-pronged air study that will deploy airplanes and ground-based instruments from the Mississippi River to the Atlantic.

The study will look at how interactions between natural and human-made emissions affect air quality -- and how they relate to climate change. Unlike much of the world, the Southeast has seen a cooling trend over the past century.

The region saw a warm period during the 1920s to 1940s, says the U.S. Global Change Research Program, followed by a downward trend through the 1960s. Temperatures have been rising again since the 1970s.

Pollutants interact with volatile organic compounds that come from plants, turning them into airborne particles called aerosols. So-called black aerosols absorb solar radiation and warm up temperatures. But lighter-colored ones, typical of the Southeast, reflect sunlight and have a cooling effect.

Light aerosols have a dark side. They can make ground-level ozone -- which cities such as Charlotte and Atlanta have wrestled with for decades -- worse.

The larger study is supported by the National Science Foundation, Environmental Protection Agency, National Oceanic and Atmospheric Administration and the Electric Power Research Institute, with researchers from 30 U.S. and international institutions.