Now US and Swedish researchers have found that interaction of soot aerosols with other pollutants, especially sulphates, can dramatically increase their effect on climate.
"While most aerosols reflect the sunlight back to space, cooling the atmosphere, soot absorbs the solar energy, heating the air," Renyi Zhang of Texas A&M University told environmentalresearchweb. "Soot can affect the tiny droplets in the clouds themselves or even prevent clouds from forming at all. We found fresh soot particles directly from sources have rather limited properties but transformation of soot in the atmosphere drastically enhances their atmospheric effects."
Zhang and colleagues from Texas A&M University, the University of Minnesota, US, and Ergonomics and Aerosol Technology, Sweden, exposed soot particles in the lab to subsaturated sulphuric acid vapour in order to mimic internal mixing in the atmosphere. The particles grew in both mass and hygroscopic size. They also showed a marked change in morphology, increasing in effective density by a factor of 3-10.
These changes increased the optical scattering of the particles by roughly tenfold and almost doubled their absorption at 80% relative humidity.
"Because of the way it collects other pollutants, particularly sulphate that is originated from power plants, soot can have much larger effects on visibility and cloud formation," added Zhang.
The results are also likely to apply to other types of particles as the adsorption of sulphuric acid was a physical process, depending only on the particle surface area.
"Representing an important mechanism of atmospheric aging, internal mixing of soot with sulphuric acid has profound implications on visibility, human health, and direct and indirect climate forcing," write the researchers in a paper in PNAS.
Now the scientists plan to use their results in atmospheric models to evaluate the effects of soot better.
"Presently our knowledge on soot is very limited because of its complex transformation in air and this work will help to reduce uncertainty in our effort to assess its atmospheric effects," said Zhang. "Overall our results provide crucial data for modelling of global climate change and local air quality."