Sulfate-coated soot boosts global warming

Ned Stafford/Hamburg, Germany

Sulfate and nitrate particles in the atmosphere are thought to help combat global warming because they reflect sunlight, but a new study suggests that when combined with soot the particles could instead enhance global warming.

Soot absorbs solar energy and ranks behind only carbon dioxide in its potential for contributing to global warming. But the already high solar energy absorption of soot aerosols is significantly increased when coated with sulfates and nitrates from the atmosphere, according to a study led by Kimberly Prather of the University of California, San Diego (UCSD). The coating acts like a lens, focusing more light into the soot particle.

Prather and Ryan Moffet, a then graduate student at UCSD, studied atmospheric aerosols in Riverside, California, and Mexico City using an aerosol time of flight mass spectrometer (ATOFMS). The instrument performs real-time measurements of physical, chemical, and optical characteristics of aerosols.

’The key to our method is the samples are not collected, they are pulled directly into the instrument from the atmosphere and analysed,’ says Prather. ’We learn immediately what is in the air. This is one of the only instruments in the world capable of acquiring this information immediately.’

Fresh airborne fractal soot particles dominate shortly after sunrise during rush hour traffic. But within hours most soot particles begin to age, developing a coating of secondary species, including sulfate, ammonium, organics, nitrate, and water through light-driven chemical reactions. The team measured the aging process of particles throughout the day, and found that a sulfate and nitrate coating enhances light absorption up to 1.6 times that of non-coated soot particles.

Some experts have suggested that flooding the stratosphere with sulfur could reflect enough sunlight to slow global warming.

Prather notes that carbon aerosols are confined mainly to the lowest layer of the Earth’s atmosphere (the troposphere), with little or no carbon in the stratosphere (the next layer up) to mix with sulfates. ’But there may be other ramifications that we haven’t thought of,’ she says. ’We understand very little about the surface chemistry that would occur on these aerosols, how long they would exist, where [or] if they would settle, how they might catalyse other processes, etc.’

George Ban-Weiss at the Carnegie Institution for Science’s department of global ecology in Stanford, California, told Chemistry World that models had already shown that soot absorption is enhanced by coatings of sulfate, nitrate or organic carbon. However, Prather’s study is the first to directly measure how soot particles are coated in the atmosphere as they age through the day.

’This is something that has been estimated by models, but has never actually been measured in the atmosphere,’ he says. ’To me, the most important finding is that soot particles age, that is, get a non-absorptive coating within a few hours in the atmosphere.’ 

But he adds: ’It should be noted that their measurements are in dirty urban environments, so it is unclear if this holds true in more pristine environments.’

Prather agrees that additional studies of the phenomenon in locations around the world will be necessary to determine the overall global climate impact, but notes that her study is a major first step toward that goal.