New wildfire soot variety could upset all global warming estimates


Subhra Priyadarshini

Soot from wildfires across the world could actually be causing far more warming than our climate models account for. Hidden away from all scientific estimates is a newly-discovered form of soot – the ‘superaggregates’ – emitted from wildfires. Scientists have found that this kind of soot causes 90 per cent more warming than conventional sub-micrometer soot particles.

Current climate models make calculations of wildfire emissions based on sub-micrometer soot particles. The new findings suggest that if we were to reassess the warming from wildfires alone, the figure would go up many times.
The Nagarhole forest wildfire of 2012 that burnt thousands of acres to ashes.

The superaggregates came into light when scientists from the Desert Research Institute in Nevada-Reno, USA, were studying the massive 2012 wildfire in the Nagarhole National Forest of Karnataka. Studying aerosol samples over the Indian Ocean at the Maldives Climate Observatory on Hanimaadhoo Island, they found a new type of soot particle almost 10 times longer than normal and far more compact in shape than the sub-microscopic variety.

“We call these particles superaggregates because of their super-micron size. Conventional soot particles from diesel vehicles, cook stoves and other ‘contained’ combustion sources are sub-micron size aggregates,” one of the lead researchers Rajan Chakrabarty, presently a faculty member at the Washington University in St. Louis told Nature India.
Though ten times longer than conventional aggregates, the superaggregates have similar mass density. “This means although larger in size, these superaggregates can remain aloft in the atmosphere for the same lifespan (approximately a week) as conventional aggregates,” Chakrabarty said. This also means they get deposited on human lungs the very same way as conventional particles.
According to the scientists, the superaggregate form of soot has not been observed from wildfires before this study.
When they analysed the radiative properties, the scientists found that compared to spherical soot particles, these superaggregates could lead to 90% more warming in the atmosphere.
After detecting soot superaggregates from the 2012 Nagarhole sanctuary fire, the scientists went back to look at smoke samples from the 2010 Millerton Lake fire in Northern California, the 2011 Las Conchas fire in New Mexico, and some more wildfires near Mexico City. Not surprisingly, they found superaggregates in those samples too.
The scientists say though wildfires contribute significantly to global soot emissions, their aerosol formation mechanisms and particle properties are poorly represented in climate models. Superaggregates – previously unrecognized pollutants – could have considerable impact on climate and human health, they add.
“The higher heating effect of these particles, compared to volume-equivalent spheres, could change current estimates of climate forcing by models,” Chakrabarty added. He said multi-front future research in this area could lead to development of mechanical filtration systems to control public health impacts of soot superaggregates during large-scale wildfires.


1. Chakrabarty, R. K. et al. Soot superaggregates from flaming wildfires and their direct radiative forcing.Sci. Rep. (2014) doi: 10.1038/srep05508

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