Researchers discover how to better estimate the role of soot in global warming

Climate change models for assessing the effects of black carbon (or soot) from sources such as motor vehicles and wildfires often lack precision. Observations in new research on soot from wildfire smoke should help make more accurate predictions of these effects.

Forest fire

The new work by researchers at Los Alamos National Laboratory should hopefully help resolve the uncertainty that currently surrounds estimating the impact of black carbon on climate change.

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“Black carbon or soot is the second most potent global warming agent after CO2 and methane, despite having a short lifespan of a few weeks,” explained corresponding author James Lee, a climate researcher at Los Angeles. Alamos, “but its impact in climate models is still very uncertain.”

This research closes the gap between the light absorption efficiency of soot and the level predicted by current models. It was published in Geophysical Research Letters.

A gap in climate change models

Carbon black is known for its ability to absorb solar radiation and convert sunlight into atmospheric heat. In addition to forest fires, it also comes from other sources, including vehicles and power plants.

As a rule, soot from smoke is mixed with other particles – for example, organic aerosols condensed into plumes. Its ability to absorb light depends on the size of these aerosols that form around its particles.

“While black carbon is commonly thought to cause warming,” Lee said, “its impact on climate is not well known because of how it coexists with other types of particles in the atmosphere”.

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Current climate models tend to overestimate the amount of radiation absorbed by black carbon. They roughly estimate the complex structure and size of the soot, disregarding differences in their organic coatings. And the results of this are predictions that are off the mark, when it comes to the climatic effects of wildfires.

Single particle modeling results tend to be more accurate. However, the researchers say it is too expensive – in computational terms – to use in Earth system models. The Los Alamos team therefore wanted to create black carbon parameters that could be used in such models without incurring very high computational costs.

Close the gap

For their research, the team sampled smoke from several wildfires that occurred over two summers in the western United States.

In total, the researchers examined about 60 million smoke particles that were collected from the 10-meter-tall tower at the Center for Aerosol-gas Forensic Experiments (CAFE) in Los Alamos. They took into account variations in the organic coatings of the black carbon particles, unlike the other models.

The team used empirical data collected by CAFE as well as existing absorption models to determine the amount of light energy absorbed by each particle. He then inferred the overall black carbon uptake of the plumes.

Results related to independent measurements of smoke properties carried out in parallel. This has not been the case with models that idealize the form of smoke mixing.

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“Wildfires emit soot and organic particles that absorb and scatter sunlight respectively to heat or cool the atmosphere with varying net effect, depending on the composition of the smoke mixture,” Manvendra Dubey said. , principal investigator at Los Alamos and director of CAFE. “This mixture evolves over time as the smoke from large megafires disperses around the world. We found a systematic relationship between the increase in soot light-absorbing efficiency with age due to the growth of organic coatings.

This work will help overcome large uncertainties and biases when estimating the effects of wildfires on climate. Researchers could use the ratio of coating material to volume of black carbon in the plume to predict an increase in black carbon uptake. This report can be incorporated into Earth system models to better determine the role of soot in climate change.

The CAFE team is working with their counterparts at Pacific Northwest National Laboratory to include their proven parameterizations in the Department of Energy’s Energy Exascale Earth System Model (E3SM).

Incorporating this finding into climate models will allow “robust estimates of soot warming from wildfires,” Dubey said. In particular, this will help improve the estimate of effects in the Arctic, where warming is four times higher than globally.

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Wildfire Smoke demonstrates significant and predictable improvements in light absorption by black carbon

Teresa H. Sadler