Climate-change models for gauging the effects of black carbon (or soot) from sources including motor vehicles and wildfires, often fall short in terms of accuracy. Observations in new research on soot from wildfire smoke should help to make more accurate predictions of these effects.
The new work by researchers at the Los Alamos National Laboratory should hopefully help to resolve the uncertainty that currently revolves around estimating the impact of black carbon on climate change.
“Black carbon or soot is the next most potent climate-warming agent after CO2 and methane, despite a short lifetime of weeks,” explained corresponding author James Lee, a Los Alamos climate researcher, “but its impact in climate models is still highly uncertain.”
This research closes the gap between soot’s light absorption efficiency and the level predicted by current models. It was issued in Geophysical Research Letters.
A gap in climate change models
Black carbon is known for its ability to absorb solar radiation and transform sunlight into atmospheric heating. In addition to wildfires, it also derives from other sources including vehicles and power plants.
Typically, soot from smoke is mixed with other particles – for example, condensed organic aerosols in plumes. Its ability to absorb light is dependent on the size of these aerosols, which form around its particles.
“While black carbon is generally thought to cause warming,” said Lee, “its climate impact is not well known because of how it co-exists with other types of particles in the atmosphere.”
Current climate models tend to assess too highly the amount of radiation that black carbon absorbs. They roughly estimate the complex structure and sizes of soot, failing to account for differences in their organic coatings. And the results of this are predictions that are off the mark, as regards climate effects of wildfires.
Results from single-particle modeling tend to be more accurate. However, researchers say it is too costly – computationally – to use in earth-system models. The Los Alamos team, therefore, wanted to create black carbon parameters that can be used in such models without incurring very high computational costs.
Closing the gap
For their research, the team sampled smoke from multiple wildfires that happened over two summers in western U.S. Samples included those from New Mexico’s Medio Fire of 2020 as well as aged plumes from the states of California and Arizona.
In all, the researchers examined an estimated 60 million smoke particles that were collected from the 10-meter tall tower of Los Alamos’ Center for Aerosol-gas Forensic Experiments (CAFE). They took into account variations in organic coatings of black carbon particles, unlike in other models.
The team used empirical data collected by CAFE together with existing models of absorption to find out the amount of light energy that each particle absorbed. It then inferred the plumes’ overall black carbon absorption.
Results tied in with independent measurements of properties of smoke made in parallel. This has not been the case with models that idealize the mixing form of smoke.
“Wildfires emit soot and organic particles that respectively absorb and scatter the sunlight to warm or cool the atmosphere to a varying net effect, depending on the composition of the smoke mixture,” said Manvendra Dubey, Los Alamos principal investigator and CAFE director. “This mixing evolves over time as smoke from large megafires disperses globally. We discovered a systematic relationship between the increase in light absorption efficiency of soot with age due to the growth in organic coatings.”
This work will help to overcome great uncertainties and biases when estimating the effects of wildfires on the climate. Researchers could use the ratio of coating material to black carbon volume in the plume to predict an increase in black carbon absorption. This ratio can be integrated into earth-system models to better determine the role of soot in climate change.
The CAFE team is working with counterparts at Pacific Northwest National Laboratory to include their proven parameterizations into the Energy Exascale Earth System Model (E3SM) of the Department of Energy.
Incorporation of this discovery into climate models will enable “robust estimates of warming by wildfire soot,” Dubey said. It will especially aid improved effect estimation in the Arctic, where warming is four times higher than in the globe.