Wildfire-smoke observations fill gap in estimating soot’s role in climate change
New research that fine-tunes the amount of sunlight absorbed by black carbon in smoke from wildfires will help clean up a long-standing flaw in Earth system models, enabling more accurate predictions of global climate change.
“Black carbon or soot is the second most potent climate warming agent after CO2 and methane, despite a short lifespan of weeks, but its impact in climate models is still very uncertain,” said James Lee, climate researcher at Los Alamos National Laboratory and corresponding author of the new study in Geophysical Survey Letters on light absorption by wildfire smoke. “Our research will remove that uncertainty.”
The Los Alamos study resolves a long-standing discrepancy between the observations of the amount of light absorbed by black carbon in smoke and the amount predicted by models given how black carbon is mixed with other material such as condensed organic aerosols present in plumes. .
The team used the Center for Aerosol-Gas Forensic Experiments (CAFÉ) multi-instrument lab in Los Alamos to sample smoke from several wildfires over two summers in the western United States, including the nearby Mid-Fire in New Mexico in 2020 and old plumes of California and Arizona.
The CAFÉ team is now working with colleagues from the Pacific Northwest National Laboratory to incorporate their validated parameterizations into the Department of Energy’s Energy Exascale Earth System Model, or E3SM. This will better evaluate climate forcing and wildfire feedback.
Black carbon emitted from vehicles, power plants, residential heating and forest fires is a powerful absorber of solar radiation and converts incoming light into atmospheric heating.
“Wildfires emit soot and organic particles that respectively absorb and scatter the sunlight to warm or cool the atmosphere with a varying net effect depending on the composition of the smoke mixture,” said Manvendra Dubey, CAFÉ Director and Project Principal Investigator at Los Alamos. “This mixing evolves over time as smoke from large megafires spreads worldwide. We discovered a systematic relationship between the increase in light absorption efficiency of soot with age due to the growth of organic coatings.”
The discovery accurately captures the complex dimensions and structure of soot currently being approximated in models, Dubey said.
“We aim to include it in climate models to provide robust estimates of warming from bushfire soot, particularly in the Arctic, which is warming four times faster than the world,” Dubey said.
“While black carbon is widely believed to cause warming,” Lee said, “its impact on climate is not well known because of the way it interacts with other types of particles in the atmosphere.”
That uncertainty stems in part from a lack of understanding of how black carbon’s light-reflecting and absorbing properties evolve as it ages and undergoes complicated chemistry in the rapidly changing atmospheric conditions as wildfire smoke spreads. The plume can linger in the upper atmosphere for months.
During that evolution, organic aerosols form and condense around black carbon particles. Some of these aerosols focus the light on the black carbon, increasing absorption, but how much light is absorbed depends on the size of the aerosols and how they cover the soot.
Climate models currently idealize this mixing state of smoke, Dubey said. Because the models do not account for the variation in organic coatings based on the size of each particle, the models overestimate how much radiation is absorbed by black carbon. This leads to major uncertainties and prejudices in the climate effects of wildfires.
Single particle modeling yields better results, but is too computationally expensive to embed in Earth system models such as E3SM. That’s why the Los Alamos researchers sought to create parameters for black carbon that could be incorporated into models of the Earth’s system without the prohibitively high computational cost of modeling large numbers of particles.
The Los Alamos team analyzed 60 million smoke particles collected from CAFÉ’s 10-meter high sampling tower. This observation method allowed them to account for variations in the amount of organic coating on each particle — the missing piece from previous models. With the empirical data collected by CAFÉ, the team used existing absorption models to determine how much light energy each particle absorbed and then deduce the plumes’ total black carbon absorption. Their results were consistent with independent measurements of smoke properties performed in parallel, while modeling based on the idealized black carbon particles did not match observations. Los Alamos results can be scaled up to represent atmospheric plumes in global climate models
The team found that they could predict the amplification of black carbon absorption from the ratio of the coating material to the volume of black carbon in the plume. That simple ratio can be incorporated into complex Earth system models for determining the climate impact of black carbon.
Study finds less impact of wildfire smoke on climate
James E. Lee et al., Wildfire Smoke shows significant and predictable improvements in black carbon absorption, Geophysical Survey Letters (2022). DOI: 10.1029/2022GL099334
Quote: Wildfire smoke observations fill gap in estimate of role of soot in climate change (2022, July 22) retrieved July 22, 2022 from https://phys.org/news/2022-07-wildfire-smoke-gap-soot- role-climate .html
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