Black carbon – or soot – emitted by vehicles, power plants, residential heating, and wildfires is a highly potent absorber of solar radiation that converts incoming light to atmospheric heating. However, since soot is usually mixed with other substances, it has been difficult to estimate its effect on the climate.
A new study led by the Los Alamos National Laboratory has investigated how soot’s light-reflecting and light-absorbing properties evolve as it ages during the dispersal of wildfire smoke, in order to predict more accurately this substance’s contribution to global warming.
“Black carbon or soot is the next most potent climate-warming agent after CO2 and methane, despite a short lifetime of weeks, but its impact in climate models is still highly uncertain. Our research will clear up that uncertainty,” said study lead author James Lee, a climate researcher at the Los Alamos National Lab.
The scientists sampled smoke from several wildfires over two summers in the Western United States, including the Medio Fire in New Mexico in 2020 and aged plumes from fires in California and Arizona. Months after these wildfires were extinguished, the smoke plumes can linger in the upper atmosphere, and form organic aerosols that condense around black carbon particles. Some of these aerosols focus light on the soot, increasing its absorption. However, the quantity of absorbed light depends on the size of the aerosols and how they coat the soot.
“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 study senior author Manvendra Dubey, an expert in climate, energy, and air quality research at the Los Alamos Lab. “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.”
By analyzing 60 million smoke particles, the researchers managed to account for variations in the amount of organic coating on each particle, and used existing absorption models to determine how much light energy each particle absorbed in order to infer the total black carbon absorption of the plumes. They found that current climate models generally overestimate how much radiation is absorbed by black carbon, leading to large uncertainties and biases in wildfire climate effects.
“When smoke plumes remain in the lower troposphere and don’t experience temperatures below freezing, black carbon absorption can be predicted by an empirical relationship related to the ratio of coating material to black carbon volume in the plume. This simple parameterization is suitable for incorporation into complex earth systems models to determine the climate impact of black carbon,” the authors concluded.
The study is published in the journal Geophysical Research Letters.