2 June 2020
adapted from the story by NOAA Communications
Smoke emitted from wildfires and agricultural burning constitutes one of the largest sources of aerosol particles to Earth's atmosphere. However, little is known about the importance of smoke on the climate system after it dissipates into remote areas of the planet.
A NOAA study published in Nature Geosciences takes a new look at this faint, old smoke and finds that it is just as important an influence on the climate as the thick plumes produced by active fires. The study draws on data from NASA's groundbreaking Atmospheric Tomography Mission (ATom), which sent NASA's DC-8, packed with the state-of-the-art instrumentation, on four pole-to-pole flights over the middle of the Atlantic and Pacific oceans to search for short-lived pollutants in the most far-flung and unstudied parts of the atmosphere.
"We found that, although we were oftentimes thousands of kilometers from active fires, one-quarter of the aerosol particles in the remote lower atmosphere originated from a fire," said lead author Gregory Schill, a CIRES scientist working at CSL. "Even after smoke from wildfires and agricultural burns dissipates into the background atmosphere, it strongly influences particulate matter concentrations throughout the global atmosphere."
Onboard the NASA DC-8 was the NOAA Particle Analysis by Laser Mass Spectrometry (PALMS) instrument, an instrument that is both highly sensitive and selective for detecting smoke particles. "What surprised us is that dilute smoke was detected almost everywhere, even over the remote southern ocean and Antarctica," said CIRES scientist Karl Froyd, the principal investigator for the PALMS instrument.
Globally, wildfires, agricultural burns and residential fires contribute roughly 37 million tons of smoke particles to the atmosphere every year. Biomass burning aerosol affects the Earth's climate directly by blocking incoming sunshine from reaching the surface, and influencing clouds, which also intercept sunlight. Despite the key roles smoke plays in modulating climate, the abundance and distribution of smoke in the atmosphere has not been not well understood, especially after smoke plumes age, are diluted into the background atmosphere, and are removed by interaction with clouds and precipitation. In a future climate projected to be both drier and warmer, wildfires are expected to increase.
Today's global climate models rely heavily on satellite remote sensing to estimate the abundance of smoke aerosols, even though dilute smoke is sometimes too faint for satellites to detect. One of the primary goals of the four ATom research flights from 2016 to 2018 was to obtain accurate measurements of smoke and other short-lived climate-influencing pollution in the background atmosphere, then use these new data to calibrate satellite estimates and improve the skill of climate models in reproducing our current climate and predicting the impacts of climate change.
Schill and co-authors, who included scientists from NASA, the National Center for Atmospheric Research, the University of Maryland and the University of Vienna, found that one prominent climate model, the Goddard Earth Observing Model, overestimated the amount of smoke aerosols by an average of more than 400 percent, largely due to underestimating removal by clouds and precipitation.
After updating the model's accounting for aerosol removal, the scientists estimate that although smoke in the background atmosphere is very dilute, it is so widespread that its impact on the Earth's energy balance equals that of all the thick, fresh smoke plumes typically seen near fires.
NOAA scientist Dan Murphy, a co-author and the Chemical Science Laboratory's Cloud & Aerosol Processes program leader, said the study provides important insights for a future that will be warmer, in many places drier, and severe droughts more common.
"If we're going to look at climate effects from wildfires and biomass burning," Murphy said, "we can't ignore the dilute smoke."
Schill, G.P., K.D. Froyd, H. Bian, A. Kupc, C. Williamson, C.A. Brock, E. Ray, R.S. Hornbrook, A.J. Hills, E.C. Apel, M. Chin, P.R. Colarco, and D.M. Murphy, Widespread biomass burning smoke throughout the remote troposphere, Nature Geosciences, doi:10.1038/s41561-020-0586-1, 2020.
Biomass burning emits ~34–41 Tg yr−1 of smoke aerosol to the atmosphere. Biomass burning aerosol directly influences the Earth's climate by attenuation of solar and terrestrial radiation; however, its abundance and distribution on a global scale are poorly constrained, particularly after plumes dilute into the background remote troposphere and are subject to removal by clouds and precipitation. Here we report global-scale, airborne measurements of biomass burning aerosol in the remote troposphere. Measurements were taken during four series of seasonal flights over the Pacific and Atlantic Ocean basins, each with near pole-to-pole latitude coverage. We find that biomass burning particles in the remote troposphere are dilute but ubiquitous, accounting for one-quarter of the accumulation-mode aerosol number and one-fifth of the aerosol mass. Comparing our observations with a high-resolution global aerosol model, we find that the model overestimates biomass burning aerosol mass in the remote troposphere with a mean bias of >400%, largely due to insufficient wet removal by in-cloud precipitation. After updating the model's aerosol removal scheme we find that, on a global scale, dilute smoke contributes as much as denser plumes to biomass burning’s scattering and absorption effects on the Earth's radiation field.