雖科學家已證實黑碳（即煤炭）為主要的光吸收與暖化媒介，然而對於野火燒盡後流入大氣的棕碳瞭解甚少，這些強化大氣層暖化的氣膠其作用存在太多不確定性。 工程與應用科學學院（School of Engineering & Applied Science）能源、環境與化學工程學系（Department of Energy, Environmental & Chemical Engineering）的氣霧科學家助理教授查克拉柏蒂（Rajan Chakrabarty）與榮譽副教授的威廉（Brent Williams）在研究室發現：棕色碳氣溶膠（brown carbon aerosol）滯留於大氣一段時間後，便會由光吸收（light-absorbing）性質轉變為光散射（light-scattering）。物質擴散之初，其煙霧顏色為棕色，且會在大氣中造成溫室效應；然而任其飛舞數日，即從棕轉白、減少約46％的光吸收，進而顯著降低暖化現象。
查克拉柏蒂、威廉與其研究團隊藉美國森林服務處（U.S. Forest Service）的協助，採集阿拉斯加各地的泥炭地燃料（peatland
時任華盛頓大學氣候變化計畫（Washington University Climate Change Program）主持人，威廉表示：「隨時間經過，煙霧產生化學變化的程度逐漸增加，且煙霧顆粒的顏色也越漸轉白。煙霧的顏色若維持棕色，透過對輻射之吸收其可持續造成暖化；倘若顏色轉白，其則散射輻射光、造成環境被冷卻。」如此結果將在氣候微調與衛星檢算上有所助益，亦能協助聯邦機構進一步了解野火燃燒的後續發展。
Doubt cast on warming
implications of brown carbon aerosol from wildfires
devastating wildfires continue to rage in the western U.S. and Canada, a team
of environmental engineers at Washington University in St. Louis have
discovered that light-absorbing organic particulate matter, also known as brown
carbon aerosol, in wildfire smoke loses its ability to absorb sunlight the
longer it remains in the atmosphere.
Rajan Chakrabarty, assistant
professor, and Brent Williams, the Raymond R. Tucker Distinguished InCEES
Career Development Associate Professor, both aerosol scientists in the
Department of Energy, Environmental & Chemical Engineering in the School of
Engineering & Applied Science, and their labs found that brown carbon
aerosol changes its properties from light-absorbing to light-scattering the
longer it remains in the atmosphere. When it is first emitted, the smoke is
brown and has a warming effect on the atmosphere. But over several days in the
atmosphere, the smoke gradually turns white and has a significantly reduced
warming effect. The resulting white smoke roughly contributes to roughly a 46
percent reduction in light absorption compared with the brown smoke.
"Our study casts doubts on
the warming implications of brown carbon," Chakrabarty said. "If this
life cycle analysis is not taken into account, climate models could very well
give rise to overestimated values of warming due to these aerosols."
Results of the research were
published in early online publication in Environmental
Science & Technology Letters Sept. 21.
While scientists already
identified black carbon, or soot, as the major light-absorbing and warming
agent, less is known about the effects of brown carbon from smoldering
wildfires on the atmosphere. There are a lot of uncertainties regarding the
role of these aerosols in enhancing atmospheric warming, the researchers said.
Chakrabarty, Williams and their
lab members made the discovery by burning peatland fuels, acquired from
different regions of Alaska with the help of the U.S. Forest Service, in their
combustion chamber. Smoldering peat fires in the Boreal forests are a major
source of organic aerosol and carbon emissions. In a unique simulated
environment, the team studied the changes in the physics, chemistry, optical
properties and composition of the brown carbon smoke over several days. By
exposing the smoke plumes to ultraviolet radiation and oxidants, such as ozone,
in a photochemical reactor, they could mimic the natural effects in the
When the smoke was first emitted
from the burns, it was a muddy brown from the way it absorbed the shorter
visible wavelengths (blue to green).
"We saw that as the smoke
ages in our simulated atmosphere, we are increasing the extent to which the
chemical changes can happen," said Williams, director of the Washington
University Climate Change Program (WUCCP). "As this chemistry occurs in
the atmosphere, the smoke particles become a lighter color as the days go on.
If the particles always stayed brown, they would continue to contribute to
warming because they absorb more radiation. If after a few days in the atmosphere
they turn white, they'll start contributing to cooling by scattering more
Chakrabarty said the results will
be beneficial to scientists for fine-tuning climate models and satellite
retrieval algorithms. It also will assist several federal agencies working to
"Wildfires are going to be
the major source of air pollution because of decades of fire suppression and
increased fuel loads," Chakrabarty said. "We have exerted control to
a certain extent on regular emissions, such as those from vehicles and
industries, but this is nature, and it's very hard to control."
Williams said the longtime land
management in the West and warming in the northern part of the country has led
to an increase of combustible fuels as climate change has become more
"Events such as droughts,
floods and invasive pests can all kill your vegetation, so you have standing,
dead vegetation with a whole forest floor full of fuel ready to go,"
Williams said. "All of our models predict that these fires are going to
get more extreme. In addition to the climate implications that were a focus of
this study, air quality management districts are also very concerned about the
health consequences for communities that will have poorer air quality because
of these forest fires."
Benjamin J. Sumlin, Apoorva Pandey, Michael J. Walker,
Robert S. Pattison, Brent J. Williams, Rajan K. Chakrabarty. Atmospheric
Photooxidation Diminishes Light Absorption by Primary Brown Carbon Aerosol from
Biomass Burning. Environmental Science & Technology Letters
2017; DOI: 10.1021/acs.estlett.7b00393