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燃燒進行式──溫室氣體的微妙變身

美西與加拿大的森林野火,終成空氣汙染的隱藏版集散地?


(圖一)野火經常發生在美西、加拿大等地,可能由人為引起、或透過光照、火山爆發等原因產生。圖片來源:Morguefile

燃後氣體變化不為人知的秘密

  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。
  查克拉柏蒂表示,未將此現象之週期分析納入考慮,意味著氣霧劑對暖化的影響極有可能被高估。「我們的研究員對棕色碳(brown carbon)帶來的暖化影響抱持懷疑。」

從虛擬天空裡看氣霧膠的暖化能力

  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 fuels),並將其置於燃燒室悶燒,而來自北方森林的泥碳火則為有機氣膠與碳逸散的來源。團隊製造特殊環境,針對棕碳煙霧進行一連串物理、化學與光學物質變化之研究。透過光化學反應堆(photochemical reactor),將煙霧暴露於紫外線及臭氧中,他們模擬出大氣層裡各式的自然反應。爾後發現──燃燒之初,其逸散出的煙霧經由吸收對短波長的可見光,其呈現泥棕色。


(圖二)燃燒後出現的煙霧經過一段時間,會由棕轉白,其對輻射的反應也將產生變化。圖片來源:Unsplash

  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。

阻火之路勢在必行

  「經過數十年的火生火滅、以及與日俱增的燃料累積,野火將可能成為空氣組成的主要來源。我們已在各式交通工具與工業用廢氣排放上做出一定程度的控制,然而來自自然環境的影響,實在太難說了。」查克拉柏蒂如此說道。
  「隨氣候變化越來越普遍,西方長期經營土地的事實、與北方城市暖化現象已助長易燃物質的增加。」威廉進一步解釋:「旱災、洪水、蟲害等對植物有害、進而造成腐敗產生易燃物,加上整座森林的「燃料」,危險可想而知。研究在在預測野火對氣候影響的極限,因群火繚繞導致的空氣品質有可能間接影響群體健康,絕對不容忽視。

資訊來源:Science Daily

Doubt cast on warming implications of brown carbon aerosol from wildfires

As 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 atmosphere.

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 radiation."

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 understand wildfires.

"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 prevalent.

"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."

Journal Reference:
1.      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
 

 
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