它要爆發了嗎？從太空監測火山噴氣

原文網址：http://www.mtu.edu/news/stories/2017/october/gonna-blow-measuring-volcanic-emissions-space.html

它要爆發了嗎？從太空監測火山噴氣

利用NASA衛星測量二氧化碳的結果，可以精確指出排放二氧化碳的人類與火山活動位置，這或許有助於監測造成氣候變遷的溫室氣體變化。

在上個月末，峇里島上的層狀火山――阿貢火山開始冒煙，山下也頻頻發生小型地震。從那時起，當局已經疏散了數千人，避免1963年阿貢火山噴發造成的慘劇再次發生，該次事件造成1000多人死亡。

火山噴發之前通常會有警訊。雖然人類難以感知火山產生的微小地震，但可以藉由地震儀偵測到。此外火山口也會冒出水蒸氣。當火山開始噴出像是二氧化碳和二氧化硫之類的氣體時，代表噴發時刻可能已經迫在眉睫。

但接近火山頂部是項非常危險的作業。利用遙測技術偵測飄往空中的二氧化碳和二氧化硫噴氣，可以在不需冒著生命危險或設備損害的情況下，大幅增進人們對於火山的瞭解。利用遙測系統監控噴氣可以避免人道災難發生，也可以防止發布錯誤的預報。

雖然阿貢火山尚未噴發(在此文章撰寫時)，但地震活動依然劇烈。峇里島當局開始揣測是否真的即將發生火山爆發；從該地區撤離的居民也渴望重回家園，而觀光活動也因此下滑。

包括密西根理工大學的火山學家Simon Carn在內的研究人員，發表了一系列文章詳細描述他們如何利用近地軌道衛星，首次測量人類和自然活動在局部地區排放的二氧化碳，其中包含刊登於期刊《科學》(Science)的論文「Spaceborne detection of localized carbon dioxide sources(利用衛星搭載儀器偵測區域二氧化碳來源)」。

本期《科學》OCO-2特集的五篇論文展現了NASA軌道碳觀測衛星2號(Orbiting Carbon Observatory-2)的能力；以及從衛星搭載感測器的觀測結果，讓我們看見碳如何使地球上的一切事物產生關聯。本研究由NASA噴射推進實驗室資助。

從太空中監測CO₂排放

Carn參與的論文是在討論研究團隊如何運用衛星搭載儀器，以公里為單位對大氣二氧化碳濃度進行高解析度和高靈敏度的測量。資料顯示衛星的感測器可以精確定位出區域大氣二氧化碳的來源。考慮到一開始得先建立的大氣二氧化碳背景值的數據量有多麼龐大，這項成果確實得來不易。

衛星利用其上裝載的光譜儀來進行測量，它可以透過人眼無法見到的光波，對反射光(輻射)進行高光譜解析度的測量。當光線穿過二氧化碳，其中有些波段會被吸收。剩餘的光線碰到海洋和地面後會反射回去。OCO-2的感測器測量反射回來的陽光就可以定量出有多少二氧化碳被吸收，使得科學家可以分離出二氧化碳的排放來源，不管它是人為或自然因素。

「這篇文章的主要重點是我們可以偵測到二氧化碳在小區域的點狀來源，而不是測量大範圍的大氣二氧化碳濃度。」地質與礦業工程科學系的副教授Carn表示，「火山可以成為區域排放二氧化碳的強力來源，但是從全球角度來看，現有證據皆顯示人類排放的二氧化碳比火山多出許多。」

OCO-2衛星的空間解析度為2.25公里，高到化學訊號不會被背景值稀釋。然而，就算OCO-2的測量能力達到空前水準，還是無法用來做為定期監測火山的工具，因為它經過地球同一地區的頻率並不夠頻繁。

Carn表示：「雖然我們證實這項技術確實可以運作，但還是需要更先進的感測器才能讓它成為進行例行性監測的工具，尤其是對於火山這種噴氣成分會迅速變化的地方。如果可以從太空定期監測火山噴出的二氧化碳，對於我們用的這項技術來說是另一項十分有用的附加功能。而且觀察結果立刻就能(在阿貢火山)派上用場。」

Carn徹底檢查衛星載具對二氧化碳的測量結果，來找出其是否有偵測到太平洋島國萬那杜上三座火山噴出的二氧化碳。其中一座雅蘇爾火山至少從1700年代就開始噴發，OCO-2測量當天其噴出的二氧化碳大約比大氣背景值高出3.4 ppm，大致等同於42,000噸。相較之下，人類每天排放的二氧化碳平均為1億噸。

OCO-2的感測器也測量了洛杉磯盆地排出的二氧化碳，結果顯示上方就像有一個二氧化碳「圓穹」籠罩一般。都市地區占了人為排放量的70%以上。

「目前地球的自然作用可以吸收人類燃燒石化燃料而排放的二氧化碳量的一半。」加州帕薩迪納NASA噴射推進實驗室OCO-2科學計畫的副主持人Annmarie Eldering表示。她也是發表在《科學》的OCO-2科學計畫整體現況的論文主要作者。「如果這些自然作用減緩下來，對移除大氣二氧化碳的助益便會減少，由溫室氣體引起的暖化現象就會加速並增強。衛星數據開始讓我們更加認識氣候如何影響碳循環，而減少我們對於兩者未來可能會如何變化的巨大不確定性。」

OCO-2對整個洛杉磯的測量結果精確到可以捕捉城市內部不同排放源的濃度差異。隨著衛星經過擁擠的城市到郊區，再離開至北方人煙稀少的沙漠，他們也跟著描繪出過程中二氧化碳濃度如何逐漸下降。

Is it gonna blow? Measuring volcanic emissions from space

Carbon dioxide measured by a NASA satellite pinpoints sources of the gas from human and volcanic activities, which may help monitor greenhouse gases responsible for climate change.

Late last month, a stratovolcano in Bali named Mount Agung began to smoke. Little earthquakes trembled beneath the mountain. Officials have since evacuated thousands of people to prevent what happened when Agung erupted in 1963, killing more than 1,000 people.

Before volcanoes erupt, there are often warning signs. Tiny earthquakes rarely felt by humans but sensed by seismographs emanate from the volcano. Plumes of water vapor rise from the crater. When the volcano begins to emit gases like carbon dioxide and sulfur dioxide, eruption may be imminent.

But getting close to the top of a volcano is dangerous work. Using remote sensing to detect rising carbon dioxide and sulfur dioxide emissions without endangering people or equipment would greatly increase human understanding of volcanoes. Remote sensing emissions could prevent humanitarian disasters—and false alarms.

Mount Agung hasn’t erupted yet (at the time this article was written), but seismic activity remains intense. Balinese officials are beginning to wonder if an eruption truly is imminent; the people who were evacuated from the area want to return to their homes and tourism is down.

Researchers including Michigan Technological University volcanologist Simon Carn have published a collection of papers including “Spaceborne detection of localized carbon dioxide sources” in the journal Science; the article details the first-known measurement of localized anthropogenic and natural carbon dioxide sources from a satellite in low-Earth orbit.

The five papers in the OCO-2 Science Special Collection showcase the abilities of NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite; measurements from the satellite’s sensors provide insights into how carbon links everything on Earth. The research is supported by NASA’s Jet Propulsion Laboratory.

Monitoring CO₂ Emissions From Space

The paper Carn co-authored discusses how the research team has taken high-resolution, sensitive spaceborne measurements of atmospheric carbon dioxide at the kilometer scale. This data reveals that the satellite’s sensors are able to pinpoint localized sources of carbon dioxide in the atmosphere—a difficult task considering the sheer amount of background carbon dioxide in the atmosphere to begin with.

The satellite uses spectrometry; the sensors onboard the satellite measure reflected sunlight—radiation—in high-spectral resolution using wavelengths undetectable to the human eye. When light passes through carbon dioxide, some is absorbed by the gas. The remaining light bounces off the ocean and the Earth. The OCO-2 sensors measure the light that bounces back to quantify what was absorbed by carbon dioxide, allowing scientists to isolate emission sources, whether human or natural.

“The main focus of the article is detecting localized, point-source emissions of carbon dioxide as opposed to measuring the broad-scale concentration in the atmosphere,” says Carn, an associate professor in the Department of Geological and Mining Engineering and Sciences. “Volcanoes can be strong, localized sources of carbon dioxide. But on a global basis, all available evidence indicates that human activities are emitting much more carbon dioxide than volcanoes.”

The OCO-2 satellite’s spatial resolution—2.25 kilometers—is high enough that chemical signals are not diluted. However, while OCO-2’s measurements are unprecedented, the satellite cannot be used as a routine volcano monitoring tool because it does not pass over the same place on the Earth frequently enough.

“This is a demonstration that the technique does work, but we need better sensors before it becomes a routine monitoring tool, especially for volcanoes where we expect rapid changes in gas emissions,” Carn says. “If we could measure volcanic carbon dioxide from space routinely, it would be a very powerful addition to the techniques we use. That kind of observation would be useful (for Agung) right now.”

Carn combed through satellite data to find detectable spaceborne carbon dioxide measurements from three volcanoes in the Pacific island nation of Vanuatu. One of these, Mount Yasur, has been erupting since at least the 1700s, and on the day of the OCO-2 measurement was emitting carbon dioxide about 3.4 parts per million above background atmospheric levels, equal to about 42 kilotons of emissions. In comparison, human emissions average 100,000 kilotons a day.

OCO-2’s sensors also measured carbon dioxide emissions over the Los Angeles basin, detecting a sort of carbon dioxide “dome”. Urban areas account for more than 70 percent of anthropogenic emissions.

“Natural processes on Earth are currently able to absorb about half of human fossil fuel emissions,” says Annmarie Eldering, OCO-2 deputy project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, and lead author of an overview paper in Science on the state of OCO-2 science. “If those natural processes falter, slowing down the helpful removal of carbon dioxide, greenhouse-gas-induced warming would accelerate and intensify. These data begin to give us a better view of how climate affects the carbon cycle, reducing the huge uncertainty around how both might change in the future.”

The OCO-2 measurements across Los Angeles were detailed enough to capture differences in concentrations within the city resulting from localized sources. They also tracked diminishing carbon dioxide concentrations as the spacecraft passed from over the crowded city to the suburbs and out to the sparsely populated desert to the north.

原始論文：Florian M. Schwandner, Michael R. Gunson, Charles E. Miller, Simon A. Carn, Annmarie Eldering, Thomas Krings, Kristal R. Verhulst, David S. Schimel, Hai M. Nguyen, David Crisp, Christopher W. O’Dell, Gregory B. Osterman, Laura T. Iraci, James R. Podolske. Spaceborne detection of localized carbon dioxide sources. Science, 2017; 358 (6360): eaam5782 DOI: 10.1126/science.aam5782

引用自：Michigan Technological University. "Is it gonna blow? Measuring volcanic emissions from space."