美國康乃爾大學和非營利組織環境保護基金（Environmental Defense Fund）的新研究發現，氨態氮肥產業的甲烷排放量被嚴重低估，實際排放量竟高於美環保署對全美所有工業過程的估計值。
氨態氮肥業是天然氣重要的下游產業，以天然氣為原料或燃料，生產氨和其他升級產品，過程中排放強效溫室氣體甲烷。
研究人員透過配備高精度甲烷感測器的Google街景車發現，氨態氮肥廠的甲烷排放量不但是該產業申報值的100倍，也遠高於環保署對全美所有工業流程甲烷排放總量的估計值。
「我們選了一個大多數人沒聽說過的小型工業來研究，結果發現它的甲烷排放量是美國所有工業生產排放的三倍，」研究共同作者、康乃爾大學土木環境工程教授艾伯森（John Albertson）說，「這表示過去的估計值與實際測量值之間有極大差距。」
艾伯森的研究領域是用移動感測辨識石油和天然氣產地以及化肥業的逸散性甲烷排放。
這篇研究「用移動感測方法估算美國氨肥業的甲烷排放量」發表在《Elementa》期刊。
近年，由於頁岩氣開採效率的提高，加上人們認為天然氣是比煤、石油、汽油或柴油乾淨的化石燃料，天然氣的使用量有所成長。
「但天然氣主要成分是甲烷，其每個分子的暖化效應比二氧化碳更強，」艾伯森說，「供應鏈中任何地方若有大量排放或洩漏，都可能使天然氣成為氣候變遷的一個重要因素。」
美國只有幾十家工廠生產氨態氮肥，通常位於公共道路附近，在下風處便可檢測出甲烷逸散。
「即使僅外洩一小部分，甲烷強大的溫室效應讓小洩漏亦不容忽視，」另一位共同作者、環境保護基金首席科學家魯德克（Joseph Rudek）說，「以20年的時間來看，甲烷的暖化潛力是二氧化碳的84倍。」
在這項研究中，Google街景車在美國中部六個代表性化肥廠附近的公共道路上行駛，測量逸散性甲烷排放，即無意間洩漏至大氣中甲烷。
研究人員解釋，此種排放可能是化肥生產過程中化學反應不完全、燃料不完全燃燒或洩漏所造成。
該團隊發現，平均來說，工廠使用的氣體有0.34％會排放到大氣中。
化肥業自主申報的甲烷排放量為每年0.2千兆克（0.2 gigagram，即200公噸）。若將這六個工廠的排放率擴大到整個產業，每年甲烷排放總量為28千兆克（即2.8萬公噸），是自主申報量的100倍。
根據美國環保署的估計，全美所有工業流程每年僅產生8千兆克的甲烷排放，遠遠不及化肥業的總排放量。
該研究的部分資金來自阿特金森永續未來中心與環境保護基金的聯合研究計畫。
Emissions of the potent greenhouse gas methane from the ammonia fertilizer industry have been greatly underestimated, finds new research from Cornell University and the nonprofit Environmental Defense Fund. The methane emissions were higher than the EPA"s estimate for all industrial processes in the United States.
The scientists estimated methane emissions from this important downstream consumer of natural gas, the ammonia fertilizer industry, which uses natural gas as a feedstock and a fuel for the production of ammonia and other upgraded products.
Using a Google Street View car equipped with a high-precision methane sensor, the researchers discovered that methane emissions from ammonia fertilizer plants were 100 times higher than the fertilizer industry"s self-reported estimate.
The methane emissions also were substantially higher than the Environmental Protection Agency"s estimate for all industrial processes in the United States.
"We took one small industry that most people have never heard of and found that its methane emissions were three times higher than the EPA assumed was emitted by all industrial production in the United States," said John Albertson, co-author and professor of civil and environmental engineering at Cornell.
"It shows us that there"s a huge gap between a priori estimates and real-world measurements," said Albertson, whose research covers mobile sensing to identify fugitive methane emissions in oil and gas production regions as well as in the fertilizer industry.
The researchers" findings are reported in "Estimation of Methane Emissions From the U.S. Ammonia Fertilizer Industry Using a Mobile Sensing Approach," published in the journal "Elementa."
The use of natural gas has grown in recent years, bolstered by improved efficiency in shale gas extraction and the perception that natural gas is a less dirty fossil fuel than coal, oil, gasoline or diesel.
"But natural gas is largely methane, which molecule-per-molecule has a stronger global warming potential than carbon dioxide," Albertson said. "The presence of substantial emissions or leaks anywhere along the supply chain could make natural gas a more significant contributor to climate change than previously thought."
Ammonia fertilizer is produced at just a few dozen plants in the United States. Factories are often located near public roadways, where emissions carried downwind can be detected, in this case by mobile sensors.
"Even though a small percentage is being leaked, the fact that methane is such a powerful greenhouse gas makes the small leaks very important," said Joseph Rudek, co-author and lead senior scientist at the Environmental Defense Fund. "In a 20-year timeframe, methane"s global warming potential is 84 times that of carbon dioxide."
For this study, the Google Street View vehicle traveled public roads near six representative fertilizer plants in the country"s midsection to quantify "fugitive methane emissions," defined as inadvertent losses of methane to the atmosphere.
The emissions were likely due to incomplete chemical reactions during fertilizer production, incomplete fuel combustion or leaks, the researchers explained.
The team discovered that, on average, 0.34 percent of the gas used in the plants is emitted to the atmosphere.
Scaling this emission rate from the six plants to the entire industry suggests total annual methane emissions of 28 gigagrams per year – 100 times higher than the fertilizer industry"s self-reported estimate of 0.2 gigagrams per year.
This figure far exceeds the EPA"s estimate that all industrial processes in the United States produce only eight gigagrams of methane emissions per year.
The research was funded in part by a grant from the Atkinson Center for a Sustainable Future"s joint research program with the Environmental Defense Fund.
