How do we know that the methane comes from the natural gas field, and not another source, like big herds of cattle, which are known to produce lots of methane? In addition to methane, my colleagues and I also measured other chemicals, including ammonia and ethane, from aboard the airplane. Cows release methane along with ammonia into the air, but no ethane. Natural gas, on the other hand, contains both methane and ethane, but no ammonia. Therefore, we can tell the source of the methane depending on whether or not there is more ammonia or ethane in the air. In each of the three natural gas fields we studied, the increase in methane in the air downwind of each field corresponded with an increase in ethane, but not ammonia. Therefore, we concluded that natural gas operations were the major source of the methane in the atmosphere.

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