But why is it that some areas are "smelly"—like rotten eggs or crude oil—and other areas are odorless, despite the presence of distinctive plumes of vapor? The answer is in the chemical makeup of the gases.

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Most of the gas emitted from Yellowstone's thermal features is steam (boiling water), which is often visible, especially on a cold morning. If we ignore the steam, the remaining gas is primarily carbon dioxide (typically > 90%) with minor additions of helium, hydrogen sulfide, nitrogen, oxygen, methane, ammonia and other trace gases.

Research on the chemistry of Yellowstone's gas emissions is driven by a need to better understand and monitor both the deep magmatic system and the overlying hydrothermal system. This research shows there are 3 primary sources of gas at Yellowstone: the deep magmatic system; shallower crustal (rock-related) sources; and the atmosphere. The gas that ultimately discharges at the surface may contain components from all of these sources.

Studies of gas emissions are further complicated by the fact that some gas components are found in more than one source. For example, helium and carbon dioxide are emitted by magma at all volcanoes, but can also be released from crustal rocks under the influence of heat. So how can you tell the difference between these sources of gas? One method is to determine the isotopic composition of some gas components which can tell the story about gas origins.

For example, prominent degassing features at the Mud Volcano thermal area north of Yellowstone Lake include large churning pools of muddy water and a long-lived a superheated fumarole—the hottest vent in the park at ~114°C (237°F). Hydrogen sulfide makes up about 0.15% of the gas emissions and accounts for the strong rotten egg odor that permeates the air. Helium concentrations at Mud Volcano are only about 0.002% of the gas, but isotopic analyses of the carbon dioxide and helium readily confirm the link to the magmatic system. In fact, the helium isotopes show that the Mud Volcano gases have the strongest magmatic signature of any of Yellowstone's thermal areas. This thermal area is located well within the Yellowstone Caldera, so the strong connection to magma is reasonable.

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In contrast, seeps at Devil's Den, below Tower Fall along the Yellowstone River, emit gas with a tar-like organic odor of hydrocarbons tinged with ammonia. South of the Tower-Roosevelt junction, similar hydrocarbon-rich gases are found at Washburn Hot springs. At these sites, carbon and helium isotopes indicate that the magmatic gases are significantly diluted by gases formed from the thermal breakdown of organic matter in crustal rocks.

Nitrogen and oxygen in Yellowstone gas emissions are primarily derived from the atmosphere. These gases dissolve in the rain water that percolates from the surface down into the hydrothermal system. They are released back to the atmosphere as the heated water boils back up to the surface, where these odorless trace gases provide hints about subsurface water flow.

Studies of gases at Yellowstone are expanding rapidly as new methods are developed for long-term measurements of steam, carbon dioxide and hydrogen sulfide. Typically, gas samples are collected sporadically. One of the first quasi-continuous gas sensors was installed near Norris Geyser Basin during the summer of 2016, and in the summer of 2017 the equipment was placed on the Central Plateau. In 2018, there are plans to install Yellowstone's first continuously operating (year-round) gas sensor at Norris. Stay tuned to Yellowstone Caldera Chronicles for details on the results from these experiments, and more information about both smelly and non-smelly gases!