The Air Gains Weight

In a rare change to one of the most basic measurements, scientists say the air around us is heavier than they had thought.

A 1969 measurement of the level of argon in the air we breathe was too low, according to a team from the Korea Research Institute of Standards and Science and the International Bureau of Weights and Measures in France.

Argon is a gas that rarely interacts chemically with anything. The 25-year-old measurement assumed that argon was 0.917 percent of the air's total composition. The new measurement, reported in a recent issue of the journal Metrologia, puts the value at 0.9332 percent.

The other contents of Earth's atmosphere are nitrogen (78 percent), oxygen (21 percent), water vapor (typically about one percent), and carbon dioxide (0.04 percent). Stuff coming in at below 0.01 percent include neon, helium, methane, hydrogen, nitrous oxide, and ozone.

The new argon results imply that the air is denser by 0.01 percent. Although such a small change would seem to be insignificant, it does affect precision measurements of mass.

Consider a pound of feathers on a scale opposite a pound of lead. In a vacuum, the scale is balanced, but this is not true when air is present. It will push up on the feathers, just as water pushes up on a floating object. Because the feathers are more "buoyant" in air than lead, the scale will tip towards the metal.

Scientists can mathematically correct for air buoyancy, but they need to know the precise fractions of air's constituents. With the old argon value, there were errors - in weighing, for instance, stainless steel - at the level of 15 parts in a billion.

To put it another way, a measurement of 1,000 tons of steel would have been off by half an ounce.

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