Earth's atmosphere contains a number of trace gases that, while only present in small amounts, play important roles in processes that affect our planet's climate. One of these gases is nitrous acid, or HONO. HONO, when reacting with sunlight, is a source of hydroxyl radicals (OH) in the lower atmosphere.

These radicals are thought to help control the level of pollutants in the lowest region of the Earth's atmosphere, known as the troposphere.

"We sometimes refer to OH as the 'detergent' of the atmosphere, because it is the main atmospheric oxidant and helps to scrub the atmosphere of nearly all reactive gases," said Glenn Wolfe of the Atmospheric Chemistry and Dynamics Lab at NASA's Goddard Space Flight Center. "OH and HO2 inter-convert rapidly, thus we usually refer to them collectively as the HOx family."

Members of the HOx family allow the atmosphere to clean itself, and can even help break down hydrocarbons released by human activities like the burning of fossil fuels.

"The main sources of HOx are: ozone + light + water; formaldehyde + light; and HONO + light," explained Wolfe. "The relative strength of these sources depends on many physical and chemical parameters, and is thus a strong function of both location and time."

Previously, scientists thought that, in some regions, reactions between HONO and sunlight were responsible for creating 80% of the OH in the troposphere. The new study provides more details about the relationship between HOx and HONO, and shows that the old theory could be very wrong.

Sampling the Atmosphere
Wolfe and his colleagues spent time on a giant Zeppelin aircraft, collecting samples and atmospheric data from the skies above northern Italy. What they found was a large amount of HONO sitting in an undisturbed layer of the troposphere. The layer is isolated from processes at the Earth's surface by temperature inversions that happen early in the day.

This layer of HONO is produced by reactions between nitrous oxides (such as NO2) and hydroperoxy radicals (HO2), and ultimately ends up consuming HOx itself.

In short, OH is produced when HONO is struck by sunlight... but the OH is then quickly mopped up to produce new HONO after it is formed. This means that, overall, HONO is not actually an important source of OH production in this region of the atmosphere after all. It's a closed cycle where HONO produces OH, and the OH is then used to re-build the HONO.

Wolfe explained that the results do not mean there is less OH in the atmosphere than previously thought. Scientists have taken direct measurements of OH in the past, so we have a good idea of how much is present. The study does help us understand where the OH comes from and how the levels of OH in the atmosphere are maintained.

"What these results offer is an explanation for a long-standing discrepancy between HONO and OH observations," said Wolfe. "In many previous studies, the amount of HONO observed would imply much more OH than was observed. By assuming HONO formation is a sink for HOx, we have "null cycle" that stabilizes the chemistry."

A Small Piece of the Puzzle
The findings concerning HONO in the troposphere could be important in understanding atmospheric pollution at a local level. In terms of the global perspective, however, Wolfe feels there are larger forces at play.

"Globally, OH production is dominated by ozone photolysis, thus HONO will have a small impact on that scale," Wolfe commented.

Astrobiologists are concerned with processes in the atmosphere that affect the habitability of planet Earth. Atmospheric chemistry is also an important field of research for astrobiologists who hope to identify inhabited planets around distant stars by observing extrasolar atmospheres.

This study may not have huge implications for understanding atmospheric chemistry at the global level on Earth and its role in habitability, but the findings are still an important piece of the larger puzzle.

"HONO is an interesting and important molecule when considering near-surface air quality," said Wolfe, "but in the long term, climate change is our greatest enemy."

The study was supported in part by NOAA and the National Science Foundation.

The paper, "Missing Gas-Phase Source of HONO Inferred from Zeppelin Measurements in the Troposphere," was published in the journal Science.