Catalyst for Coastal Smog Shows Up Inland

Reducing smog in urban areas just grew a bit more challenging. Researchers have discovered a surprising amount of nitryl chloride in the air over Boulder, Colorado. The compound, which contributes to smog, was supposed to exist only in coastal areas—and Boulder is 1400 kilometers from the ocean. If confirmed, the findings mean that environmental regulators across the land must contend with a previously unknown source of pollution.

Certain molecules react easily with sunlight and change the composition of the troposphere, the lowest kilometer of the atmosphere. One such molecule is nitryl chloride. Sunlight breaks it into nitrogen oxide and chlorine, and in turn the chlorine reacts with organic compounds in the atmosphere to make ozone, O3, one of the constituents of smog. Normally, smog needs lots of sunlight to form, which is why it builds up during the day and diminishes at night. But nitryl chloride can kick-start the process.

Until now, scientists had thought nitryl chloride produced ozone only in coastal areas, where the salt in seawater is lofted into the air by wind and waves. Chlorine from the salt quickly reacts with certain atmospheric nitrogen oxides, which are present only at night, to form nitryl chloride. The reactions were thought to be irrelevant for inland areas, which lack significant sources of chlorine.

That assumption started to change in February 2008, when atmospheric scientist Joel Thornton of the University of Washington, Seattle, joined his colleague Steven Brown of the National Oceanic and Atmospheric Administration in Boulder to prepare for an upcoming ocean research cruise. "When we decided to test our instrument for nitryl chloride detection," Thornton said, "we found that this chlorine atom precursor is produced in Boulder at levels similar to coastal regions. This was quite surprising."

Last February, Thornton, Brown, and the rest of the team started studying the phenomenon. At a site just west of Boulder, they measured nitryl chloride quantities round the clock for 2 weeks. The team reports in tomorrow's issue of Nature that nitryl chloride levels at the site were nearly half the peak amounts in coastal areas. That's enough to contribute significantly to ozone production.

The chlorine in the nitryl chloride could be coming from windblown dust salty soils, coal-fired power plants, salt applied to icy roads, or forest and brush fires. Whatever the source, Thornton says, the findings reveal a previously unknown smog-producer in urban areas. But nitryl chloride may have a good side: it helps to break down the greenhouse gas methane. Consequently, Thornton says, "we need to develop a strong understanding of the air quality and climate implications of our findings."

Nitryl chloride adds a new factor to inland air chemistry, says atmospheric chemist Geoffrey Tyndall of the National Center for Atmospheric Research, also in Boulder. The chemical can start contributing to smog as soon as the sun rises, he says, so it's going to have to be studied more carefully when officials draft ozone regulations. "It could be a big player, particularly in the northeastern part of the country, which has much more vehicle traffic," he says.