Measuring up
Measuring up "Nitryl chloride has been speculated to exist for decades, but only within the past five years have we developed the ability to measure it," Joel Thornton of the University of Washington, US, told environmentalresearchweb., March 30, 2010

By using a chemical ionization mass spectrometer, Thornton and colleagues from the University of Washington; the US National Oceanic and Atmospheric Administration; and the Cooperative Institute for Research in Environmental Studies, Boulder, US, found that the concentration of nitryl chloride at Boulder, Colorado, roughly one mile above sea level, was 500 parts per trillion. This is about the same as the concentration that was measured above the ocean during a subsequent research cruise between Long Island Sound, US, Norway and Iceland.

"In February 2008, we were in Boulder, Colorado, installing our instruments in a shipping container to prepare for a chlorine-related study on a research cruise across the Atlantic," said Thornton. "While in Boulder, we decided to test our instrument developed for measuring a potentially important chlorine atom precursor – nitryl chloride. We found that this chlorine atom precursor is produced in Boulder, 900 miles from any coast, at levels similar to coastal regions. This was initially quite surprising and frankly completely unexpected."

Once back from the research cruise, the team carried out more measurements, this time from a park 150 feet above the city, a location removed from any obvious sources of chloride. Information from air-quality monitoring in national parks around the US indicated that nitryl chloride is present there too.

"Chlorine atoms are highly reactive, impacting the lifetime of hydrocarbons such as the greenhouse gas methane, and they are thought to enhance smog formation in coastal regions, but their sources in the air remain poorly characterized," said Thornton. "Most chlorine in the first kilometre or two of the atmosphere is emitted by sea spray, though in that form – sodium chloride – it is relatively inert."

Reaction of air pollutants with sea spray can transform the chloride into more reactive compounds, such as nitryl chloride (ClNO2), that liberate chlorine atoms when hit by sunlight, says Thornton. "Formation of chlorine atom precursors is expected in coastal settings where there is a lot of sea spray and often a lot of pollution," he added.

Following their discovery at Boulder, the researchers believe that night-time chemistry involving oxides of nitrogen is producing nitryl chloride at locations away from the coast. The exact source of the chloride is unclear; besides sea spray, chloride can enter the atmosphere from sources such as coal and biomass burning, chemicals used on icy winter roads, and the giant salt flats in Nevada and Utah. Nitrogen oxides, meanwhile, are produced by combustion and by lightning strikes.

"Long-term air-quality data sets suggest this chemistry is widespread during the night-time in polluted air across the US," said Thornton. "We think that this chemistry can also explain a significant fraction of chlorine atoms thought to be produced in the air globally."

The findings require a rethink of the role of night-time chemistry in the formation of smog in polluted regions and its connection to climate-forcing agents, say the researchers. Typically reactions overnight are believed to be a net sink for the pollutants that contribute to smog and haze formation. But if the production of nitryl chloride is as widespread as the team's findings indicate, there are two implications for air quality.

"First, nitryl chloride releases both nitrogen dioxide and chlorine atoms when hit by sunlight," said Thornton. "Together these will kick-start the smog formation cycles so that they start earlier in the day and therefore proceed for longer, resulting in more ozone and more oxidants, which form haze particles. Thus, smog formation may be more efficient than we thought."

There are also implications for climate, since human-induced nitrogen-oxide emissions are known to increase haze particle formation and boost destruction rates of greenhouse gases, such as methane. "Our findings suggest that these 'cooling' effects may be stronger than currently thought," said Thornton.

"We're just in the beginning stages of this research," he added. "Much remains to do, not least of which is to develop a stronger and more detailed understanding of the air quality and climate implications of our findings. For this, we must continue to confirm the presence of this chemistry in other regions and incorporate what we understand about it into computer models of air quality and climate."

The researchers reported their work in Nature.