A team led by James Roberts, from the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colorado, has developed a new mass spectrometer capable of measuring nitryl chloride (ClNO2) – a chemical that encourages the formation of ozone.
This compound is created when nitrogen oxides – from ship exhausts and city smoke – mix with aerosol particles containing chloride, such as sea salt spray. Until recently there was no way of measuring nitryl chloride, so nobody knew how much was floating around.
Cruising along the south-east coast of the US, Roberts and his colleagues recorded unexpectedly high levels of nitryl chloride near the cities of Houston and Miami.
We saw nitryl chloride levels over one part per billion on several occasions, more than 20 times greater than previous estimates from numerical models," says Roberts.
The highest levels of nitryl chloride occurred at night, when nitrogen oxides – from ship exhaust plumes and industrial pollution – mixed with chlorine from sea salt spray. During the day the action of sunlight on nitryl chloride breaks it down into chlorine atoms and nitrogen dioxide.
These reactive chlorine atoms play a key role in encouraging ozone to form. In the lower atmosphere ozone is a major pollutant, causing respiratory problems and increasing human mortality rates.
"Such chemistry could occur in any urban coastal regions, potentially leading to a globally significant effect," says Lucy Carpenter, an atmospheric chemist at the University of York, UK.
Roberts suggests that areas at risk are likely to include southern California and the eastern seaboard of the US, much of the Mediterranean region and large parts of southern Asia. But as yet it is hard to judge how widespread the pollution is.
"Extrapolating the results to the global scale is problematic. It is possible, if not likely, that the overall importance of nitryl chloride is limited to heavily polluted conditions relatively close to major nitrogen oxide sources, such as those investigated in this study," says Bill Keene, an atmospheric scientist at the University of Virginia.
But even if the problem is largely confined to industrial ports, that doesn't mean it should be ignored.
"These results reinforce the need to control nitrogen oxide emissions to the greatest reasonable extent," says Alex Pszenny, from the University of New Hampshire.