High levels of nitryl chloride in the polluted subtropical marine boundary layer

The cycling of halogen compounds in the lower atmosphere is poorly understood. It is known that halogens such as chlorine, bromine and iodine are converted from halides, which are relatively inert, to reactive radicals. These reactive radicals can affect ozone production and destruction, aerosol formation and the lifetimes of important trace gases such as methane, mercury and naturally occurring sulphur compounds. However, the processes by which halides are converted to reactive halogens are uncertain. Here, we report atmospheric measurements of nitryl chloride, an active halogen, along the southeast coastline of the United States and near Houston, Texas. We show that the main source of nitryl chloride is the night-time reaction of dinitrogen pentoxide with chloride-containing aerosol. The levels observed are much greater than earlier estimates based on numerical models and are sufficiently large to affect oxidant photochemistry in areas where nitrogen oxides and aerosol chloride sources coexist, such as urban areas and ship engine exhaust plumes.