15 April 2008
Scientists at the ESRL Chemical Sciences Division, Pacific Marine Environmental Laboratory, and the University of New Hampshire have made the first real-world observations of a potentially important source of active halogens in the coastal troposphere, nitryl chloride (ClNO2). The chemical forms at night from the interaction of chemicals derived from anthropogenic nitrogen oxides (NOx) pollution and sea salt. Sunlight breaks down the ClNO2 during the daytime, releasing chlorine atoms (Cl) that then take part in reactions that form ozone. The authors concluded that in addition to sea salt, many other particles containing chloride also lead to ClNO2 production. The authors published their findings online in the April 6 edition of Nature Geoscience. Significant press coverage of the story has occurred, including stories by ABC News, New Scientist (Sea breezes carry unhealthy whiff of ozone), Chemistry World, and the Houston Chronicle.
Background: Chemically active halogens (free radicals such as chlorine atoms, bromine atoms, or oxides ClO, BrO, and IO) are important in the chemistry of the lower atmosphere, especially oxidant chemistry at midlatitudes. However, the detailed chemical processes that convert and cycle halogens in the lower atmosphere are still quite uncertain. Nitryl chloride is a potentially important source of active halogens that, until this study, had not been directly observed in the atmosphere. The authors made their ClNO2 observations in the Gulf of Mexico near Houston with a chemical-ionization mass spectrometry technique deployed on the NOAA research vessel Ronald H. Brown during the 2006 Texas Air Quality Study. They also measured simultaneously the amount of dinitrogen pentoxide (N2O5) as well as other trace gases, so that they could decipher the chemical processes that form the ClNO2 as well as the subsequent chemistry that leads to ozone formation.
Significance: The levels of ClNO2 observed in this study are much greater than earlier estimates based on numerical models. The results indicated that the ClNO2 chemistry could affect oxidant formation in areas where NOx and sea-salt chloride sources exist. Just over half of the global population resides within 200 km of a coastline, where such processes could enhance the photochemical production of ozone. Climate-related effects could also occur through the radiative forcing by ozone, as well as through the interaction of released halogens with sulfur chemistry in the marine atmosphere. This research contributes to the Air Quality Program of NOAA's Weather and Water Goal.
Osthoff1,2, H.D., J.M. Roberts1, A.R. Ravishankara1,3, E.J. Williams1, 2, B.M. Lerner1,2, R. Sommariva1,2, T.S. Bates4, D. Coffman4, P.K. Quinn4, J.E. Dibb5, H. Stark1,2, J.B. Burkholder1, R.K. Talukdar1,2, J. Meagher1, F.C. Fehsenfeld1,2, and S.S. Brown1, High levels of nitryl chloride in the polluted subtropical marine boundary layer, Nature Geosciences, doi:10.1038/ngeo177, 2008.