Observations of I₂ at a remote marine site

Abstract. Inorganic iodine plays a significant role in the photochemistry of the marine boundary layer, but the sources and cycling of iodine are not well understood. We report the first I₂ observations in marine air that is not impacted by coastal macroalgal emissions or sea ice chemistry. The data clearly demonstrate that the very high I₂ levels previously reported for coastal air are not representative of open ocean conditions. In this study, gas phase I₂ was measured at the Cape Verde Atmospheric Observatory, a semi-remote site in the eastern tropical Atlantic, using atmospheric pressure chemical ionization tandem mass spectrometry. Atmospheric I₂ levels typically increased beginning at sunset, leveled off after midnight, and then rapidly decreased at sunrise. There was also a smaller midday maximum in I₂ that was probably caused by a measurement artifact. Ambient I₂ mixing ratios ranged from <0.02–0.6 pmol mol⁻¹ in May 2007 and <0.03–1.67 pmol mol⁻¹ in May 2009. The sea-air flux implied by the nighttime buildup of I₂ is too small to explain the observed daytime IO levels at this site. Iodocarbon measurements made in this region previously are also insufficient to explain the observed 1–2 pmol mol⁻¹ of daytime IO. The observations imply the existence of an unknown daytime source of gas phase inorganic iodine. Carpenter et al. (2013) recently proposed that sea surface emissions of HOI are several times larger than the flux of I₂. Such a flux could account for both the nighttime I₂ and the daytime IO observations.