A nocturnal atmospheric loss of CH₂I₂ in the remote marine boundary layer

Ocean emissions of inorganic and organic iodine compounds drive the biogeochemical cycle of iodine and produce reactive ozone-destroying iodine radicals that influence the oxidizing capacity of the atmosphere. Di-iodomethane (CH₂I₂) and chloro-iodomethane (CH₂ICl) are the two most important organic iodine precursors in the marine boundary layer. Ship-borne measurements made during the TORERO (Tropical Ocean tRoposphere Exchange of Reactive halogens and Oxygenated VOC) field campaign in the east tropical Pacific Ocean in January/February 2012 revealed strong diurnal cycles of CH₂I₂ and CH₂ICl in air and of CH₂I₂ in seawater. Both compounds are known to undergo rapid photolysis during the day, but models assume no night-time atmospheric losses. Surprisingly, the diurnal cycle of CH₂I₂ was lower in amplitude than that of CH₂ICl, despite its faster photolysis rate. We speculate that night-time loss of CH₂I₂ occurs due to reaction with NO₃ radicals. Indirect results from a laboratory study under ambient atmospheric boundary layer conditions indicate a k _CH2I2+NO3 of ≤4 × 10^-13 cm³ molecule^-1 s^-1; a previous kinetic study carried out at ≤100 Torr found k _CH2I2+NO3 of 4 × 10^-13 cm³ molecule^-1 s^-1. Using the 1-dimensional atmospheric THAMO model driven by sea-air fluxes calculated from the seawater and air measurements (averaging 1.8 +/- 0.8 nmol m^-2 d^-1 for CH₂I₂ and 3.7 +/- 0.8 nmol m^-2 d^-1 for CH₂ICl), we show that the model overestimates night-time CH₂I₂ by >60 % but reaches good agreement with the measurements when the CH₂I₂ + NO₃ reaction is included at 2-4 × 10^-13 cm³ molecule^-1 s^-1. We conclude that the reaction has a significant effect on CH₂I₂ and helps reconcile observed and modeled concentrations. We recommend further direct measurements of this reaction under atmospheric conditions, including of product branching ratios.