Coastal iodine emissions. 1. Release of I₂ by laminaria digitata in chamber experiments

Tidally exposed macroalgae emit large amounts of I₂ and iodocarbons that produce hotspots of iodine chemistry and intense particle nucleation events in the coastal marine boundary layer. Current emission rates are poorly characterized, however, with reported emission rates varying by 3 orders of magnitude. In this study, I₂ emissions from 25 Laminaria digitata samples were investigated in a simulation chamber using incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). The chamber design allowed gradual extraction of seawater to simulate tidal emersion of algae. Samples were exposed to air with or without O₃ and to varying irradiances. Emission of I₂ occurred in four distinct stages: (1) moderate emissions from partially submerged samples; (2) a strong release by fully emerged samples; (3) slowing or stopping of I₂ release; and (4) later pulses of I₂ evident in some samples. Emission rates were highly variable and ranged from 7 to 616 pmol min^-1 gFW^-1 in ozone-free air, with a median value of 55 pmol min^-1 gFW ^-1 for 20 samples.