Spatial and temporal variability of iodine in aerosol

In this work we describe the compilation and homogenization of an extensive dataset of aerosol total iodine field observations in the period between 1963 and 2018 and we discuss its spatial and temporal trends. Total iodine in aerosol shows a distinct latitudinal dependence, with an enhancement towards the northern hemisphere (NH) tropics and lower values towards the poles. Longitudinally, there is some indication of a wave-one profile in the Tropics, which peaks in the Atlantic and shows a minimum in the Pacific, following the well-known wave-one longitudinal variation of tropical tropospheric ozone. These spatial trends result from the global distribution of the main oceanic iodine source to the atmosphere (the reaction of surface ozone with aqueous iodide on the sea water-air interface). New data from Antarctica show that the south polar seasonal variation of iodine in aerosol mirrors that observed previously in the Arctic, with two equinoctial maxima and the dominant maximum occurring in spring. While no clear seasonal variability is observed in NH middle latitudes, there is an indication of different seasonal cycles in the NH tropical Atlantic and Pacific. A weak positive long-term trend is observed in the tropical annual averages, which is consistent with an enhancement of the anthropogenic ozone-driven global oceanic source of iodine over the last 50 years.