Lofted Dust Initiates Iodine-Induced Ozone Loss

Significant enhancements of iodine have been observed in Sahara dust events in form of methyl iodide (CH₃I) and iodine monoxide (IO), but atmospheric models currently do not consider dust to be a source of iodine in part due to sparse field observations. Dust plumes are often accompanied by significant ozone loss, which is commonly attributed to reactive uptake on dust surfaces. This presentation uses aircraft observations of “mini ozone holes” near the west coast of South America (TORERO field campaign), which are associated with dust layers originating from the Atacama and Sechura Deserts. Ozone depletion in these layers are found to be widespread, i.e., thousands of km along the coast, up to 6 km altitude, and 500 km over the open ocean. IO concentrations inside these layers exceed background IO concentrations in the free troposphere by as much as a factor 6, indicating vigorous gas-phase ozone destruction by iodine chemistry. We use field measurements of IO- and BrO radicals, CH₃I, NO₂, photolysis frequencies, aerosol size distributions, and high-spectral resolution lidar in conjunction with cloud resolving back trajectories and chemical model simulations to estimate the iodine source from dust, and investigate the mechanisms responsible for the “mini ozone holes” (e.g., reactive ozone uptake, odd oxygen uptake on dust surfaces, gas-phase iodine chemistry). The atmospheric relevance of dust as a source for iodine, sink for ozone, and implications for ozone burden are discussed, which impact air quality and climate.