Mini Ozone Holes Due to Dust Release of Iodine in the Remote Tropical Free Troposphere

Abstract:

Significant enhancements of iodine have been observed in Sahara dust events in form of methyl iodide (CH₃I) and iodine monoxide (IO) radicals, but atmospheric models currently do not consider dust as a source of iodine. Dust plumes are often accompanied by significant ozone loss, which is commonly attributed to reactive uptake of O₃ and other odd oxygen species (i.e., N₂O₅, HNO₃) on dust surfaces. However, laboratory experiments struggle to reproduce the large reactive uptake coefficients needed to explain field observations, and do not consider iodine chemistry. We present first observations of "mini ozone holes" in the remote (Southern Hemisphere) tropical free troposphere west of South America (TORERO field campaign), and show IO is elevated in otherwise unpolluted (low NO_x) dust layers that originate from the Atacama and Sechura Deserts. Ozone concentrations inside these elevated dust layers are often 10-20 ppb, and as low as 3 ppb. Ozone depletion is found to be widespread, i.e., dust layers extend thousands of km along the coast, up to 6 km altitude, and 500 km over the open ocean. IO radical concentrations inside decoupled dust layers are higher than in the marine boundary layer, and exceed nearby free tropospheric IO background concentrations by as much as a factor 6, indicating vigorous gas-phase ozone destruction by iodine chemistry inside the dust layers. We use field measurements of IO and BrO radicals, CH₃I, NO₂, photolysis frequencies, aerosol size distributions, and high-spectral resolution lidar aboard the NSF/NCAR Gulfstream 5 aircraft, 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". The implications for surface air quality, oxidative capacity, and climate are briefly discussed.