Iodide

The deposition of ozone to seawater is an important ozone sink. Despite constituting as much as a third of the total ozone deposition, it receives significantly less attention than the deposition to terrestrial ecosystems. Models have typically calculated the deposition rate based on a resistance-in-series model with a uniform waterside resistance. This leads to models having an essentially uniform deposition velocity of approximately 0.05 cm s −1 to seawater, which is significantly higher than the limited observational dataset. Following from Luhar et al. (2018) we include a representation of the oceanic deposition of 5 ozone into the GEOS-Chem model of atmospheric chemistry and transport based on its reaction with sea-surface iodide. The updated scheme halves the calculated annual area-weighted mean deposition velocity to water from 0.0464 cm s −1 (25 th and 75 th percentiles of 0.0461 cm s −1 and 0.0471 cm s −1 respectively), to 0.0231 cm s −1 (25 th and 75 th percentiles of 0.0121 cm s −1 and 0.0303 cm s −1 respectively). The calculated ozone deposition velocity varies from 0.009 cm s −1 in polar waters to 0.040 cm s −1 at the tropics. This improves comparisons to observations. The variability is driven mainly by the temperature-10 dependant rate constant for the reaction between iodide and ozone, the temperature dependence of the solubility and variations in the ocean iodide concentration. The calculated annual deposition flux of ozone to the ocean is reduced from 222 Tg yr −1 to 112 Tg yr −1 , and overall deposition of ozone to all surface types reduces from 862 Tg y −1 to 758 Tg y −1. Tropospheric ozone burdens and global mean OH increase from 324 Tg to 328 Tg, and from 1.17 × 10 6 molec cm −3 to 1.18 × 10 6 molec cm −3 , respectively. 34% of surface grid boxes experience a 10% or greater increase in ozone concentration. Comparisons between 15 observations of surface ozone and the model are improved with the new parameterization notably around the Southern Ocean. Process level representation of oceanic deposition of ozone thus appears essential for representing the concentration of surface ozone over the planet.