[A-AS11] Atmospheric Chemistry

[AAS11-12] Evaluation of processes driving iodine monoxide and ozone variability over the Western Pacific Warm Pool

*Takashi Sekiya¹, Hisahiro Takashima^2,1, Yugo Kanaya¹, Fumikazu Taketani¹, Kengo Sudo^3,1, Martina M. Friedrich⁴, Michel Van Roozendael⁴ (1.Japan Agency for Marine-Earth Science and Technology, 2.Faculty of Science, Fukuoka University, 3.Graduate School of Environmental Studies, Nagoya University, 4.Royal Belgian Institute for Space Aeronomie)

Keywords:Troposheric ozone, Iodine chemistry, Chemical transport model, Ship-borne observations

Iodine chemistry exerts a nonnegligible influence on tropospheric ozone depletion over oceanic regions, which is a potent greenhouse gas following CO₂ and CH₄. The impact of iodine on tropospheric ozone has been extensively studied using three-dimensional chemical transport models (CTMs). However, CTMs fail to capture negative relationships between iodine monoxide (IO) and ozone over the Western Pacific Warm Pool (WPWP) observed by ship-borne observations (Takashima et al., 2022), because the factors governing the IO-ozone relationships remain uncertain. This study examines the impact of fine-scale meteorological variability and ozone-independent iodine oceanic sources on the IO and ozone concentrations through ship-borne observations and high-resolution CTMs during November–December 2014. The high (0.56°)-resolution model demonstrates a negative correlation between IO and ozone at the observation locations (r = -0.45), which is more closely aligned with that derived from the ship-borne observation data (r = -0.70) than the coarse (2.8°)-resolution model (r = 0.08). Sensitivity analysis indicates that a contrast in the correlation between the 0.56° and 2.8° resolutions emerges from the interplay of fine-scale atmospheric transport and chemistry processes, rather than from fine-scale atmospheric transport solely or ozone-dependent oceanic iodine release. This interplay leads to a greater ozone loss mediated by the iodine cycle at 0.56° resolution (by 0.56 ppbv day^-1) compared to that at 2.8° resolution (by 0.20 ppbv day^-1), due to the reduced mixing with air outside the boundary layer over the WPWP at finer resolution. Furthermore, incorporating ozone-independent iodine sources such as photolysis of CH₂I₂, CH₂IBr, and CH₂ICl enhances the IO-ozone anti-correlation coefficient (r = -0.50). These findings highlight the critical roles of interplay of the fine-scale atmospheric transport and chemistry processes and oceanic ozone-independent iodine sources in the co-variability of IO and ozone over the WPWP.