Abstract The ocean is a large but uncertain sink of tropospheric ozone. Ozone deposition is controlled partly by its reactions with marine substances, but in situ evidence of this marine biogeochemical control remains sparse. Here we report a novel measurement of ozone uptake efficiency (OUE) from a trans‐Atlantic cruise (50°N to 45°S). Observed OUE for surface waters varied two‐fold and the implied chemical deposition velocity varied from 0.012 to 0.034 cm s−1. Iodide accounted for on average 2/3 of total OUE, with generally higher contributions in tropical waters. The residual OUE, generally higher in temperate waters and positively correlated with biological proxies, was likely due to marine organics. OUE was also measured for 1,000 m waters, which were likely devoid of iodide but contained biologically refractory organics. Unexpectedly, these waters were rather reactive toward ozone, suggesting that surface organics that affect ozone uptake are not all freshly produced by marine biota.

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