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Abstract. Constraints from ozone ( O3 ) observations over oceansare needed in addition to those from terrestrial regions to fully understandglobal tropospheric chemistry and its impact on the climate. Here, weprovide a large data set of ozone and carbon monoxide (CO) levels observed(for 11 666 and 10 681 h, respectively) over oceans. The data set is derivedfrom observations made during 24 research cruise legs of R/V Mirai during 2012 to2017, in the Southern, Indian, Pacific, and Arctic oceans, covering theregion from 67 ∘  S to 75 ∘  N. The data are suitable forcritical evaluation of the over-ocean distribution of ozone derived fromglobal atmospheric chemistry models. We first give an overview of thestatistics in the data set and highlight key features in terms ofgeographical distribution and air mass type. We then use the data set toevaluate ozone mixing ratio fields from the tropospheric chemistryreanalysis version 2 (TCR-2), produced by assimilating a suite of satelliteobservations of multiple species into a global atmospheric chemistry model,namely CHASER. For long-range transport of polluted air masses fromcontinents to the oceans, during which the effects of forest fires andfossil fuel combustion were recognized, TCR-2 gave an excellent performancein reproducing the observed temporal variations and photochemical buildup of O3 when assessed from ΔO3∕ΔCO ratios. For cleanmarine conditions with low and stable CO mixing ratios, two focused analyseswere performed. The first was in the Arctic (> 70 ∘  N)in September every year from 2013 to 2016; TCR-2 underpredicted O3 levels by 6.7 ppbv (21 %) on average. The observed vertical profiles from O3 soundings from R/V Mirai during September 2014 had less steep verticalgradients at low altitudes (> 850 hPa) than those obtained byTCR-2. This suggests the possibility of a more efficient descent of the O3 -rich air from above than assumed in the models. For TCR-2 (CHASER),dry deposition on the Arctic ocean surface might also have beenoverestimated. In the second analysis, over the western Pacific equatorialregion (125–165 ∘  E, 10 ∘  S to 25 ∘  N), theobserved O3 level more frequently decreased to less than 10 ppbv incomparison to that obtained with TCR-2 and also those obtained in most ofthe Atmospheric Chemistry Climate Model Intercomparison Project (ACCMIP)model runs for the decade from 2000. These results imply loss processes thatare unaccounted for in the models. We found that the model's positive biaspositively correlated with the daytime residence times of air masses over aparticular grid, namely 165–180 ∘  E and 15–30 ∘  N; anadditional loss rate of 0.25 ppbv h −1 in the grid best explained thegap. Halogen chemistry, which is commonly omitted from currently usedmodels, might be active in this region and could have contributed toadditional losses. Our open data set covering wide ocean regions iscomplementary to the Tropospheric Ozone Assessment Report data set, whichbasically comprises ground-based observations and enables a fully globalstudy of the behavior of O3 .

atmospheric chemistry and physics

Ozone and carbon monoxide observations over open oceans on R/V &amp;lt;i&amp;gt;Mirai&amp;lt;/i&amp;gt; from 67° S to 75° N during 2012 to 2017: testing global chemical reanalysis in terms of Arctic processes, low ozone levels at low latitudes, and pollution transport