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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 14 Jan 2019

Research article | 14 Jan 2019

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

Ozone and carbon monoxide observations over open oceans on R/V Mirai 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

Yugo Kanaya1, Kazuyuki Miyazaki1, Fumikazu Taketani1, Takuma Miyakawa1, Hisahiro Takashima1,2, Yuichi Komazaki1, Xiaole Pan1,3, Saki Kato2, Kengo Sudo1,4, Jun Inoue5, Kazutoshi Sato6, and Kazuhiro Oshima1,7 Yugo Kanaya et al.
  • 1Japan Agency for Marine–Earth Science and Technology (JAMSTEC), Yokohama 2360001 Japan
  • 2Fukuoka University, Fukuoka 8140133, Japan
  • 3now at: Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 4Nagoya University, Nagoya 4648601, Japan
  • 5National Institute of Polar Research, Tachikawa 1908518, Japan
  • 6Kitami Institute of Technology, Kitami 0908507, Japan
  • 7now at: Institute of Environmental Sciences, Rokkasho 0393212, Japan

Abstract. Constraints from ozone (O3) observations over oceans are needed in addition to those from terrestrial regions to fully understand global tropospheric chemistry and its impact on the climate. Here, we provide 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 derived from observations made during 24 research cruise legs of R/V Mirai during 2012 to 2017, in the Southern, Indian, Pacific, and Arctic Oceans, covering the region from 67° S to 75° N. The data are suitable for critical evaluation of the over-ocean distribution of ozone derived from chemical transport models. We first give an overview of the statistics in the data set and highlight key features in terms of geographical distribution and air mass type. We then use the data set to evaluate ozone concentration fields from Tropospheric Chemistry Reanalysis version 2 (TCR-2), produced by assimilating a suite of satellite observations of multiple species into a chemical transport model, namely CHASER. For long-range transport of polluted air masses from continents to the oceans, during which the effects of forest fires and fossil fuel combustion were recognized, TCR-2 gave an excellent performance in reproducing the observed temporal variations and photochemical buildup of O3 when assessed from ΔO3 / ΔCO ratios. For clean marine conditions with low and stable CO concentrations, two focused analyses were 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 ppb (21 %) on average. The observed vertical profiles from O3 soundings from R/V Mirai during September 2014 had less steep vertical gradients at low altitudes (> 850 hPa) than those obtained TCR-2. This suggests the possibilities of more efficient descent of the O3-rich air from above or less efficient dry deposition on the surface than were assumed in the model. In the second analysis, over the western Pacific equatorial region (125–165° E, 10° S to 25° N), the observed O3 level frequently decreased to less than 10 ppb in comparison to that obtained with TCR-2, and also those obtained in most of the Atmospheric Chemistry Climate Model Intercomparison Project (ACCMIP) model runs for the decade from 2000. These results imply loss processes that are unaccounted for in the models. We found that the model’s positive bias positively correlated with the daytime residence times of air masses over a particular grid, namely 165–180° E and 15–30° N; an additional loss rate of 0.25 ppb h−1 in the grid best explained the gap. Halogen chemistry, which is commonly omitted from currently used models, might be active in this region and could have contributed to additional losses. Our open data set covering wide ocean regions is complementary to the Tropospheric Ozone Assessment Report data set, which basically comprises ground-based observations, and enables a fully global study of the behavior of O3.

Yugo Kanaya et al.
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Yugo Kanaya et al.
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Short summary
Ozone and carbon monoxide levels were uniquely observed (for > 10 000 h) over oceans from 67° S to 75° N. Tropospheric Chemistry Reanalysis ver. 2 reproduced observed evolution of pollution plumes from continents but underpredicted and overpredicted ozone levels in the Arctic and in the western Pacific equatorial region, respectively. Processes to explain the gaps are proposed, including halogen-mediated destruction in the low latitudes. Our open data set will complement the TOAR data collection.
Ozone and carbon monoxide levels were uniquely observed (for  10 000 h) over oceans from 67° S...