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
Authors: Kanaya Yugo , Miyazaki Kazuyuki , Taketani Fumikazu , Miyakawa Takuma , Takashima Hisahiro , Komazaki Yuichi , Pan Xiaole , Kato Saki , Sudo Kengo , Sekiya Takashi , Inoue Jun , Sato Kazutoshi , Oshima Kazuhiro 
Published: June 2019

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
global atmospheric chemistry 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 mixing ratio fields from the tropospheric chemistry
reanalysis version 2 (TCR-2), produced by assimilating a suite of satellite
observations of multiple species into a global atmospheric chemistry 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 mixing ratios, 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 ppbv (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 by
TCR-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 been
overestimated. In the second analysis, over the western Pacific equatorial
region (125–165∘ E, 10∘ S to 25∘ N), the
observed O3 level more frequently decreased to less than 10 ppbv 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 ppbv 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.