Multi-axis differential optical absorption spectroscopy (MAX-DOAS) and direct sun NO₂ vertical column network data are used to investigate the accuracy of tropospheric NO₂ column measurements of the GOME-2 instrument on the MetOp-A satellite platform and the OMI instrument on Aura. The study is based on 23 MAX-DOAS and 16 direct sun instruments at stations distributed worldwide. A method to quantify and correct for horizontal dilution effects in heterogeneous NO₂ field conditions is proposed. After systematic application of this correction to urban sites, satellite measurements are found to present smaller biases compared to ground-based reference data in almost all cases. We investigate the seasonal dependence of the validation results as well as the impact of using different approaches to select satellite ground pixels in coincidence with ground-based data. In optimal comparison conditions (satellite pixels containing the station) the median bias between satellite tropospheric NO₂ column measurements and the ensemble of MAX-DOAS and direct sun measurements is found to be significant and equal to −34 % for GOME-2A and −24 % for OMI. These biases are further reduced to −24 % and −18 % respectively, after application of the dilution correction. Comparisons with the QA4ECV satellite product for both GOME-2A and OMI are also performed, showing less scatter but also a slightly larger median tropospheric NO₂ column bias with respect to the ensemble of MAX-DOAS and direct sun measurements.

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.

Nitrogen dioxide (NO₂) profile retrievals were performed by ship-borne Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) using a compact/low-power spectrometer on the Japanese research vessel Kaiyo during two ocean cruises around Japan and Japan–Bali (Indonesia)–Indian Ocean. DOAS analysis using a 425–450 nm fitting window revealed a clear land–ocean contrast in NO₂ differential slant column densities (DSCDs) but poor fitting results and negative values, especially at low elevation angles at low latitudes (<~20° N). The poor fitting resulted in sparse NO₂ volume mixing ratio (VMR) data for the 0–1 km layer after applying our vertical profile retrieval method. In contrast, NO₂ VMRs retrieved using fitting results from 460–490 nm are positive even at low latitudes, while they are reasonably similar to those obtained from 425–450 nm at mid-latitudes. Because NO₂ DSCD for 425–450 nm shows a negative correlation with water vapor (H₂O) DSCD, the poor fitting appears to be due primarily to interference by H₂O. We analyzed a 338–370 nm fitting window, which is free from H₂O, and found good agreement between NO₂ VMRs retrieved from 460–490 nm and 338–370 nm, even at low latitudes, at NO₂ VMRs higher than ~0.2 ppbv. The results indicate that the background value of NO₂ VMR over the western Pacific and Indian Ocean during the cruises was less than ~0.2 ppbv, with occasional enhancement to levels of ~0.2–0.4 ppbv.

We introduce the new Global Ozone Monitoring Experiment-2 (GOME-2) daily and monthly level-3 product of total column ozone (O₃), total and tropospheric column nitrogen dioxide (NO₂), total column water vapour, total column bromine oxide (BrO), total column formaldehyde (HCHO), and total column sulfur dioxide (SO₂) (daily products https://doi.org/10.15770/EUM_SAF_AC_0048, AC SAF, 2023 a ; monthly products https://doi.org/10.15770/EUM_SAF_AC_0049, AC SAF, 2023 b ). The GOME-2 level-3 products aim to provide easily translatable and user-friendly data sets to the scientific community for scientific progress as well as to satisfy public interest. The purpose of this paper is to present the theoretical basis as well as the verification and validation of the GOME-2 daily and monthly level-3 products. The GOME-2 level-3 products are produced using the overlapping area-weighting method. Details of the gridding algorithm are presented. The spatial resolution of the GOME-2 level-3 products is selected based on the sensitivity study. The consistency of the resulting level-3 products among three GOME-2 sensors is investigated through time series of global averages, zonal averages, and bias. The accuracy of the products is validated by comparison to ground-based observations. The verification and validation results show that the GOME-2 level-3 products are consistent with the level-2 data. Small discrepancies are found among three GOME-2 sensors, which are mainly caused by the differences in the instrument characteristic and level-2 processor. The comparison of GOME-2 level-3 products to ground-based observations in general shows very good agreement, indicating that the products are consistent and fulfil the requirements to serve the scientific community and general public.

We investigated long-term observations of the tropospheric nitrogen dioxide vertical column density (NO₂ TropVCD) from the Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) network in Russia and ASia (MADRAS) from 2007 to 2017 at urban (Yokosuka and Gwangju) and remote (Fukue and Cape Hedo) sites in East Asia. The monthly mean in the NO₂ TropVCD from MAX-DOAS measured at ~13:30 local time, which is the Ozone Monitoring Instrument (OMI) overpass time, shows good agreement with OMI data during summer, but differences between the two datasets increase in winter. The Theil-Sen slope of the long-term trend indicate a relatively rapid and gradual reduction in NO₂ at Yokosuka and two remote sites (Fukue and Cape Hedo), respectively, regardless of the season except for fall at Fukue, but significant changes in NO₂ are not observed at Gwangju, Korea. In contrast, OMI satellite data reveal an increase in the NO₂ TropVCD at all sites except for Yokosuka, where a decreasing trend common to MAX-DOAS is found, suggesting that the results from satellites need to be cautiously used for investigating long-term trends in less polluted or remote areas. Using backward trajectories, potential source regions are identified for the two urban sites. The spatial distribution from OMI data shows good agreement with the potential source regions at Yokosuka. The potential source regions in Gwangju are identified as the National Industrial Complex in Yeosu and Gwangyang, while the transport route is not clearly visible with OMI data because of their low sensitivity in less polluted areas. The proposed approach is suitable for identifying potential source areas that might not be recognized by satellite observations.

Iodine compounds destroy ozone (O₃) in the global troposphere and form new aerosols, thereby affecting the global radiative balance. However, few reports have described the latitudinal distribution of atmospheric iodine compounds. This work reports iodine monoxide (IO) measurements taken over unprecedented sampling areas from the Arctic to the Southern Hemisphere and spanning sea surface temperatures (SSTs) of approximately 0 to 31.5 ^∘C. The highest IO concentrations were observed over the Western Pacific warm pool (WPWP), where O₃ minima were also measured. There, a negative correlation was found between O₃ and IO mixing ratios at extremely low O₃ concentrations. This correlation is not explained readily by the O₃-dependent oceanic fluxes of photolabile inorganic iodine compounds, which is the dominant source in recent global-scale chemistry transport models representing iodine chemistry. Actually, the correlation rather implies that O₃-independent pathways can be similarly important in the WPWP. The O₃-independent fluxes result in a 15 % greater O₃ loss than that estimated for O₃-dependent processes alone. The daily O₃ loss rate related to iodine over the WPWP is as high as approximately 2 ppbv (parts per billion by volume) despite low O₃ concentrations of approximately 10 ppbv, with the loss being up to 100 % greater than that without iodine. This finding suggests that warming SST driven by climate change might affect the marine atmospheric chemical balance through iodine–ozone chemistry.