This study presents the results from the Tropospheric Chemistry Reanalysis version 2 (TCR-2) for the period 2005–2018 at 1.1^∘ horizontal resolution obtained from the assimilation of multiple updated satellite measurements of ozone, CO, NO₂, HNO₃, and SO₂ from the OMI, SCIAMACHY, GOME-2, TES, MLS, and MOPITT satellite instruments. The reanalysis calculation was conducted using a global chemical transport model MIROC-CHASER and an ensemble Kalman filter technique that optimizes both chemical concentrations of various species and emissions of several precursors, which was efficient for the correction of the entire tropospheric profile of various species and its year-to-year variations. Comparisons against independent aircraft, satellite, and ozonesonde observations demonstrate the quality of the reanalysis fields for numerous key species on regional and global scales, as well as for seasonal, yearly, and decadal scales, from the surface to the lower stratosphere. The multi-constituent data assimilation brought the model vertical profiles and interhemispheric gradient of OH closer to observational estimates, which was important in improving the description of the oxidation capacity of the atmosphere and thus vertical profiles of various species. The evaluation results demonstrate the capability of the chemical reanalysis to improve understanding of the processes controlling variations in atmospheric composition, including long-term changes in near-surface air quality and emissions. The estimated emissions can be employed for the elucidation of detailed distributions of the anthropogenic and biomass burning emissions of co-emitted species (NO_x, CO, SO₂) in all major regions, as well as their seasonal and decadal variabilities. The data sets are available at https://doi.org/10.25966/9qgv-fe81 (Miyazaki et al., 2019 a).

Efforts to stem the transmission of coronavirus disease 2019 (COVID-19) led to rapid, global ancillary reductions in air pollutant emissions. Here, we quantify the impact on tropospheric ozone using a multiconstituent chemical data assimilation system. Anthropogenic NO_x emissions dropped by at least 15% globally and 18 to 25% regionally in April and May 2020, which decreased free tropospheric ozone by up to 5 parts per billion, consistent with independent satellite observations. The global total tropospheric ozone burden declined by 6TgO₃ (∼2%) in May and June 2020, largely due to emission reductions in Asia and the Americas that were amplified by regionally high ozone production efficiencies (up to 4 TgO₃/TgN). Our results show that COVID-19 mitigation left a global atmospheric imprint that altered atmospheric oxidative capacity and climate radiative forcing, providing a test of the efficacy of NO_x emissions controls for co-benefiting air quality and climate.

Global and regional sources and sinks of carbon across the earth's surface have been studied extensively using atmospheric carbon dioxide (CO₂) observations and atmospheric chemistry-transport model (ACTM) simulations (top-down/inversion method). However, the uncertainties in the regional flux distributions remain unconstrained due to the lack of high-quality measurements, uncertainties in model simulations, and representation of data and flux errors in the inversion systems. Here, we assess the representation of data and flux errors using a suite of 16 inversion cases derived from a single transport model (MIROC4-ACTM) but different sets of a priori (bottom-up) terrestrial biosphere and oceanic fluxes, as well as prior flux and observational data uncertainties (50 sites) to estimate CO₂ fluxes for 84 regions over the period 2000–2020. The inversion ensembles provide a mean flux field that is consistent with the global CO₂ growth rate, land and ocean sink partitioning of −2.9 ± 0.3 (± 1σ uncertainty on the ensemble mean) and −1.6 ± 0.2 PgC yr⁻¹, respectively, for the period 2011–2020 (without riverine export correction), offsetting about 22 %–33 % and 16 %–18 % of global fossil fuel CO₂ emissions. The rivers carry about 0.6 PgC yr⁻¹ of land sink into the deep ocean, and thus the effective land and ocean partitioning is −2.3 ± 0.3 and −2.2 ± 0.3, respectively. Aggregated fluxes for 15 land regions compare reasonably well with the best estimations for the 2000s (∼ 2000–2009), given by the REgional Carbon Cycle Assessment and Processes (RECCAP), and all regions appeared as a carbon sink over 2011–2020. Interannual variability and seasonal cycle in CO₂ fluxes are more consistently derived for two distinct prior fluxes when a greater degree of freedom (increased prior flux uncertainty) is given to the inversion system. We have further evaluated the inversion fluxes using meridional CO₂ distributions from independent (not used in the inversions) aircraft and surface measurements, suggesting that the ensemble mean flux (model–observation mean ± 1σ standard deviation = −0.3 ± 3 ppm) is best suited for global and regional CO₂ flux budgets than an individual inversion (model–observation 1σ standard deviation = −0.35 ± 3.3 ppm). Using the ensemble mean fluxes and uncertainties for 15 land and 11 ocean regions at 5-year intervals, we show promise in the capability to track flux changes toward supporting the ongoing and future CO₂ emission mitigation policies.

Trends and variability in tropospheric hydroxyl (OH) radicals influence budgets of many greenhouse gases, air pollutant species, and ozone depleting substances. Estimations of tropospheric OH trends and variability based on budget analysis of methyl chloroform (CH₃CCl₃) and process‐based chemistry transport models often produce conflicting results. Here we use a previously tested transport model to simulate atmospheric CH₃CCl₃ for the period 1985–2018. Based on mismatches between model output and observations, we derive consistent anomalies in the inverse lifetime of CH₃CCl₃ (K_G) using measurements from two independent observational networks (National Oceanic and Atmospheric Administration and Advanced Global Atmospheric Gases Experiment). Our method allows a separation between “physical” (transport, temperature) and “chemical” (i.e., abundance) influences on OH + CH₃CCl₃ reaction rate in the atmosphere. Small increases in K_G due to “physical” influences are mostly driven by increases in the temperature‐dependent reaction between OH and CH₃CCl₃ and resulted in a smoothly varying increase of 0.80% decade⁻¹. Chemical effects on K_G, linked to global changes in OH sources and sinks, show larger year‐to‐year variations (∼2%–3%), and have a negative correlation with the El Niño Southern Oscillation. A significant positive trend in K_G can be derived after 2001, but it persists only through 2015 and only if we assume that CH₃CCl₃ emissions decayed more slowly over time than our best estimate suggests. If global CH₃CCl₃ emissions dropped below 3 Gg year⁻¹ after 2015, recent CH₃CCl₃ measurements indicate that the 2015–2018 loss rate of CH₃CCl₃ due to reaction with OH is comparable to its value 2 decades ago.

With combined use of the ocean–atmosphere simulation models and field observation data, we evaluate the parameters associated with the total caesium-137 amounts of the direct release into the ocean and atmospheric deposition over the western North Pacific caused by the accident of Fukushima Daiichi nuclear power plant (FNPP) that occurred in March 2011. The Green's function approach is adopted for the estimation of two parameters determining the total emission amounts for the period from 12 March to 6 May 2011. It is confirmed that the validity of the estimation depends on the simulation skill near FNPP. The total amount of the direct release is estimated as 5.5–5.9 × 10¹⁵ Bq, while that of the atmospheric deposition is estimated as 5.5–9.7 × 10¹⁵ Bq, which indicates broader range of the estimate than that of the direct release owing to uncertainty of the dispersion widely spread over the western North Pacific.

We characterise a transboundary ozone pollution outbreak transported across East Asia in early May 2009 using new multispectral satellite observations of lowermost tropospheric ozone (located below 3 km altitude) in synergy with other satellite data and models. Our analysis is focused on the daily evolution of ozone pollution plumes initially formed over the North China Plain (NCP) and their transport pathways over northern China, Korea, Japan and the surrounding seas. A main aspect of the study is an estimation of the contribution of photochemical production of ozone during transport using the ratio of ozone to carbon monoxide enhancements with respect to background levels derived from satellite data and also from chemistry–transport models.A key contribution of the analysis is the use of new satellite data offering unprecedented skills to observe the horizontal distribution of lowermost tropospheric ozone over East Asia on a daily basis, with a multispectral approach called IASI + GOME2 (combining Infrared Atmospheric Sounding Interferometer observations in the IR and Global Ozone Monitoring Experiment-2 measurements in the UV). These satellite observations are in good agreement with ozonesondes, with low mean biases (3 %), a precision of about 16 %, a correlation coefficient of 0.85 and practically the same standard deviation for a comparison based on 2 years of data from 46 launching stations distributed worldwide, during all seasons. A similar agreement is also found over East Asia. Moreover, IASI + GOME2 offers a unique capacity for observing the evolution of near-surface ozone during pollution outbreaks (with 5 % bias and 0.69 correlation), according to a comparison with surface in situ measurements during two major ozone events over several Japanese islands. Single-band ozone retrievals, such as those from IASI in the thermal infrared, do not capture such variability.Using IASI + GOME2, we show that (i) ozone pollution plumes are transported by an anticyclonic circulation around the Yellow Sea from the NCP to northern China, Korea and Japan, collocated with carbon monoxide plumes; (ii) over northern China the plume splits into two pollution filaments with one mixing with freshly emitted pollutants; and (iii) ozone is produced every day of the event, accounting for an enhancement in concentration during transport across East Asia of up to ∼ 84 % with respect to that produced over NCP. This estimation is done according to monotonically increasing values during 7 days of the ratio of ozone to carbon monoxide enhancements within the transported pollution plumes from about ∼ 0.25 over the NCP to ∼ 0.46 over the Pacific south of Japan.