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A study was conducted to compare the δ(O2/N2) scales used by four laboratories engaged in atmospheric δ(O2/N2) measurements. These laboratories are the Research Institute for Environmental Management Technology, Advanced Industrial Science and Technology (EMRI/AIST); the National Institute for Environmental Studies (NIES); Tohoku University (TU); and Scripps Institution of Oceanography (SIO). Therefore, five high-precision standard mixtures for the O2 molar fraction gravimetrically prepared by the National Metrology Institute of Japan, AIST (NMIJ/AIST) with a standard uncertainty of less than 5 per meg (0.001 ‰) were used as round-robin standard mixtures. EMRI/AIST, NIES, TU, and SIO reported the analyzed values of the standard mixtures on their own δ(O2/N2) scales, and the values were compared with the δ(O2/N2) values gravimetrically determined by NMIJ/AIST (the NMIJ/AIST scale). The δ(O2/N2) temporal drift in the five standard mixtures during the intercomparison experiment from May 2017 to March 2020 was corrected based on the δ(O2/N2) values analyzed before and after the laboratory measurements by EMRI/AIST. The scales are compared based on offsets in zero and span. The relative span offsets of EMRI/AIST, TU, NIES, and SIO scales against the NMIJ/AIST scale were -0.11%±0.10%, -0.10%±0.13%, 3.39 %±0.13 %, and 0.93 %±0.10 %, respectively. The largest offset corresponded to a 0.30 Pg yr−1 decrease and increase in global estimates for land biospheric and oceanic CO2 uptakes based on trends in atmospheric CO2 and δ(O2/N2). The deviations in the measured δ(O2/N2) values on the laboratory scales from the NMIJ/AIST scale are 65.8±2.2, 425.7±3.1, 404.5±3.0, and 596.4±2.4 per meg for EMRI/AIST, TU, NIES, and SIO, respectively. The difference between atmospheric δ(O2/N2) values observed at Hateruma Island (HAT; 24.05∘ N, 123.81∘ E), Japan, by EMRI/AIST and NIES were reduced from -329.3±6.9 to -6.6±6.8 per meg by converting their scales to the NMIJ/AIST scale.

Systematic measurements of the atmospheric Ar∕N2 ratio have been made at ground-based stations in Japan and Antarctica since 2012. Clear seasonal cycles of the Ar∕N2 ratio with summertime maxima were found at middle- to high-latitude stations, with seasonal amplitudes increasing with increasing latitude. Eight years of the observed Ar∕N2 ratio at Tsukuba (TKB) and Hateruma (HAT), Japan, showed interannual variations in phase with the observed variations in the global ocean heat content (OHC). We calculated secularly increasing trends of 0.75 ± 0.30 and 0.89 ± 0.60 per meg per year from the Ar∕N2 ratio observed at TKB and HAT, respectively, although these trend values are influenced by large interannual variations. In order to examine the possibility of the secular trend in the surface Ar∕N2 ratio being modified significantly by the gravitational separation in the stratosphere, two-dimensional model simulations were carried out by arbitrarily modifying the mass stream function in the model to simulate either a weakening or an enhancement of the Brewer–Dobson circulation (BDC). The secular trend of the Ar∕N2 ratio at TKB, corrected for gravitational separation under the assumption of weakening (enhancement) of BDC simulated by the 2-D model, was 0.60 ± 0.30 (0.88 ± 0.30) per meg per year. By using a conversion factor of 3.5 × 10−23 per meg per joule by assuming a one-box ocean with a temperature of 3.5 ∘C, average OHC increase rates of 17.1 ± 8.6 ZJ yr−1 and 25.1 ± 8.6 ZJ yr−1 for the period 2012–2019 were estimated from the corrected secular trends of the Ar∕N2 ratio for the weakened- and enhanced-BDC conditions, respectively. Both OHC increase rates from the uncorrected- and weakened-BDC secular trends of the Ar∕N2 ratio are consistent with 12.2 ± 1.2 ZJ yr−1 reported by ocean temperature measurements, while that from the enhanced-BDC is outside of the range of the uncertainties. Although the effect of the actual atmospheric circulation on the Ar∕N2 ratio is still unclear and longer-term observations are needed to reduce uncertainty of the secular trend of the surface Ar∕N2 ratio, the analytical results obtained in the present study imply that the surface Ar∕N2 ratio is an important tracer for detecting spatiotemporally integrated changes in OHC and BDC.

Time series of the atmospheric O2∕N2 ratio and CO2 mole fraction of flask samples obtained from the National Institute for Environmental Studies' (NIES's) flask sampling network are presented. The network includes two ground sites, Hateruma island (HAT; 24.05 ∘ N, 123.81 ∘ E) and Cape Ochiishi (COI; 43.17 ∘ N, 145.50 ∘ E), and cargo ships regularly sailing in the western Pacific. Based on temporal changes in fossil-fuel-derived CO2 emissions, global atmospheric CO2 burden and atmospheric potential oxygen (APO), which were calculated from the observed O2∕N2 ratio and CO2 mole fraction according to APO = O2 + 1.1 × CO 2 , we estimated the global carbon sinks of the ocean and land biosphere for a period of more than 15 years. In this carbon budget calculation, we adopted a correction for the time-varying ocean O2 outgassing effect with an average of 0.54 PgC yr −1 for 2000–2016. The outgassing effect, attributed mainly to global ocean warming, was evaluated under the assumption that the net ocean gas flux is proportional to the change in the ocean heat content for the 0–2000 m layer. The resulting oceanic and land biotic carbon sinks were 2.6±0.7 and 1.5±0.9 PgC yr −1 , respectively, for a 17-year period (2000–2016) and 2.4±0.7 and 1.9±0.9 PgC yr −1 , respectively, for a 14-year period (2003–2016). Despite the independent approaches, the sink values of this study agreed with those estimated by the Global Carbon Project (GCP) within a difference of about ±0.4 PgC yr −1 . We examined the carbon sinks for an interval of 5 years to assess the temporal trends. The pentad (5-year) ocean sinks showed an increasing trend at a rate of 0.08±0.02 PgC yr −2 during 2001–2014, while the pentad land sinks showed an increasing trend at a rate of 0.23±0.04 PgC yr −2 for 2001–2009 and a decreasing trend at a rate of - 0.22 ± 0.04 PgC yr −2 during 2009–2014. Although there is good agreement in the trends of the pentad sinks between this study and that of GCP, the increasing rate of the pentad ocean sinks of this study was about 2 times larger than that of GCP.

A study was conducted to compare the δ(O2/N2) scales used by four laboratories engaged in atmospheric δ(O2/N2) measurements. These laboratories are the Research Institute for Environmental Management Technology, Advanced Industrial Science and Technology (EMRI/AIST), the National Institute for Environmental Studies (NIES), Tohoku University (TU), and Scripps Institution of Oceanography (SIO). Therefore, five high-precision standard mixtures for O2 molar fraction gravimetrically prepared by the National Metrology Institute of Japan (NMIJ), AIST (NMIJ/AIST) with a standard uncertainty of less than 5 per meg were used as round-robin standard mixtures. EMRI/AIST, NIES, TU, and SIO reported the analysed values of the standard mixtures on their own δ(O2/N2) scales, and the values were compared with the δ(O2/N2) values gravimetrically determined by NMIJ/AIST (the NMIJ/AIST scale). The δ(O2/N2) temporal drift in the five standard mixtures during the inter-comparison experiment was corrected based on the δ(O2/N2) values analysed before and after the experiments by EMRI/AIST. The scales are compared based on offsets in zero and span. The span offsets from the NMIJ/AIST scale ranged from −0.17 % to 3.3 %, corresponding with the difference of 0.29 Pg yr−1 in the estimates for land biospheric and oceanic CO2 uptakes. The zero offsets from the NMIJ/AIST scale are −581.0 ± 2.2, −221.4 ± 3.1, −243.0 ± 3.0, and −50.7 ± 2.4 per meg for EMRI/AIST, TU, NIES, and SIO, respectively. The atmospheric δ(O2/N2) values observed at Hateruma Island (HAT; 24.05° N, 123.81° E), Japan, by EMRI/AIST and NIES became comparable by converting their scales to the NMIJ/AIST scale.

The inverse analysis was used to estimate the fire carbon emission in Equatorial Asia induced by the big El Niño in 2015. This inverse analysis is unique because it extensively used high-precision atmospheric mole fraction data of carbon dioxide (CO2) from the commercial aircraft observation project. By comparisons with independent shipboard observations, especially carbon monoxide (CO) data, the validity of the estimated fire-induced carbon emission was elucidated. The best estimate, which used both aircraft and shipboard CO2 observations, indicated 273 Tg C for fire emission during September–October 2015. This two-month-long emission accounts for 75 % of the annual total fire emission and 45 % of the annual total net carbon flux within the region, indicating that fire emission is a dominant driving force of interannual variations of carbon fluxes in Equatorial Asia. Several sensitivity experiments demonstrated that aircraft observations could measure fire signals, though they showed a certain degree of sensitivity to prior fire-emission data. The inversions coherently estimated smaller fire emissions than the priors, partially because of the small contribution of peatland fires, indicated by enhancement ratios of CO and CO2 observed by the ship. In the future warmer climate condition, Equatorial Asia would experience more severe droughts and have risks for releasing a large amount of carbon into the atmosphere. Therefore, the continuation of aircraft and shipboard observations is fruitful for reliable monitoring of carbon fluxes in Equatorial Asia.

Systematic measurements of the atmospheric Ar/N2 ratio have been made at ground-based stations in Japan and Antarctica since 2012. Clear seasonal cycles of the Ar/N2 ratio with summertime maxima were found at middle to high latitude stations, with seasonal amplitudes increasing with increasing latitude. Eight years of the observed Ar/N2 ratio at Tsukuba and Hateruma, Japan showed not only secular increasing trends, but also interannual variations in phase with the observed variations in the global ocean heat content (OHC). The observed secular trend of the Ar/N2 ratio was 0.75±0.30 per meg yr-1. Sensitivity test by using a 2-dimensional model with the Brewer-Dobson circulation (BDC) scenarios indicated the possibility of the secular trend in the surface Ar/N2 ratio being modified significantly by the gravitational separation in the stratosphere. The secular trend of the Ar/N2 ratio, corrected for gravitational separation under the assumption of weakening of BDC simulated by the 2D model, was 0.60±0.30 per meg yr-1. By using a conversion factor of 3.5x10-23 per meg J-1 by assuming a 1-box ocean with a temperature of 3.5 °C, then an average OHC increase rate of 17.1±8.6 ZJ yr-1 for the period 2012–2019 was estimated from the corrected secular trend of the Ar/N2 ratio. This value is consistent with 12.2±1.2 ZJ yr-1 reported by ocean temperature measurements. The effect of the actual atmospheric circulation on the Ar/N2 ratio is still unclear, however the analytical results obtained in the present study imply that the surface Ar/N2 ratio is an important tracer for detecting spatiotemporally-integrated changes in OHC and BDC.

The Grater Tokyo Area is the most populated (38 million) metropolitan area in the world. To capture fossil fuel carbon dioxide (CO2) emissions from the Grater Tokyo Area, we performed ground-based atmospheric observations for measuring concentrations of CO2, radiocarbon in CO2 (14CO2), oxygen (O2) and carbon monoxide (CO) at Tokyo Skytree (TST, with high altitude (250m) inlet) and Yoyogi (YYG, turbulentCO2 flux measurement site located in resident area) in Tokyo and at National Institute for Environmental Studies (NIES, suburb/rural area) in Ibaraki, Japan. The 14CO2 measurement was used for separating the fossil fuel CO2 emissions from the biotic emissions. Results from 14CO2 measurements showed that a ratio of fossil fuel-derived CO2 to the variation of CO2 concentrations was 71% in average for winter both at TST and YYG but varied from 44% to 92%, indicating significant contribution of biotic CO2 in Tokyo. The O2:CO2 exchange ratio (oxidation ratio, OR) was used for the partitioning of CO2 into emissions from gas fuels and gasoline. We observed larger OR in winter than in summer (due to both wintertime increases of fossil fuel combustion and summertime terrestrial biospheric activities) at TST and YYG and larger OR in the morning and late evening in winter due to increase of gas fuel combustion at YYG. We showed that the O2 concentrations might be also used as a proxy for continuous monitoring of fossil fuel CO2 content by assuming typical ratio of gas fuels and gasoline combustions. The presenter will introduce the related projects including development of building/road-scale dynamic CO2 mapping and grid-based CO2 emission inventory with high special resolution in Tokyo.

Extremely high levels of column-averaged dry-air mole fractions of atmospheric methane (XCH4) were detected in August and September 2013 over northeast Asia (∼ 20 ppb above the averaged summertime XCH4 over 2009–2012, after removing a long-term trend), as being retrieved from the Short-Wavelength InfraRed (SWIR) spectral data observed with the Thermal And Near-infrared Sensor for carbon Observation – Fourier Transform Spectrometer (TANSO-FTS) onboard Greenhouse Gases Observing Satellite (GOSAT). Similar enhancements of XCH4 were also observed by the ground-based measurements at two Total Carbon Column Observing Network (TCCON) sites in Japan. The analysis of surface CH4 concentrations observed at three monitoring sites around the Japan archipelago suggest that the extreme increase of XCH4 has occurred in a limited area. The model analysis was conducted to investigate this anomalously high XCH4 event, using an atmospheric transport model. The results indicate that the extreme increase of XCH4 is attributed to the anomalous atmospheric pressure pattern over East Asia during the summer of 2013, which effectively transported the CH4-rich air to Japan from the strong CH4 source areas in east China. The two Japanese TCCON sites, ∼ 1000 km east–west apart each other, coincidentally located along the substantially CH4-rich air flow from east China. This analysis demonstrates the capability of GOSAT to monitor an XCH4 event on a synoptic scale. We anticipate that the synoptic information of XCH4 from GOSAT data contributes to improve our understanding of regional carbon cycle and the regional flux estimation.

Atmospheric oxygen (O2) and carbon dioxide (CO2) variations in the North Pacific were measured aboard a cargo ship, the New Century 2 (NC2), while it cruised between Japan and the United States between December 2015 and November 2016. A fuel cell analyzer and a nondispersive infrared analyzer were used for the measurement of O2 and CO2, respectively. To achieve parts-per-million precision for the O2 measurements, we precisely controlled the flow rates of the sample and reference air introduced into the analyzers and the outlet pressure. A relatively low airflow rate (10 cm3 min−1) was adopted to reduce the consumption rate of the reference gases. In the laboratory, the system achieved measurement precisions of 3.8 per meg for δ(O2 ∕ N2), which is commonly used to express atmospheric O2 variation, and 0.1 ppm for the CO2 mole fraction. After the in situ observation started aboard NC2, we found that the ship's motion caused false wavy variations in the O2 signal with an amplitude of more than several tens of ppm and a period of about 20 s. Although we have not resolved the problem at this stage, hourly averaging considerably suppressed the variation associated with ship motion. Comparison between the in situ observation and flask sampling of air samples aboard NC2 showed that the averaged differences (in situ–flask) and the standard deviations (±1σ) are −2.8 ± 9.4 per meg for δ(O2 ∕ N2) and −0.02 ± 0.33 ppm for the CO2 mole fraction. We compared 1 year of in situ data for atmospheric potential oxygen (APO; O2 +1.1 × CO2) obtained from the broad middle-latitude region (140° E–130° W, 29° N–45° N) with previous flask sampling data from the North Pacific. This comparison showed that longitudinal differences in the seasonal amplitude of APO, ranging from 51 to 73 per meg, were smaller than the latitudinal differences.

We present a comprehensive estimate of nitrous oxide (N2O) emissions using observations and models from 1995 to 2008. High-frequency records of tropospheric N2O are available from measurements at Cape Grim, Tasmania; Cape Matatula, American Samoa; Ragged Point, Barbados; Mace Head, Ireland; and at Trinidad Head, California using the Advanced Global Atmospheric Gases Experiment (AGAGE) instrumentation and calibrations. The Global Monitoring Division of the National Oceanic and Atmospheric Administration/Earth System Research Laboratory (NOAA/ESRL) has also collected discrete air samples in flasks and in situ measurements from remote sites across the globe and analyzed them for a suite of species including N2O. In addition to these major networks, we include in situ and aircraft measurements from the National Institute of Environmental Studies (NIES) and flask measurements from the Tohoku University and Commonwealth Scientific and Industrial Research Organization (CSIRO) networks. All measurements show increasing atmospheric mole fractions of N2O, with a varying growth rate of 0.1–0.7% per year, resulting in a 7.4% increase in the background atmospheric mole fraction between 1979 and 2011. Using existing emission inventories as well as bottom-up process modeling results, we first create globally gridded a priori N2O emissions over the 37 years since 1975. We then use the three-dimensional chemical transport model, Model for Ozone and Related Chemical Tracers version 4 (MOZART v4), and a Bayesian inverse method to estimate global as well as regional annual emissions for five source sectors from 13 regions in the world. This is the first time that all of these measurements from multiple networks have been combined to determine emissions. Our inversion indicates that global and regional N2O emissions have an increasing trend between 1995 and 2008. Despite large uncertainties, a significant increase is seen from the Asian agricultural sector in recent years, most likely due to an increase in the use of nitrogenous fertilizers, as has been suggested by previous studies.

In situ observation of the atmospheric CO2, CH4, and CO mixing ratios at Hateruma Island (HAT, 24.05° N, 123.80° E) often show synoptic-scale variations with correlative elevations during winter, associated with air transport from the East Asian countries. We examine winter (November– March) trends in ΔCH4 / ΔCO2, ΔCO / ΔCO2, and ΔCO / ΔCH4 observed at Hateruma over the period 1999 to 2010. To investigate the relationship between the East Asian emissions and the short-term variations in the atmospheric mixing ratios, we use the FLEXPART Lagrangian particle dispersion model (LPDM). The observed ratios ΔCH4 / ΔCO2 and ΔCO / ΔCO2 both show an overall gradual decrease over the study period due to a recent rapid increase in fossil fuel consumption in China. We note, however, that the decreasing rates of ΔCH4 / ΔCO2 and ΔCO / ΔCO2 show gradual decrease and increase, respectively, during the entire observation periods used in this study. The ΔCO / ΔCH4 slope, on the other hand, shows an increasing trend during 1999–2004 but a decrease during 2005–2010. Calculation of the concentration footprint for the atmospheric observation at HAT by using the FLEXPART LPDM indicates that most of the short-term variations are caused by emission variations from northern and eastern China. Combined with a set of reported emission maps, we have estimated the temporal changes in the annual CH4 and CO emissions from China under the assumption that the estimate of the fossil-fuel-derived CO2 emissions based on the energy statistics are accurate. The estimated annual CH4 emissions, corresponding to nonseasonal sources or anthropogenic sources without rice fields, show a nearly constant value of 39 ± 7 TgCH4 yr−1 during 1998–2002, and then gradually increase to 46 ± 8 TgCH4 yr−1 in 2009/2010. The estimated annual CO emissions increase from 134 ± 32 TgCO yr−1 in 1998/1999 to 182 ± 42 TgCO yr−1 in 2004/2005, level off after 2005, and then slightly decrease to less than 160 TgCO yr−1 in 2008–2010.

We examined potential interferences from water vapor and atmospheric background gases (N2, O2, and Ar), and biases by isotopologues of target species, on accurate measurement of atmospheric CO2 and CH4 by means of wavelength-scanned cavity ring-down spectroscopy (WS-CRDS). Changes of the background gas mole fractions in the sample air substantially impacted the CO2 and CH4 measurements: variation of CO2 and CH4 due to relative increase of each background gas increased as Ar < O2 < N2, suggesting similar relation for the pressure-broadening effects (PBEs) among the background gas. The pressure-broadening coefficients due to variations in O2 and Ar for CO2 and CH4 are empirically determined from these experimental results. Calculated PBEs using the pressure-broadening coefficients are linearly correlated with the differences between the mole fractions of O2 and Ar and their ambient abundances. Although the PBEs calculation showed that impact of natural variation of O2 is negligible on the CO2 and CH4 measurements, significant bias was inferred for the measurement of synthetic standard gases. For gas standards balanced with purified air, the PBEs were estimated to be marginal (up to 0.05 ppm for CO2 and 0.01 ppb for CH4) although the PBEs were substantial (up to 0.87 ppm for CO2 and 1.4 ppb for CH4) for standards balanced with synthetic air. For isotopic biases on CO2 measurements, we compared experimental results and theoretical calculations, which showed excellent agreement within their uncertainty. We derived instrument-specific water correction functions empirically for three WS-CRDS instruments (Picarro EnviroSense 3000i, G-1301, and G-2301), and evaluated the transferability of the water correction function from G-1301 among these instruments. Although the transferability was not proven, no significant difference was found in the water vapor correction function for the investigated WS-CRDS instruments as well as the instruments reported in the past studies within the typical analytical precision at sufficiently low water concentrations (

Pollution events extracted from the in situ observations of atmospheric CO2 and O2 mixing ratios at Hateruma Island (HAT, 24° N, 124° E) during the period from October 2006 and December 2008 are examined. The air mass origins for the pollution events are categorized by using back trajectory analysis, and the oxidative ratios (OR = −O2:CO2 molar exchange ratio) for selected pollution events are calculated. We find that there is a significant difference in the average oxidative ratios between events from China (OR = 1.14 ± 0.12, n = 25) and Japan/Korea (OR = 1.37 ± 0.15, n = 16). These values are in a good agreement with the national average oxidative ratios for the emissions from fossil fuel burning and cement production (FFBC) in China (ORFFBC = 1.11 ± 0.03) and Korea/Japan (ORFFBC = 1.36 ± 0.02). Compared with the observation, simulations of the atmospheric O2 and CO2 mixing ratios using Lagrangian particle dispersion models do a good job in reconstructing the average oxidative ratio of the pollution events originating in China but tend to underestimate for events originating in Japan/Korea. A sensitivity test suggests that the simulated atmospheric oxidative ratios at HAT are especially sensitive to changes in Chinese fuel mix.

In-situ observations of atmospheric CO2 and CH4 at Hateruma Island (24.05° N, 123.80° E, 47 m a.s.l), Japan shows large synoptic scale variations during a 6-month period from November to April, when the sampled air is predominantly of continental origin due to the Asian winter monsoon. Synoptic scale variations are extracted from the daily averaged values for the years between 1996 and 2007, along with the annual standard deviations (σCO2 and σCH4 for CO2 and CH4, respectively) for the relevant 6-month period. During this 6-month period the absolute mixing ratios of CO2 and CH4 at Hateruma are also elevated compared to those at two sites in the central North Pacific Ocean. The temporal change in σCO2 shows a systematic increase over the 12-year period, with elevated excursions in 1998 and 2003; there is no clear increase in σCH4. We also find that the σCO2/σCH4 ratio increases gradually from 1996 to 2002 and rapidly after 2002 without any extreme deviations that characterised σCO2. The σCO2/σCH4 ratio correlates closely with the recent rapid increase in fossil carbon emissions from China, as indicated in the Carbon Dioxide Information Analysis Center (CDIAC) database. This methodology can be applied to multiple chemical tracers of sufficient lifetime, for tracking overall changes in regional emissions.

Simultaneous ground-based measurements of ozone (O3) and carbon monoxide (CO) were conducted in March 2005 as part of the East Asian Regional Experiment (EAREX) 2005 under the umbrella of the Atmospheric Brown Clouds (ABC) project. Multiple air quality monitoring networks were integrated by performing intercomparison of individual calibration standards and measurement techniques to ensure comparability of ambient measurements, along with providing consistently high time-resolution measurements of O3 and CO at the surface sites in East Asia. Ambient data collected from eight surface stations were compared with simulation results obtained by a regional chemistry transport model to infer recent changes in CO emissions from East Asia. Our inverse estimates of the CO emissions from China up to 2005 suggested an increase of 16% since 2001, in good agreement with the recent MOPITT satellite observations and the bottom-up estimates up to 2006. The O3 enhancement relative to CO in continental pollution plumes traversed in the boundary layer were examined as a function of transport time from the Asian continent to the western Pacific Ocean. The observed ΔO3/ΔCO ratios show increasing tendency during eastward transport events due likely to en-route photochemical O3 formation, suggesting that East Asia is an important O3 source region during spring.