[1] We present an analysis of interannual variability (IAV) and trends in atmospheric methane (CH₄) mixing ratios over the western Pacific between 55°N and 35°S from 1994 to 2010. Observations were made by the Center for Global Environmental Research (CGER) of the National Institute for Environmental Studies (NIES), using voluntary observation ships sailing between Japan and Australia/New Zealand and between Japan and North America, sampling background maritime air quasi‐monthly (∼10 times per year) with high latitudinal resolution. In addition, simulations of CH₄ were performed using NIES atmospheric transport model. A large CH₄ increase was observed in the tropics (10°N–5°S) during 1997 (between 15 ± 3 and 19 ± 3 ppb yr⁻¹) and during 1998 for other regions (40°N–50°N: 10 ± 2–16 ± 1 ppb yr⁻¹; 10°S–25°S: 12 ± 2–22 ± 4 ppb yr⁻¹). The CH₄ increase leveled off from 1999 to 2006 at all latitudes. The CH₄ growth rate was enhanced in 2007 (25°N–50°N: 10 ± 1–12 ± 3 ppb yr⁻¹; 15°S–35°S: 7 ± 1–8 ± 1 ppb yr⁻¹) but diminished thereafter; however, a large CH₄ growth (10 ± 1–17 ± 1 ppb yr⁻¹) was observed in 2009 over the northern tropics (0°–15°N). These observations, combined with the simulation results, suggest that to explain the CH₄ increase in 2007 would require an increase in surface emissions of ∼20 ± 3 Tg‐CH₄ yr⁻¹ globally and an increase in the Northern Hemisphere (NH) of 4–7 ± 3 Tg‐CH₄ yr⁻¹ more than that in the Southern Hemisphere (SH), assuming no change in OH concentrations; alternatively, a decrease in OH concentrations of 4.5 ± 0.6%–5.5 ± 0.5% yr⁻¹ globally would be required if we assume no change in surface emissions. Over the western Pacific, the IAV in CH₄ within the northern tropics was characterized by a high growth rate in mid‐1997 and a reduced growth in 2007. The present data indicate that these events were strongly influenced by the IAV in atmospheric circulation associated with El Niño and La Niña events. Our observations captured the CH₄ anomaly in 1997 associated with forest fires in Indonesia. The IAV and trends in CH₄ as seen by our data sets capture the global features of background CH₄ levels in the northern midlatitudes and the SH, and regional features of CH₄ variations in the western tropical Pacific.

Atmospheric concentrations of the greenhouse gases carbon dioxide and nitrous oxide have increased substantially because of human activities. However, their sources in South Asia, which contribute strongly to the accelerating global growth of carbon dioxide and nitrous oxide, are poorly quantified. Here, we present aircraft measurements with high temporal and vertical resolution up to 20 km during the Asian summer monsoon where rapid upward transport of surface pollutants to greater altitudes occurs. Using Lagrangian model simulations, we successfully reconstruct observed carbon dioxide profiles leading to an improved understanding of the vertical structure of carbon dioxide in the Asian monsoon region. We show that spatio-temporal patterns of carbon dioxide on the Indian subcontinent driven by regional flux variations rapidly propagate to approximately 13 km with slower ascent above. Enhanced carbon dioxide compared to the stratospheric background can be detected up to 20 km. We suggest that the propagation of these signals from the surface to the stratosphere can be used to evaluate transport models and assess carbon dioxide fluxes in South Asia.

Emissions of greenhouse gases (GHGs) from the Indian subcontinent have increased during the last 20 years along with rapid economic growth; however, there remains a paucity of GHG measurements for policy-relevant research. In northern India and Bangladesh, agricultural activities are considered to play an important role in GHG concentrations in the atmosphere. We performed weekly air sampling at Nainital (NTL) in northern India and Comilla (CLA) in Bangladesh from 2006 and 2012, respectively. Air samples were analyzed for dry-air gas mole fractions of CO₂, CH₄, CO, H₂, N₂O, and SF₆ and carbon and oxygen isotopic ratios of CO₂ (δ¹³C-CO₂ and δ¹⁸O-CO₂). Regional characteristics of these components over the Indo-Gangetic Plain are discussed compared to data from other Indian sites and Mauna Loa, Hawaii (MLO), which is representative of marine background air. We found that the CO₂ mole fraction at CLA had two seasonal minima in February–March and September, corresponding to crop cultivation activities that depend on regional climatic conditions. Although NTL had only one clear minimum in September, the carbon isotopic signature suggested that photosynthetic CO₂ absorption by crops cultivated in each season contributes differently to lower CO₂ mole fractions at both sites. The CH₄ mole fraction of NTL and CLA in August–October showed high values (i.e., sometimes over 4000 ppb at CLA), mainly due to the influence of CH₄ emissions from the paddy fields. High CH₄ mole fractions sustained over months at CLA were a characteristic feature on the Indo-Gangetic Plain, which were affected by both the local emission and air mass transport. The CO mole fractions at NTL were also high and showed peaks in May and October, while CLA had much higher peaks in October–March due to the influence of human activities such as emissions from biomass burning and brick production. The N₂O mole fractions at NTL and CLA increased in June–August and November–February, which coincided with the application of nitrogen fertilizer and the burning of biomass such as the harvest residues and dung for domestic cooking. Based on H₂ seasonal variation at both sites, it appeared that the emissions in this region were related to biomass burning in addition to production from the reaction of OH and CH₄. The SF₆ mole fraction was similar to that at MLO, suggesting that there were few anthropogenic SF₆ emission sources in the district. The variability of the CO₂ growth rate at NTL was different from the variability in the CO₂ growth rate at MLO, which is more closely linked to the El Niño–Southern Oscillation (ENSO). In addition, the growth rates of the CH₄ and SF₆ mole fractions at NTL showed an anticorrelation with those at MLO, indicating that the frequency of southerly air masses strongly influenced these mole fractions. These findings showed that rather large regional climatic conditions considerably controlled interannual variations in GHGs, δ¹³C-CO₂, and δ¹⁸O-CO₂ through changes in precipitation and air mass.

The influence of the COVID‐19 pandemic on urban CO₂ emissions was investigated using atmospheric observations in the residential area (approximately 4 × 10⁵ m²) in Tokyo, Japan. The measured CO₂ flux decreased by 20% ± 3% in April–May 2020 compared to the same period during the past few years. The exchange ratio of O₂ and CO₂ revealed the breakdown of emission sources; the decrease of flux was due to the decrease in liquid fuel consumption, while household gas fuel consumption was slightly increased; this was also supported by the simultaneously measured CO:CO₂ concentration ratio. A human‐activity analysis in the flux footprint area supported the results of the observation; car traffic decreased and household gas consumption increased.