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Long‐Term Measurements of CO2, CH4, and Isotopic Ratios of CO2 in the Western Pacific: Trends, Variations, and Implications.

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Title: Long‐Term Measurements of CO2, CH4, and Isotopic Ratios of CO2 in the Western Pacific: Trends, Variations, and Implications.
Authors: Ou‐Yang, Chang‐Feng1,2 (AUTHOR) cfouyang@ncu.edu.tw, Wang, Jia‐Lin1,2 (AUTHOR), Lin, Chia‐Ching3 (AUTHOR), Chiu, Chia‐Yang3 (AUTHOR), Liu, Wen‐Tzu4 (AUTHOR), Lan, Xin5,6 (AUTHOR), Neff, Don6 (AUTHOR), Miller, John B.6 (AUTHOR), Michel, Sylvia Englund7 (AUTHOR), Vaughn, Bruce H.7 (AUTHOR), Chiu, Yu‐Chen8 (AUTHOR), Yu, Jhih‐Yuan8 (AUTHOR), Cheng, Chun‐Chu8 (AUTHOR), Schnell, Russell C.6 (AUTHOR), White, James W. C.7 (AUTHOR), Lin, Neng‐Huei1,2,3 (AUTHOR) nhlin@cc.ncu.edu.tw
Source: Journal of Geophysical Research. Atmospheres. 4/28/2025, Vol. 130 Issue 8, p1-19. 19p.
Subject Terms: *Biomass burning, *Radiative forcing, *Greenhouse gases, *Air masses, Atmospheric composition
Abstract: Carbon dioxide (CO2) and methane (CH4) are major contributors to climate change, increasing radiative forcing and global warming. This study investigates temporal variations of CO2 and CH4 levels, along with CO2 isotopic ratios, at two western Pacific background sites: Lulin Atmospheric Background Station (LLN, 2,862 m ASL) in central Taiwan since August 2006, representing the free troposphere, and Dongsha Island (DSI, 8 m ASL) in the South China Sea since March 2010, representing sea‐level conditions. Mean growth rates of CO2, CH4, and δ13C‐CO2 at LLN, DSI, and Mauna Loa (MLO) were found similar over the monitoring period until March 2019, whereas δ18O‐CO2 showed unclear trend. Seasonal patterns at LLN and DSI reflect distinct influences from different regions. At LLN, daytime photosynthesis reduced CO2, whereas CH4 rose in the afternoon caused by the influences associated with upslope valley breezes. We also observed an enlarging diurnal amplitude of CH4 with a significant large growth rate of 9.2 ppb yr−1 in the afternoon, which was plausibly owing to the intensified upslope winds at LLN. Based on the measurements associated with backward trajectories during nighttime, Southeast Asian biomass burning enhanced 1.5 ppm (+0.4%) of CO2 and 15.4 ppb (+0.8%) of CH4 at LLN in spring. Concentration weighted trajectory identifies potential source areas along the westerlies passing through southern China and Thailand, with reduced contributions from southern Southeast Asia as a result of mixing with the air in the South China Sea. Plain Language Summary: Carbon dioxide (CO2) and methane (CH4) are major drivers of global warming, with growing attention focused on anthropogenic emissions in East Asia owing to its rapid increase of population and consumption of energy and food. Furthermore, biomass burning in Southeast Asia is recognized as a potential regional source of greenhouse gases. The emitted air pollutants can travel with westerlies, affecting the atmospheric composition in the Pacific. This study investigates the long‐term variations of CO2, CH4, and CO2 isotopes at two background sites in the western Pacific as the gateway between source and receptor regions in the East Asia. The seasonal features at each site are largely influenced by the origins of incoming air masses, resulting in different annual amplitudes and time‐lags in seasonal maxima and minima of individual species. Daily cycles of CO2 and CH4 are investigated using real‐time measurements at Lulin Atmospheric Background Station (LLN) as a high‐altitude station in central Taiwan, suggesting an enlarging CH4 diurnal amplitude likely driven by intensified upslope winds. The impact of Southeast Asian biomass burning on CO2 and CH4 levels in the western Pacific is also assessed at LLN, complemented by concentration weighted trajectory (CWT) analysis to identify the source regions. Key Points: CO2, CO2 isotopic ratios, and CH4 trends and seasonality were investigated at LLN and DSI as two background sites in the western PacificThe changing diurnal amplitudes in CH4 imply increasingly intensifying upslope winds at LLNThe impact of Southeast Asian biomass burning on CO2 and CH4 in the Pacific is evaluated through measurements conducted at LLN [ABSTRACT FROM AUTHOR]
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Abstract:Carbon dioxide (CO2) and methane (CH4) are major contributors to climate change, increasing radiative forcing and global warming. This study investigates temporal variations of CO2 and CH4 levels, along with CO2 isotopic ratios, at two western Pacific background sites: Lulin Atmospheric Background Station (LLN, 2,862 m ASL) in central Taiwan since August 2006, representing the free troposphere, and Dongsha Island (DSI, 8 m ASL) in the South China Sea since March 2010, representing sea‐level conditions. Mean growth rates of CO2, CH4, and δ13C‐CO2 at LLN, DSI, and Mauna Loa (MLO) were found similar over the monitoring period until March 2019, whereas δ18O‐CO2 showed unclear trend. Seasonal patterns at LLN and DSI reflect distinct influences from different regions. At LLN, daytime photosynthesis reduced CO2, whereas CH4 rose in the afternoon caused by the influences associated with upslope valley breezes. We also observed an enlarging diurnal amplitude of CH4 with a significant large growth rate of 9.2 ppb yr−1 in the afternoon, which was plausibly owing to the intensified upslope winds at LLN. Based on the measurements associated with backward trajectories during nighttime, Southeast Asian biomass burning enhanced 1.5 ppm (+0.4%) of CO2 and 15.4 ppb (+0.8%) of CH4 at LLN in spring. Concentration weighted trajectory identifies potential source areas along the westerlies passing through southern China and Thailand, with reduced contributions from southern Southeast Asia as a result of mixing with the air in the South China Sea. Plain Language Summary: Carbon dioxide (CO2) and methane (CH4) are major drivers of global warming, with growing attention focused on anthropogenic emissions in East Asia owing to its rapid increase of population and consumption of energy and food. Furthermore, biomass burning in Southeast Asia is recognized as a potential regional source of greenhouse gases. The emitted air pollutants can travel with westerlies, affecting the atmospheric composition in the Pacific. This study investigates the long‐term variations of CO2, CH4, and CO2 isotopes at two background sites in the western Pacific as the gateway between source and receptor regions in the East Asia. The seasonal features at each site are largely influenced by the origins of incoming air masses, resulting in different annual amplitudes and time‐lags in seasonal maxima and minima of individual species. Daily cycles of CO2 and CH4 are investigated using real‐time measurements at Lulin Atmospheric Background Station (LLN) as a high‐altitude station in central Taiwan, suggesting an enlarging CH4 diurnal amplitude likely driven by intensified upslope winds. The impact of Southeast Asian biomass burning on CO2 and CH4 levels in the western Pacific is also assessed at LLN, complemented by concentration weighted trajectory (CWT) analysis to identify the source regions. Key Points: CO2, CO2 isotopic ratios, and CH4 trends and seasonality were investigated at LLN and DSI as two background sites in the western PacificThe changing diurnal amplitudes in CH4 imply increasingly intensifying upslope winds at LLNThe impact of Southeast Asian biomass burning on CO2 and CH4 in the Pacific is evaluated through measurements conducted at LLN [ABSTRACT FROM AUTHOR]
ISSN:2169897X
DOI:10.1029/2024JD042255