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

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.