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

Abstract Carbon dioxide (CO 2 ) and methane (CH 4 ) are major contributors to climate change, increasing radiative forcing and global warming. This study investigates temporal variations of CO 2 and CH 4 levels, along with CO 2 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 CO 2 , CH 4 , and δ 13 C‐CO 2 at LLN, DSI, and Mauna Loa (MLO) were found similar over the monitoring period until March 2019, whereas δ 18 O‐CO 2 showed unclear trend. Seasonal patterns at LLN and DSI reflect distinct influences from different regions. At LLN, daytime photosynthesis reduced CO 2 , whereas CH 4 rose in the afternoon caused by the influences associated with upslope valley breezes. We also observed an enlarging diurnal amplitude of CH 4 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 CO 2 and 15.4 ppb (+0.8%) of CH 4 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.