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 delta 13C-CO2 at LLN, DSI, and Mauna Loa (MLO) were found similar over the monitoring period until March 2019, whereas delta 18O-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.