Long-term measurements of tropospheric carbon dioxide and its isotopes in coastal East Asia

Many studies have reported that Asian continental outflow has significantly impacted on global atmospheric chemistry, affecting air quality over the western Pacific, even as far as North America. Taiwan is situated at an ideal location in the western North Pacific for monitoring biomass burning and Asian dust coincided with polluted air masses originating from Southeast Asia and Asian continent. A high-elevation baseline station, Lulin Atmospheric Background Station (LABS; 23.47°N, 120.87°E; 2,862 m a.s.l.), was thus established in April 2006 to investigate the characteristics of trace gases and atmospheric transport patterns in the lower free troposphere. In March 2010, as part of GMD's Cooperative Air Sampling Network, flask air sampling at Dongsha Island (DSI; 20.70°N, 116.73°E; 3 m a.s.l.) was launched to measure surface greenhouse gases (GHGs) in the South China Sea. As reported by IPCC in 2013, CO2 is the largest contributor of the atmospheric well-mixed GHGs, accounting for 64.3% of the total global radiative forcing from 1750 to 2011. The annual mean CO2 mixing ratio reached the milestone of 400 ppm at LABS (402.6±3.1 ppm) in 2015 and at DSI (401.7±4.7 ppm) in 2014. Vegetation growth in spring drew down CO2 mixing ratios and elevated its stable isotope ratio (δ13C of CO2) at LABS and DSI in summer. By the continuous measurement of CO2 using a cavity ring-down spectroscopy, a daily minimum with a lager standard deviation was observed at LABS during daytime when photosynthesis was active. The CO2 increased at an average rate of +2.69±2.30 ppm at LABS and +2.10±1.32 ppm at DSI during the measurement period, whereas the δ13C isotope ratio of CO2 decreased at an average rate of -0.060±0.099 ‰ at LABS and -0.021±0.053 ‰ at DSI. Increasing trends for the δ18O isotope ratio of CO2 were found at both LABS and DSI in recent years.