Temporal variations in atmospheric CO₂ on Rishiri Island in 2006–2013: responses of the interannual variation in amplitude to climate and the terrestrial sink in East Asia

Abstract. Surface observation of the atmospheric CO₂ mixing ratio implies the combined influences of both natural fluctuations and anthropogenic activities on the carbon cycle. Atmospheric CO₂ has been measured on Rishiri Island in the outflow region of Eurasia since May 2006. We report the first 7 year temporal atmospheric CO₂ variations from diurnal to interannual scales. In the diurnal scale, an obvious cycle appeared as a minimum in the afternoon and maximum at midnight in the summer months. Seasonally, the maximum CO₂ concentration appeared around the beginning of April, while the minimum appeared around the middle of August. A mean growing season length of ~126 days was estimated. In the period from 2007 to 2012, the peak-to-peak amplitude increased until 2009 and decreased thereafter, with a mean value of 19.7 ppm. In the long term, atmospheric CO₂ is increasing by a mean growth rate of 2.1 ppm year⁻¹. Investigations on the driving climatic factors on the interannual variation in amplitude indicated that temperature in East Asia (40–60° N, 90–150° E) affected the CO₂ amplitude by affecting the seasonal maximum, with a time lag of 1–2 years. On the contrary, precipitation did not likely affect CO₂ amplitudes. The amplitude also responded to a natural carbon source/sink variation in East Asia. We suggest that temperature in the first year would affect carbon sinks in the second year in the fetch regions, which further affect CO₂ amplitude mainly through ecosystem respiration. Circulation changes also likely contributed to the decreasing amplitude since 2009, as indicated by the simultaneous decrease in the ²²²Rn concentration in spring and summer.