Quantifying the contributions of fossil fuel emissions and East Asian summer monsoon to Chinese regional background atmosphere by tree-ring Δ¹⁴C

Four time series of annual tree-ring Δ¹⁴C from Changbai Mountain (CBM), Nanwutai (NWT), Lin'an (LAN), and Dinghu Mountain (DHM) regional background sites in China spanning the last ca. 70 years were measured, with the objectives to reveal their temporal-spatial variations and to quantify the contributions of regional fossil fuel CO₂ (CO_2ff) emissions and East Asian summer monsoon (EASM). The temporal-spatial Δ¹⁴C variations at these sites are mainly influenced by atmospheric nuclear tests and fossil fuel emissions. The decreases in the main Δ¹⁴C peaks in 1964 from high latitudinal site to low latitudinal site reflect the influence of global atmospheric nuclear tests, while the incomplete high-low latitudinal decreases in the small Δ¹⁴C peaks in 1959 and the relative high values during 1965–1973 reflect part influences of regional atmospheric nuclear tests. Except CBM, the values at NWT, LAN and DHM are generally lower than the Northern Hemisphere background Δ¹⁴C since 1985, due to CO_2ff emissions. CO_2ff inputs to the regional atmospheric CO₂ are lower (2.8 ± 2.1 ppm) during 1985–1999 and higher (8.0 ± 2.1 ppm) during 2000–2019. 1 Mt CO_2ff emissions from Northeast China, Shaanxi Province, Yangtze River Delta Economic Region and Guangdong Province make the atmospheric Δ¹⁴C approximately decrease by 0.12 ± 0.01 ‰ at CBM, 0.30 ± 0.02 ‰ at NWT, 0.07 ± 0.00 ‰ at LAN and 0.24 ± 0.01 ‰ at DHM, respectively. Estimated contributions of EASM air masses using bomb-¹⁴C are 41.0 % ∼ 55.4 % at these sites in the summer of 1964–1966. This study highlights the importance of long-term observations of atmospheric Δ¹⁴C at the regional background sites.