Fig. 3: Source transitions revealed by BC isotope signals since China’s clean air actions.

The term “Source Transition” refers to the major shifts in the relative contributions of biomass burning, coal combustion and liquid fossil fuel combustion to BC aerosol over the past decade, as revealed by Δ¹⁴C and δ¹³C measurements. The f_bb is calculated by Eq. (1) in Method. The expected Δ¹⁴C and δ¹³C endmember range for biomass, liquid fossil and coal fossil (Supplementary Table 7) are shown as green, grey and yellow rectangles, respectively. The circle area shows BC aerosol concentrations in µg m^–3 (see scales with black circles on the bottom-right side). a The shifts of carbon isotope signatures in North China (Tianjin (BTH) and Beijing (BJ)) and East China (Shanghai (SH) and Shanghai Chongming Island (YRE)) since 2012. The 2012–2014 BTH data from refs. ^33,34. and 2013 winter Beijing data from ref. ³³. b Seasonal isotope signatures of BC at Tianjin (BTH). The black, green, red and blue respectively represent winter, spring, summer and autumn in the current campaigns. The numerical suffix indicates the campaign year. For example, ‘winter 20’ refers to 2020 winter (December 2020, January and February 2021). c Seasonal isotope signatures of BC at Kaifeng in the Central China Plains. d The shifts of BC isotope signatures in wintertime haze events at Hefei (HF) and Shanghai (SH) in East China during China’s clean air actions. YRE data from ref. ⁴. and urban Shanghai data from refs. ^4,33. e Seasonal isotope signatures of BC at Yantai on the east coast of Shandong Peninsula. f Seasonal isotope signatures of BC at Hefei in the western YRD. Uncertainties are less than 0.2‰ for δ¹³C and 5‰ for Δ¹⁴C (corresponding to <1% for f_bb) and thus not shown here because they are much smaller than the diameter of the circles (all dual-isotope data are provided in Supplementary Table 8).