Fig. 3: Comparison of CO_2ff estimations using various background selections.

a–d CO_2ff estimations for Xi’an¹⁸, Beijing⁴², Shenzhen (this study), and Hengshan (this study), respectively, based on regional (Huangzhong (HZ)²⁵, Shangdianzi (SDZ)¹¹, Nanling (NL, this study)), latitudinal (NH-1, NH-2, NH-3)¹⁰, and European (EU)^23,24 background references. Each box plot represents the distribution of CO_2ff values for a specific time period and background type. The colored boxes indicate the interquartile range (25th–75th percentile), with whiskers extending to 1.5 times the interquartile range (IQR). Horizontal black lines within the boxes represent the median values, black squares denote the mean values, and black stars indicate outliers. The blue dashed lines indicate the fluctuations in mean values, while the pink dashed lines depict the changes in median values. Time periods vary by panel and reflect available data ranges for each location and background. Each time segment is represented by a distinct box color: pink for the earliest period, blue for the second period, yellow for the third period, and green for the most recent period.