Figure 2
From: Assessment of regional air quality by a concentration-dependent Pollution Permeation Index
Pollution Permeation Index (PPI) of China’s four regions in 2015.
(a) Jing-Jin-Ji (Beijing-Tianjin-Hebei), (b) Yangtze River Delta (YRD), (c) Pearl River Delta (PRD), and (d) Cheng-Yu (Chengdu-Chongqing). The specific PPI values are presented in Table S3.
