Fig. 7.

Correlation coefficients between NDVI and air pollutants in different regions of mainland China after the city was blocked. The correlation coefficient ranges from − 1 to 1. The closer the correlation coefficient is to 1, the stronger the positive correlation; the closer the correlation coefficient to − 1, the stronger the negative correlation; the closer the correlation coefficient is to 0, indicating that there is almost no correlation between the two. a North China; b Northeast China; c East China; d Central China; e South China; f Southwest China; g Northwest China