Analysis of spatio-temporal variability of aerosol optical depth with empirical orthogonal functions in the Changjiang River Delta, China

This work aims to analyze the spatial and temporal variability of aerosol optical depth (AOD) from 2000 to 2012 in the Changjiang River Delta (CRD), China. US Terra satellite moderate resolution imaging spectroradiometer (MODIS) AOD and ngstrom exponent (alpha) data constitute a baseline, with the empirical orthogonal functions (EOFs) method used as a major data analysis method. The results show that the maximum value of AOD observed in June is 1.00 +/- 0.12, and the lowest value detected in December is 0.40 +/- 0.05. AOD in spring and summer is higher than in autumn and winter. On the other hand, the alpha-value is lowest in spring (0.86 +/- 0.10), which are affected by coarse particles. High alpha-value appears in summer (1.32 +/- 0.05), which indicate that aerosols are dominated by fine particles. The spatial distribution of AOD has a close relationship with terrain and population density. Generally, high AODs are distributed in the low-lying plains, and low AODs in the mountainous areas. The spatial and temporal patterns of seasonal AODs show that the first three EOF modes cumulatively account for 77% of the total variance. The first mode that explains 67% of the total variance shows the primary spatial distribution of aerosols, i.e., high AODs are distributed in the northern areas and low AODs in the southern areas. The second mode (7%) shows that the monsoon climate probably plays an important role in modifying the distribution of aerosols, especially in summer and winter. In the third mode (3%), this distribution of aerosols usually occurs in spring and winter when the prevailing northwestern or western winds could bring aerosol particles from the inland areas into the central regions of the CRD.