## Results  
### Changes in Aerosols  
Figure 1a shows that aerosol loading increased and that the change in the average AOD detected by CALIPSO (approximately 0.13) was more intense than that detected by MODIS (approximately 0.05) during the monsoon season. Many factors are involved in contributing to the difference between MODIS and CALIPSO AOD, including cloud contamination, aerosol types, and vertical distribution. For example, the MODIS is more sensitive to cloud contamination compared with CALIPSO AOD, generally resulting in artificially high AOD retrieval by MODIS, due to the undetected thin clouds [33]. However, for biomass burning regions, CALIPSO AOD is significantly higher than MODIS AOD due to the strong absorption of smoke to solar radiation [34]. Moreover, the aerosol vertical distribution also importantly affects MODIS AOD retrieval, caused by the insufficient ability for MODIS to determine the vertical information of the aerosol layer [35]. Irrespective of the difference between MODIS and CALIPSO AOD, the CALICP observations are consistent with the MODIS results for the geographical patterns and seasonal variations [36; 37]. The change in the average AOD detected by CALIPSO exhibited a higher significance level (\(p\) = 0.07) than that detected by MODIS (\(p\) = 0.22). This result is has important contributions from the extensive cloud contamination on MODIS during the monsoon season [33]. Additionally, the consistently robust and rapid increase in aerosols in South Asia have verified by satellite and ground-based observations from many previous studies [38; 39; 40]. [PERSON] [39] found that the positive trends in AOD are found over the main continental regions of South Asia based on ground-based radiometric measurements. [PERSON], et al. [40] exhibited overall increasing trends in AOD (7.69%) with a high significance based on Aerosol Robotic Network (AERONET) measurements  
Figure 1: Temporal variations in spatial average AOD from (**a**) CALIPSO and MODIS during the monsoon season and (**b**) the vertical average aerosol extinction coefficient at 532 nm in South Asia from 2006 to 2015, respectively. The dashed lines indicate the linear fit line of the solid line with the same color; \(p\) is the significance level. from 2001 to 2010 in northern India, which are consistent with satellite observations (MODIS) of increasing AOD and enhanced anthropogenic emissions over India. Moreover, [PERSON], et al. [(41)] also reported a high degree of correlation (above 0.7) between the inter-comparison of MODIS AOD and AERONET data in India. Figure 1b depicts the change in the vertically-averaged aerosol extinction coefficient at 532 nm during the study period. This profile is calculated by averaging all extinction coefficient values at the presence of aerosol in each point with a vertical interval of 240 m within the given spatial-temporal range, thereby indicating the total ability of aerosols to vertically scatter and absorb solar radiation. Moreover, to highlight the most probable altitude of the aerosol layer, we removed the points in which the occurrence frequency is one order of magnitude less than the maximum occurrence frequency of aerosol in vertical profiles. Figure 1b displays that the aerosol mostly occurs under 5 km m.s.l, and its extinction coefficient is exponentially weakened with the increase in altitude in South Asia. This result is consistent with previous work [(42; 43)]. Moreover, the vertical aerosol extinction coefficient increases distinctly during the monsoon season in South Asia from 2006 to 2015, especially below 2 km. Moreover, the sum of dust, smoke, and polluted dust aerosol loadings detected by CALIPSO is near the total loading of aerosol (orange and blue lines in Figure 1a). This result indicates that aerosols mostly consist of dust, smoke, and polluted dust during the monsoon season in South Asia, hence attaining consistency with previous work [(44)].