Spatiotemporal variation and provincial scale differences of the AOD across China during 2000–2021.

The spatiotemporal variation of the aerosol optical depth (AOD) over China was studied using MODIS data 
retrieved with the MAIAC algorithm. Time series were extended to July 2021. The study area is defined as the 
mainland of South-Eastern China Below the Hu line, with Hebei as the most northerly province (in short SECBH) 
and includes 16 provinces, 4 central government-controlled municipalities and one autonomous region, all 
referred to here as provinces. The extended time series and provincial-scale averaging of the AOD provide more 
detailed and new information on the spatiotemporal variation across the SECBH. Differences in temporal 
behaviour are observed between the north, central-south and south of the SEBCH. The AOD increased in southern 
China before it increased in the northern provinces. The AOD was high between 2006 and 2014, with strong 
interannual fluctuations in response to the emission reduction policy opposed by economic developments, and 
decreased significantly from 2013/2014, in response to the Clean Air Action Plan (2013–2017). However, during 
the period 2017–2020 the AOD stabilized and fluctuated within 10% of the average value for these years which 
was similar to, and in part of the study area lower than, the AOD in 2000. Exceptions are Guangxi, Guangdong, 
Jiangxi and part of Shandong, where the AOD in 2020 was slightly higher than in 2000. The extension of the 
AOD time series until July 2021 shows that earlier derived trends do not continue and the emission reduction 
policy needs to be extended to achieve the earlier air quality goals.