Spatial and seasonal variations of aerosols over China from two decades of multi-satellite observations – Part 1: ATSR (1995–2011) and MODIS C6.1 (2000–2017)
Authors: Sogacheva Larisa , de Leeuw Gerrit , Rodriguez Edith , Kolmonen Pekka , Georgoulias Aristeidis K. , Alexandri Georgia , Kourtidis Konstantinos , Proestakis Emmanouil , Marinou Eleni , Amiridis Vassilis , Xue Yong , van der A Ronald J. 
Published: August 2018

Abstract. Aerosol optical
depth (AOD) patterns and interannual and seasonal variations over China are
discussed based on the AOD retrieved from the Along-Track Scanning Radiometer
(ATSR-2, 1995–2002), the Advanced ATSR (AATSR, 2002–2012) (together ATSR) and
the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra
satellite (2000–2017). The AOD products used were the ATSR Dual View (ADV)
v2.31 AOD and the MODIS/Terra Collection 6.1 (C6.1) merged dark target (DT) and
deep blue (DB) AOD product. Together these datasets provide an AOD time
series for 23 years, from 1995 to 2017. The difference between the AOD values
retrieved from ATSR-2 and AATSR is small, as shown by pixel-by-pixel and
monthly aggregate comparisons as well as validation results. This allows for
the combination of the ATSR-2 and AATSR AOD time series into one dataset without offset
correction. ADV and MODIS AOD validation results show similar high correlations with the
Aerosol Robotic Network (AERONET) AOD (0.88 and 0.92, respectively), while
the corresponding bias is positive for MODIS (0.06) and negative for ADV
(−0.07). Validation of the AOD products in similar conditions, when ATSR and
MODIS/Terra overpasses are within 90 min of each other and when both
ADV and MODIS retrieve AOD around AERONET locations, show that ADV performs
better than MODIS in autumn, while MODIS performs slightly better in spring
and summer. In winter, both ADV and MODIS underestimate the AERONET AOD. Similar AOD patterns are observed by ADV and MODIS in annual and seasonal
aggregates as well as in time series. ADV–MODIS difference maps show that
MODIS AOD is generally higher than that from ADV. Both ADV and MODIS show
similar seasonal AOD behavior. The AOD maxima shift from spring in the south to summer along the eastern coast further north. The agreement between sensors regarding year-to-year AOD changes is quite good.
During the period from 1995 to 2006 AOD increased in the southeast (SE) of China. Between
2006 and 2011 AOD did not change much, showing minor minima in 2008–2009.
From 2011 onward AOD decreased in the SE of China. Similar patterns
exist in year-to-year ADV and MODIS annual AOD tendencies in the overlapping
period. However, regional differences between the ATSR and MODIS AODs are
quite large. The consistency between ATSR and MODIS with regards to the AOD
tendencies in the overlapping period is rather strong in summer, autumn and
overall for the yearly average; however, in winter and spring, when there is a
difference in coverage between the two instruments, the agreement between
ATSR and MODIS is lower. AOD tendencies in China during the 1995–2017 period will be discussed in more
detail in Part 2 (a following paper: Sogacheva et al., 2018), where a method to combine AOD time series from ADV and
MODIS is introduced, and combined AOD time series are analyzed.