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. Aerosol opticaldepth (AOD) patterns and interannual and seasonal variations over China arediscussed based on the AOD retrieved from the Along-Track Scanning Radiometer(ATSR-2, 1995–2002), the Advanced ATSR (AATSR, 2002–2012) (together ATSR) andthe MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terrasatellite (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) anddeep blue (DB) AOD product. Together these datasets provide an AOD timeseries for 23 years, from 1995 to 2017. The difference between the AOD valuesretrieved from ATSR-2 and AATSR is small, as shown by pixel-by-pixel andmonthly aggregate comparisons as well as validation results. This allows forthe combination of the ATSR-2 and AATSR AOD time series into one dataset without offsetcorrection. ADV and MODIS AOD validation results show similar high correlations with theAerosol Robotic Network (AERONET) AOD (0.88 and 0.92, respectively), whilethe corresponding bias is positive for MODIS (0.06) and negative for ADV( −0.07 ). Validation of the AOD products in similar conditions, when ATSR andMODIS/Terra overpasses are within 90 min of each other and when bothADV and MODIS retrieve AOD around AERONET locations, show that ADV performsbetter than MODIS in autumn, while MODIS performs slightly better in springand summer. In winter, both ADV and MODIS underestimate the AERONET AOD. Similar AOD patterns are observed by ADV and MODIS in annual and seasonalaggregates as well as in time series. ADV–MODIS difference maps show thatMODIS AOD is generally higher than that from ADV. Both ADV and MODIS showsimilar 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. Between2006 and 2011 AOD did not change much, showing minor minima in 2008–2009.From 2011 onward AOD decreased in the SE of China. Similar patternsexist in year-to-year ADV and MODIS annual AOD tendencies in the overlappingperiod. However, regional differences between the ATSR and MODIS AODs arequite large. The consistency between ATSR and MODIS with regards to the AODtendencies in the overlapping period is rather strong in summer, autumn andoverall for the yearly average; however, in winter and spring, when there is adifference in coverage between the two instruments, the agreement betweenATSR and MODIS is lower. AOD tendencies in China during the 1995–2017 period will be discussed in moredetail in Part 2 (a following paper: Sogacheva et al., 2018), where a method to combine AOD time series from ADV andMODIS is introduced, and combined AOD time series are analyzed.

atmospheric chemistry and physics

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)