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)

Abstract

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(ATSR-2, -2002, the Advanced ATSR (AATSR, 2002(AATSR, -2012) (together ATSR) and the MODerate resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(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. 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.

Two decades of satellite observations of AOD over mainland China using ATSR-2, AATSR and MODIS/Terra: data set evaluation and large-scale patterns

“…However, Che et al (2016) likely used an older version of the ADV-retrieved data set than the one produced by v2.31 which is used in the current work. The v2.31 AOD is substantially different from the earlier version as shown in Sogacheva et al (2017), especially for high AOD regions. (Che et al (2016) do not mention which ADV version was used, but at their submission date the latest updates described in Sogacheva et al, 2017 had not been implemented).…”

“…The data were produced using ADV version 2.31 which includes cloud postprocessing as described in Sogacheva et al (2017).…”

“…Hence, a period of 17 years of ATSR data is available. With the focus on aerosols over land, the aerosol optical depth (AOD) at a wavelength of 550 nm (AOD550, from here on referred to as AOD, unless specified otherwise) retrieved using the ATSR dual view algorithm (ADV; Kolmonen et al, 2016;Sogacheva et al, 2017) was used. This data set is further extended to 2015 by using AOD data available from Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Terra satellite, Collection 6 Figure 1.…”

“…However, that approach may fail over very bright surfaces such as snow, ice or some desert areas. ATSR AOD data retrieved using the ADV algorithm have been successfully applied in many studies (e.g., Veefkind et al, 1998Veefkind et al, , 1999Veefkind et al, , 2000Robles-Gonzalez et al, 2000, 2006Schmid et al, 2003;Sundström et al, 2012;Holzer-Popp et al, 2013;Virtanen et al, 2014;Rodriguez et al, 2015;Sogacheva et al, 2015Sogacheva et al, , 2017Popp et al, 2016). By combining the ATSR-2 and AATSR data sets, a unique time series of AOD over land, from June 1995 to April 2012, offers the opportunity to analyze the temporal variation of the AOD and possibly detect trends.…”

“…Seasonal variations of AOD are presented and briefly discussed, as well as the two-decade time series of AOD. In a second paper (Sogacheva et al, 2018), the seasonal and long-term variations for different regions in China will be presented.…”