Investigations into the development of a satellite-based aerosol climate data record using ATSR-2, AATSR and AVHRR data over north-eastern China from 1987 to 2012

Abstract

Abstract. Satellites provide information on the temporal and spatial\ndistributions of aerosols on regional and global scales. With the same\nmethod applied to a single sensor all over the world, a consistent data set\nis to be expected. However, the application of different retrieval\nalgorithms to the same sensor and the use of a series of different sensors\nmay lead to substantial differences, and no single sensor or algorithm is\nbetter than any other everywhere and at all times. For the production of\nlong-term climate data records, the use of multiple sensors cannot be\navoided. The Along Track Scanning Radiometer (ATSR-2) and the Advanced ATSR\n(AATSR) aerosol optical depth (AOD) data sets have been used to provide a\nglobal AOD data record over land and ocean of 17\xa0years (1995–2012), which is\nplanned to be extended with AOD retrieved from a similar sensor. To\ninvestigate the possibility of extending the ATSR data record to earlier years,\nthe use of an AOD data set from the Advanced Very High Resolution Radiometer\n(AVHRR) is investigated. AOD data sets used in this study were retrieved\nfrom the ATSR sensors using the ATSR Dual View algorithm ADV version 2.31, developed\nby Finnish Meteorological Institute (FMI), and from the AVHRR sensors using\nthe aerosol optical depth over land (ADL) algorithm developed by RADI/CAS. Together, these data sets cover a\nmulti-decadal period (1987–2012). The study area includes two contrasting\nareas, both in regards to aerosol content and composition and surface\nproperties, i.e. a region over north-eastern China, encompassing a highly\npopulated urban/industrialized area (Beijing–Tianjin–Hebei) and a sparsely\npopulated mountainous area. Ground-based AOD observations available from ground-based sun photometer AOD\ndata in AERONET and CARSNET are used as a reference, together with broadband\nextinction method (BEM) data at Beijing to cover the time before sun\nphotometer observations became available in the early 2000s. In addition,\nMODIS-Terra C6.1 AOD data are used as a reference data set over the wide area\nwhere no ground-based data are available. All satellite data over the study\narea were validated against the reference data, showing the qualification of\nMODIS for comparison with ATSR and AVHRR. The comparison with MODIS shows\nthat AVHRR performs better than ATSR in the north of the study area\n(40 ∘ \u2009N), whereas further south ATSR provides better results. The\nvalidation against sun photometer AOD shows that both AVHRR and ATSR\nunderestimate the AOD, with ATSR failing to provide reliable results in the\nwintertime. This is likely due to the highly reflecting surface in the dry\nseason, when AVHRR-retrieved AOD traces both MODIS and reference AOD data\nwell. However, AVHRR does not provide AOD larger than about 0.6 and hence is\nnot reliable when high AOD values have been observed over the last decade.\nIn these cases, ATSR performs much better for AOD up to about 1.3.\nAVHRR-retrieved AOD compares favourably with BEM AOD, except\nfor AOD higher than about 0.6. These comparisons lead to the conclusion that\nAVHRR and ATSR AOD data records each have their strengths and weaknesses\nthat need to be accounted for when combining them in a single multi-decadal\nclimate data record.