Abstract
Abstract. Satellites provide information on the temporal and spatial distributions of aerosols on regional and global scales. With the same method applied to a single sensor all over the world, a consistent data set is to be expected. However, the application of different retrieval algorithms to the same sensor and the use of a series of different sensors may lead to substantial differences, and no single sensor or algorithm is better than any other everywhere and at all times. For the production of long-term climate data records, the use of multiple sensors cannot be avoided. The Along Track Scanning Radiometer (ATSR-2) and the Advanced ATSR (AATSR) aerosol optical depth (AOD) data sets have been used to provide a global AOD data record over land and ocean of 17 years (1995–2012), which is planned to be extended with AOD retrieved from a similar sensor. To investigate the possibility of extending the ATSR data record to earlier years, the use of an AOD data set from the Advanced Very High Resolution Radiometer (AVHRR) is investigated. AOD data sets used in this study were retrieved from the ATSR sensors using the ATSR Dual View algorithm ADV version 2.31, developed by Finnish Meteorological Institute (FMI), and from the AVHRR sensors using the aerosol optical depth over land (ADL) algorithm developed by RADI/CAS. Together, these data sets cover a multi-decadal period (1987–2012). The study area includes two contrasting areas, both in regards to aerosol content and composition and surface properties, i.e. a region over north-eastern China, encompassing a highly populated urban/industrialized area (Beijing–Tianjin–Hebei) and a sparsely populated mountainous area. Ground-based AOD observations available from ground-based sun photometer AOD data in AERONET and CARSNET are used as a reference, together with broadband extinction method (BEM) data at Beijing to cover the time before sun photometer observations became available in the early 2000s. In addition, MODIS-Terra C6.1 AOD data are used as a reference data set over the wide area where no ground-based data are available. All satellite data over the study area were validated against the reference data, showing the qualification of MODIS for comparison with ATSR and AVHRR. The comparison with MODIS shows that AVHRR performs better than ATSR in the north of the study area (40∘ N), whereas further south ATSR provides better results. The validation against sun photometer AOD shows that both AVHRR and ATSR underestimate the AOD, with ATSR failing to provide reliable results in the wintertime. This is likely due to the highly reflecting surface in the dry season, when AVHRR-retrieved AOD traces both MODIS and reference AOD data well. However, AVHRR does not provide AOD larger than about 0.6 and hence is not reliable when high AOD values have been observed over the last decade. In these cases, ATSR performs much better for AOD up to about 1.3. AVHRR-retrieved AOD compares favourably with BEM AOD, except for AOD higher than about 0.6. These comparisons lead to the conclusion that AVHRR and ATSR AOD data records each have their strengths and weaknesses that need to be accounted for when combining them in a single multi-decadal climate data record.
Highlights
Aerosol particles are important atmospheric constituents that play significant roles in many processes, such as atmospheric chemistry, the absorption and scattering of solar radiation, and the lifetime of cloud and precipitation systems (Boucher et al, 2013; Koren et al, 2014; Guo et al, 2014, 2016a, 2018)
Advanced Very High Resolution Radiometer (AVHRR) aerosol optical depth (AOD) data are available over the study area from the aerosol optical depth over land (ADL) algorithm for the years 1983–2014 (Xue et al, 2017) and from ATSR-2 and Advanced ATSR (AATSR) using the ADV algorithm for the years 1995–2003 (ATSR-2) and 2002– 2012 (AATSR), with some gaps as described in de Leeuw et al (2018)
The consistency between ATSR-2 and AATSR AOD is addressed in Sogacheva et al (2018a), who show that no systematic differences occur over China
Summary
Aerosol particles are important atmospheric constituents that play significant roles in many processes, such as atmospheric chemistry, the absorption and scattering of solar radiation, and the lifetime of cloud and precipitation systems (Boucher et al, 2013; Koren et al, 2014; Guo et al, 2014, 2016a, 2018). Observations of the concentrations of trace gases and aerosols are publicly available, as several observational networks have been established, such as NASA’s AERONET (AErosol RObotic NETwork; Holben et al, 1998), with observations mainly in the east of China, CARE-China (Xin et al, 2015), the Chinese Aerosol Remote Sensing Network (CARSNET; Che et al, 2009, 2015) and SONET (Sun-sky radiometer Observation NETwork; Li et al, 2018) Most of these observations started in the last decade and very few, if any, historical data on a large scale are available for the construction of the long time series needed to show the evolution of pollutant concentrations over many years and analyse the effects of different contributions. In another study combining satellite data with ground-based observations, the role of precursor gases (volatile organic compounds, VOCs, in particular) and photochemical reactions in the formation of aerosols (PM2.5) were revealed (Bai et al, 2018)
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