Abstract
With ten-year (2004–2013) ground-based observations of Beijing Forest (BJF) and Beijing City (BJC) sites in North China, we validated the high-quality MODerate resolution Imaging Spectroradiometer (MODIS) Collection 5 (C5) and Collection 6 (C6) Aerosol Optical Depth (AOD) products’ precision and discussed the sensors degradation issues. The annual mean AOD and Angstrom exponent (α) were 0.20 ± 0.02 and 0.83 ± 0.15 in the background over the past ten years, and they were 0.59 ± 0.07 and 1.13 ± 0.08 in the urban, respectively. Ground-based AOD had both slightly declining trends, with variations of 0.023 and 0.057 over the past decade in the background and urban, respectively. There were large differences among the eight kinds of MODIS AOD products (Terra vs. Aqua, C5 vs. C6, DT (Deep Target) vs. DB (Deep Blue), and DTDB in the background and urban areas), but all the products’ monthly errors had larger variations in the spring and summer, and smaller ones in the autumn and winter. In the background, more than 62% of DT matchups for C5 and C6 products were within NASA’s expected error (EE) envelope. In the urban, 69%~72% of C6 DB retrievals were falling within EE envelope. The new dataset named C6 DTDB had better performance in the background, whereas it overestimated by 37%~41% in the urban caused by surface reflectivity estimation error. The range of monthly average error varied from −0.21 to 0.28 in the background and from −0.63 to 0.48 in the urban. From the background to the urban areas, the retrieval errors of Terra and Aqua had slightly increased by 0.0023~0.0158 and 0.0011~0.0124 per year, respectively, which implied that the two MODIS instruments had degraded slowly.
Highlights
As we all know, aerosols are important components in the Earth system [1], and have significant influences on global climate [2], air quality [3] and human health [4] through the direct and indirect radiation forcing [5,6]
There are two separate types of MODerate resolution Imaging Spectroradiometer (MODIS) Aerosol Optical Depth (AOD) retrieval algorithms over land, one is Dark Target (DT) algorithm, which retrieves over dark land [14,15,16,17], another one is Deep Blue (DB) algorithm, which is developed initially to work on bright surfaces [18,19,20]
In the latest MODIS Collection 6 (C6) product, DT algorithm, is based on the same methodology as Collection 5 (C5) DT but with some important modifications such as updating cloud mask to retrieve aerosol product in heavy smoke conditions, adjusting the aerosol model which is a function of seasons and locations, revising Quality Assurance (QA) logic and so on
Summary
Aerosols are important components in the Earth system [1], and have significant influences on global climate [2], air quality [3] and human health [4] through the direct and indirect radiation forcing [5,6]. 2016, 8, 754 aerosol retrieval algorithms are identical for Terra and Aqua MODIS radiation data, the aerosol products of the two sensors are kind of different due to the disparities in spectral channels, samples selection time, radiometric calibration and other factors [11,12,13]. In the latest MODIS Collection 6 (C6) product, DT algorithm, is based on the same methodology as Collection 5 (C5) DT but with some important modifications such as updating cloud mask to retrieve aerosol product in heavy smoke conditions, adjusting the aerosol model which is a function of seasons and locations, revising Quality Assurance (QA) logic and so on. The annual mean values of AOD and α at BJF±were and ±
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