Abstract
Assessments of long-term changes of air quality and global radiative forcing at a large scale heavily rely on satellite aerosol optical depth (AOD) datasets, particularly their temporal binning products. Although some attempts focusing on the validation of long-term satellite AOD have been conducted, there is still a lack of comprehensive quantification and understanding of the representativeness of satellite AOD at different temporal binning scales. Here, we evaluated the performances of the Moderate Resolution Imaging Spectroradiometer (MODIS) AOD products at various temporal scales by comparing the MODIS AOD datasets from both the Terra and Aqua satellites with the entire global AErosol RObotic NETwork (AERONET) observation archive between 2000 and 2017. The uncertainty levels of the MODIS hourly and daily AOD products were similarly high, indicating that MODIS AOD retrievals could be used to represent daily aerosol conditions. The MODIS data showed the reduced quality when integrated from the daily to monthly scale, where the relative mean bias (RMB) changed from 1.09 to 1.21 for MODIS Terra and from 1.04 to 1.17 for MODIS Aqua, respectively. The limitation of valid data availability within a month appeared to be the primary reason for the increased uncertainties in the monthly binning products, and the monthly data associated uncertainties could be reduced when the number of valid AOD retrievals reached 15 times in one month. At all three temporal scales, the uncertainty levels of satellite AOD products decreased with increasing AOD values. The results of this study could provide crucial information for satellite AOD users to better understand the reliability of different temporal AOD binning products and associated uncertainties in their derived long-term trends.
Highlights
Atmospheric aerosols impact radiative energy transfer, serving as one of the most important climatic forcing factors [1] and contribute to climate change [2]
The representativeness and validity of the Moderate Resolution Imaging Spectroradiometer (MODIS) C6.1 Aerosol optical depth (AOD) products were evaluated via comparison with the ground truths obtained from the AErosol RObotic NETwork (AERONET) observations
Taking AERONET as the reference data, the AOD estimates of MODIS Aqua outperformed those of Terra, especially regarding the relative mean bias (RMB) values
Summary
Atmospheric aerosols impact radiative energy transfer, serving as one of the most important climatic forcing factors [1] and contribute to climate change [2]. Atmospheric aerosols can scatter radiation back to space and reduce the amount of radiation received by the Earth; and as an indirect impact, they can alter cloud properties and modulate the amount of radiation scattered and absorbed by clouds [3]. These direct and indirect impacts of aerosols are closely linked to aerosol optical properties such as absorption and scattering coefficients [4,5]. Aerosol optical depth (AOD) is one of the most important parameters for quantifying aerosol loading [5,6], which has been widely used in regional air quality-related applications [7–9], aerosol radiative forcing estimations [10], regional and global climate change modeling [11], and public health studies [12]. The data from AERONET sites still seem to be rather limited in terms of capturing the extensive and continuous spatiotemporal heterogeneities and complex nature of global aerosol distributions [15,16]
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