Conceptualizing How Severe Haze Events Are Impacting Long-Term Satellite-Based Trend Studies of Aerosol Optical Thickness over Asia
Error budgets derived for aerosol trend analysis from satellite-based datasets always consider sensor calibration, cloud contamination, and sampling scale. Here, we also consider and characterize an additional uncertainty induced by severe haze events. Such events were evaluated using MODerate resolution Imaging Spectroradiometer (MODIS) daily aerosol optical thickness (AOT) products (Terra MOD04 and Aqua MYD04) and AErosol RObotic Network (AERONET) AOT measurements over two regions subject to relatively high anthropogenic pollution loadings (and, as such, those conditions whereby the hygroscopic enhancement of local particulate mass is more likely): Beijing, China; and Kanpur, India. Further, the data are analyzed for trend analysis using two methods: linear and non-linear regression techniques. The latter, considered using the relatively new ensemble empirical mode decomposition (EEMD) methodology, allows for better representation of trends in non-linear time series, which is more practical for considering aerosol global trends overall. Our work shows that the severe haze events exhibit a significant impact on the AOT trends derived from these two regions. AOT trends from both the Terra and Aqua MODIS platforms over Kanpur are consistent with and without haze AOT using the linear method. Slight decreasing AOT trends were observed at Beijing from Terra and Aqua using both linear and non-linear methods. Case studies show the practical influence of severe haze events on the over- and under-estimate of MODIS AOT in these urban areas and how ground-based instrumentation critically assist interpretation of satellite-based aerosol observations.
KeywordsHaze Aerosol optical thickness Aerosol optical properties Aerosol trends
The authors thank the anonymous reviewers for valuable suggestions and comments. We also thank the NASA Goddard Space Flight Center and Langley Data Centers for MODIS and CALIPSO data, visibility and relative humidity data from the National Centers for Environmental Information, and the NASA AERONET project and principal investigators/staff responsible for establishing and maintaining the AERONET sites used here. This research was supported in part by the grant of General Research Fund (project id: 15205515), the grant HKU9/CRF/12G of Collaborative Research Fund from the Research Grants Council of Hong Kong; the grant PolyU 1-ZVAJ from the Faculty of Construction and Environment, the Hong Kong Polytechnic University.
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