Abstract
In urban environments, aerosol distributions may change rapidly due to building and transport infrastructure and human population density variations. The recent availability of medium resolution Landsat-8 and Sentinel-2 satellite data provide the opportunity for aerosol optical depth (AOD) estimation at higher spatial resolution than provided by other satellites. A year of 30 m Landsat-8 and 10 m Sentinel-2A AOD data retrieved using the Land Surface Reflectance Code (LaSRC) were compared with coincident ground-based Aerosol Robotic Network (AERONET) Version 3 AOD data for 20 Chinese cities. Stringent selection criteria were used to select contemporaneous data - only satellite and AERONET data acquired within 10 minutes were considered. The average satellite retrieved AOD over a 1470 m × 1470 m window centered on each AERONET site was derived to capture fine scale urban AOD variations. AERONET Level 1.5 (cloud-screened) and Level 2.0 (cloud-screened and also quality assured) data were considered. For the 20 urban AERONET sites in 2016 there were 106 (Level 1.5) and 67 (Level 2.0) Landsat-8 AERONET AOD contemporaneous data pairs, and 118 (Level 1.5) and 89 (Level 2.0) Sentinel-2A AOD data pairs. The greatest AOD values (>1.5) occurred in Beijing, suggesting that the Chinese capital was one of the most polluted cities in China in 2016. The LaSRC Landsat-8 and Sentinel-2A AOD retrievals agreed well with the AERONET AOD data (linear regression slopes > 0.96; coefficient of determination r2 > 0.90; root mean square deviation < 0.175) and demonstrate that the LaSRC is an effective and applicable medium resolution AOD retrieval algorithm over urban environments. The Sentinel-2A AOD retrievals had better accuracy than the Landsat-8 AOD retrievals, which is consistent with previously published research. The implications of the research and the potential for urban aerosol monitoring by combining the freely available Landsat-8 and Sentinel-2 satellite data are discussed.
Highlights
In much of Asia, rapid economic growth has resulted in increased air pollution [1,2,3,4]
Due to the stringent selection criteria (Section 3.2), six of the 26 Aerosol Robotic Network (AERONET) sites had no remaining contemporaneous data, leaving a total of 20 urban AERONET sites considered in the reminder of this study
Researchers have compared coarser spatial resolution but near daily Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth (AOD) retrievals with AERONET AOD using temporal selection criteria of ±30 min and spatial window side dimensions varying from 9 km to 50 km [9,20,55,56,57,58] and ±7.5 min with a 10 km spatial window dimension [59]
Summary
In much of Asia, rapid economic growth has resulted in increased air pollution [1,2,3,4]. The increased occurrence of urban haze, characterized by high concentrations of fine particulate matter with diameter less than 2.5 μm (PM2.5), has received growing attention due to human health and quality of life concerns [5,6]. In situ air quality measurements provide high temporal resolution observations but can be difficult to extrapolate spatially. The Moderate Resolution Imaging Spectroradiometer (MODIS) AOD product is derived over land using four aerosol models and fixed blue, red and 2.1 μm wavelength reflectance ratios [9]. Satellite based AOD products have been developed and used to estimate the concentration of fine particulate matter for epidemiological and air quality research [10,11,12,13]. Coarse resolution products are less suitable for studying aerosols in urban environments where aerosol distributions may change at finer spatial scales due to factors including spatial variations in building and transport infrastructure and human population densities [22,23,24]
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