Validation and Comparison of MODIS C6.1 and C6 Aerosol Products over Beijing, China

Xinpeng Tian,Jing Wei,Qiang Liu,Xiuhong Li

doi:10.3390/rs10122021

Abstract

The operational Moderate Resolution Imaging Spectroradiometer (MODIS) Aerosol Products (APs) have provided long-term and wide-spatial-coverage aerosol optical properties across the globe, such as aerosol optical depth (AOD). However, the performance of the latest Collection 6.1 (C6.1) of MODIS APs is still unclear over urban areas that feature complex surface characteristics and aerosol models. The aim of this study was to validate and compare the performance of the MODIS C6.1 and C6 APs (MxD04, x = O for Terra, x = Y for Aqua) over Beijing, China. The results of the Dark Target (DT) and Deep Blue (DB) algorithms were validated against Aerosol Robotic Network (AERONET) ground-based observations at local sites. The retrieval uncertainties and accuracies were evaluated using the expected error (EE: ±0.05 + 15%) and the root-mean-square error (RMSE). It was found that the MODIS C6.1 DT products performed better than the C6 DT products, with a greater percentage (by about 13%–14%) of the retrievals falling within the EE. However, the DT retrievals collected from two collections were significantly overestimated in the Beijing region, with more than 64% and 48% of the samples falling above the EE for the Terra and Aqua satellites, respectively. The MODIS C6.1 DB products performed similarly to the C6 DB products, with 70%–73% of the retrievals matching within the EE and estimation uncertainties. Moreover, the DB algorithm performed much better than DT algorithm over urban areas, especially in winter where abundant missing pixels were found in DT products. To investigate the effects of factors on AOD retrievals, the variability in the assumed surface reflectance and the main optical properties applied in DT and DB algorithms are also analyzed.

Highlights

Tropospheric aerosols, known as particulate matter (PM), originate from both natural and anthropogenic processes
The results showed that the MxD04 C6/C6.1 Dark Target (DT) algorithm greatly overestimated the aerosol optical depth (AOD) compared with Aerosol Robotic Network (AERONET) measurements and had a significant number of missing pixels
The C6.1 DT was able to reduce the number of errors and retrieved AOD better than the C6 DT

Summary

Introduction

Tropospheric aerosols, known as particulate matter (PM), originate from both natural and anthropogenic processes. The properties of aerosol are routinely measured by ground-based monitoring stations, e.g., the Aerosol Robotic Network (AERONET) While these stations provide fine spectral and high-precision aerosol information at point locations, satellite remote sensing can provide much more information about an aerosol’s spatial distribution. The MODIS C6 APs were fully validated regionally and globally, and most of the studies for C6.1 are focused on the merged DT and DB AOD products [9,10], which is not enough for the newest C6.1 APs. In the recent years, the aerosol effects have been significant and diversified in Beijing, a city that has suffered severe deterioration in air quality and an increase in air pollution. The effects of the variability of the assumed surface reflectance, and the accuracy of the SSA parameter on aerosol retrievals were considered and discussed

AERONET

Spatial and Temporal Matching

Findings

Conclusions