Abstract
Abstract. Aerosol optical depth (AOD) product retrieved from MODerate Resolution Imaging Spectroradiometer (MODIS) measurements has greatly benefited scientific research in climate change and air quality due to its high quality and large coverage over the globe. However, the current product (e.g., Collection 6) over land needs to be further improved. The is because AOD retrieval still suffers large uncertainty from the surface reflectance (e.g., anisotropic reflection) although the impacts of the surface reflectance have been largely reduced using the Dark Target (DT) algorithm. It has been shown that the AOD retrieval over dark surface can be improved by considering surface bidirectional distribution reflectance function (BRDF) effects in previous study. However, the relationship of the surface reflectance between visible and shortwave infrared band that applied in the previous study can lead to an angular dependence of the AOD retrieval. This has at least two reasons. The relationship based on the assumption of isotropic reflection or Lambertian surface is not suitable for the surface bidirectional reflectance factor (BRF). However, although the relationship varies with the surface cover type by considering the vegetation index NDVISWIR, this index itself has a directional effect and affects the estimation of the surface reflection, and it can lead to some errors in the AOD retrieval. To improve this situation, we derived a new relationship for the spectral surface BRF in this study, using 3 years of data from AERONET-based Surface Reflectance Validation Network (ASRVN). To test the performance of the new algorithm, two case studies were used: 2 years of data from North America and 4 months of data from the global land. The results show that the angular effects of the AOD retrieval are largely reduced in most cases, including fewer occurrences of negative retrievals. Particularly, for the global land case, the AOD retrieval was improved by the new algorithm compared to the previous study and MODIS Collection 6 DT algorithm, with the increase of 2.0 and 4.5 % AOD retrievals falling within the expected accuracy envelope ±(0.05 + 15 %), respectively. This implies that the users can get more accurate data without angular bias, i.e., more meaningful AOD data.
Highlights
IntroductionAerosols from natural sources (e.g., volcanic ash, sea spray aerosol and dust) and human activities (e.g., industrial emission, forest fire smoke and fossil fuel burning aerosol) play a key role in climate, environment and human health
Aerosols from natural sources and human activities play a key role in climate, environment and human health
In which 2 years (2008 and 2010) of data are from North America (25–65◦ N, 135–60◦ W) and 4 months (January and July in 2008 and 2010, respectively) of data are from the global land areas
Summary
Aerosols from natural sources (e.g., volcanic ash, sea spray aerosol and dust) and human activities (e.g., industrial emission, forest fire smoke and fossil fuel burning aerosol) play a key role in climate, environment and human health. Aerosols have a significant impact on climate due to their direct and indirect effects (Kaufman et al, 2002; IPCC, 2013). Most of the aerosols affect the climate by cooling the atmosphere through reflecting solar radiation into outer space, whereas absorbing aerosols (e.g., black carbon) warm the atmosphere; this is called the direct effect of aerosol. The net effect of aerosols is cooling. As for indirect effects on climate, aerosols can play a role as cloud condensation nuclei and influence the formation and albedo of the cloud
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
