Abstract
Abstract. We diagnose the potential causes for the Multi-angle Imaging SpectroRadiometer's (MISR) persistent high aerosol optical depth (AOD) bias at low AOD with the aid of coincident MODerate-resolution Imaging Spectroradiometer (MODIS) imagery from NASA's Terra satellite. Stray light in the MISR instrument is responsible for a large portion of the high AOD bias in high-contrast scenes, such as broken-cloud scenes that are quite common over ocean. Discrepancies among MODIS and MISR nadir-viewing blue, green, red, and near-infrared images are used to optimize seven parameters individually for each wavelength, along with a background reflectance modulation term that is modeled separately, to represent the observed features. Independent surface-based AOD measurements from the AErosol RObotic NETwork (AERONET) and the Marine Aerosol Network (MAN) are compared with MISR research aerosol retrieval algorithm (RA) AOD retrievals for 1118 coincidences to validate the corrections when applied to the nadir and off-nadir cameras. With these corrections, plus the baseline RA corrections and enhanced cloud screening applied, the median AOD bias for all data in the mid-visible (green, 558 nm) band decreases from 0.006 (0.020 for the MISR standard algorithm (SA)) to 0.000, and the RMSE decreases by 5 % (27 % compared to the SA). For AOD558 nm < 0.10, which includes about half the validation data, 68th percentile absolute AOD558 nm errors for the RA have dropped from 0.022 (0.034 for the SA) to < 0.02 (~ 0.018).
Highlights
The research aerosol retrieval algorithm (RA) for the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR) is used to analyze regional wildfire smoke, desert dust, urban pollution, volcanic ash, and other individual events and to test algorithm modifications that might be applied to the MISR standard aerosol retrieval algorithm (SA) that generates the operational product for the entire MISR data set (e.g., Kahn et al, 2001; Kahn and Limbacher, 2012; Limbacher and Kahn, 2014)
In Limbacher and Kahn (2014), we showed that a small positive bias remained in the RA at low aerosol optical depth (AOD) over ocean (∼ 0.01 for the green at AOD < 0.10), even with all the adjustments that were implemented in that study
We identify here the following types of stray light as contributing to, and possibly accounting fully for, the observed bias in TOA reflectance in high-contrast scenes that produces the AOD overestimation: primary ghosting convolved with background reflectance modulation, a smaller secondary ghosting term that acts on scene halves, blurring of contrast features, and possibly a uniform veiling-light term that is too small www.atmos-meas-tech.net/8/2927/2015/
Summary
The research aerosol retrieval algorithm (RA) for the NASA Earth Observing System’s Multi-angle Imaging SpectroRadiometer (MISR) is used to analyze regional wildfire smoke, desert dust, urban pollution, volcanic ash, and other individual events and to test algorithm modifications that might be applied to the MISR standard aerosol retrieval algorithm (SA) that generates the operational product for the entire MISR data set (e.g., Kahn et al, 2001; Kahn and Limbacher, 2012; Limbacher and Kahn, 2014). Considerable effort has produced a MISR Level 1 product with about 3 % absolute radiometric accuracy, and generally even better band-toband and camera-to-camera relative calibration (Bruegge et al, 2004, 2007; Diner et al, 2004; Kahn et al, 2005; Lyapustin et al, 2007; Lallart et al, 2008), there remain some artifacts in the radiometry that have not been characterized quantitatively (e.g., Bruegge et al, 2004) These can affect both the AOD (including a generally high mid-visible AOD bias of ∼ 0.02 for low-AOD cases over dark water) and especially the aerosol type results (e.g., Kahn et al, 2010; Limbacher and Kahn, 2014). Further description of the globally distributed, AERONET/MAN coincident data set used here is given in Limbacher and Kahn (2014)
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