Title: Improved retrieval of aerosol optical depth by satellite
Description: Atmospheric aerosols are of major concern for public health and climate change, but their sources and atmospheric distributions remain poorly constrained. Satellite-borne radiometers offer a new constraint on aerosol sources and processes by providing global aerosol optical depth (AOD) retrievals. However, quantitative evaluation of chemical transport models (CTMs) with AOD products retrieved from satellite backscattered reflectances can be compromised by inconsistent assumptions of aerosol optical properties and errors in surface reflectance estimates. We present an improved AOD retrieval algorithm for the MODIS satellite instrument using locally derived surface reflectances and CTM aerosol optical properties. Assuming negligible atmospheric reflectance at 2.13in cloud-free conditions, we derive 0.47/2.13 and 0.65/2.13 surface reflectance ratios at 1°×1.25° horizontal resolution for the continental United States in summer 2004 from the subset of top-of-atmosphere (TOA) reflectance data with minimal aerosol reflectance. We find higher ratios over arid regions than those assumed in the operational MODIS AOD retrieval algorithm, explaining the high AOD bias found in these regions. We simulate TOA reflectances for each MODIS scene using local aerosol optical properties from the GEOS-Chem CTM, and fit these reflectances to the observed MODIS TOA reflectances for a best estimate of AODs for each scene. Comparison with coincident ground-based (AERONET) AOD observations in the western and central United States during the summer of 2004 shows considerable improvement over the operational MODIS AOD products in this region. We find the AOD retrieval is more accurate at 0.47 than at 0.65 μm because of the higher signal to noise ratio, and that the correlation between MODIS and AERONET AODs improves as averaging time increases.

We further improve the AOD retrieval method using an extensive ensemble of aircraft, ground-based, and satellite aerosol observations during the ICARTT field campaign over eastern North America in summer 2004. The aircraft measurements show narrower aerosol size distributions than those typically assumed in models, and correcting this leads to higher model and satellite retrieved AODs. We find that single scattering albedos calculated assuming externally mixed aerosol more closely represent observations than those calculated assuming internal mixing. Our improved MODIS AOD retrieval compares well to the ground-based AERONET data (R = 0.84, slope = 1.02), significantly improving on the MODIS operational products. Inference of surface PM 2.5 from our MODIS AOD retrieval shows good correlation to the EPA-AQS data (R = 0.78) but a high regression slope (slope = 1.48). The high slope is seen in all AOD inferred PM2 5 concentrations (AERONET: slope = 2.04; MODIS c005: slope = 1.51) and is caused by an overestimate of PM2 5 over strong source regions which could reflect the mid day peak in sulfate concentrations corresponding to AOD sampling times.