Comparison of MODIS ocean aerosol retrievals with ship-based sun photometer measurements from the Around the America's expedition

The Around the America's expedition was a 25,000 mile sailing circumnavigation of the American coastlines that took place from June 2009 to June 2010. The wide geographical span of the voyage provided a chance to analyze oceanic aerosol optical properties; covering regions where such measurements are not frequently taken. These were obtained utilizing a Microtops II Sun photometer. In this study we compare these measurements with the ocean aerosol product from the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua and Terra platforms. Results for aerosol optical depth (AOD) show a strong relationship between both measurements, with most values from MODIS falling within published expectations. However, when compared to Microtops, a positive bias is seen in low AOD measurements from MODIS. A possible relationship between these discrepancies in measurements and surface wind speed is found; with a tendency for overestimation occurring at wind speeds near and over 6 m/s, while underestimation is observed for calmer conditions. For Ångström exponent it is found that higher differences in measurements occur at low AOD. No apparent relationship between these differences and wind speed was found.