Comparison of Moderate Resolution Imaging Spectroradiometer ocean aerosol retrievals with ship-based Sun photometer measurements from the Around the Americas expedition

The Around the Americas expedition was a 25,000 mile sailing circumnavigation of the North and South American continents, in coastal waters, that took place from June 2009 to June 2010. The broad geographical span of the voyage made it possible to measure marine aerosol optical depths in regions where surface measurements are not frequently taken. These were measured with a handheld 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, MODIS values are biased high relative to surface observations for small optical depth values. There appears to be a relationship between these discrepancies in measurements and surface wind speed, with a group of values showing overestimation at wind speeds near and over 6 m/s and a second, smaller group showing underestimation for calmer conditions. For derived Angstrom exponents, it is found that higher differences occur at low AOD. No relationship between these differences and wind speed is found.