African dust deposition to Florida: Temporal and spatial variability and comparisons to models

Mineral dust plays an important role in ocean biogeochemistry as a source of Fe which in some regions is a limiting micronutrient. Ocean models often use the output of dust transport models to estimate dust-Fe deposition. However, models have not been adequately tested, because of the dearth of long-term dust deposition measurements. Here we present the results of a 3 year deposition study in a nine-station network in Florida which is impacted by African dust every year, and we compare these measurements with estimates from global dust models. Wet deposition (WD) and bulk deposition (BD) rates of soil related elements (Al, Fe, and Mn) were highly correlated and remarkably uniform across the state; they exhibited an extremely strong summer maximum that closely matched concurrently measured dust concentrations in Miami. The average dust WD across the network was about 150 mu g cm(-2) yr(-1), and the BD rate was 200 mg mu g cm(-2) yr(-1). Dry deposition (DD), defined as the difference between BD and WD, was a minor component, about 20-30% of the total. In a comparison with nine dust models in the Aerosol Comparisons between Observations and Models project (AeroCom) database, models correctly characterized the seasonal cycle of deposition but most substantially underestimated summer deposition. The ratios of WD to DD in the models varied greatly, ranging from about 1: 1 to 30: 1 in contrast to the Florida Atmospheric Mercury Study station ratio, which was about 3:1 to 4:1. These results show a clear need for more dust deposition measurements in regions dominated by oceanic air masses and for a better understanding of the treatment of deposition processes in models.