Visible Reflectance Spectroscopy on a Buoy-Mounted Aerosol Sampler: Development of a Sensor for Quantifying the Deposition of Mineral Dust to the Oceans

Iron deficiency has been shown to limit the growth of phytoplankton in the high-nutrient, low-chlorophyll regions of the Equatorial Pacific, Southern Ocean, and Subarctic North Pacific, and is a potentially limiting nutrient for nitrogen-fixing phytoplankton, which represent an important component of primary production in oceans. The major source of iron to the open ocean is the atmospheric transport and deposition of iron-bearing soil dust from the arid regions of North Africa, central Asia and Australia. A buoy-mounted aerosol sampler deployed on the Bermuda Testbed Mooring collected a time series of samples allowing quantification of the deposition of African aerosol iron to the Sargasso Sea. We are developing a sensor using visible reflectance spectroscopy (VRS) that can be integrated with this sampler to make real-time measurements of the concentration of iron in aerosol dust over the oceans. In VRS, a white light source illuminates the sample and the spectrum of the reflected light is recorded. Analyses of the spectra include calculating percent color and the first derivative of the spectrum. As the amount of iron in the sample increases, the percentage of red increases. The first derivatives of the spectra contain peaks which can identify iron minerals (e.g., hematite & goethite). The ability to couple aerosol dust analysis with in-water iron measurements will increase our understanding of how iron enters the oceans and what role it plays in the marine biosphere