Dataset: GT10-11 - N African Mineral Dust Composition
Deployment: KN204-01

Sample Collection
Aerosol samples were collected on acid-washed Whatman 41 (W41) cellulose ester filter papers deployed in one of two high-volume aerosol samplers (model 5170-VBL, Tisch Environmental) deployed simultaneously operating at approximately 1.2 cubic meters air per minute. The aerosol samplers were deployed on the ship’s flying bridge (14 m above sea level) as high off the water as possible. Contamination from the ship’s stack exhaust was avoided by controlling aerosol sampling with respect to wind sector and wind speed using an anemometer interfaced with a Campbell Scientific CR800  datalogger and PC. The samplers were allowed to run when the wind was +/- 60 degrees from the bow and > 0.5 m/s. When the wind failed to meet these two criteria, the motors were shut off automatically and not allowed to restart until the wind met both the speed and direction criteria for 5 continuous minutes. The anemometer was deployed nearby on a separate pole in 'free air' where turbulence from the wind crossing the bow did not cause the wind vane to wobble excessively.

Trace Metal Determination
For determinations of total aerosol trace metal composition, the W41 filter discs were digested in tightly capped Teflon vials using sequential additions of concentrated acid solutions [nitric acid (Optima) and hydrofluoric acid (Optima)] at 150 degrees C (Morton et al., 2013, accepted). Any solution remaining after each digestion step was heated to dryness. After the final digestion and dry down, the samples were re-dissolved with 0.32 M nitric acid for analysis using inductively-coupled plasma mass spectrometry (ICP-MS; Agilent 7500CS and Thermo Element 2). All filter digestions were performed under a Class 100 laminar flow hood at the National High Magnetic Field Laboratory at Florida State University (FSU).

Filters for both the total and soluble aerosol concentration measurements were stored frozen prior to processing.

Blank solutions for the acid digestions were prepared by digesting W41 discs that had been deployed in the aerosol samplers for 1 h while not in operation. For example, blanks accounted for 65.6 ± 0.52 ng m-3 air Fe (representing an average of 0.09 ± 0.001% of the Fe sample concentrations). Trace metal concentrations in the blanks were subtracted from all acid-digested filter samples. Leachate blanks were prepared by passing 100 mL of deionized water through W41 filters that had also been deployed in the same way as the acid digestion blanks. Blank leachates averaged 0.004 ± 0.001 ng m-3 air dissolved Fe (representing an average of 1.6 ± 0.38% of the Fe sample concentrations) and were subsequently subtracted from all leachate sample concentrations. Relative standard deviations of replicate analyses averaged 7% for total Fe concentrations. Replicate analyses of soluble Fe produced relative standard deviations of 10%.

References:
Buck, C.S., et al. 2010. The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: observations from the A16N CLIVAR/CO2 repeat hydrography section. Marine Chemistry 120, 57-70. doi: 10.1016/j.marchem.2008.08.003.
Buck, C. S., Landing, W.M.,  Resing,  J. A.,  Lebon, G. T. 2006. Aerosol iron and aluminum solubility in the northwest Pacific Ocean: Results from the 2002 IOC cruise. Geochemistry, Geophysics, Geosystems. 7. doi: 10.1029/2005GC000977.
Morton, P.L. et al. 2013. Methods for the sampling and analysis of marine aerosols: results from the 2008 GEOTRACES aerosol intercalibration experiment. Limnology and Oceanography Methods 11:62-78. doi: 10.4319/lom.2013.11.62.