Dissolution of aerosol iron in the surface waters of the North Pacific and North Atlantic oceans as determined by a semicontinuous flow-through reactor method

Aerosol iron dissolution in oceanic waters is determined with a semicontinuous batch-leaching method. In this procedure, aerosol samples that were collected from the marine boundary layer were leached in an all-Teflon flow-through reaction chamber by multiple aliquots of similar to 40 mL 0.4 mu m filtered oceanic surface water collected using ultraclean techniques. Each aliquot of seawater is permitted to leach the aerosol sample in the reaction chamber for a predetermined time before the seawater solution is separated from the aerosol sample by a 0.4 mu m Nucleopore filter membrane. A new aliquot of seawater is then added to the leaching chamber to continue leaching the same aerosol particles retained on the filter. The procedure is repeated with progressive increases in leaching time. The method allows the Fe released from the aerosol particles to be continuously removed from the system before saturating the complexation capacity of the seawater-leaching solution or adsorbing onto the leaching chamber wall. This method results in accurate measurements of seawater-dissolvable labile aerosol Fe that is independent of the total aerosol Fe concentrations in the sample. The aerosol Fe solubility determined using this method ranges from 3.5 +/- 1.5% for the North Atlantic during September 2005 to 5.7 +/- 2.0% for the North Pacific during April 2005 and April 2006.