(Sampling and Digestion methods are reproduced directly from http://bco-dmo.org/dataset/3865.)

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 meters 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 meters per second (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. Filters for total aerosols were stored frozen prior to processing.

Sample Digestion:
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). 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).

Blank solutions for the acid digestions were prepared by digesting W41 discs that had been deployed in the aerosol samplers for 1 hour while not in operation. For example, blanks accounted for 65.6 ± 0.52 nanograms per cubic meter (ng m-3) air Fe (representing an average of 0.09 ± 0.001% of the Fe sample concentrations).

Iron Isotope Analysis:
At the University of South Carolina (USC), aliquots of total-digested aerosol filters were evaporated to dryness, spiked with an 57-58 Fe double spike, and purified by AGMP-1 Column Chemistry. Methods followed Conway et al. (2013). Following purification, iron stable isotope ratios were determined by Neptune multi-collector ICPMS. Fe was analzyed using HR mode. Each sample was analyzed twice and the mean isotope ratios are expressed here relative to international isotope standard IRMM-014. Instrumental and procedural fractionation were corrected for using a double spike reduction scheme. Samples were corrected for blank contribution based on the measured mass of Fe and measured Fe isotope signatures of blank filters (see Conway et al., 2019 for more details).

We express uncertainty conservatively (2 sigma) as 0.05 permil (1 sigma is 0.025) based on replicate seawater and aerosol sample measurements (see Conway et al., 2019) .