Composite samples of bulk aerosol were collected atop the 23 meter height sampling tower at Tudor Hill, Bermuda, on an approximately weekly basis from November 2018 through March 2020, bracketing the four BAIT project cruises. A high-volume (~700 L min^-1) air sampler was used to collect aerosols on acid-cleaned Whatman-41 cellulose filters (8"x10", 20 µm nominal pore size), which collect particles as small as 1 µm with >90% efficiency (Stafford and Ettinger, 1972). The coastal Tudor Hill site faces into the prevailing southwesterly winds, so aerosols were only collected during winds >1 m s^-1 from the 210°-315° sector in order to avoid local sources. Sample filters were stored in zip-lock polyethylene bags in a vacuum desiccator at room temperature. For analysis of total aerosol iron, 1/16th portions of the aerosol sample filters were digested with a mixture of ultrapure concentrated nitric and hydrofluoric acids and hydrogen peroxide (Fisher Optima) in Teflon vessels (Morton et al., 2013), using a microwave heating system (CEM MARS 6), then evaporated on a hot plate and diluted to volume with 2% (v/v) ultrapure nitric acid. Replicate 1/16 portions of the aerosol filters were also subjected to a flow-through leaching procedure modified from Buck et al. (2006). Briefly, the aerosol filter portions were placed atop an acid-washed 0.2 µm pore polycarbonate membrane filter loaded into a perfluoroalkoxy (PFA) resin filtration tower (Savillex), leached with 250 mL of high-purity deionized water (DIW, Barnstead Nanopure, >18.2 MΩ-cm resistivity), and the leachate acidified to 0.4% (v/v) with 6 M ultrapure hydrochloric acid (Fisher Optima) for analysis of "DIW-soluble aerosol iron". The same filter portions were then subjected to a batch leaching procedure modified from Kadko et al. (2019) using 25% acetic acid (HOAc, Fisher Optima) and 0.02 M hydroxylamine hydrochloride (Sigma) at 90°C, and the supernatant leachate was evaporated and then diluted in 2% ultrapure nitric acid (Fisher Optima) for analysis of "HOAc-soluble aerosol iron". Field blanks for the aerosols (an acid-cleaned filter) were deployed on the Tudor Hill tower and processed in the same manner as samples, but without operating the aerosol sampler pump.

Iron concentrations in aerosol digest solutions and aerosol leachate solutions were determined by ICP-MS without preconcentration, using calibration standards prepared in 2% ultrapure nitric acid (Fisher Optima, for aerosol digests and HOAc leachates) or 0.4% ultrapure hydrochloric acid (Fisher Optima, for DIW leachates) and yttrium as an internal standard. Field blank values for total aerosol iron, DIW-soluble aerosol iron and HOAc-soluble aerosol iron were 0.105, 0.018 and 0.028 µmol per filter, respectively, which equate to atmospheric loadings of 0.035, 0.006 and 0.009 nmol m^-3 for a typical sampled air volume of around 3,000 m³. Limits of detection for iron in blank-corrected aerosol samples were estimated from three times the standard deviation on the mean of field blank values; the thus-defined limits of detection were 0.159, 0.011 and 0.011 µmol per filter, for total aerosol iron, DIW-soluble aerosol iron and HOAc-soluble aerosol iron, respectively. Overall uncertainty on the total aerosol iron concentrations is less than ±25% (one-sigma), based on analyses of duplicate filter digests.  Repeat ICP-MS analyses of iron in the aerosol leachate samples indicate an analytical uncertainty of less than ±5% (one-sigma). In the absence of duplicate samples for the aerosol leaches, we assume an overall analytical uncertainty of less than ±25% (one-sigma) for DIW-soluble and HOAc-soluble aerosol iron, similar to that estimated for total aerosol iron.

Problems/Issues:
No aerosol samples were collected during the period of 16 September to October 14, 2019, owing to the passage of Hurricane Humberto and subsequent loss of electrical power at the Tudor Hill sampling site.