Composite samples of bulk aerosol were collected atop the 23 meter height sampling tower at Tudor Hill, Bermuda (https://bios.asu.edu/tudorhill/facility-description), on an approximately weekly basis from November 2018 through March 2020, bracketing the four BAIT project cruises. A high-volume (~700 Liters per minute) air sampler was used to collect aerosoils 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 (e.g., 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 aluminum, 1/16 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. The efficiency of this aerosol digestion procedure was verified using Arizona Test Dust (ISO 12103-1, A1 Ultrafine Test Dust, Powder Technologies, Inc.), which the GEOTRACES program recommends as a reference material for assessing digestion methodology for analysis of total aerosol trace elements. In digests of subsamples of this material, we measured 63878 ± 4485 ppm aluminum (n = 3), which compares well with the stated composition of 66153 ppm. 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 filtrastion 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 aluminum". 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 aluminum". 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.
Aluminum concentrations in aerosol digest solutions and aerosol leachate solutions were determined by inductively coupled plasma mass spectrometry 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 aluminum, DIW-soluble aerosol aluminum and HOAc-soluble aerosol aluminum were 0.132, 0.627 and 0.033 µmol per filter, respectively, which equate to atmospheric loadings of 0.044, 0.209 and 0.011 nmol m-3 for a typical sampled air volume of around 3,000 m3. Limits of detection for aluminum 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 for total aerosol aluminum, DIW-soluble aerosol aluminum and HOAc-soluble aerosol aluminum were 0.063, 0.378 and 0.008 µmol per filter, respectively, which equate to atmospheric loadings of 0.021, 0.126 and 0.003 nmol m-3 for a typical sampled air volume of around 3,000 m3. Overall uncertainty on the total aerosol aluminum concentrations is less than ±10% (one-sigma), based on duplicate, separate-day analyses of filter digests. In the absence of duplicate samples for the aerosol leaches, we assume an overall analytical uncertainty of less than ±10% (one-sigma) for DIW-soluble and HOAc-soluble aerosol aluminum, similar to that estimated for total aerosol aluminum.
