Sampling was performed during the PYROWIND oceanographic campaign in the Atlantic Ocean (April 27th – May 19th 2022), on board of the Spanish R/V Hespérides. The research cruise covered an 8,000-km transect, from Brazil (Salvador) to the Strait of Gibraltar. Subsurface water samples (n = 52) were continuously collected through the pump-underway ship intake system during navigation. Field and procedural blanks were introduced to assess possible sample contamination. In the field, a petri dish with a polycarbonate filter (0.45 μm) was left open at the working station (field blank) for each set of samples to assess airborne contamination (n = 52). The blank remained open during the duration of the samples' preparation and removal. Procedural blanks (all reagents and materials with no sample; 1 blank per 10 samples) were run parallelly to the samples, following the same steps. Particles were quantified and analyzed using ATR-FTIR (≥ 315 μm) and μFTIR (10–315 μm). The large fraction (≥ 315 μm) was analyzed on a stereomicroscope (Leica Z16 APO) equipped with a camera (Canon EOS 650D). The filter was horizontally examined from the top left to the bottom right and every relevant particle was photographed. Each particle was counted and classified according to their type. The chemical analysis was carried out using ATR-FTIR (Agilent 630 Cary ATR-FTIR spectroscopy). The small fraction (10–315 μm) was analyzed using μFTIR (Agilent 620 FTIR microscope with a FPA detector and a Cary 670 FTIR spectrometer) and examined with siMPle (Primpke et al., 2020).

Empty cells represent the samples/blanks that were not measured for a specific parameter (e.g. since we collected 52 samples in total and only 15 were analyzed for the small fraction [< 315 μm], the remaining 37 samples did not go through the small fraction analysis, which are the ones with empty cells).