Microlitter size from tropical to temperate regions of the Atlantic Ocean was assessed using the pump-underway ship intake system. 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 were continuously collected through the pump-underway ship intake system during navigation. The water was collected by a pump (BKMKC.1011, TECNUIM) with a Teflon core located in the keel of the vessel at about 4.5 m deep, with a flow rate of 4 m³/h. The pump-underway system had a tap connection to the wet laboratory, to which the filtering device was connected to a hose. The filtering device consisted of a flowmeter attached to a funnel, which enclosed a 63 μm stainless-steel sieve, in order to avoid airborne contamination. The flow ran 24/7 and was divided into samples of 4.15 ± 1.47 m³ (10 L/min; n = 52). All 52 samples were analyzed for the large fraction (≥ 315 μm) and 15 were analyzed for the small fraction (10–315 μm). The large fraction (≥ 315 μm; n = 52) 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. The size of the particles was determined with ImageJ (Schneider et al., 2012). The small fraction (10–315 μm; n = 15) was analyzed using μFTIR (Agilent 620 FTIR microscope with a FPA detector and a Cary 670 FTIR spectrometer) and measured with siMPle (open-source software; https://simple-plastics.eu/). Since fragments dominated the small fraction (10–315 μm; n = 15), the dataset focuses on the size of the fragments obtained through the small fraction analysis.

i-plastic project is funded by JPI-Oceans through support by the national funding agencies: Brazil: Fundação Cearense de Apoio ao Desenvolvimento Científico e Tecnológico (FUNCAP); Italy: Ministero dell'Università e della Ricerca (MUR); Portugal: Fundação para a Ciência e a Tecnologia; Spain: Agencia Estatal de Investigacíon (AEI; Project reference: AEI/10.13039/501100011033)