Original data for article: Microplastics accumulate to thin layers in the stratified Baltic Sea

<strong>Description of the data and files</strong>: The data was created in 2019-2020 and opened in 2021. The dataset consists of two Excel files: BalticSeaMIF_sampling.xlsx contains information about sampling, including coordinates of the sampling sites, depths and volumes of the samples, and as background information CTD (conductivity-temperature-depth) measurements from each sampling site. BalticSeaMIF_rawdata.xlsx contains raw data from the FPA-FTIR measurements. Raw data includes numbers, mass estimations, polymer types, and sizes of microplastics. Additionally, calculations for the background corrected numbers and mass estimations of microplastics per sample are provided. <strong>Creation and processing of the data</strong>: Data is created by measuring pre-treated and filtered water samples with FPA-FTIR and analysing the spectral data with siMPle (Primpke et al. <strong>2020</strong>, <em>Applied Spectroscopy</em>, 74, 1127-1138, https://doi.org/10.1177/0003702820917760). Please see the related article for more details about the methods and results. <strong>Purpose and use of the data</strong>: Data is published under Creative Commons CC-BY 4.0 license. Data may be useful for comparisons between datasets and meta-analyses of microplastic concentrations in various environments.