<p>Current knowledge about the environmental loading of microplastics via atmospheric deposition is predominantly based on number-based data which has been used to estimate mass deposition. The aim of this study was to determine mass-based atmospheric deposition rates, with an emphasis on uncertainties related to the sampling and analysis of inhalable microplastics in a semi-urban environment in Helsinki, Finland. Atmospheric deposition samples were collected during a six-week sampling campaign with four parallel samplers. Eight common polymers – polycaprolactam, polycarbonate, polyethylene, polymethylmethacrylate, polypropylene, polystyrene, polyethylene terephthalate, and styrene-butadiene rubber – were analysed by pyrolysis-gas chromatography-mass spectrometry. The total mass deposition rates of the studied polymers ranged between 50 ± 10 and 100 ± 23&#xa0;µg d^− 1 m^− 2. The most abundant polymer types were polyethylene, polyethylene terephthalate, and polycaprolactam, which accounted for up to 95% of the plastic polymer mass in atmospheric deposition. High reproducibilities were observed between the parallel samplers. Potential matrix interferences were identified and acknowledged in quantification of polyethylene, polyethylene terephthalate, and polyvinyl chloride. Direct mass-based determination of microplastic deposition rates reduces the uncertainties associated with sample pretreatment and conversion from number-based data.</p>