Airborne micro-nanoplastics (MNPs) have become one of the important and emergent constituents of atmospheric aerosols, garnering significant attention in recent times. However, the absence of any standard methods for collecting, sample preparation, and analysis as well as an open database for comparison and compilation of results, has given rise to numerous questions and challenges.

Between August 2022 and January 2023, air samples were collected in Porto, Portugal. To achieve this objective, two types of passive collectors were utilized: the NILU Precipitation Collector for collecting wet atmospheric particulate fallout, and the Atmospheric Microplastic Collector for capturing microplastics in wet and dry deposition. As passive deposition does not provide size-resolving particle sampling, size-fractioning filtration process was adopted after collecting the samples from the air. Different types of sieves and filters, with various porosities, were used to separate particles of different sizes. Sieves with mesh sizes of 125, 63, and 25 μm were employed to retain larger particles, while filters with pore sizes of 12, 0.45, and 0.22 μm were used to retain smaller particles. These smaller particles include the respirable fraction of microplastics, which are categorized based on classification for respirable matter PM₁₀ and PM_2.5. Optical microscopy was used to quantify microplastics after the filtration process. To ensure accuracy in quantification, recovery and blank tests were conducted. The analysis revealed that the respirable fraction size accounted for 25% of the total MNPs, and the quantity of microplastics with sizes ranging from 12 to 0.45 μm in the atmosphere of Porto was more than 400 microplastics per square meter per day (MP/m²/day).