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First direct evidence of microplastics in the atmosphere at high altitude. Air mass trajectory analyses showed long-distance transport. Urban areas could be sources of microplastics ending up in distant areas. Atmospheric long-range transport of microplastics is a global pollution issue. Nowadays, there is no direct evidence about the presence of microplastics (MPs) in the atmosphere above ground level. Here, we investigated the occurrence, chemical composition, shape, and size of MPs in aircraft sampling campaigns flying within and above the planetary boundary layer (PBL). The results showed that MPs were present with concentrations ranging from 1.5 MPs m−3 above rural areas to 13.9 MPs m−3 above urban areas. MPs represented up to almost one third of the total amount of microparticles collected. Fourier Transform Infrared Spectroscopy allowed identifying seven types of MPs with the highest diversity corresponding to urban areas. Atmospheric transport and deposition simulations were performed using the HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. Air mass trajectory analyses showed that MPs could be transported more than 1000 km before being deposited. This pioneer study is the first evidence of the microplastic presence above PBL and their potential long-range transport from their point of release even crossing distant borders. C212 airborne accesswas generously provided by INTA, coordinated by the Aerial Platformfor Research teamandwith the logistic and operational support of Group 47 of the Spanish Air Force. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model. We also acknowledge support from the EnviroPlaNet Network Thematic Newtork of Micro- and Nanoplastics in the Environment (RED2018-102345-T; Ministerio de Ciencia, Innovacion y Universidades). Wethank the financial support provided by the Spanish Ministerio de Ciencia, Innovacion y Universidades (CTM2016-74927-C2-1-R/2-R, CGL2015-69758-P, CGL2017-92086-EXP, RTI2018-094867-B-I00) and National Institute for Aerospace Technology (PAI/APL/001/09). Peerreview

The atmosphere is an extreme environment where organisms are subject to low temperatures and high radiation. Many of the microorganisms detected there appear in resistant forms or show mechanisms of adaptation designed to withstand these extreme conditions. Airborne microorganisms may play an important role in the global climate system, biogeochemical cycling, and health. Dust storms are the atmospheric phenomenon that move more topsoil through the Earth’s atmosphere, and numerous microorganisms attached to dust particles are thus transported. The Iberian Peninsula is periodically affected by this phenomenon as African dust frequently reaches southern Europe and the Mediterranean basin. There are numerous methods for sampling airborne microbes, but factors such as low biomass and high variability of the atmosphere render them not yet sufficiently efficient. Very few studies have been conducted directly in the atmosphere via sampling using airborne platforms. The National Institute for Aerospace Technology has two CASA C-212-200 aircraft that have been suitably modified to operate as airborne research platforms. These aircraft are a unique tool for the study of atmospheric microbial diversity and the different environments where they can be found. A study of the airborne microbial diversity in a Saharan dust event from four aerobiology sampling flights is provided in advance.