Microplastics in high-mountain air at the Altzomoni Observatory, Mexico: evidence of regional transport shaped by...

High-altitude environments are often treated as pristine reference sites, yet they can be intermittently influenced by regional anthropogenic outflow when boundary-layer growth and mountain-valley circulations enhance vertical and upslope transport. We present the first characterization of airborne microplastics (MPs) at the Altzomoni Atmospheric Observatory (4000 m a.s.l., between the Popocatépetl and Iztaccíhuatl volcanoes, Mexico), one of the highest-elevation atmospheric monitoring stations worldwide. Using a diurnal sampling design, micro-FTIR particle identification, Weather Research and Forecasting (WRF) simulations of Planetary Boundary Layer (PBL) evolution, and X-ray fluorescence elemental analysis of PM₁₀, we evaluated how boundary-layer dynamics modulate the arrival of MPs and co-transported aerosol. Airborne MPs increased from 0.41 (0.11) MPs m^-3 in the morning to 0.66 (0.14) MPs m^-3 in the afternoon. WRF results attributed this rise to the transition from strongly suppressed PBL conditions at night to active upslope currents that transport pollution from the city during the day. The particle morphology was dominated by fibers, including MPs and artificial non-plastic particles and polymer composition were led by polyester and polyethylene, consistent with the anthropogenic signature. Elemental analysis showed a mixed mineral-anthropogenic aerosol, with robust afternoon enhancements in sulfur and lead relative to morning conditions. The concurrent afternoon enrichment of MPs and anthropogenic elemental tracers supports boundary-layer-driven regional transport as a mechanism enabling the vertical reach of mixed anthropogenic aerosol to high altitude. These findings demonstrate strong time-of-day dependence in MPs exposure, underscoring the importance of resolving diurnal variability when assessing atmospheric MPs transport in remote environments.