Vertical concentrations gradients and transport of airborne microplastics in wind tunnel experiments

Abstract. Microplastics are an ubiquitous man-made material in the environment, including the atmosphere. Little work focused on the atmospheric transport mechanisms of microplastics and its dispersion, despite being a potential pollutant. We study the vertical transport of airborne microplastics in a wind tunnel as a controllable environment with neutral stability, to identify the necessary conditions for long-range atmospheric transport of microplastics. An ultrasonic disperser generated airborne water droplets from a suspension of polystyrene microspheres (MP) with a diameter of 0.51 μm. The water droplets were injected into the airflow, evaporating and releasing single airborne MPs. The disperser allowed for time-invariant and user-controlled concentrations of MP in the wind tunnel. MP were injected at 27, 57, and 255 mm above ground. A single GRIMM R11 optical particle counter (OPC) and three Alphasense OPCs measured time-averaged MP concentration profiles (27, 57, and 157 mm, above ground). These were combined with turbulent airflow characteristics measured by a hot-wire probe to estimate vertical particle fluxes using the flux-gradient similarity theory. The GRIMM R11 OPC measured vertical concentration profiles by moving its sampling tube vertically. The three Alphasense OPCs measured particle concentrations simultaneously at three distinct heights. Results show that maximum concentrations are not measured at the injection height, but are shifted to the surface by gravitational settling. The MP experience higher gravitational settling while they are part of the larger water droplets. For the lowest injection at 27 mm, the settling leads to smaller MP concentrations in the wind tunnel, as MPs are lost to deposition. Increasing the wind speed decreases the loss of MP by settling, but settling is present until our maximum friction velocity of 0.14 ms^-1. For the highest injection at 255 mm and laminar flow, the settling resulted in a net MP emission, challenging the expectation of a net MP deposition for high injection. Turbulent flows reverse the MP concentration profile giving a net MP deposition with deposition velocities of 3.7 ± 1.9 cm s^-1. Recognizing that microplastics share deposition velocities with mineral particles bridges the gap in understanding their environmental behavior. The result supports the use of existing models to evaluate the transport of microplastics in the accumulation mode. The similar deposition velocities imply, that atmospheric transported microplastics can be found in the same places as mineral particles.