Supplementary material to "Atmospheric deposition of microplastics: a sampling and analytical method including the associated measurement uncertainties"

S1 Validation of the wet and dry sampling setups based on spike-recovery tests using red and blue polyethylene (PE) spheresThe potential for particle losses from the setups for collecting wet and dry deposition was assessed by conducting spike-recovery tests in the laboratory and in the field (at the NABEL station in Duebendorf) using red and blue spherical PE particles with nominal diameters in the range of 53-63 µm.The procedure for spiking these reference particles was as 5 follows.Between 100 and 300 red or blue PE spheres were deposited on a glass slide using a metallic needle.An image of the glass slide was recorded with an automated optical microscope (VHX-7000, Keyence, Japan) and the number of particles were counted automatically based on a colour thresholding feature built into the microscope's software.For dry deposition samples, the glass slide was rinsed over a glass dish with ethanol using a pressurized aluminium spray to transfer the particles on the slide to the glass dish below.For wet deposition samples, first the lid of the filtration device was remove d, 10 and the outlet was connected to a vacuum pump that was switched on.Then, the glass slide was rinsed over the opening of the filtration device with ethanol using a pressurized aluminium spray bottle such that particles on the glass slide would be transferred into the filtration device and settle onto the 15 µm steel mesh fitted into it.In each case, the glass slide was inspected under the optical microscope afterwards to check if any particles remained on the glass slide.Any remaining particles were deducted from the initial particle count.15 For testing the aluminium filtration device used for collecting wet deposition samples, spike -recovery experiments (n = 6) were first carried out in the laboratory to check whether particles could be lost e.g. in the gaps between individual components of the filtration device.To do so, between 100 and 300 red PE spheres were counted and added to the filtration device's 15 µm mesh.The filtration device was disassembled, and the steel mesh was carefully removed using metal tweezers.The steel mesh was placed onto the stage of the optical microscope and an optical image of the steel mesh was 20 recorded.The red PE spheres deposited on the steel mesh were counted.The number of red PE spheres remaining on the steel mesh was compared to the initial number of red PE spheres to compute the recovery of the red PE surrogates.Next, similar spike-recovery experiments were performed in the field (n = 3).The filtration device was mounted onto the sampler located in the field.In the laboratory, between 100 and 300 red PE spheres were added to a glass slide and transferred to a clean 50 mL glass beaker.The beaker was covered with an aluminium foil, transported to the field, and the 25 contents of the beaker were poured through the sampler's opening while simultaneously rinsing the inner surface of the beaker with ethanol to ensure that all particles were transferred.The on-site filtration system was switched on such that the particles would flow through the sampler and deposit onto the steel mesh fitted into the filtration device.The filtration de vice was then dismounted from the sampler, closed and transported back to the laboratory.There, the filtration device was disassembled, and the steel mesh was removed using metal tweezers.The number of red PE particles on the steel mesh were 30 counted under an optical microscope.The recoveries of red PE particles were recorded.