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The micro-orifice uniform deposit impactor–droplet freezingtechnique (MOUDI-DFT) combines particle collection by inertial impaction(via the MOUDI) and a microscope-based immersion freezing apparatus (theDFT) to measure atmospheric concentrations of ice nucleating particles(INPs) as a function of size and temperature. In the first part of thisstudy we improved upon this recently introduced technique. Using opticalmicroscopy, we investigated the non-uniformity of MOUDI aerosol deposits atspatial resolutions of 1, 0.25 mm, and for some stages when necessary 0.10mm. The results from these measurements show that at a spatial resolution of1 mm and less, the concentration of particles along the MOUDI aerosoldeposits can vary by an order of magnitude or more. Since the total area ofa MOUDI aerosol deposit ranges from 425 to 605 mm<sup>2</sup> and the areaanalyzed by the DFT is approximately 1.2 mm<sup>2</sup>, this non-uniformity needsto be taken into account when using the MOUDI-DFT to determine atmosphericconcentrations of INPs. Measurements of the non-uniformity of the MOUDIaerosol deposits were used to select positions on the deposits that hadrelatively small variations in particle concentration and to build substrateholders for the different MOUDI stages. These substrate holders improvereproducibility by holding the substrate in the same location for eachmeasurement and ensure that DFT analysis is only performed on substrateregions with relatively small variations in particle concentration. Inaddition, the deposit non-uniformity was used to determine correctionfactors that take the non-uniformity into account when determiningatmospheric concentrations of INPs. In the second part of this study, theMOUDI-DFT utilizing the new substrate holders was compared to thecontinuous flow diffusion chamber (CFDC) technique of Colorado State University. Theintercomparison was done using INP concentrations found by the twoinstruments during ambient measurements of continental aerosols. Resultsfrom two sampling periods were compared, and the INP concentrationsdetermined by the two techniques agreed within experimental uncertainty. Theagreement observed here is commensurate with the level of agreement found inother studies where CFDC results were compared to INP concentrationsmeasured with other methods

atmospheric measurement techniques

The micro-orifice uniform deposit impactor–droplet freezing technique (MOUDI-DFT) for measuring concentrations of ice nucleating particles as a function of size: improvements and initial validation