Abstract
Abstract. The micro-orifice uniform deposit impactor–droplet freezing technique (MOUDI-DFT) combines particle collection by inertial impaction (via the MOUDI) and a microscope-based immersion freezing apparatus (the DFT) to measure atmospheric concentrations of ice nucleating particles (INPs) as a function of size and temperature. In the first part of this study we improved upon this recently introduced technique. Using optical microscopy, we investigated the non-uniformity of MOUDI aerosol deposits at spatial resolutions of 1, 0.25 mm, and for some stages when necessary 0.10 mm. The results from these measurements show that at a spatial resolution of 1 mm and less, the concentration of particles along the MOUDI aerosol deposits can vary by an order of magnitude or more. Since the total area of a MOUDI aerosol deposit ranges from 425 to 605 mm2 and the area analyzed by the DFT is approximately 1.2 mm2, this non-uniformity needs to be taken into account when using the MOUDI-DFT to determine atmospheric concentrations of INPs. Measurements of the non-uniformity of the MOUDI aerosol deposits were used to select positions on the deposits that had relatively small variations in particle concentration and to build substrate holders for the different MOUDI stages. These substrate holders improve reproducibility by holding the substrate in the same location for each measurement and ensure that DFT analysis is only performed on substrate regions with relatively small variations in particle concentration. In addition, the deposit non-uniformity was used to determine correction factors that take the non-uniformity into account when determining atmospheric concentrations of INPs. In the second part of this study, the MOUDI-DFT utilizing the new substrate holders was compared to the continuous flow diffusion chamber (CFDC) technique of Colorado State University. The intercomparison was done using INP concentrations found by the two instruments during ambient measurements of continental aerosols. Results from two sampling periods were compared, and the INP concentrations determined by the two techniques agreed within experimental uncertainty. The agreement observed here is commensurate with the level of agreement found in other studies where CFDC results were compared to INP concentrations measured with other methods.
Highlights
Ice formation in the atmosphere can occur via two different processes: homogeneous and heterogeneous nucleation
Particle concentrations at a spatial resolution of 0.10 mm are shown only for stages 6– 8 and only for the region of the aerosol deposit that corresponds to the region analyzed in the Droplet freezing technique (DFT) experiments when using substrate holders in the Micro-orifice uniform deposit impactor (MOUDI)
The MOUDI-DFT is a recent approach to measuring concentrations of ice nucleating particles (INPs) as a function of size in the atmosphere
Summary
Ice formation in the atmosphere can occur via two different processes: homogeneous and heterogeneous nucleation. In addition to the approaches mentioned above, Huffman et al (2013) recently introduced the micro-orifice uniform deposit impactor–droplet freezing technique (MOUDI-DFT) for measuring the concentration of INPs as a function of size. This technique addresses some of the limitations of previous size-resolving instrumentation. The ice nucleating properties of collected particles are determined in the laboratory by a microscope-based droplet freezing technique (the DFT) that is capable of measuring the concentrations of INPs in the immersion mode to a temperature of approximately −37 ◦C (Koop et al, 1998; Chernoff and Bertram, 2010; Haga et al, 2013, 2014; Wheeler et al, 2015), which is roughly the homogeneous freezing temperature of water droplets 100 μm in diameter (Pruppacher and Klett, 1997). As highlighted by DeMott et al (2011), intercomparison studies of INP instrumentation are important for finding potential biases or deficiencies present in the methods, relating independent data sets, and identifying where efforts for instrument improvement should be focused
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
