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

Abstract

Abstract. The micro-orifice uniform deposit impactordroplet 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 mm 2 and the area analyzed by the DFT is approximately 1.2 mm 2 , 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.

The Horizontal Ice Nucleation Chamber (HINC): INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch

“…To quantify INP concentrations in the range between 253 and 273 K, a technique that is sensitive at warmer temperatures must be used, such as offline techniques of drop freezing (e.g., Mason et al, 2015;Conen et al, 2015). However, improving the LOD of online counters can also be achieved by means of using an aerosol concentrator upstream of an INP counter.…”

“…Typically, filter sampling with subsequent offline freezing methods, and online measurements with continuous-flow-diffusion chambers (CFDCs) are used as INP measurement techniques. For filter sampling, aerosols are collected for a certain time and known air volume, after which the collected particulate is cooled and exposed to controlled temperature and RH conditions (e.g., Bigg, 1967;Santachiara et al, 2010;Conen et al, 2011;Bingemer et al, 2012;Ardon-Dryer and Levin, 2014;Knopf et al, 2014;Mason et al, 2015). Filter techniques observe the onset freezing temperature of a sample with a very large number of particles resulting in a very sensitive detection limit.…”

Comparative measurements of ambient atmospheric concentrations of ice nucleating particles using multiple immersion freezing methods and a continuous flow diffusion chamber

“…The second immersion freezing method involved freezing of droplets grown on substrate-collected particles in a temperature-and humidity-controlled flow cell (Mason et al, 2015) and is referred to as the droplet freezing technique (DFT). A micro-orifice uniform deposit impactor (MOUDI; MSP Corp.) was used to size-select particles from known volumes of air onto a substrate for direct DFT analysis in a number of cases (MOUDI-DFT, Mason et al, 2015).…”

“…To quantify the initial stage of ice nucleation in the atmosphere, multiple sampling techniques are now being used in field studies (Hader et al, 2014;Mason et al, 2015;DeMott et al, 2015;Stopelli et al, 2015;Boose et al, 2016;Schrod et al, 2016Schrod et al, , 2017. Since these various measurements are being used as bases for developing numerical model parameterizations for different emission sources, their comparability should be assessed.…”

The WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) and application for ambient dust

“…The dust was transported from the Sahara and North Africa. Size-segregated ambient dust particles were collected on cyclopore polycarbonate filters using a micro-orifice uniform deposit impactor (MOUDI; MSP Corporation model 110-R, Marple et al, 1991), which operated at 30 L min −1 and for 24 h, similarly to Huffman et al (2013) and Mason et al (2015). The 95.6 ± 9.6 6.0 × 10 −10 248.8 3.2 89.4 ± 10.8 2.6 × 10 −10 248.7 MOUDI has eleven stages with cut points (D 50 ) of 0.056, 0.10, 0.18, 0.32, 0.56, 1.0, 1.8, 3.2, 5.6, 10 and 18 µm.…”