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

摘要

A number of new measurement methods for ice nucleating particles (INPs) havebeen introduced in recent years, and it is important to address how thesemethods compare. Laboratory comparisons of instruments sampling major INPtypes are common, but few comparisons have occurred for ambient aerosolmeasurements exploring the utility, consistency and complementarity ofdifferent methods to cover the large dynamic range of INP concentrations thatexists in the atmosphere. In this study, we assess the comparability of fouroffline immersion freezing measurement methods (Colorado State University icespectrometer, IS; North Carolina State University cold stage, CS; NationalInstitute for Polar Research Cryogenic Refrigerator Applied to Freezing Test,CRAFT; University of British Columbia micro-orifice uniform depositimpactor–droplet freezing technique, MOUDI-DFT)and an online method (continuous flow diffusion chamber, CFDC) used in a manner deemed to promote/maximizeimmersion freezing, for the detection of INPs in ambient aerosols at differentlocations and in different sampling scenarios. We also investigated thecomparability of different aerosol collection methods used with offlineimmersion freezing instruments. Excellent agreement between all methods couldbe obtained for several cases of co-sampling with perfect temporal overlap.Even for sampling periods that were not fully equivalent, the deviationsbetween atmospheric INP number concentrations measured with different methodswere mostly less than 1 order of magnitude. In some cases, however, thedeviations were larger and not explicable without sampling and measurementartifacts. Overall, the immersion freezing methods seem to effectivelycapture INPs that activate as single particles in the modestly supercooledtemperature regime ( −20 °C), although more comparisons areneeded in this temperature regime that is difficult to access with onlinemethods. Relative to the CFDC method, three immersion freezing methods thatdisperse particles into a bulk liquid (IS, CS, CRAFT) exhibit a positive biasin measured INP number concentrations below −20 °C, increasingwith decreasing temperature. This bias was present but much less pronouncedfor a method that condenses separate water droplets onto limited numbers ofparticles prior to cooling and freezing (MOUDI-DFT). Potential reasons forthe observed differences are discussed, and further investigations proposedto elucidate the role of all factors involved.