Abstract
Ice-nucleating particles (INPs) are of atmospheric importance because they catalyse the freezing of supercooled cloud droplets, strongly affecting the lifetime and radiative properties of clouds. There is a need to improve our knowledge of the global distribution of INPs, their seasonal cycles and long-term trends, but our capability to make these measurements is limited. Atmospheric INP concentrations are often determined using assays involving arrays of droplets on a cold stage, but such assays are frequently limited by the number of droplets that can be analysed per experiment, often involve manual processing (e.g. pipetting of droplets), and can be susceptible to contamination. Here, we present a microfluidic platform, the LOC-NIPI (Lab-on-a-Chip Nucleation by Immersed Particle Instrument), for the generation of water-in-oil droplets and their freezing in continuous flow as they pass over a cold plate for atmospheric INP analysis. LOC-NIPI allows the user to define the number of droplets analysed by simply running the platform for as long as required. The use of small (∼100 μm diameter) droplets minimises the probability of contamination in any one droplet and therefore allows supercooling all the way down to homogeneous freezing (around -36 °C), while a temperature probe in a proxy channel provides an accurate measure of temperature without the need for temperature modelling. The platform was validated using samples of pollen extract and Snomax®, with hundreds of droplets analysed per temperature step and thousands of droplets being measured per experiment. Homogeneous freezing of purified water was studied using >10 000 droplets with temperature increments of 0.1 °C. The results were reproducible, independent of flow rate in the ranges tested, and the data compared well to conventional instrumentation and literature data. The LOC-NIPI was further benchmarked in a field campaign in the Eastern Mediterranean against other well-characterised instrumentation. The continuous flow nature of the system provides a route, with future development, to the automated monitoring of atmospheric INP at field sites around the globe.
Highlights
Paper be in the range of 102 to 103 cm−3 in the mid-latitudes, the concentration of Ice-nucleating particles (INPs) that can trigger ice nucleation at, for example, −20 °C are typically only 10−4 to 10−2 cm−3.Heterogeneous ice nucleation in the atmosphere is thought to proceed via two main pathways: immersion mode, in which INPs are immersed within a liquid droplet, and deposition mode, in which water vapour deposits as ice onto an INP, and may involve liquid condensates in pores, which freezes.[9,10] It has been suggested that immersion freezing may be the most common glaciation pathway in clouds containing supercooled liquid water.[11,12]A range of instruments have been developed to study INPs in terms of their ice-nucleating activity and to measure their concentration in samples of air.[13,14,15,16,17] Two commonly used and complementary types of instruments are continuous flow diffusion chambers (CFDCs)[18] and cold stage-based droplet freezing assays.[19]
Heterogeneous ice nucleation in the atmosphere is thought to proceed via two main pathways: immersion mode, in which Ice-nucleating particles (INPs) are immersed within a liquid droplet, and deposition mode, in which water vapour deposits as ice onto an INP, and may involve liquid condensates in pores, which freezes.[9,10]
Droplets containing an aqueous suspension of INPs are arrayed on a cold stage and cooled at a defined rate until all of the droplets have frozen
Summary
Paper be in the range of 102 to 103 cm−3 in the mid-latitudes, the concentration of INPs that can trigger ice nucleation at, for example, −20 °C are typically only 10−4 to 10−2 cm−3.Heterogeneous ice nucleation in the atmosphere is thought to proceed via two main pathways: immersion mode, in which INPs are immersed within a liquid droplet, and deposition mode, in which water vapour deposits as ice onto an INP, and may involve liquid condensates in pores, which freezes.[9,10] It has been suggested that immersion freezing may be the most common glaciation pathway in clouds containing supercooled liquid water.[11,12]A range of instruments have been developed to study INPs in terms of their ice-nucleating activity and to measure their concentration in samples of air.[13,14,15,16,17] Two commonly used and complementary types of instruments are continuous flow diffusion chambers (CFDCs)[18] and cold stage-based droplet freezing assays.[19].
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