Photolysis of hydrogen peroxide purifies water and may make it suitable for droplet freezing experiments

Abstract:

Purifying water is an imperative task to minimize interferences from nonhomogeneous freezing in ice nucleation (IN) measurements. Currently, a majority of droplet freezing assays suffers from persistent background freezing contributions in part due to pre-existing impurity in water, resulting in a wide range of 50% frozen fraction (FF₅₀) from -20 °C to -35.7 °C (Hiranuma et al., 2019 Fig. S10). Polen et al. (2018) discusses the impact irregularities on surfaces, droplet surface area, and experimental preparation techniques can have on freezing temperatures. Additionally, water quality can create variation in experimental outcomes (Polen et al., 2018), which is why we utilize HPLC-grade water over other options. The only exception is in the case of facilitating extremely small volume of deionized water droplets (i.e., 0.6 nL), where we virtually observe a 1:1 match between homogeneous freezing measurement and classical nucleation theory (CNT) (Reicher, Segev, and Rudich, 2018). However, such a rigorous technique is still limited and available at only a few places across the world. Thus, the water background issue persists for a majority of researchers in the atmospheric IN research community. Here, we present that photo-dissociation of aqueous hydrogen peroxide (H₂O₂) through Haber-Weiss reactions (Lunak, and Sedlak, 1992) helps purify water and realize homogeneous freezing according to CNT in a reasonable manner. In this work, we generated our H₂O₂-treated droplets through the following protocol:

1. Making H₂O₂ suspensions by mixing H₂O₂ (35 vol%) with HPLC-grade water for various concentrations ranging from 0.5 vol% to 17.5%,
2. Applying >20 minutes of boiling and several hours (≥5 hours) of full-spectra UV-light radiation to the suspensions for H2O2 residual neutralization (Hill et al., 2014; Schiebel, 2017), and
3. Simmering and storing the suspensions at 4°C afterwards for >12 hours.

Once this suspension treatment was completed, we produced 3 µL droplets for negative control water freezing tests. From our preliminary results, we found that (1) there was a systematic shift of FF₅₀ towards low temperature as a function of H₂O₂ concentration and (2) only 1 vol% H₂O₂ suspensions were optimized to CNT for their FF₅₀ (Koop and Murray, 2016). The boiling- and UV-treatment time may be another important variable controlling its FF₅₀ and freezing spectral shape. Further, we have examined if our H₂O₂ suspension impacts IN propensity of some test materials (e.g., illite NX, cellulose, and Snomax). Overall, our findings and water purification protocol may be a meaningful addition to commonly used water droplet freezing assays.

References

Hill, T. et al.: Measurement of ice nucleation-active bacteria on plants and in precipitation by quantitative pcr, Appl. Environ. Microbiol., 80, 1256–1267, 2013.

Hiranuma, N. et al.: A comprehensive characterization of ice nucleation by three different types of cellulose particles immersed in water, Atmos. Chem. Phys., 19, 4823–4849, 2019.

Lunak, S., Sedlak, P.: Photoinitiated reactions of hydrogen peroxide in the liquid phase, Photochem. Photobiol. A: Chem., 68, 1–33, 1992.

Polen et al.: Cleaning up our water: reducing interferences from nonhomogeneous freezing of “pure” water in droplet freezing assays of ice-nucleating particles, Atmos. Meas. Tech., 11, 5315–5334, 2018.

Reicher, N., Segev, L., and Rudich, Y.: The WeIzmann Supercooled Droplets Observation on a Microarray (WISDOM) and application for ambient dust, Atmos. Meas. Tech., 11, 233–248, 2018.

Schiebel, T.: Ice nucleation activity of soil dust aerosols, Ph.D. dissertation, Karlsruhe Institute of Technology, 137 pp., 2017.