Hydrogen Peroxide Treatment to Study Atmospheric Immersion Freezing Ability of Inorganic Particles

Atmospheric ice-nucleating particles (INPs) are an important subset of suspended solid in the air, and they can modulate clouds, precipitation, and the Earth system. Hydrogen peroxide (H₂O₂) treated droplet freezing assays are a common practice in atmospheric ice nucleation (IN) research to measure the immersion freezing abilities of inorganic INPs of chemically heterogeneous samples. The reproducibility of these assays is dependent on the exclusion of organic INPs, which can be presumably decomposed by H₂O₂, and solely assessing inorganic INPs. However, no observational assessments are available for the impact of H₂O₂ stabilizer on ice nucleation, verification of organic INP removal, and test of leftover H₂O₂ on ice nucleation suppression. Here we conducted a study to validate H₂O₂ treatment technique is useful for organic INP removal in droplet freezing assay using reference and environmental samples of known compositions. Our results indicate that virtually all organic INPs can be removed above freezing temperatures of -15 °C. Further, the impurities identified and/or remaining H₂O₂ negligibly act as IN-active compounds in our samples, causing no or negligible heterogeneous freezing. The removal of the H₂O₂ stabilizer compounds through our treatment has shown that homogeneous freezing is possible when following this procedure. Thus, it is useful for droplet freezing assays to quantify inorganic INPs and further aid in reducing experimental errors due to background contaminants in test water.