Sources of organic ice nucleating particles in soils

Abstract

Abstract. Soil organic matter (SOM) may be a significant source of atmospheric ice nucleating particles (INPs), especially of those active > −15 • C. However, due to both a lack of investigations and the complexity of the SOM itself, the identities of these INPs remain unknown. To more comprehensively characterize organic INPs we tested locally representative soils in Wyoming and Colorado for total organic By contrast, lysozyme, which digests bacterial cell walls, only reduced INPs active at ≥ −7.5 or ≥ −6 • C, depending on the soil. The known IN bacteria were not detected in any soil, using PCR for the ina gene that codes for the active protein. We directly isolated and photographed two INPs from soil, using repeated cycles of freeze testing and subdivision of droplets of dilute soil suspensions; they were complex and apparently organic entities. Ice nucleation activity was not affected by digestion of Proteinase K-susceptible proteins or the removal of entities composed of fulvic and humic acids, sterols, or aliphatic alcohol monolayers. Organic INPs active colder than −10 to −12 • C were resistant to all investigations other than heat, oxidation with H 2 O 2 , and, for some, digestion with papain. They may originate from decomposing plant material, microbial biomass, and/or the humin component of the SOM. In the case of the latter then they are most likely to be a carbohydrate. Reflecting the diversity of the SOM itself, soil INPs have a range of sources which occur with differing relative abundances.

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

“…Even amongst standard immersion freezing methods, uncertainties of a factor of a few n INPs (T ) and 2-4 • C are likely common on the basis of this study and may be the best that can be achieved. Application of size selection to immersion freezing collections for comparison to MOUDI-DFT data (especially at lower temperatures) and CFDC data; information on INP compositions inferred under all sampling scenarios to help constrain influences of various types (e.g., methods of Hill et al, 2016); and an intercomparison of all methods vs. a cloud parcel simulation chamber, considered as a de facto standard, would all assist resolution and improvement of understanding of measurement discrepancies.…”

“…2.2) were also cooled, and correction for any frozen aliquots in the pure water control vs. temperature was made in all cases, similar to the CS method. Binomial sampling confidence intervals (95 %) were determined for IS data, as described in Hill et al (2016).…”

Ice-nucleating particles in Canadian Arctic sea-surface microlayer and bulk seawater

“…This temperature was chosen because some biological materials have been shown to lose their ice nucleation activity following heating to 95 • C (Christner et al, 2008), possibly due to denaturation of the tertiary structure of ice nucleating proteins (Hill et al, 2016). Samples of microlayer and bulk seawater were put into polypropylene tubes, sealed with lids, and heated to 100 • C in a heating block (Accublock, Labnet, S/N: D1200) for an hour, then cooled to room temperature for approximately 30 min before freezing measurements.…”

Ice-nucleating particle concentrations unaffected by urban air pollution in Beijing, China

“…This results in a significant impact on the extent, lifetime, formation of precipitation and radiative properties of clouds . Currently, four mechanisms are proposed for heterogeneous ice nucleation in mixed-phase clouds: deposition ice nucleation, condensation freezing, immersion freezing and contact freezing (Vali et al, 2015;Hoose and Möhler, 2012). It is under discussion whether condensation freezing is different from immersion freezing on a fundamental level (Wex et al, 2014) and whether at least some of the observed deposition ice nucleation can be attributed to pore condensation and freezing (Marcolli, 2014).…”