Biological residues define the ice nucleation properties of soil dust

Abstract

Abstract. Soil dust is a major driver of ice nucleation in clouds leading to precipitation. It consists largely of mineral particles with a small fraction of organic matter constituted mainly of remains of micro-organisms that participated in degrading plant debris before their own decay. Some micro-organisms have been shown to be much better ice nuclei than the most efficient soil mineral. Yet, current aerosol schemes in global climate models do not consider a difference between soil dust and mineral dust in terms of ice nucleation activity. Here, we show that particles from the clay and silt size fraction of four different soils naturally associated with 0.7 to 11.8 % organic carbon (w/w) can have up to four orders of magnitude more ice nucleation sites per unit mass active in the immersion freezing mode at −12 • C than montmorillonite, the nucleation properties of which are often used to represent those of mineral dusts in modelling studies. Most of this activity was lost after heat treatment. Removal of biological residues reduced ice nucleation activity to, or below that of montmorillonite. Desert soils, inherently low in organic content, are a large natural source of dust in the atmosphere. In contrast, agricultural land use is concentrated on fertile soils with much larger organic matter contents than found in deserts. It is currently estimated that the contribution of agricultural soils to the global dust burden is less than 20 %. Yet, these disturbed soils can contribute ice nuclei to the atmosphere of a very different and much more potent kind than mineral dusts.

Atmospheric ice nucleators active ≥ −12 °C can be quantified on PM<sub>10</sub> filters

“…2). These results support, but do not prove until the source of IN are specifically identified, the proposition by Conen et al (2011) that larger numbers of IN per unit mass of soil dust may be found in cooler regions and where soils have larger concentrations of organic matter, compared to warmer regions or where soils have lower organic matter concentrations, such as desert soils. At the same sampling site (Jungfraujoch), Kamphus et al (2010) previously analysed single cloud ice residues by time-of-flight-massspectrometry.…”

“…We selected eight filters collected between 10 June and 11 July 2010 which had sampled air masses of different geographical origin. Conen et al (2011) suggested that larger numbers of IN per unit mass of soil dust may be found in colder, compared to warmer regions. Our filter selection served to test whether -in principle -such differences may be detectable through the analysis of PM 10 filters.…”

Immersion freezing of ice nucleation active protein complexes

“…In this context, it is worth noting that Kleber et al (2007) found INA protein complexes to be well preserved -and maybe accumulated -when being connected to mineral surfaces. Also Conen et al (2011) found that organic matter consisting of remains of micro-organisms that took part in the degradation of plant debris is responsible for enhancing the ice nucleation of atmospheric mineral dust.…”

Refreeze experiments with water droplets containing different types of ice nuclei interpreted by classical nucleation theory

“…This together with the heterogeneity of the sample and the jumps and trends observed for the time sequences of some samples supports the notion that ice nucleation occurs at specific sites on the sample surface. However, it is not clear whether these active sites originate from a specific mineral component or even biogenic components in the dust sample (Conen et al, 2011;Tobo et al, 2014;O'Sullivan et al, 2014). Moreover, the activity of sites could be influenced by coagulation or the breakup of aggregates (Emersic et al, 2015).…”