Contribution of Soil Organic Matter to the Ice Nucleation Activity of Arable Soil Dust Aerosol Particles

Only a minor fraction of atmospheric aerosol particles acts as a trigger for heterogeneous ice formation in clouds. Nevertheless, the activity of these ice nucleating particles (INPs) controls primary ice formation followed by secondary ice multiplication processes and thereby markedly influences the cloud radiative properties as well as the initiation of precipitation. Soil mineral dust aerosol INPs, mostly from deserts, are widely acknowledged to be important for ice formation in clouds. On the other hand, recent investigations have shown that agricultural soil dust has an enhanced ice nucleation activity up to a factor of 10 compared to desert dust, especially at temperatures above -26 °C (Steinke, 2013; Steinke et al., submitted). This enhancement appears to be caused by very ice-active primary biological particles, such as bacteria, fungal spores and pollen, and their cell-free INP proteins (Hoose and Möhler, 2012; Fröhlich-Nowoisky et al., 2012; Pummer et al., 2015). In addition, arable soil dust aerosol particles contain a considerably higher amount of organic matter, derived from plants and the soil microflora, compared to desert dust particles, which further increases its ice nucleation activity (O’Sullivan et al., 2014; Tobo et al., 2014; Hill et al., 2016).

To investigate and quantify the ice nucleation activity of soil dust, we conducted a series of laboratory measurements with five different soil dust samples at the cloud simulation chamber AIDA (Aerosol Interactions and Dynamics in the Atmosphere). The chamber operates under atmospherically relevant conditions over wide ranges of temperature, pressure and humidity. By controlled adiabatic expansions, the ascent of an air parcel in the troposphere is simulated. In combination with supplementary INP measurements, we thus attained a robust data set on the ice nucleation activity of soil dust aerosol over a wide temperature range in the immersion freezing regime.

In addition, the role of primary biological particles and organic matter was examined in more detail to investigate the origin of the enhanced ice nucleation activity of arable soil dust. For example, the soil dust samples were exposed to dry heat for pyrolysis of organic compounds. Additional instrumentation allowed us to characterize the untreated and the heat treated total aerosol particles as well as the ice residuals received from immersion freezing and deposition nucleation experiments.

References

Fröhlich-Nowoisky et al. (2012): Biogeography in the air: fungal diversity over land and oceans.

Hill et al. (2016): Sources of organic ice nucleating particles in soils.

Hoose and Möhler (2012): Heterogeneous ice nucleation on atmospheric aerosols: a review of results from laboratory experiments.

O’Sullivan et al. (2014): Ice nucleation by fertile soil dusts: relative importance of mineral and biogenic components.

Pummer et al. (2015): Ice nucleation by water-soluble macromolecules.

Steinke (2013): Ice nucleation properties of mineral dusts, PhD thesis.

Steinke et al. (submitted): Ice nucleation activity of agricultural soil dust aerosols from Mongolia, Argentina and Germany.

Tobo et al. (2014): Organic matter matters for ice nuclei of agricultural soil origin.