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Abstract:
Ice nucleating particles (INPs) are a rare but important type of atmospheric aerosol particles, contributed mostly by mineral dust particles and by biogenic macromolecules expressed by a range of different microorganisms. INPs’ ability to nucleate ice in cloud droplets already far above their homogeneous freezing temperature (at ~ -38°C) influences for instance cloud radiative processes and precipitation formation.
Despite a recent surge of research on INPs, neither are all important sources of atmospheric INP sufficiently known, nor their atmospheric abundance which varies with location and season.
Here we examine the heat sensitivity of some types of INP of biogenic origin by exposing them to a range of different heating temperatures (60°C, 85°C, and 90°C) for one hour. The heating is expected to destroy proteinaceous ice active macromolecules. The ice activity of different samples was examined before and after heating.
Examined samples included birch pollen (Betula pendula), fungi (Mortierella alpina, Fusarium acuminatum), the bacteria Pseudomonas syringae (from a commercially available SNOMAX sample) and aspen leaves (from Populus tremuloides) which had been sampled and freeze-dried decades ago. We compare their heat sensitivity to that of INPs from airborne aerosol samples collected on filters in summer months at Villum Research Station (VRS) in North Greenland, which were exposed to the same heating procedure.
For samples from F. acuminatum and P. syringae, a continuing decrease in ice activity (expressed as INP per sample mass) was observed for each of the heating steps. The decrease was larger than one order of magnitude for each heating step across the examined temperature range (roughly -5°C to -25°C). For the B. pendula sample, highly ice active macromolecules inducing ice nucleation at > -10°C were already destroyed by heating to 60°C, while the signal below -15°C was changed much less by any of the heating steps. The M. alpina sample showed no change in ice activity after heating to 60°C, but a strong decrease across the examined temperature range after heating further to 85°C, and some additional decrease (roughly one order of magnitude) after heating to 90°C. The aspen leave samples showed no noticeable reaction to heating at freezing temperatures below -15°C, but behaved similar to the M. alpina sample at -10°C.
Interestingly, for freezing temperatures > -10°C, INP concentrations of VRS summer samples also showed no or only a small decrease in ice activity upon heating to 60°C, similar as the M. alpina and aspen leave samples. Also similar to these two, VRS samples showed a very pronounced decrease upon heating to 85°C and some further decrease upon heating to 90°C. This is interesting in the light that recent research suggested that M. alpina, together with the bacteria species Pantoea ananatis, are likely sources of the INPs present in a aspen leave sample of the same batch as the one examined here. Combining these findings, we speculate that M. alpina may be of considerable importance as terrestrially sourced atmospheric INPs for regions even including the summer Arctic.
