Ice-Nucleating Activation Capacity of Natural-Origin Aerosols in Sierra Nevada National Park (Spain)

Aerosol particles capable of acting as ice-nucleating particles (INPs) play a key role in Earth’s radiative forcing by controlling ice crystal formation and, consequently, the microphysical and optical properties of clouds. At high-mountain sites characterized by near-pristine conditions, natural aerosols become particularly important and may dominate key atmospheric processes. Among these, pollen particles have been shown to exert a non-negligible regional impact on ice nucleation (Prenni et al., 2009), together with re-suspended local soil dust, which often exhibits higher activity than transported mineral dust (O’Sullivan et al., 2014). Wind-blown snow particles represent an additional natural aerosol in high-mountain environments during the snow season, increasingly affected by the production of artificial snow (Baloh et al., 2019). The present study focuses on the characterization of regional natural INPs in the Sierra Nevada environment.

Dominant pollen types in the region are Olea, Pinus, Cupressaceae and Quercus (Cariñanos et al., 2025). Pollen samples collected directly from the vegetation, together with soil samples collected at different elevations in the Sierra Nevada slope - to account for the influence of wind-driven aerosol transport - and snow samples were analysed for their INP ability. Ice-nucleating activity was analysed using GRAINS (Bazo et al., 2025), an immersion droplet freezing array with 100 µL droplets. To assess the contribution of heat-labile components to ice nucleation, all samples were subjected to heat treatment at 95 °C for 30 minutes and subsequently reanalyzed.

Figure 1 shows the INP spectra of Pinus pollen suspension prepared at a concentration of 1 mg mL⁻¹. The suspension was obtained by dispersing 20 mg of sieved (2 mm) pollen in 20 mL of ultrapure water, followed by agitation, filtration with a 0.45 µm syringe filter, and a resting period of 1 h at 4 °C. The sample was then divided into two laboratory tubes, one analysed directly and the other analysed after heat treatment. The Pinus suspension activates at approximately −12 °C, a temperature influenced by the intrinsic ice-nucleating activity of the sample and experimental factors (droplet volume and suspension concentration). A reduction in activity is observed from −17.5 °C onward after heat treatment, likely associated with the removal of heat-labile compounds. This behavior is consistent with previous studies (Duan et al., 2023), although comparison across the literature remains challenging due to differences in methodology.

Figure 1: INP spectra (normalized by droplet volume) of Pinus.

In this study we will jointly present the overall impact of natural-origin particles in high-mountain sites, that highlight the ice-nucleating relevance of local natural aerosols and provide insight into the role of heat-sensitive components in their activity. This is particularly relevant for disentangling the respective influences of natural and anthropogenic aerosols on aerosol–cloud interaction (ACI) processes.

This work was supported by MIXDUST project (PID2024.160280NB.I00) and NUCLEUS project (PID2021-128757OB-I00) funded by MCIU/ AEI/10.13039/501100011033 and "ERDF/EU".

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