Wind-dispersed pollen grains emitted from vegetation are directly injected into the atmosphere being an important source of natural aerosols globally. These coarse particles of pollen can rupture into smaller particles, known as subpollen particles (SPPs), that may act as cloud condensation nuclei (CCN) and affect the climate. In this study, we characterize and investigate the ability of SPPs of 10 Mediterranean-climate pollen types to activate as CCN. A continuous flow CCN counter (CCNC) was used to measure the activation of size-selected (80, 100 and 200 nm dry mobility diameter) particles at different supersaturations (SS). Hygroscopicity parameter (κ) for each SPP type and size has been calculated using κ-Köhler theory. Organic chemical speciation and protein content has been determined to further characterize pollen solutions. Furthermore, the surface activity of SPPs has also been investigated by using pendant drop tensiometry. All studied SPP samples show critical supersaturation (SSCrit) values that are atmospherically relevant SS conditions. Hygroscopicity κ values are in the range characteristic of organic compounds (0.1–0.3). We found that organic speciation and protein content vary substantially among pollen types, with saccharides and fatty acids being the only organic compounds found in all pollen types. A clear relationship between SPP activation and its organic composition was not observed. This study also reveals that all SPPs investigated reduce the surface tension of water at high concentrations but at diluted concentrations (such as those of activation in the CCNC), the water surface tension value is a good approximation in Köhler theory. Overall, this analysis points out that pollen particles might be an important source of CCN in the atmosphere and should be considered in aerosol-cloud interactions processes., This work was supported by BioCloud project (RTI2018.101154.A.I00) funded by MCIN/AEI/10.13039/501100011033, FEDER “Una manera de hacer Europa” and NUCLEUS project (PID2021-128757OB-I00) funded by MCIN/AEI/10.13039/501100011033 and NextGenerationEU/PRTR. This work received support from the European Union's Horizon 2020 research and innovation program through projects ACTRIS.IMP (grant agreement No 871115) and ATMO_ACCESS (grant agreement No 101008004), by the Spanish Ministry of Science and Innovation through projects ELPIS (PID2020-120015RB-I00) and ACTRIS-España (CGL2017-90884REDT)). By the Junta de Andalucía Excellence, project ADPANE (P20-00136), AEROPRE (P-18-RT-3820) and by University of Granada Plan Propio through Visiting Scholars (PPVS2018-04), Singular Laboratory (LS2022-1) programs and Pre-Competitive Research Projects Pre-Greenmitigation3 (PP2022.PP34). Funding for open access charge, University of Granada/CBUA. Andrea Casans is funded by Spanish ministry of research and innovation under the predoctoral program FPI (PRE2019-090827) funded by MCIN/AEI/10.13039/501100011033, FSE “El FSE invierte en tu futuro”. Fernando Rejano is funded by Spanish ministry of universities through predoctoral grant FPU19/05340. Juan Andrés Casquero-Vera is funded by FJC2021-047873-I, MCIN/AEI/10.13039/501100011033 and NextGenerationEU/PRTR. Elisabeth Andrews is funded in part by NOAA cooperative agreements NA17OAR4320101. Thanks to the NOAA Global Monitoring Laboratory for the use of the CCN counter., Peer reviewed

Detailed knowledge on the formation of new aerosol particles in the atmosphere from precursor gases, and their subsequent growth, commonly known as new particle formation (NPF) events, is one of the largest challenges in atmospheric aerosol science. High pre-existing particle loadings are expected to suppress the formation of new atmospheric aerosol particles due to high coagulation and condensation (CS) sinks. However, NPF events are regularly observed in conditions with high concentrations of pre-existing particles and even during intense desert dust intrusions that imply discrepancies between the observations and theory. In this study, we present a multi-site analysis of the occurrence of NPF events under the presence of desert dust particles in dust-influenced areas. Characterization of NPF events at five different locations highly influenced by desert dust outbreaks was done under dusty and non-dusty conditions using continuous measurements of aerosol size distribution in both fine and coarse size fractions. Contrary to common thought, our results show that the occurrence of NPF events is highly frequent during desert dust outbreaks, showing that NPF event frequencies during dusty conditions are similar to those observed during non-dusty conditions. Furthermore, our results show that NPF events also occur during intense desert dust outbreaks at all the studied sites, even at remote sites where the amounts of precursor vapours are expected to be low. Our results show that the condensation sink associated with coarse particles (CSC) represents up to the 60 % of the total CS during dusty conditions, which highlights the importance of considering coarse-fraction particles for NPF studies in desert-dust-influenced areas. However, we did not find a clear pattern of the effect of desert dust outbreaks on the strength of NPF events, with differences from site to site. The particle growth rate (GR) did not present a clear dependence on the CS during dusty and non-dusty conditions. This result, together with the fact that desert dust has different effects on the growth and formation rates at each site, suggests different formation and growth mechanisms at each site between dusty and non-dusty conditions, probably due to differences in precursor vapours' origins and concentrations as well as changes in the oxidative capacity of pre-existing particles and their effectiveness acting as CS. Further investigation based on multiplatform measurement campaigns and chamber experiments with state-of-the-art gaseous and particulate physical and chemical properties measurements is needed to better understand the role of catalyst components present in desert dust particles in NPF. Finally, our results reveal a significant impact of NPF events on the cloud condensation nuclei (CCN) budget during desert dust outbreaks at the studied sites. Therefore, since desert dust contributes to a major fraction of the global aerosol mass load, and since there is a foreseeable increase in the frequency, duration and intensity of desert dust episodes due to climate change, it is imperative to improve our understanding of the effect of desert dust outbreaks on NPF and the CCN budget for better climate change prediction., Juan Andrés Casquero-Vera is funded by FJC2021-047873-I, by MCIN/AEI/10.13039/501100011033 and NextGenerationEU/PRTR, by Spanish Ministry of Universities and the European Union – NextGenerationEU, and by the Academy of Finland through ACCC Flagship (Atmosphere and Climate Competence Center, project no. 337549). This research has been partially supported by the Spanish Ministry of Science and Innovation (grant nos. PID2020-120015RB-I00, RED2022-134824-E, PID2019-108990RB-I00 and PID2021-128757OB-I00 funded by MCIN/AEI/10.13039/501100011033) and by the European Union's Horizon 2020 Research and Innovation programme through projects ACTRIS IMP (grant no. 871115) and ATMO ACCESS (grant no. 101008004) and via Horizon Europe through “Non-CO2 Forcers and their Climate, Weather, Air Quality and Health Impacts”, FOCI (project no. 101056783). This research was also partially supported by Plan Propio of University of Granada through EMERALD project (PPJIA2022-15) and the Singular Laboratory AGORA programme (LS2022-1) and Scientific Units of Excellence Program (grant no. UCE-PP2017-02) and by the Consejo Superior de Investigaciones Científicas (CSIC) under project 202030E261. Measurements at Hada Al Sham were funded by the Deanship of Scientific Research (DSR; grant no I-122-430) at King Abdulaziz University (KAU)., Peer reviewed

This work investigates scattering matrix elements during different Saharan dust outbreaks over Granada (southeast Spain) in 2022 using a polarized imaging nephelometer (PI-Neph) capable of measuring continuously the phase function (F11) and the polarized phase function (-F12/F11) at three different wavelengths (405, 515 and 660 nm) in the range 5–175°. The focus is on two extreme dust events (PM10 > 1000 µg m‑3) in March 2022. During the peaks of these events F11 and -F12/F11 show the classical patterns observed for dust samples in laboratory measurements available in the Granada–Amsterdam Light Scattering Database at all wavelengths. However, for the moments prior to and after the peaks the results reveal important sensitivity in -F12/F11 at 405 nm. For the other wavelengths, however, this difference in -F12/F11 is not evident. Moreover, no remarkable changes are found in F11, which is always characterized by strong predominance of forward scattering. The analyses of more frequent and moderate events recorded in summer 2022 (PM10 between 50 and 100 µg m‑3) revealed F11 and -F12/F11 patterns like those observed prior to and after the extreme events. The combination of PI-Neph measurements with additional in situ instrumentation allowed a typing classification that revealed the peaks in the extreme dust events as pure dust, while for the rest of cases it remarked a mixture of dust with urban background pollution. In addition, simulations with the Generalized Retrieval of Atmosphere and Surface Properties (GRASP) code explain the different patterns in -F12/F11, with changes in the refractive indexes and with the different contributions of the fine and coarse mode. © Author(s) 2025, This work was supported mainly by the Horizon Europe program under the Marie Skłodowska-Curie Staff Exchange Actions with the project GRASP-SYNERGY (grant agreement no. 101131631). The work was also funded by the European Union's Horizon 2020 research and innovation program through projects ACTRIS.IMP (grant agreement no. 871115) and ATMO_ACCESS (grant agreement no. 101008004); by the Spanish Ministry of Science and Innovation through projects ELPIS (PID2020-12001-5RB-I00), MULHACEN (PID2021-128008OB-I00) and NUCLEUS (PID2021-128757OB-I00), funded by MICIU/AEI/10.13039/501100011033 and ERDF through “A way of making Europe” and ACTRIS-España (RED2022-134824-E); and by the University of Granada Plan Propio through the Excellence Research Unit Earth Science and Singular Laboratory AGORA (LS2022-1) programs. Elena Bazo received funding by MICIU/AEI/10.13039/501100011033 and the ESF+ through FPI fellowship PRE2022-101272. Francisco José García-Izquierdo acknowledges financial support from the grant PID2021-123370OB-I00 (CATS), funded by MCIN/AEI/10.13039/501100011033. We are very thankful to the Air Quality Service of Junta de Andalucía for supplying the PM10 data., Peer reviewed