Diego Aliaga

New particle formation (NPF) is the phenomenon wherein gaseous precursors form critical clusters of barely a few nanometres in diameter, after which, under favourable conditions, these particles can grow to climate-relevant sizes. Here we present measurements from 2022 to 2024 of particle and ion number size distributions from the Zeppelin Observatory (ZEP), an Arctic research station situated on the western edge of Svalbard atop a mountain. NPF events begin in April and continue occurring into November. The events at the start of the NPF season (i.e. April/May) are considerably stronger (i.e. a larger production of nucleation mode particles) compared with other months of the year. The peaks in NPF strength coincide with peaks in the accumulated solar insolation experienced by arriving air masses. During the summer period NPF events occur on 20 %–40 % of days each month; however, there is a consistent decline starting in June. We show that the combined influence of solar insolation and the surface area of pre-existing aerosols (i.e. condensation sink, CS) is a strong predictor for the likelihood of NPF. We develop a simplified predictive model which matches the frequency of NPF events identified via the classification schemes used in this study; we show that the ratio of solar insolation over CS corresponds well to the frequency of NPF events ( R ² of 0.78). We show that NPF events occur during the polar night (i.e. when the sun does not pass above the horizon) and that these events are linked to high-altitude air masses. Furthermore, we detail the likely geographic origins of nucleation mode particles as measured at ZEP. We show that NPF events are considerably more likely to originate from marine regions towards the west of Svalbard, particularly the Greenland Sea, which is the marine region with the greatest likelihood of originating air masses linked to an NPF day. We also show that NPF events lead to an increase in the number of Aitken mode particles, indicating that potentially a significant proportion of the Aitken mode particles originate from NPF. Of the measured NPF events, 37 % exhibited nucleation mode particles that grew beyond 25 nm (a diameter representing the minimum activation diameter for particles to act as cloud condensation nuclei). Overall, we present a concise picture of the life cycle of nucleation mode particles in the Arctic, including the effect wet scavenging has in reducing the condensation sink, which in turn promotes the occurrence of NPF events.

Aerosol particles modulate Earth's radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analyzed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency and intensity of new particle formation in the European FT. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.

New particle formation (NPF) is a crucial process that significantly affects the number of atmospheric particles, forming a substantial portion of the total aerosol population. Therefore, it has important implications for both human health and climate. While extensive research has been conducted in rural areas of the Po Valley, Italy, there is a substantial lack of continuous measurements with state-of-the-art instruments in Milan, one of the most industrialized and densely populated cities in the region. This study aims to address this gap by analysing one year of detailed particle number size distribution measurements between 1.2 and 480 nm at an urban background site in Milan. These data were used to examine the occurrence and characteristics of NPF and to identify how the meteorological and air pollution conditions affect it. We show that a cleaner atmosphere, meaning lower concentrations of air pollutants and lower condensation sink, and a higher ventilation promote NPF. Detailed modelling of the air masses history further revealed that a longer residence time in the Po Valley and a greater exposure to anthropogenic emissions inhibit NPF. Furthermore, we show that strong winds, particularly from the northwest sector (e.g., Foehn winds), facilitate NPF, likely by reducing the condensation sink for precursor vapours. This locates Milan among the urban sites where atmospheric cleaning enhances NPF, providing insights for urban air quality management.

Aerosol properties, loading, trends, and variability in the upper troposphere are key to understanding the evolving state of the atmosphere and the role of aerosols in climate and cloud processes. However, long-term in-situ aerosol observations at high altitudes remain scarce worldwide, particularly in the Global South. This observational gap limits our ability to develop a global perspective on aerosol sources, processes, and impacts within the climate system.Here we present 13 years (2012–2024) of continuous aerosol-related measurements conducted at the world’s highest Global Atmosphere Watch (GAW) station, located on Mount Chacaltaya (CHC) in the central Andes of Bolivia at an elevation of 5.2 km a.s.l. This dataset is one of the longest in existence on the South American continent and therefore provides a unique opportunity to evaluate trends in aerosol concentrations and properties. These trends and properties are influenced by, for example, biomass burning in the Amazon, the transport of pollution from the conurbation of La Paz and El Alto, located 18 km to the south, and the subsidence of air masses from the upper troposphere.We focus on particle number size distributions (PNSD), equivalent black carbon (eBC), and related meteorological and chemical tracers, including water vapor mixing ratio (WVMR) and carbon monoxide (CO). We characterize aerosol properties and loading by combining traditional time-series analysis (e.g., separation by hour of day, season, and year) with an unsupervised k-means clustering approach that disentangles the dominant atmospheric regimes influencing aerosol properties at CHC. The clustering uses PNSD, eBC, and WVMR as input variables and identifies seven distinct categories of days, hereafter referred to as atmospheric regimes, which represent significantly different source regions and aerosol processing pathways (e.g., cloud processing, wet deposition, and new particle formation). The performance of the clustering is evaluated using independent tracers, namely CO concentrations and HYSPLIT back trajectories. For each regime, the individual days grouped within it exhibit internally consistent CO levels and air-mass provenance that are clearly distinct from those of other regimes. This result is particularly encouraging given that neither CO nor back trajectories were included as inputs to the clustering algorithm.One regime is particularly noteworthy, representing a persistent free-tropospheric state characterized by extremely low WVMR, CO, and eBC, along with signatures of early-morning new particle formation. We find that the concentration of particles in this regime has significantly decreased over the 13-year period which indicates a declining upper-tropospheric particle concentration. A second notable regime is associated with biomass burning. We find that its occurrence has increased over time, from ~10% of days during the biomass-burning season (August–November) in the first years to ~50% in the last years. This suggests an increment on the number of biomass burning episodes measured at the station. Additional categories capture aerosol–cloud processing during Amazonian boundary-layer uplift, local eBC influence from the La Paz–El Alto metropolitan area, and strong nucleation under dry, coastal/Altiplano air masses. Overall, these results emphasize a region in rapid change and the importance and utility of long-term measurements in under sampled areas.

Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.

Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.

Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.

Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.

New Particle Formation (NPF) is a crucial process that significantly affects the number of atmospheric particles, forming a substantial portion of the total aerosol population. Therefore, it has important implications for both human health and climate. While extensive research has been conducted in rural areas of the Po Valley, Italy, there is a substantial lack of continuous measurements with state-of-the-art instruments in Milan, one of the most industrialized and densely populated cities in the region. This study aims to address this gap by analysing one year of detailed particle number size distribution measurements between 1.2 and 480 nm at an urban background site in Milan. These data were used to examine the occurrence and characteristics of NPF and to identify how the meteorological and air pollution conditions affect it. We show that a cleaner atmosphere, meaning lower concentrations of air pollutants and lower condensation sink, and a higher ventilation promote NPF. Detailed modelling of the air masses history further revealed that a longer residence time in the Po Valley and a greater exposure to anthropogenic emission sources inhibit NPF. Furthermore, we show that strong winds, particularly from the northwest sector (e.g., Foehn winds), facilitate NPF, likely by reducing the condensation sink for precursor vapours. This locates Milan among the urban sites where atmospheric cleaning enhances NPF, providing insights for urban air quality management.

Aerosol particles modulate Earth’s radiation budget and cloud microphysics, yet the processes that control their formation in the free troposphere (FT) are still poorly understood. Monitoring aerosol size distributions and new particle formation (NPF) in this region is crucial to understanding secondary aerosol production, growth dynamics, and their broader climatic implications. We analysed approximately two years of size-resolved aerosol and ion measurements from two high-altitude GAW/ACTRIS stations, Monte Cimone (2165 m a.s.l., GAW ID: CMN) and Jungfraujoch (3580 m a.s.l., GAW ID: JFJ), to characterise aerosol populations and the frequency-intensity of new particle formation in the European free troposphere. Three different NPF classification methods were applied and compared to assess event frequency and characteristics at both sites. Particles larger than 25 nm exhibited marked seasonal variability, largely influenced by boundary layer dynamics. In contrast, the overall abundance of freshly nucleated particles remained relatively stable throughout the year, being significantly perturbed only during NPF events. Interestingly, despite a consistently higher background of freshly nucleated particles at JFJ, NPF events were more frequent and more intense at CMN. CMN displayed higher particle formation and growth rates, likely due to its lower elevation and proximity to the polluted Po Valley, leading to a stronger influence from boundary layer emissions. In contrast, JFJ, located in a cleaner high-Alpine environment, experienced fewer anthropogenic influences and less intense nucleation events. At both sites, a low condensation sink before NPF onset was identified as a critical factor that favours nucleation.