Heikki Junninen

Particle nucleation from trace atmospheric vapours is important for climate since it gives rise to more than half of global cloud condensation nuclei. Sulfuric acid (H₂SO₄) has long been recognised to drive particle nucleation in the atmosphere and, more recently, highly oxygenated products of biogenic vapours—in particular monoterpenes such as α-pinene (C₁₀H₁₆)—have also been shown to nucleate under atmospheric conditions, without requiring additional vapours. This raises the question of whether a nucleation synergy exists between α-pinene oxygenated organic molecules (AP-OOM) and H₂SO₄, as has been suggested by early studies. Here we report new particle formation from AP-OOM and H₂SO₄ in the absence of base vapours such as ammonia (NH₃), measured in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber at cool boundary layer temperatures of −10 °C and +5 °C. We find that AP-OOM nucleation rates increase strongly when H₂SO₄ concentrations exceed around 10⁶ cm⁻³. The enhancement is synergistic and cannot be explained as a simple linear addition of independent chemical systems. Above this threshold, the nucleation rate depends approximately linearly on H₂SO₄ concentration, in contrast with the strong sensitivity to H₂SO₄ for H₂SO₄–NH₃ nucleation. Nucleation rates are 10–100-fold higher in the presence of ions from galactic cosmic rays or from the CERN pion beam. Based on these measurements, we have parameterised a temperature-dependent H₂SO₄-AP-OOM nucleation rate in the absence of base vapours and implemented it in the EMAC (ECHAM/MESSy Atmospheric Chemistry) Earth system model. In comparison with a parameterisation developed in an earlier study [Riccobono et al., Science, 2014, 344, 717–721.], the new parameterisation indicates sharply reduced nucleation rates in the boundary layer over warm regions, and increased rates over northern boreal forests.

The aim of the present study was to create a nanofibrous membrane functionalized with a combination of Cu and Zn nanoclusters and evaluate its antiviral passivating effects against both the influenza A virus (IAV) and SARS-CoV-2, as we previously demonstrated that nanofibrous materials modified with Cu nanoclusters can passivate SARS-CoV-2 virions. In contrast, Zn nanoclusters have a more potent virucidal activity against IAV virions. Poly(vinylidene fluoride) (PVDF) membranes activated with Cu and Zn nanoclusters were successfully fabricated by electrospinning a PVDF solution in dimethylacetamide (DMA) with the addition of 2.0 wt% Cu(NO3)2·2.5H2O and 2.0 wt% ZnCl2. Characterization of modified materials was performed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, mercury intrusion porosimetry, thermogravimetric analysis, contact angle measurement, and microwave plasma atomic emission spectroscopy. In addition, filtration efficiency was determined using particles with diameters ranging from 100 nm to 3000 nm. Tissue culture infectious dose 50 assay (TCID50) was employed to determine virucidal activity against IAV (H1N1) and SARS-CoV-2 (Wuhan-Hu-1). The prepared nanofibrous material showed high filtration efficiency and strong virucidal activity against IAV and SARS-CoV-2.

Mercury is a persistent pollutant with significant public health impacts in polar regions where fish consumption drives human exposure. Atmospheric oxidation pathways control where mercury deposits globally, but the lack of molecular-level observations of oxidized mercury products has hindered the validation of proposed chemical mechanisms. Here, we show the in-situ online detection of individual mercuric halides (HgCl₂, BrHgCl, HgBr₂, ClHgI, BrHgI, and HgI₂) in the polar boundary layer using atmospheric pressure chemical ionization mass spectrometry. Our observations identify HgBr₂ as the dominant oxidized mercury species at both poles, while HgCl₂ and other halides were also observed in Antarctica. The observed speciation diverges from current model predictions, which favor HgCl₂ and HOHgBr as dominant oxidized forms. Our results show that real-time molecular measurements can substantially advance global mercury monitoring and improve the chemical models used to assess environmental policies and predict deposition patterns.

Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2 and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹ at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_x at upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre.

The ability of anthropogenic aerosols to freeze supercooled cloud droplets remains debated. In this work, we present observational evidence for the glaciation of supercooled liquid-water clouds at industrial aerosol hot spots at temperatures between −10° and −24°C. Compared with the nearby liquid-water clouds, shortwave reflectance was reduced by 14% and longwave radiance was increased by 4% in the glaciation-affected regions. There was an 8% reduction in cloud cover and an 18% reduction in cloud optical thickness. Additionally, daily glaciation-induced snowfall accumulations reached 15 millimeters. Glaciation events downwind of industrial aerosol hot spots indicate that anthropogenic aerosols likely serve as ice-nucleating particles. However, rare glaciation events downwind of nuclear power plants indicate that factors other than aerosol emissions may also play a role in the observed glaciation events.

Pure biogenic new particle formation (NPF) induced by highly oxygenated organic molecules (HOMs) could be an important mechanism for pre-industrial aerosol formation. However, it has not been unambiguously confirmed in the ambient due to the scarcity of truly pristine continental locations in the present-day atmosphere or the lack of chemical characterization of NPF precursors. Here, we report ambient observations of pure biogenic HOM-driven NPF over a peatland in southern Finland. Meteorological decoupling processes formed an “air pocket” (i.e., a very shallow surface layer) at night and favored NPF initiated entirely by biogenic HOM from this peatland, whose atmospheric environment closely resembles that of the pre-industrial era. Our study sheds light on pre-industrial aerosol formation, which represents the baseline for estimating the impact of present and future aerosol on climate, as well as on future NPF, the features of which may revert toward pre-industrial–like conditions due to air pollution mitigation.

Particulate matter (PM) is a major air pollutant that can have adverse effects on human health, especially for vulnerable populations such as children, the elderly, and those with respiratory or cardiovascular conditions. This study presents a method for prediction of particulate matter concentration with aerodynamic diameter smaller then 10 μm (PM₁₀) in an urban environment. Meteorological data and vehicle flow data from an urban road in Curitiba, Brazil, were used. The air quality was analyzed in two monitoring points located 1 km apart, the sampling points are named Politécnico and Perkons, where SDS011 optical sensors were installed. The prediction was based on the machine learning algorithm Random Forest (RF). The baseline concentration was a dataset from historical records of particulate matter measurements from monitoring stations in Curitiba. Several scenarios were tested and it was concluded that the daily time scale presents the best performance in PM₁₀ prediction, with 80.42% accuracy, using the baseline and PM₁₀ Perkons as descriptors. The most important meteorological variables for the prediction were: air temperature (°C), wind speed (m/s), and wind gust (m/s). Throughout the day there were two peaks with large amounts of pollutants in the air, near 8:00 am and 6:00 pm, times when there are the largest flows of vehicles circulating on the road. The Random Forest algorithm proved to be a good estimator of PM concentration, which is a proxy for air pollution.

Highly oxygenated organic molecules (HOMs) are a major source of new particles affecting Earth’s climate^1,2. HOM production from the oxidation of volatile organic compounds (VOCs) occurs during both day and night, and can lead to new particle formation (NPF)^3,4. However, NPF involving organic vapors has been reported much more often during daytime^3-6 than during nighttime^7,8. Here, we show that the nitrate radicals (NO₃) - which arise predominantly at night – inhibit NPF during the oxidation of monoterpenes based on three lines of observational evidence: NPF experiments in the CLOUD chamber at CERN; radical chemistry experiments using an oxidation flow reactor; and field observations in a wetland that occasionally exhibits nocturnal NPF. Nitrooxy-peroxy radicals formed from NO₃ chemistry suppress the production of ultra-low volatility organic compounds (ULVOCs) responsible for biogenic NPF, which are covalently bound RO₂ dimer association products. The ULVOC yield of α-pinene in the presence of NO3 is one-fifth of that resulting from ozone chemistry alone. Even trace amounts of NO₃ radicals, at sub parts per trillion level, suppress the NPF rate by a factor of 4. Ambient observations further confirm that when NO₃ chemistry is involved, monoterpene NPF is completely turned off. Our results explain the frequent absence of nocturnal biogenic NPF in monoterpene-rich environments.