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Wetlands cover only 3g% of the global land surface area, but boreal wetlands are experiencing an unprecedented warming of four times the global average. These wetlands emit isoprene and terpenes (including monoterpenes (MT), sesquiterpenes (SQT), and diterpenes (DT)), which are climate-relevant highly reactive biogenic volatile organic compounds (BVOCs) with an exponential dependence on temperature. In this study, we present ecosystem-scale eddy covariance (EC) fluxes of isoprene, MT, SQT, and DT (hereafter referred to together as terpenes) at Siikaneva, a boreal fen in southern Finland, from the start to the peak of the growing season of 2021 (19 May 2021 to 28 June 2021). These are the first EC fluxes reported using the novel state-of-the-art Vocus proton transfer reaction mass spectrometer (Vocus-PTR) and the first-ever fluxes reported for DTs from a wetland. Isoprene was the dominant compound emitted by the wetland, followed by MTs, SQTs, and DTs, and they all exhibited a strong exponential temperature dependence. The Q10 values, the factor by which terpene emissions increases for every 10gg C rise in temperature, were up to five times higher than those used in most BVOC models. During the campaign, the air temperature peaked above 31gg C on 21-22 June 2021, which is abnormally high for boreal environments, and the maximum flux for all terpenes coincided with this period. We observed that terpene emissions were elevated after this abnormally "high-temperature stress period", indicating that past temperatures alter emissions significantly. The standardized emission factor (EF) of the fen for isoprene (EFiso) was 11.1g±g 0.3gnmolgm-2gs-1, which is at least two times higher than in previous studies and as high as the emission factors typical for broadleaf and other forests in the lower latitudes. We observed EFMT of 2.4g±g 0.1gnmolgm-2gs-1, EFSQT of 1.3g±g0.03gnmolgm-2gs-1, higher than typical for needle leaf and broadleaf tree functional types, and EFDT of 0.011g±g0.001gnmolgm-2gs-1. We also compared the landscape average emissions to the model of emissions of gases and aerosols from nature (MEGAN) v2.1 and found that the emissions were underestimated by over 9 times for isoprene, over 300 times for MTs, and 800 times for SQTs. Our results show that due to very high EFs and high sensitivity to increasing temperatures, these high-latitude ecosystems can be a large source of terpenes to the atmosphere, and anthropogenic global warming could induce much higher BVOC emissions from wetlands in the future., Lejish Vettikkat was financially supported by the UEF EPHB doctoral degree program. This work was financially supported by the Academy of Finland Flagship program (grant nos. 337550 and 337549) and Academy of Finland project no. 310682. Simon Schallhart was supported by Academy of Finland project no. 323255. Tuukka Petäjä was supported by Academy of Finland project nos. 1325681 and 1328616. Elisa Männistö and Eeva-Stiina Tuittila were supported by Academy of Finland project no. 330840. Roger Seco was supported by a Ramón y Cajal grant (RYC2020-029216-I) funded by MCIN/AEI/10.13039/501100011033 and by “ESF Investing in your future”, as well as project PID2021-122892NA-I00 funded by MCIN/AEI and by “ERDF A way of making Europe”. IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (MCIN/AEI, Project CEX2018-000794-S). Alex B. Guenther was supported by the US National Science Foundation award ANS-2041250.

The SPectrometer for Ice Nuclei (SPIN) has been modified to access ice nucleation at low temperatures. The modification consists of a reconfiguration of components from SPIN's cooling system to provide refrigerant with a low boiling point to the chamber. We describe the setup modification and determine the temperature and humidity range accessible to experiments. The modification extends the measurement range of SPIN to 208 K, which enables measurements in the temperature regime relevant for ice formation in cirrus clouds. This addition of low-temperature capability allows for far more comprehensive measurements of the temperature- and humidity-dependent ice nucleation of test substances, to investigate fundamentals of ice nucleation mechanisms. We present exemplary data of heterogeneous ice nucleation on silver iodide and homogeneous ice nucleation in solution droplets to demonstrate the usefulness of the modified SPIN setup for precision measurements to detect discrepancies between experiments and widely used theories.

One barrier to predicting biogenic secondary organic aerosol (SOA) formation in a changing climate can be attributed to the complex nature of plant volatile emissions. Plant volatile emissions are dynamic over space and time, and change in response to environmental stressors. This study investigated SOA production from emissions of healthy and aphid-stressed Scots pine saplings via dark ozonolysis and photooxidation chemistry. Laboratory experiments using a batch reaction chamber were used to investigate SOA production from different plant volatile mixtures. The volatile mixture from healthy plants included monoterpenes, aromatics, and a small amount of sesquiterpenes. The biggest change in the volatile mixture for aphid-stressed plants was a large increase (from 1.4 to 7.9 ppb) in sesquiterpenes-particularly acyclic sesquiterpenes, such as the farnesene isomers. Acyclic sesquiterpenes had different effects on SOA production depending on the chemical mechanism. Farnesenes suppressed SOA formation from ozonolysis with a 9.7-14.6% SOA mass yield from healthy plant emissions and a 6.9-10.4% SOA mass yield from aphid-stressed plant emissions. Ozonolysis of volatile mixtures containing more farnesenes promoted fragmentation reactions, which produced higher volatility oxidation products. In contrast, plant volatile mixtures containing more farnesenes did not appreciably change SOA production from photooxidation. SOA mass yields ranged from 10.8 to 23.2% from healthy plant emissions and 17.8-26.8% for aphid-stressed plant emissions. This study highlights the potential importance of acyclic terpene chemistry in a future climate regime with an increased presence of plant stress volatiles.

HIF prolyl 4-hydroxylases (HIF-P4Hs, also known as PHDs and EGLNs) are crucial enzymes that modulate the hypoxia inducible factor (HIF) response and help to maintain cellular oxygen homeostasis. This function is especially well-known for cytoplasmic or nuclear enzymes HIF-P4H-1–3 (PHDs 1–3, EGLNs 2, 1 and 3, respectively), but the physiological role is still obscure for a fourth suggested HIF-P4H, P4H-TM that is a transmembrane protein and resides in the endoplasmic reticulum. Recently however, both experimental and clinical evidence of the P4H-TM involvement in CNS physiology has emerged. In this study, we first investigated the expression pattern of P4H-TM in the mouse brain and found a remarkably selective abundance in brains areas that are involved in social behaviors and anxiety including amygdala, lateral septum and bed nucleus of stria terminalis. Next, we performed behavioral assays in P4h-tm−/− mice to investigate a possible phenotype associated to these brain areas. In locomotor activity tests, we found that P4h-tm−/− mice were significantly more active than their wild-type (WT) littermate mice, and habituation to test environment did not abolish this effect. Instead, spatial learning and memory seemed normal in P4h-tm−/− mice as assessed by Morris swim task. In several tests assessing anxiety and fear responses, P4h-tm−/− mice showed distinct courageousness, and they presented increased interaction towards fellow mice in social behavior tests. Most strikingly, P4h-tm−/− mice practically lacked behavioral despair response, a surrogate marker of depression, in forced swim and tail suspension tests. Instead, mutant mice of all other Hif-p4h isoforms lacked such a behavioral phenotype. In summary, this study presents a remarkable anatomy-physiology association between the brain expression of P4H-TM and the behavioral phenotype in P4h-tm−/− mice. Future studies will reveal whether P4H-TM may serve as a novel target for anti-depressant and anti-anxiety pharmacotherapy.

Iron is very important element for functioning of the brain. Its concentration changes with aging the brain or during disease. The aim of our work was the histological examination of content of ferritin and free iron (unbound) in brain cortex in association with Aβ plaques from their earliest stages of accumulation in amyloid plaque forming APP/PS1 transgenic mice. Light microscopy revealed the onset of plaques formation at 8-monthage. Detectable traces of free iron and no ferritin were found around plaques at this age, while the rate of their accumulation in and around Aβ plaques was elevated at 13 months of age. Ferritin accumulated mainly on the edge of Aβ plaques, while the smaller amount of free iron was observed in the plaque-free tissue, as well as in and around Aβ plaques. We conclude that free iron and ferritin accumulation follows the amyloid plaques formation. Quantification of cortical iron and ferritin content can be an important marker in the diagnosis of Alzheimer’s disease.

The SPectrometer for Ice Nuclei (SPIN) has been modified to access ice nucleation at low temperatures. The modification consists of a reconfiguration of components from SPIN's cooling system to provide refrigerant with a low boiling point to the chamber. We describe the setup modification and determine the temperature and humidity range accessible to experiments. The modification extends the measurement range of SPIN by more than 20 K to the temperature regime relevant for ice formation in cirrus clouds. This addition of low temperature capability allows for far more comprehensive measurements of the temperature- and humidity- dependent ice nucleation of test substances, to investigate fundamentals of ice nucleation mechanisms. We present exemplary data of heterogeneous ice nucleation on silver iodide and homogeneous ice nucleation in solution droplets to demonstrate the usefulness of the modified SPIN setup for precision measurements to detect discrepancies between experiments and widely used theories.

BackgroundAlzheimer’s disease (AD) is the most common neurodegenerative disease and type 2 diabetes (T2D) plays an important role in conferring the risk for AD. Although AD and T2D share common features, the common molecular mechanisms underlying these two diseases remain elusive.MethodsMice with different AD- and/or tauopathy-linked genetic backgrounds (APPswe/PS1dE9, Tau P301L and APPswe/PS1dE9/Tau P301L) were fed for 6 months with standard diet or typical Western diet (TWD). After behavioral and metabolic assessments of the mice, the effects of TWD on global gene expression as well as dystrophic neurite and microglia pathology were elucidated. Consequently, mechanistic aspects related to autophagy, cell survival, phagocytic uptake as well as Trem2/Dap12 signaling pathway, were assessed in microglia upon modulation of PI3K-Akt signaling. To evaluate whether the mouse model-derived results translate to human patients, the effects of diabetic phenotype on microglial pathology were assessed in cortical biopsies of idiopathic normal pressure hydrocephalus (iNPH) patients encompassing β-amyloid pathology.ResultsTWD led to obesity and diabetic phenotype in all mice regardless of the genetic background. TWD also exacerbated memory and learning impairment in APPswe/PS1dE9 and Tau P301L mice. Gene co-expression network analysis revealed impaired microglial responses to AD-related pathologies in APPswe/PS1dE9 and APPswe/PS1dE9/Tau P301L mice upon TWD, pointing specifically towards aberrant microglial functionality due to altered downstream signaling of Trem2 and PI3K-Akt. Accordingly, fewer microglia, which did not show morphological changes, and increased number of dystrophic neurites around β-amyloid plaques were discovered in the hippocampus of TWD mice. Mechanistic studies in mouse microglia revealed that interference of PI3K-Akt signaling significantly decreased phagocytic uptake and proinflammatory response. Moreover, increased activity of Syk-kinase upon ligand-induced activation of Trem2/Dap12 signaling was detected. Finally, characterization of microglial pathology in cortical biopsies of iNPH patients revealed a significant decrease in the number of microglia per β-amyloid plaque in obese individuals with concomitant T2D as compared to both normal weight and obese individuals without T2D.ConclusionsCollectively, these results suggest that diabetic phenotype in mice and humans mechanistically associates with abnormally reduced microglial responses to β-amyloid pathology and further suggest that AD and T2D share overlapping pathomechanisms, likely involving altered immune function in the brain.

Type 2 diabetes (T2D) increases the risk of Alzheimer’s disease (AD). Even though these two diseases share common molecular pathways, the mechanisms remain elusive. To shed light into these mechanisms, mice with different AD- and/or tauopathy-linked genetic backgrounds were utilized; APPswe/PS1dE9 (A+Tw), Tau P301L (AwT+), and APPswe/PS1dE9/Tau P301L (A+T+). Feeding these mice with typical Western diet (TWD) led to obesity and diabetic phenotype as compared to respective mice with a standard diet. TWD also exacerbated memory and learning impairment in A+Tw and AwT+, but not in A+T+ mice. Furthermore, RNA sequencing of mouse hippocampal samples revealed altered responses to AD-related pathologies in A+Tw and A+T+ mice upon TWD, pointing specifically towards aberrant microglial functionality and PI3K-Akt signaling. Accordingly, fewer microglia alongside an increased number of dystrophic neurites around β-amyloid plaques, and impaired PI3K-Akt signaling, were discovered in the hippocampus of TWD mice. Mechanistic elucidation revealed that disruption of the PI3K-Akt signaling pathway by pharmacological or genetic approaches significantly decreased the phagocytic uptake and proinflammatory response as well as increased the activity of Syk-kinase upon ligand-induced activation of Trem2/Dap12 signaling in mouse microglia. Finally, characterization of microglial pathology in cortical biopsies of idiopathic normal pressure hydrocephalus (iNPH) patients harboring β-amyloid plaques revealed a significant decrease in the number of microglia per β-amyloid plaque in obese iNPH patients with T2D as compared to both normal weight and obese iNPH patients without T2D. Collectively, these results suggest that the peripheral diabetic phenotype in mice and humans associates with reduced microglial response to β-amyloid pathology.

During the summer 2013 Southern Aerosol and Oxidant Study (SOAS) field campaign in a rural site in the southeastern United States, the effect of hygroscopicity and composition on the phase state of atmospheric aerosol particles dominated by the organic fraction was studied. The analysis is based on hygroscopicity measurements by a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA), physical phase state investigations by an Aerosol Bounce Instrument (ABI) and composition measurements using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). To study the effect of atmospheric aging on these properties, an OH-radical oxidation flow reactor (OFR) was used to simulate longer atmospheric aging times of up to 3 weeks. Hygroscopicity and bounce behavior of the particles had a clear relationship showing higher bounce at elevated relative humidity (RH) values for less hygroscopic particles, which agrees well with earlier laboratory studies. Additional OH oxidation of the aerosol particles in the OFR increased the O : C and the hygroscopicity resulting in liquefying of the particles at lower RH values. At the highest OH exposures, the inorganic fraction starts to dominate the bounce process due to production of inorganics and concurrent loss of organics in the OFR. Our results indicate that at typical ambient RH and temperature, organic-dominated particles stay mostly liquid in the atmospheric conditions in the southeastern US, but they often turn semisolid when dried below ∼ 50 % RH in the sampling inlets. While the liquid phase state suggests solution behavior and equilibrium partitioning for the SOA particles in ambient air, the possible phase change in the drying process highlights the importance of thoroughly considered sampling techniques of SOA particles.

The relationship between aerosol hygroscopicity and cloud droplet activation was studied at the Puijo measurement station in Kuopio, Finland, during the autumn 2014. The hygroscopic growth of 80, 120 and 150 nm particles was measured at 90 % relative humidity with a hygroscopic tandem differential mobility analyzer. Typically, the growth factor (GF) distributions appeared bimodal with clearly distinguishable peaks around 1.0–1.1 and 1.4–1.6. However, the relative contribution of the two modes appeared highly variable reflecting the probable presence of fresh anthropogenic particle emissions. The hygroscopicity-dependent activation properties were estimated in a case study comprising four separate cloud events with varying characteristics. At 120 and 150 nm, the activation efficiencies within the low- and high-GF modes varied between 0–34 and 57–83 %, respectively, indicating that the less hygroscopic particles remained mostly non-activated, whereas the more hygroscopic mode was predominantly scavenged into cloud droplets. By modifying the measured GF distributions, it was estimated how the cloud droplet concentrations would change if all the particles belonged to the more hygroscopic group. According to κ-Köhler simulations, the cloud droplet concentrations increased up to 70 % when the possible feedback effects on effective peak supersaturation (between 0.16 and 0.29 %) were assumed negligible. This is an indirect but clear illustration of the sensitivity of cloud formation to aerosol chemical composition., published version, peerReviewed

Accumulation of β-amyloid (Aβ) and phosphorylated tau in the brain are central events underlying Alzheimer's disease (AD) pathogenesis. Aβ is generated from amyloid precursor protein (APP) by β-site APP-cleaving enzyme 1 (BACE1) and γ-secretase-mediated cleavages. Ubiquilin-1, a ubiquitin-like protein, genetically associates with AD and affects APP trafficking, processing and degradation. Here, we have investigated ubiquilin-1 expression in human brain in relation to AD-related neurofibrillary pathology and the effects of ubiquilin-1 overexpression on BACE1, tau, neuroinflammation, and neuronal viability in vitro in co-cultures of mouse embryonic primary cortical neurons and microglial cells under acute neuroinflammation as well as neuronal cell lines, and in vivo in the brain of APdE9 transgenic mice at the early phase of the development of Aβ pathology. Ubiquilin-1 expression was decreased in human temporal cortex in relation to the early stages of AD-related neurofibrillary pathology (Braak stages 0-II vs. III-IV). There was a trend towards a positive correlation between ubiquilin-1 and BACE1 protein levels. Consistent with this, ubiquilin-1 overexpression in the neuron-microglia co-cultures with or without the induction of neuroinflammation resulted in a significant increase in endogenously expressed BACE1 levels. Sustained ubiquilin-1 overexpression in the brain of APdE9 mice resulted in a moderate, but insignificant increase in endogenous BACE1 levels and activity, coinciding with increased levels of soluble Aβ40 and Aβ42. BACE1 levels were also significantly increased in neuronal cells co-overexpressing ubiquilin-1 and BACE1. Ubiquilin-1 overexpression led to the stabilization of BACE1 protein levels, potentially through a mechanism involving decreased degradation in the lysosomal compartment. Ubiquilin-1 overexpression did not significantly affect the neuroinflammation response, but decreased neuronal viability in the neuron-microglia co-cultures under neuroinflammation. Taken together, these results suggest that ubiquilin-1 may mechanistically participate in AD molecular pathogenesis by affecting BACE1 and thereby APP processing and Aβ accumulation.

Secondary organic aerosol (SOA) impact climate by scattering and absorbing radiation and contributing to cloud formation. SOA models are based on studies of simplified chemical systems that do not account for the chemical complexity in the atmosphere. This study investigated SOA formation from a mixture of real Scots pine (Pinus sylvestris) emissions including a variety of monoterpenes and sesquiterpenes. SOA generation was characterized from different combinations of volatile compounds as the plant emissions were altered with an herbivore stress treatment. During active herbivore feeding, monoterpene and sesquiterpene emissions increased, but SOA mass yields decreased after accounting for absorption effects. SOA mass yields were controlled by sesquiterpene emissions in healthy plants. In contrast, SOA mass yields from stressed plant emissions were controlled by the specific blend of monoterpene emissions. Conservative estimates using a box model approach showed a 1.5- to 2.3-fold aerosol enhancement when the terpene complexity was taken into account. This enhancement was relative to the commonly used model monoterpene, “α-pinene”. These results suggest that simplifying terpene complexity in SOA models could lead to underpredictions in aerosol mass loading.

Article, Boreal forests are a major source of climate-relevant biogenic secondary organic aerosols (SOAs) and will be greatly influenced by increasing temperature. Global warming is predicted to not only increase emissions of reactive biogenic volatile organic compounds (BVOCs) from vegetation directly but also induce large-scale insect outbreaks, which significantly increase emissions of reactive BVOCs. Thus, climate change factors could substantially accelerate the formation of biogenic SOAs in the troposphere. In this study, we have combined results from field and laboratory experiments, satellite observations and global-scale modelling in order to evaluate the effects of insect herbivory and large-scale outbreaks on SOA formation and the Earth's climate. Field measurements demonstrated 11-fold and 20-fold increases in monoterpene and sesquiterpene emissions respectively from damaged trees during a pine sawfly (Neodiprion sertifer) outbreak in eastern Finland. Laboratory chamber experiments showed that feeding by pine weevils (Hylobius abietis) increased VOC emissions from Scots pine and Norway spruce seedlings by 10–50 fold, resulting in 200–1000-fold increases in SOA masses formed via ozonolysis. The influence of insect damage on aerosol concentrations in boreal forests was studied with a global chemical transport model GLOMAP and MODIS satellite observations. Global-scale modelling was performed using a 10-fold increase in monoterpene emission rates and assuming 10 % of the boreal forest area was experiencing outbreak. Results showed a clear increase in total particulate mass (local max. 480 %) and cloud condensation nuclei concentrations (45 %). Satellite observations indicated a 2-fold increase in aerosol optical depth over western Canada's pine forests in August during a bark beetle outbreak. These results suggest that more frequent insect outbreaks in a warming climate could result in substantial increase in biogenic SOA formation in the boreal zone and, thus, affect both aerosol direct and indirect forcing of climate at regional scales. The effect of insect outbreaks on VOC emissions and SOA formation should be considered in future climate predictions., published version, http://purl.org/eprint/status/PeerReviewed

The Puijo aerosol–cloud observation station is a unique measurement site for its location in the mixed region between the boreal forestland and the municipality of Kuopio, Finland. A measurement campaign was carried out at the station during fall 2010. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-Tof-AMS) was deployed to characterize the atmospheric submicron aerosols. Positive matrix factorization (PMF) was applied to the unified high-resolution mass spectra organic species with NO+ and NO2+ ions to discover the intrinsic relationships between the organic and inorganic species and their daily cycles. On average, the submicron aerosols in this study were dominated by organic and sulfate species, composing 48.2 and 28.7% of total observed aerosol mass, respectively, with smaller contributions from ammonium (9.3%), nitrate (4.9%), chloride (0.8%) and BC (8.1%). The sources of these species included the primary emissions originating from the city area, secondary formation from both natural and anthropogenic emissions and regional transport. The PMF analysis succeeded in separating the mixed organic and inorganic spectra into three distinct organic and one inorganic factors. For organic factors, the semi-volatile oxygenated organic aerosol (SVOOA) and low-volatility oxygenated OA (LVOOA) accounted for 54.8 and 36.3% of total organic masses, respectively, while the hydrocarbon-like organic aerosol (HOA) accounted for 8.9% of total organics, with its main source from urban emissions. The inorganic factor is identified as NH4NO3, comprising 6.9% of the fitted aerosol mass by PMF. Based on the PMF results, the nitrate species were separated into organic and inorganic components, with the organic nitrates contributing one-third of the total nitrate mass. The results highlight both anthropogenic and biogenic emissions as important atmospheric aerosol sources in a forest–urban mixed region.

During the summer 2013 SOAS field campaign in a rural site in the Southeastern United States, the effect of hygroscopicity and composition on the phase state of atmospheric aerosol particles dominated by the organic fraction was studied. The analysis is based on hygroscopicity measurements by a Hygroscopic Tandem Differential Mobility Analyzer (HTDMA), physical phase state investigations by an Aerosol Bounce Instrument (ABI), and composition measurements using a high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS). To study the effect of atmospheric aging on these properties an OH-radical oxidation flow reactor (OFR) was used to simulate longer atmospheric aging times of up to three weeks. Hygroscopicity and bounce behavior of the particles had a clear relationship showing higher bounce at elevated RH values for less hygroscopic particles which agrees well with earlier laboratory studies. Additional OH oxidation of the aerosol particles in the OFR increased the O:C and the hygroscopicity resulting in liquefying of the particles at lower RH values. At the highest OH exposures, the inorganic fraction starts to dominate the bounce process due to production of inorganics and concurrent loss of organics in the OFR. At typical ambient RH and temperature, organic dominated particles stay mostly liquid in the atmospheric conditions in the Southeastern US, but they often turn semisolid when dried below ~ 50 % RH in the sampling inlets. While the liquid phase state suggests solution behavior and instantaneous equilibrium partitioning for the SOA particles in ambient air, the possible phase change in the drying process highlights the importance of thoroughly considered sampling techniques of SOA particles.

The relationship between aerosol hygroscopicity and cloud droplet activation was studied at the Puijo measurement station in Kuopio, Finland, during the autumn 2014. The hygroscopic growth of 80, 120 and 150 nm particles was measured at 90 % relative humidity with a hygroscopic tandem differential mobility analyzer. Typically, the growth factor (GF) distributions appeared bimodal with clearly distinguishable peaks around 1.0‒1.1 and 1.4‒1.6. However, the relative contribution of the two modes appeared highly variable reflecting the varying presence of fresh anthropogenic particle emissions. The hygroscopicity-dependent activation properties were estimated in a case study comprising three separate cloud events with varying characteristics. At 120 and 150 nm, the activation efficiencies within the low- and high-GF modes varied between 0‒0.33 and 0.66‒0.86, respectively, indicating that the less hygroscopic particles remained almost non-activated, whereas the more hygroscopic mode was predominantly scavenged into cloud droplets. By modifying the measured GF distributions, it was estimated how the cloud droplet concentrations would change if all the particles belonged to the more hygroscopic group. According to the κ-Köhler simulations, the cloud droplet concentrations increased up to 70 % with increasing hygroscopicity. This is an indirect but clear illustration of the sensitivity of cloud formation to aerosol chemical composition.

New particle formation in the atmosphere produces around half of the cloud condensation nuclei that seed cloud droplets. Such particles have a pivotal role in determining the properties of clouds and the global radiation balance. Dunne et al. used the CLOUD (Cosmics Leaving Outdoor Droplets) chamber at CERN to construct a model of aerosol formation based on laboratory measured nucleation rates. They found that nearly all nucleation involves either ammonia or biogenic organic compounds. Furthermore in the present day atmosphere cosmic ray intensity cannot meaningfully affect climate via nucleation.Science this issue p. 1119Fundamental questions remain about the origin of newly formed atmospheric aerosol particles because data from laboratory measurements have been insufficient to build global models. In contrast gas phase chemistry models have been based on laboratory kinetics measurements for decades. We built a global model of aerosol formation by using extensive laboratory measurements of rates of nucleation involving sulfuric acid ammonia ions and organic compounds conducted in the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber. The simulations and a comparison with atmospheric observations show that nearly all nucleation throughout the present day atmosphere involves ammonia or biogenic organic compounds in addition to sulfuric acid. A considerable fraction of nucleation involves ions but the relatively weak dependence on ion concentrations indicates that for the processes studied variations in cosmic ray intensity do not appreciably affect climate through nucleation in the present day atmosphere.U http://science.sciencemag.org/content/sci/354/6316/1119.full.pdf

About half of present-day cloud condensation nuclei originate from atmospheric nucleation, frequently appearing as a burst of new particles near midday1. Atmospheric observations show that the growth rate of new particles often accelerates when the diameter of the particles is between one and ten nanometres2,3. In this critical size range, new particles are most likely to be lost by coagulation with pre-existing particles4, thereby failing to form new cloud condensation nuclei that are typically 50 to 100 nanometres across. Sulfuric acid vapour is often involved in nucleation but is too scarce to explain most subsequent growth5,6, leaving organic vapours as the most plausible alternative, at least in the planetary boundary layer7,8,9,10. Although recent studies11,12,13 predict that low-volatility organic vapours contribute during initial growth, direct evidence has been lacking. The accelerating growth may result from increased photolytic production of condensable organic species in the afternoon2, and the presence of a possible Kelvin (curvature) effect, which inhibits organic vapour condensation on the smallest particles (the nano-Köhler theory)2,14, has so far remained ambiguous. Here we present experiments performed in a large chamber under atmospheric conditions that investigate the role of organic vapours in the initial growth of nucleated organic particles in the absence of inorganic acids and bases such as sulfuric acid or ammonia and amines, respectively. Using data from the same set of experiments, it has been shown15 that organic vapours alone can drive nucleation. We focus on the growth of nucleated particles and find that the organic vapours that drive initial growth have extremely low volatilities (saturation concentration less than 10−4.5 micrograms per cubic metre). As the particles increase in size and the Kelvin barrier falls, subsequent growth is primarily due to more abundant organic vapours of slightly higher volatility (saturation concentrations of 10−4.5 to 10−0.5 micrograms per cubic metre). We present a particle growth model that quantitatively reproduces our measurements. Furthermore, we implement a parameterization of the first steps of growth in a global aerosol model and find that concentrations of atmospheric cloud concentration nuclei can change substantially in response, that is, by up to 50 per cent in comparison with previously assumed growth rate parameterizations., Nature, 533 (7604), ISSN:0028-0836, ISSN:1476-4687

According to epidemiological studies, type-2 diabetes increases the risk of Alzheimer’s disease. Here, we induced hyperglycaemia in mice overexpressing mutant amyloid precursor protein and presenilin-1 (APdE9) either by cross-breeding them with pancreatic insulin-like growth factor 2 (IGF-2) overexpressing mice or by feeding them with high-fat diet. Glucose and insulin tolerance tests revealed significant hyperglycaemia in mice overexpressing IGF-2, which was exacerbated by high-fat diet. However, sustained hyperinsulinaemia and insulin resistance were observed only in mice co-expressing IGF-2 and APdE9 without correlation to insulin levels in brain. In behavioural tests in aged mice, APdE9 was associated with poor spatial learning and the combination of IGF-2 and high-fat diet further impaired learning. Neither high-fat diet nor IGF-2 increased β-amyloid burden in the brain. In male mice, IGF-2 increased β-amyloid 42/40 ratio, which correlated with poor spatial learning. In contrast, inhibitory phosphorylation of glycogen synthase kinase 3β, which correlated with good spatial learning, was increased in APdE9 and IGF-2 female mice on standard diet, but not on high-fat diet. Interestingly, high-fat diet altered τ isoform expression and increased phosphorylation of τ at Ser202 site in female mice regardless of genotype. These findings provide evidence for new regulatory mechanisms that link type-2 diabetes and Alzheimer pathology.

The first wetting layer on solid nanoparticles has direct implications on the roles these particles play in industrial processes and technological applications as well as in the atmosphere. We present a technique for online measurements of the adsorption of the first few water layers onto insoluble aerosol nanoparticles. Atomized fumed silica nanoparticles were dispersed from aqueous suspension and their hygroscopic growth factors (HGF) and number of the adsorbed water layers at subsaturated conditions were measured using a nanometer hygroscopic tandem differential mobility analyzer (HTDMA). Particle morphology was characterized by electron microscopy and particle density was determined by mobility analysis. The HGFs of the size-selected particles at mobility diameters from 10 to 50 nm at 90% relative humidity (RH) varied from 1.05 to 1.24, corresponding to 2–6 layers of adsorbed water. The morphology of the generated fumed silica nanoparticles varied from spheres at 8–10 nm to agglomerates at larger diam...

Biogenic volatile organic compounds (VOCs) are a significant source of global secondary organic aerosol (SOA); however, quantifying their aerosol forming potential remains a challenge. This study presents smog chamber laboratory work, focusing on SOA formation via oxidation of the emissions of two dominant tree species from boreal forest area, Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies), by hydroxyl radical (OH) and ozone (O[subscript 3]). Oxidation of α-pinene was also studied as a reference system. Tetramethylethylene (TME) and 2-butanol were added to control OH and O[subscript 3] levels, thereby allowing SOA formation events to be categorized as resulting from either OH-dominated or O[subscript 3]-initiated chemistry. SOA mass yields from α-pinene are consistent with previous studies while the yields from the real plant emissions are generally lower than that from α-pinene, varying from 1.9% at an aerosol mass loading of 0.69 μg m[superscript −3] to 17.7% at 26.0 μg m[superscript −3]. Mass yields from oxidation of real plant emissions are subject to the interactive effects of the molecular structures of plant emissions and their reaction chemistry with OH and O[subscript 3], which lead to variations in condensable product volatility. SOA formation can be reproduced with a two-product gas-phase partitioning absorption model in spite of differences in the source of oxidant species and product volatility in the real plant emission experiments. Condensable products from OH-dominated chemistry showed a higher volatility than those from O[subscript 3]-initiated systems during aerosol growth stage. Particulate phase products became less volatile via aging process which continued after input gas-phase oxidants had been completely consumed., Academy of Finland Centre of Excellence program (project no. 1118615), Academy of Finland (decision no. 110763), Academy of Finland (decision no. 131019), Academy of Finland (decision no. 218115), Academy of Finland (decision no. 111543), Academy of Finland (decision no. 123466), Sixth Framework Programme (European Commission), United States. Dept. of Energy. Office of Biological and Environmental Research, Saastamoinen Foundation, United States. Dept. of Energy (grant DE-FG-02-05ER63997)

Traffic-related aerosol particles are ubiquitous in the urban atmosphere. As they are produced at ground level, they can also cause adverse health effects to urban dwellers. However, knowledge of the formation, transformation and chemically resolved size distribution of urban ultrafine particles is incomplete. Thus, more of these measurements are needed for better assessment of ambient air quality and its potential health effects. The particle number concentration, aerosol black carbon (BC) concentration and size distribution of traffic-related aerosols were measured near two major roads in Kuopio, Finland, from 16 June to 5 July, 2004. Furthermore, the properties of roadside aerosol particles were examined with the Tandem Differential Mobility Analyzer technique (TDMA). A suite of TDMA instruments relying on water (hygroscopic TDMA) and ethanol (organic TDMA) condensation as well as heating (volatility TDMA) were deployed to study the composition of the nucleation and Aitken mode particles ( D p = 10–50 nm) formed from vehicle exhaust. The results show that a simple three-component model was able to reproduce characteristic insoluble, organic and water-soluble volume fractions. Insoluble constituents were dominant in the Aitken mode particles, whereas organic compounds dominated the nucleation mode sizes. On average, only a small volume fraction was water-soluble, but a clear external mixing was observed particularly when enough time was allowed after the tail pipe emissions. The contribution of the insoluble material was seen to increase as a function of particle size, being typically less than 10% at 10 nm and between 20 and 50% at 50 nm, in contrast to the organic fraction, which decreased from about 80% at nucleation mode size range to 50–60% at 50 nm.

Measurements of particle formation following the gas phase oxidation of volatile organic compounds (VOCs) emitted by Scots pine (Pinus sylvestris L.) seedlings are reported. Particle formation and condensational growth both from ozone (O3) and hydroxyl radical (OH) initiated oxidation of pine emissions (about 20-120 ppb) were investigated in a smog chamber. During experiments, tetramethylethylene (TME) and 2-butanol were added to control the concentrations of O3 and OH. Particle formation and condensational growth rates were interpreted with a chemical kinetic model. Scots pine emissions mainly included α-pinene, β-pinene, Δ3-carene, limonene, myrcene and β-phellandrene, composing more than 95% of total emissions. Modeled OH concentrations in the O3- and OH-induced experiments were on the order of ~106 molecules cm−3. Our results demonstrate that OH-initiated oxidation of VOCs plays an important role in the nucleation process during the initial new particle formation stage. The highest average particle formation rate of 360 cm−3 s−1 was observed for the OH-dominated nucleation events and the lowest formation rate of less than 0.5 cm−3 s−1 was observed for the case with only O3 present as an oxidant. In contrast to the particle formation process, ozonolysis of monoterpenes appears to be much more efficient to the aerosol growth process following nucleation. Higher contributions of more oxygenated products to the SOA mass loadings from OH-dominated oxidation systems were found as compared to the ozonolysis systems. Comparison of mass and volume distributions from the aerosol mass spectrometer and differential mobility analyzer yields estimated SOA effective densities of 1.34±0.06 g cm−3 for the OH+O3 oxidation systems and 1.38±0.03 g cm−3 for the O3 dominated chemistry.

Monoterpenes are biogenic volatile organic compounds (BVOCs) which play an important role in plant adaptation to stresses, atmospheric chemistry, plant–plant and plant–insect interactions. In this study, we determined whether ozonolysis can influence the monoterpenes in the headspace of cabbage. The monoterpenes were mixed with an air-flow enriched with 100, 200 or 400 ppbv of ozone (O 3 ) in a Teflon chamber. The changes in the monoterpene and O 3 concentrations, and the formation of secondary organic aerosols (SOA) were determined during ozonolysis. Furthermore, the monoterpene reactions with O 3 and OH were modelled using reaction kinetics equations. The results showed that all of the monoterpenes were unequally affected: α -thujene, sabinene and d -limonene were affected to the greatest extend, whereas the 1,8-cineole concentration did not change. In addition, plant monoterpene emissions reduced the O 3 concentration by 12–24%. The SOA formation was dependent on O 3 concentration. At 100 ppbv of O 3 , virtually no new particles were formed but clear SOA formation was observed at the higher ozone concentrations. The modelled results showed rather good agreements for α -pinene and 1,8-cineole, whereas the measured concentrations were clearly lower compared to modelled values for sabinene and limonene. In summary, O 3 -quenching by monoterpenes occurs beyond the boundary layer of leaves and results in a decreased O 3 concentration, altered monoterpene profiles and SOA formation.

The increasing use of mobile phones by children and teenagers has raised concerns about their safety. Addressing such concerns is difficult, because no data are available on possible effects from long-term exposure to radiofrequency (RF) fields during the development of the nervous system. Possible morphological and functional changes were evaluated in the central nervous system of young male Wistar rats exposed to 900 MHz mobile phone signal for 2 h/day on 5 days/week. After 5 weeks of exposure at whole-body average specific energy absorption rates of 0.3 or 3.0 W/kg or sham exposure, six rats per group were examined histologically, and the remaining 18 rats per group were subjected to behavioral tests. No degenerative changes, dying neurons, or effects on the leakage of the blood-brain barrier were detected. No group differences were observed in the open-field test, plus maze test or acoustic startle response tests. In the water maze test, however, significantly improved learning (P = 0.012) and memory (P = 0.01) were detected in rats exposed to RF fields. The results do not indicate a serious threat to the developing brain from mobile phone radiation at intensities relevant to human exposure. However, the interesting finding of improved learning and memory warrants further studies.

The effects of local pollutant sources and particle chemical composition on aerosol–cloud interactions were investigated by measuring cloud interstitial and total aerosol size distributions, particle chemical composition and hygroscopic growth factors and cloud droplet size distributions on an observation tower, with a special focus on comparing clean air masses with those affected by local sources. The polluted air masses contained more particles than the clean air masses in all size classes, excluding the accumulation mode. This was caused by cloud processing, which was also observed for the polluted air but to a lesser extent. Some, mostly minor, differences in the particle chemical composition between the air masses were observed. The average size and number concentration of activating particles were quite similar for both air masses, producing average droplet populations with only minor distinctions. As a case study, a long cloud event was analyzed in detail regarding emissions from local sources, including a paper mill and a heating plant. Clear differences in the total and accumulation mode particle concentrations, particle hygroscopicity and chemical composition during the cloud event were observed. Particularly, larger particles, higher hygroscopicities and elevated amounts of inorganic constituents, especially SO4, were linked with the pollutant plumes. In the air masses affected by traffic and domestic wood combustion, a bimodal particle hygroscopicity distribution was observed, indicating externally mixed aerosol. The variable conditions during the event had a clear impact on cloud droplet formation.

Sulphuric acid, ammonia, amines, and oxidised organics play a crucial role in nanoparticle formation in the atmosphere. In this study, we investigate the composition of nucleated nanoparticles formed from these compounds in the CLOUD (Cosmics Leaving Outdoor Droplets) chamber experiments at CERN (Centre européen pour la recherche nucléaire). The investigation was carried out via analysis of the particle hygroscopicity, ethanol affinity, oxidation state, and ion composition. Hygroscopicity was studied by a hygroscopic tandem differential mobility analyser and a cloud condensation nuclei counter, ethanol affinity by an organic differential mobility analyser and particle oxidation level by a high-resolution time-of-flight aerosol mass spectrometer. The ion composition was studied by an atmospheric pressure interface time-of-flight mass spectrometer. The volume fraction of the organics in the particles during their growth from sizes of a few nanometers to tens of nanometers was derived from measured hygroscopicity assuming the Zdanovskii–Stokes–Robinson relationship, and compared to values gained from the spectrometers. The ZSR-relationship was also applied to obtain the measured ethanol affinities during the particle growth, which were used to derive the volume fractions of sulphuric acid and the other inorganics (e.g. ammonium salts). In the presence of sulphuric acid and ammonia, particles with a mobility diameter of 150 nm were chemically neutralised to ammonium sulphate. In the presence of oxidation products of pinanediol, the organic volume fraction of freshly nucleated particles increased from 0.4 to ~0.9, with an increase in diameter from 2 to 63 nm. Conversely, the sulphuric acid volume fraction decreased from 0.6 to 0.1 when the particle diameter increased from 2 to 50 nm. The results provide information on the composition of nucleated aerosol particles during their growth in the presence of various combinations of sulphuric acid, ammonia, dimethylamine and organic oxidation products.

We investigated hygroscopicity of nucleated nanoparticles derived from dimethylamine and α-pinene with sulfuric acid during CLOUD 7 (Cosmic Leaving OUtdoor Droplets) campaign at CERN. The hygroscopicity of nucleated nanoparticles from 10 to 20 nm in mobility diameter was measured with a nano tandem differential mobility analyzer (nano-TDMA). Here, we present preliminary results from the CLOUD 7 experiments.

The application of seed-protecting pesticides is often a prerequisite for raising legumes in the tropics. However, these chemicals may influence the development of root nodule symbiosis. In the present study, high concentrations of Gaucho insecticide (imidacloprid) and Vitavax-300 fungicide (carboxin and captan) clearly inhibited the growth of root nodule bacterium under laboratory conditions. However, they did not effect to the nodulation or biomass production of Arachis pintoi, Arachis hypogaea, Mucuna pruriens or Desmodium ovalifolium raised in a green house in eastern Costa Rica. Explanations for these results are discussed., Tuhohyönteiset ja -sienet aiheuttavat vuosittain suuria taloudellisia menetyksiä kasvinviljelylle eri puolilla tropiikkia. Runsaasti ravinteita sisältävät palkokasvien siemenet saattavat tuhoutua kylvön jälkeen nopeasti, ellei niitä käsitellä kasvinsuojeluaineilla. Kasvinsuojeluaineet saattavat kuitenkin vaikuttaa ehkäisevästi palkokasvien juurinystyräsymbioosin kehittymiseen. Käsillä olevassa työssä tutkittiin kahden Väli-Amerikassa yleisesti käytetyn kasvinsuojeluaineen (fungisidi Vitavax-300: tehoaineina karboksiini ja kaplaani sekä insektisidi Gaucho: tehoaineena imidaklopridi) vaikutuksia maapähkinän juurinystyrästä eristettyyn ritsobiin sekä neljän eri palkokasvin juurinystyröiden kehittymiseen. Kumpikin tutkittu kasvinsuojeluaine keskeytti korkeina pitoisuuksina juurinystyräbakteerin kasvun laboratorio-olosuhteissa. Paksuilla kasvinsuojeluainekerroksilla kuorrutetuista siemenistä kehittyneet taimet nystyröityivät kasvihuoneessa kuitenkin yhtä hyvin kuin käsittelemättömät vertailutaimet. Kasvinsuojeluaineiden vaikutusalue ei siis ylettynyt juurinystyröiden muodostumisalueelle. Kirjallisuudesta löytyi viitteitä, joiden mukaan eräiden palkokasvien itävän siemenen alkeisjuuren eritteet saattavat neutraloida pestisidien tehoaineita. Kirjallisuusviitteiden mukaan pestisidien vaikutus saattaa myös kohdistua erityisesti ritsosfäärissä juurinystäbakteereita syöviin alkueläimiin, jolloin juurinystyräbakteerin lisääntymismahdollisuudet paranevat.

Brain-derived neurotrophic factor (BDNF) importantly regulates learning and memory and supports the survival of injured neurons. Reduced BDNF levels have been detected in the brains of Alzheimer's disease (AD) patients but the exact role of BDNF in the pathophysiology of the disorder remains obscure. We have recently shown that reduced signaling of BDNF receptor TrkB aggravates memory impairment in APPswe/PS1dE9 (APdE9) mice, a model of AD. The present study examined the influence of Bdnf gene deficiency (heterozygous knockout) on spatial learning, spontaneous exploratory activity and motor coordination/balance in middle-aged male and female APdE9 mice. We also studied brain BDNF protein levels in APdE9 mice in different ages showing progressive amyloid pathology. Both APdE9 and Bdnf mutations impaired spatial learning in males and showed a similar trend in females. Importantly, the effect was additive, so that double mutant mice performed the worst. However, APdE9 and Bdnf mutations influenced spontaneous locomotion in contrasting ways, such that locomotor hyperactivity observed in APdE9 mice was normalized by Bdnf deficiency. Obesity associated with Bdnf deficiency did not account for the reduced hyperactivity in double mutant mice. Bdnf deficiency did not alter amyloid plaque formation in APdE9 mice. Before plaque formation (3 months), BDNF protein levels where either reduced (female) or unaltered (male) in the APdE9 mouse cortex. Unexpectedly, this was followed by an age-dependent increase in mature BDNF protein. Bdnf mRNA and phospho-TrkB levels remained unaltered in the cortical tissue samples of middle-aged APdE9 mice. Immunohistological studies revealed increased BDNF immunoreactivity around amyloid plaques indicating that the plaques may sequester BDNF protein and prevent it from activating TrkB. If similar BDNF accumulation happens in human AD brains, it would suggest that functional BDNF levels in the AD brains are even lower than reported, which could partially contribute to learning and memory problems of AD patients.

We introduce a four-year (2006–2010) continuous data set of aerosol optical properties at Puijo in Kuopio, Finland. We study the annual and diurnal variation of the aerosol scattering and absorption coefficients, hemispheric backscattering fraction, scattering Ångström exponent, and single scattering albedo, whose averages over this period were 11.1 Mm−1 (at 550 nm), 1.5 Mm−1 (at 670 nm), 0.13, 1.9, and 0.83, respectively. The scattering coefficient peaked in the spring and autumn, being 2–4 times those in the summer and winter. An exception was the summer of 2010, when the the scattering coefficient was elevated to ~300 Mm−1 by the plumes from forest fires in Russia. The absorption coefficient peaked in the winter with values of 2–3 times those in the summer. The single scattering albedo was lowest in the winter when more biomass burning derived, soot-containing aerosols were present. The optical properties varied also with wind direction and time of the day, indicating the effect of the local pollutant sources and the age of the particles. Peak values in the single scattering albedo were observed when the wind blew from a paper mill and from the sector without local pollutant sources. These observations were linked to the sulphate-rich aerosol from the paper mill and the oxygenated organics in the aged aerosol, which both are known to increase the scattering characteristics of aerosols. Changes in the single scattering albedo in the morning and afternoon in the summertime were linked to the increased traffic density at these hours. The scattering and absorption coefficients were found to be decreased by clouds. The effect was stronger for the scattering than absorption, indicating preferential activation of the more hygroscopic aerosol with higher scattering characteristics. What happens to the aerosol optical properties during a cloud event when the air masses come from different directions with different local sources, is under a more detailed inspection. Also, more aerosol mass spectrometry data will be analyzed in order to strengthen our knowledge about the role of the chemical composition of the aerosol particles in their activation into cloud droplets.

The field phase of the present study was carried out at Bura, eastern Kenya. The introduced, drought resistantProsopis juliflora seedlings were watered at four different irrigation levels. 43 sample trees with 1772 root nodules were dug up and investigated. Several trees lacked viable root nodules especially in nonirrigated plots. Regression analysis indicated significant negative correlation between the mortality of the root nodules and the level of irrigation. Rhizobium strains, which were isolated from the sample trees, were capable of nodulating indigenousAcacia senegal.

was isolated from the root nodules of L. seedlings, grown in a growth chamber, after inoculation with soil originating from an old east Kenyan casuarina forest. Optimum pH for the growth of the two isolates ranged from 6.4 to 6.9. The optimum temperature for their growth was 32°C. The growth of these cultures ceased at NaCl concentration above 2%. The influence of the isolates on the growth of the host plant was determined in a growth chamber experiment in which an American strain (HFPCc13) was used as a reference. The biomass of the inoculated seedlings was 2.4â4.1 fold those of the non-inoculated control seedlings at the end of the 7-month experiment.FrankiaCasuarina equisetifoliaFrankia

Metsänhoidon jäljet näkyvät talousmetsien rakenteessa: nuoria ja keski-ikäisiä metsiä ja metsäkuvioiden reunavyöhykkeitä on paljon. Vanhat metsät ovat pienialaisia, ne sijaitsevat hajallaan ja vanhojen lehtipuiden osuus niissä on vähäinen. Metsäluonnossa tapahtuneet elinympäristömuutokset suosivat joitakin eliölajeja, toiset eliölajit puolestaan taantuvat muutosten vuoksi. Oheisessa katsauksessa esitetään arvio metsänhoidon eri toimenpiteistä hyötyvistä sekä niistä kärsivistä eliöryhmistä. Katsauksessa on käsitelty erikseen nisäkkäät, linnut, selkärangattomat, sienet ja kasvit. Laaditut taulukot perustuvat eri eliöryhmien asiantuntijoilta (yht. 17) saatuun arvioon.

We present the first laboratory experiments of aerosol formation from oxidation of volatile organic species emitted by living plants, a process which for half a century has been known to take place in the atmosphere. We have treated white cabbage plants with methyl jasmonate in order to induce the production of monoterpenes and certain less-volatile sesqui- and homoterpenes. Ozone was introduced into the growth chamber in which the plants were placed, and the subsequent aerosol formation and growth of aerosols were monitored by measuring the particle size distributions continuously during the experiments. Our observations show similar particle formation rates as in the atmosphere but much higher growth rates. The results indicate that the concentrations of nonvolatile oxidation products of plant released precursors needed to induce the nucleation are roughly an order-of-magnitude higher than their concentrations during atmospheric nucleation events. Our results therefore suggest that if oxidized organics are involved in atmospheric nucleation events, their role is to participate in the growth of pre-existing molecular clusters rather than to form such clusters through homogeneous or ion-induced nucleation.