Positive and negative ions produced by radioactive sources and corona discharges in gases find a number of applications, including charging aerosol particles prior to their measurement by electrical and/or electrical mobility techniques. The degree to which these ions can charge aerosol particles depends on their mobility and mass; properties that are strongly affected by the composition of the carrier gas and the impurities that it contains. We show that when the purity of the carrier gas is increased, the mobility of both positive and negative ions increases by more than 50%, whereas the respective masses reduce by more than 50%. In most cases, the dominant positive species is N ₄ ⁺, whereas NO ₂ ^- and NO ₃ ^- prevail for the negative polarity. Differences in ion mobility and mass resulting from the two ionization methods (i.e., radioactive source and corona discharges) remain limited. When volatile methyl siloxanes (VMS) are introduced deliberately to the gas, the mobility of the cations decreases by 39% and their mass increases by 385%, while the dominant mobility and mass peaks of the negative ions remains almost unaffected. Interestingly, introduction of VMS also leads to consistent and reproducible positive ion properties across all variations of the experiments, which can be especially relevant for charging aerosol particles in a reproducible manner. Taken together, the new measurements we report in this paper corroborate prior knowledge that the composition and purity of the carrier gas strongly influence the properties of positive and negative ions generated in aerosol neutralizers, and provide new evidence regarding their evolution in the presence of impurities.

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The increasing use of silver nanoparticles (AgNPs) in various products leads to their presence in the aquatic environment. The dissolution of AgNPs is an important property that has a direct impact on human health and the natural environment. Understanding the dissolution behaviour of nanoparticles in liquid suspensions is essential for predicting their potential toxic effect in organisms, ranging from viruses and bacteria to humans. Moreover, the dissolution rate of nanoparticles can explain some of their disinfecting properties, which are important for sanitation. The objective of this study is to determine the dissolution behaviour of AgNPs in pure water and to improve our understanding of its most fundamental principles. Dissolution constants of AgNPs found in literature span over a wide range, indicating that improvement of the measuring method is needed. AgNPs in this study were produced in a principally impurity-free way, from the gas phase, after which they were transferred into liquid solutions. The purity of the particles produced in this study is in principle higher compared to those used until now, which allows for higher precision in determining the dissolution constant. Measuring the silver ion concentration in the resulting liquid solutions (i.e., after introducing the AgNPs in the solution) with an ICP-MS at specific time intervals gave direct information on the dissolution kinetics. The experiments were repeated with particles having diameters from 7 to 12 nm and, as expected, dissolution kinetics were found to be highly dependent on particle size. The determined dissolution constants are in the same order of magnitude as the values reported in literature. To further improve the reliability of the measurements, the experiment needs to be repeated using different methods for transferring the particles into the liquid, given that the used bubbling method showed deficiencies.

In the present research, the activation parameterization method introduced by Nenes and Seinfeld (2003) was compared and evaluated to a remote sensing-based method by Rusli, Donovan & Russchenberg (2017) for determining the cloud drop number concentration. Both methods have fundamentally different approaches for indirectly determining the cloud droplet number concentration. The parameterization method is based on the Köhler Theory, in which the activation process of particles contained in a rising parcel is modelled for predicting the number concentrations of cloud droplets. The remote sensing method, on the other hand, applies theories about particle-light interactions. Since the remote sensing method determines the cloud droplet concentrations in a more direct manner than the parameterization method, it is regarded here as the reference. An agreement was found between the two models, with a relative error of cloud droplet number concentrations between 41.1% and 78.0%, which lead to errors of the cloud’s scattering intensity in the range of 13% and 26%. Despite some discrepancies between the obtained droplet concentrations, the parameterization model shows similar trends to the remote sensing observations. It was found that the updraft velocity that is needed as input variable for the parameterization model has the largest influence on the model’s prediction of droplets concentrations, and that it is likely to be an important cause for the seen discrepancies. Furthermore, the present research shows how assumptions were made on the size distribution input variable used in the parameterization model, which were not available from observations.

Air quality monitoring using airborne platforms is rapidly gaining ground as unmanned aerial vehicles (UAVs) are becoming easier, less expensive, and safer to operate on a routine basis. To facilitate measurements of key atmospheric properties, however, efforts are still required in developing/testing miniaturized instruments for use onboard UAVs. Here, we test two commercially available cost-effective/lightweight optical particle counters (OPCs; Alphasense Model N2) capable of measuring the size distributions of airborne particles having diameters from 380 nm to 17 μm. Tests were made against a reference and recently calibrated OPC (Grimm Model 1.109) using monodisperse polystyrene spheres. All instruments were placed in a chamber in which the temperature and pressure varied in the ranges of –5 to 23°C and 0.7 to 1.0 atm, respectively; conditions typically encountered during UAV flights. Agreement in the particle number concentrations measured by the Alphasense and the Grimm OPCs was within 40%, under all experimental conditions used in this work, when particles having sizes >1 μm were employed during the tests. Deviations higher than 50%, however, were observed when the instruments were tested with 1.0- and 0.8-μm polysterene spheres. The particle sizes reported by both Alphasense OPCs were within ± 5% with respect to the nominal polysterene spheres’ size under all operating pressures and temperatures down to 5°C. At lower temperatures, the sizing accuracy of one of the two Alphasense OPCs degraded significantly. While our findings support that the Alphasense OPCs can be used at low temperature/pressure conditions, they should be carefully tested prior the measurements to ensure good performance.

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