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.

Accurate predictions of the extinction and scattering properties of the atmosphere are important for climate research and interpreting satellite data. This study introduces a model (called the H-model) that calculates the scattering coefficients and scattering enhancement factors based on in situ measurements of the dried ambient aerosol. A disadvantage of using dried aerosol measurements is that they do not correspond with the ambient conditions, as they are measured at a relative humidity below 40% and thus the particles are assumed to contain no water. Measurements of aerosol chemical composition do not contain water mass concentrations and measurements of the particle size distribution do not include water. To solve this problem, the H-model uses ISORROPIA, a thermodynamic equilibrium model, to estimate the expected amount of aerosol water content and growth factor g(RH) of aerosol particles for any given temperature and relative humidity (RH). With this information, the conversion between dry and enhanced relative humidity can be made. The chemical composition measurements can be complemented with the estimated aerosol water concentrations and the particle size distribution can be recalculated based on the growth factor for any given RH. In addition, the growth factor is also calculated by using k-Köhler theory and compared to the results of ISORROPIA. The findings of this sub-study show that the growth factors calculated by both approaches (ISORROPIA and k-Köhler theory) are similar as they significantly correlate. ISORROPIA, however, is more sensitive to small chemical changes which makes it more appropriate for the H-model. The calculated growth factors are used in the H-model to estimate changes in the chemical composition and particle size distribution of the aerosol particles at enhanced relative humidity. Subsequently, the H-model uses MIE theory to estimate the scattering properties of the particles at a specific relative humidity. By doing so, the scattering properties can be calculated at dry and enhanced RH, making it possible to calculate scattering enhancement factors. Finally, the H-model is validated by comparing the calculated scattering properties to measured scattering properties of a (humidified) nephelometer. To do so, in situ measurements from the CINDI campaign in 2009 and the TROLIX campaign in 2019 at Cabauw are used. The findings of this validation show that the results from the H-model do not yet accurately match the measurements. That being said, a strong correlation is observed between the calculated and the measured scattering properties. This shows that the H-model is able to capture changes in the particle size distribution and chemical composition while calculating the enhancement factors. It can be concluded that the results from the H-model are promising but need further work to close the gap between the calculations and measurements. The H-model makes multiple simplifications and assumptions which could be improved upon, thereby increasing the precision of the results as well. Furthermore, to fully conclude the findings of this study, the measurements of the SMPS and the nephelometers should be calibrated. A better statement can then be made about the accuracy of the comparison between the scattering properties calculated by the H-model and measured by the nephelometers.

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.

Lidar systems can provide vertically resolved measurements of the physical and optical properties of the atmospheric particles with high spatiotemporal resolution. In this study, four case studies were analyzed and studied in order to identify the dominant aerosol type over the city of Nicosia in Cyprus: 05 April and 07 April 2015 and 17 April and 21 April 2016. More specifically, vertical profiles of the extinction and backscatter coefficient, Ångström exponent, particle linear depolarization ratio and lidar ratio were manually retrieved from the lidar signal for each case study. For the first case, the main aerosol load was observed in the very lower atmosphere, between 0.25 and 1.25 km. The observed aerosol optical properties indicated the strong presence of maritime aerosols. In the second case study, the aerosol layer was thicker and it was observed between 1.6 and 7 km. Analysis of the optical properties showed that the predominant aerosol types within the PBL were mainly maritime aerosols and aerosols from local sources while higher in the atmosphere the aerosol load consisted of dust mixtures. The third case study, on 17th April 2016, was a relatively clean day and a thin aerosol layer was observed between 0.25 and 2.0 km consisting mainly of maritime and urban aerosols. 21st of April 2016, which was the fourth case study, was a dust event case study. The aerosol load, accumulated between 2.0 and 5.0 km, was characterized as a purely dust layer. The results obtained with the manual retrieval method, were compared with automatically retrieved profiles (provided by the Leibniz Institute for Tropospheric Research) and are in good agreement. More specifically, for the vertically resolved extinction profiles at 532 nm the correlation coefficient values ranged between 0.9784 and 1. The correlation coefficients for the backscatter profiles at 532 nm and 1064 nm ranged from 0.9975 to 0.9986 and from 0.9937 to 0.9996 respectively. The linear particle depolarization ratio profiles also correlated less well with the automatically retrieved one with R ranging from 0.8781 to 0.9889 while similarly, for the volume depolarization ratio profiles the correlation coefficient ranged between 0.7054 to 0.9603.