On the Surface Interaction of Aerosol Particles

Aerosol particles interact with surfaces in different environments and applications. The knowledge of fundamental surface interaction phenomena can be used to investigate the structural properties of particles, describe the performance of different instruments, and develop new applications. In this thesis, particle rebound from a surface as well as the transfer of the electric charge during rebound and resuspension were studied. The research was based on the experimental and numerical characterization of these surface interaction phenomena. Along with the fundamental understanding obtained for the interaction between aerosol particles and surfaces, new methods were developed and applied for specific aerosols, such as fungal spores.The critical velocity of rebound, which is the smallest incident velocity for a particle to rebound, was determined in a wide particle size range combining the separate groups of previous results. The main instrument developed during the research of this thesis, a variable nozzle area impactor (VNAI), was used throughout the wide size range for different particle and surface materials. The particle material was found to have a significant effect on the critical velocity of rebound. On the contrary, the effect of the surface material was observed to be much smaller, at least for the hard surface materials studied in the experiments. Also the critical velocity for oblique impacts in the impactor was characterized. It was found that the critical velocity of rebound significantly decreased with the increasing obliquity of the impact. Based on the results of this thesis, the effect of materials and obliquity on the critical velocity can be estimated quantitatively and exploited in the development of new applications, such as impactors and collectors.The triboelectric charge transfer during rebound was characterized for nanoparticles with a new simplified method developed for an electrical low pressure impactor (ELPI). In addition, the charge transfer was measured for fungal spores and standard test dusts during rebound and resuspension. The results of this thesis showed that the triboelectric charging of particles during resuspension correlated with the triboelectric charging during rebound. Spores of different fungal species were found to obtain both positive and negative charges from substrates of different materials. The information on the triboelectric charging and rebound behavior can be used for predicting the particle dynamics in different environments and designing new applications, for instance, in ventilation and filtration systems

Nanohiukkasten pomppaaminen ja varauksen siirto sähköisessä alipaineimpaktorissa

Ilmakehän aerosolihiukkasten tutkimuksessa on hiljattain noussut esille uudenlainen tarve mitata hiukkasten olomuotoa. Pienten hiukkasten olomuodon mittaaminen on lähtökohtaisesti haastavaa, mutta eräs potentiaalinen menetelmä perustuu niiden pomppaamiseen impaktorissa. Tässä työssä selvitettiin sähköisen alipaineimpaktorin ELPI:n mahdollisuuksia alle 200 nm hiukkasten pomppaamisen mittaamisessa. Koska ELPI mittaa hiukkasia sähköisesti, oli pomppaamisen rinnalla huomioitava myös hiukkasten kontaktissa impaktorialustaan tapahtuva varauksensiirto. Hiukkasten pomppaamiselle ja varauksensiirrolle ELPI:ssä luotiin laskennallinen malli, joka perustuu aiemmin julkaistuihin, alun perin yli mikrometrin kokoisille hiukkasille suunnattuihin, teorioihin sekä alipaineimpaktorin virtausmallinnukseen. Teoreettisen ja laskennallisen lähestymistavan vastapainona oli kokeellinen osio, jossa tehtiin mittauksia erityisesti pomppaamiseen soveltuvilla puhtailla teräsalustoilla varustetulla ELPI:llä sekä yksiasteimpaktorilla. Yksiasteimpaktorilla suoritetut mittaukset olivat luonteeltaan lähellä perustutkimusta, ja niissä pyrittiin selvittämään nanohiukkasten käyttäytymistä suhteessa pomppaamisen ja varauksensiirron teorioihin. ELPI:llä toteutetuissa mittauksissa puolestaan testattiin mallin toimivuutta ja niin sanottujen tunnuslukujen käyttökelpoisuutta. Tunnuslukumenetelmän tavoitteena oli määritellä yksinkertaiset virtajakaumista laskettavat tunnusluvut, jotka kuvaisivat hiukkasten pomppaamisen ja varauksensiirron voimakkuutta. ELPI:lle tehty malli kuvasi hyvin hiukkasten käyttäytymistä laitteessa. Etenkin hiukkasjakaumalle tehtävä sovitus, jossa mallinnetut virrat sovitetaan mitattuihin ELPI:n virtoihin kolmen vapaan parametrin avulla, osoittautui toimivaksi tavaksi saada tietoa hiukkasten ominaisuuksista. Tunnuslukumenetelmä ei kyennyt yhtä tarkkaan analyysiin, varsinkaan hiukkaskoon muuttuessa. Yksiasteimpaktorilla tehdyistä mittauksista saatu informaatio tuki pääosin mallin teoreettisia oletuksia ja vahvisti perustietämystä nanohiukkasten varauksensiirrosta. Vaikka laajasta tutkimusalueesta jäi myös paljon avoimia kysymyksiä, antavat tämän työn tulokset hyvän pohjan niin ELPI:n käytölle kuin mahdollisten uusien sovellusten kehittämiselle aerosolihiukkasten pomppaamisen ja varauksensiirron saralla. /Kir1

Effect of radiation interaction and aerosol processes on ventilation and aerosol concentrations in a real urban neighbourhood in Helsinki

Large-eddy simulation (LES) is an optimal tool to examine aerosol particle concentrations in detail within urban neighbourhoods. The concentrations are a complex result of local emissions, meteorology, aerosol processes and local mixing conditions due to thermal and mechanical effects. Despite this, most studies have focused on simplification of the affecting processes such as examining the impact of local mixing in idealised street canyons or treating aerosols as passive scalars. The aim of this study is to include all these processes into LES using the PALM model system and to examine the importance of radiative heating and aerosol processes in simulating local aerosol particle concentrations and different aerosol metrics within a realistic urban neighbourhood in Helsinki under morning rush hour with calm wind conditions. The model outputs are evaluated against mobile laboratory measurements of air temperature and total particle number concentration (Ntot) as well as drone measurements of lung-deposited surface area (LDSA). The inclusion of radiation interaction in LES has a significant impact on simulated near-surface temperatures in our study domain, increasing them on average from 8.6 to 12.4 ∘C. The resulting enhanced ventilation reduces the pedestrian-level (4 m) Ntot by 53 %. The reduction in Ntot due to aerosol processes is smaller, only 18 %. Aerosol processes particularly impact the smallest particle range, whereas radiation interaction is more important in the larger particle range. The inclusion of radiation interaction reduces the bias between the modelled and mobile-laboratory-measured air temperatures from −3.9 to +0.2 ∘C and Ntot from +98 % to −13 %. With both aerosol and radiation interaction on, the underestimation is 16 %, which might be due to overestimation of the ventilation. The results show how inclusion of radiative interaction is particularly important in simulating PM2.5, whereas aerosol processes are more important in simulating LDSA in this calm wind situation.</p

Determinants of spatial variability of air pollutant concentrations in a street canyon network measured using a mobile laboratory and a drone

Urban air pollutant concentrations are highly variable both in space and time. In order to understand these variabilities high-resolution measurements of air pollutants are needed. Here we present results of a mobile laboratory and a drone measurements made within a street-canyon network in Helsinki, Finland, in summer and winter 2017. The mobile lab-oratory measured the total number concentration (N) and lung-deposited surface area (LDSA) of aerosol particles, and the concentrations of black carbon, nitric oxide (NOx) and ozone (O3). The drone measured the vertical profile of LDSA. The main aims were to examine the spatial variability of air pollutants in a wide street canyon and its immediate surroundings, and find the controlling environmental variables for the observed variability's.The highest concentrations with the most temporal variability were measured at the main street canyon when the mo-bile laboratory was moving with the traffic fleet for all air pollutants except O3. The street canyon concentration levels were more affected by traffic rates whereas on surrounding areas, meteorological conditions dominated. Both the mean flow and turbulence were important, the latter particularly for smaller aerosol particles through LDSA and N. The formation of concentration hotspots in the street network were mostly controlled by mechanical processes but in winter thermal processes became also important for aerosol particles. LDSA showed large variability in the profile shape, and surface and background concentrations. The expected exponential decay functions worked better in well -mixed conditions in summer compared to winter. We derived equation for the vertical decay which was mostly con-trolled by the air temperature. Mean wind dominated the profile shape over both thermal and mechanical turbulence. This study is among the first experimental studies to demonstrate the importance of high-resolution measurements in understanding urban pollutant variability in detail.Peer reviewe

Nonvolatile ultrafine particles observed to form trimodal size distributions in non-road diesel engine exhaust

Some recent findings regarding the negative health effects of particulate matter increase the relevance of the detailed characteristics of particulate emissions from different sources and especially the nonvolatile fraction of particles. In this study, the nonvolatile fraction of ultrafine particulate emissions from a non-road diesel engine was studied. The measurements were carried out in an engine laboratory and the exhaust sample was taken from the engine-out location with various steady state driving modes. Four different fuels, including fossil fuel, soybean methyl ester (SME), rapeseed methyl ester (RME), and renewable paraffinic diesel (RPD), were used. In the sampling system, the sample was diluted and led through a thermodenuder removing the volatile fraction of particles. The measured particle size distributions of nonvolatile particles were found to be trimodal. Based on the size distribution data as well as the morphology and elemental composition of particles in transmission electron microscopy (TEM) samples, we were able to draw conclusions from the most probable origin of the different particle modes, and the modes were named accordingly. From larger to smaller in particle size, the modes were a soot mode, lubricating oil originated core (LC) mode, and a fuel originated core (FC) mode. All of these three modes were detected with every driving mode, but differences were seen, for example, between different fuels. In addition, a trade-off was observed in the concentrations of the LC mode and the soot mode as a function of the engine torque.© 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.fi=vertaisarvioitu|en=peerReviewed

Dispersion of a Traffic Related Nanocluster Aerosol Near a Major Road

Traffic is a major source of ultrafine aerosol particles in urban environments. Recent studies show that a significant fraction of traffic-related particles are only few nanometers in diameter. Here, we study the dispersion of this nanocluster aerosol (NCA) in the size range 1.3-4 nm. We measured particle concentrations near a major highway in the Helsinki region of Finland, varying the distance from the highway. Additionally, modelling studies were performed to gain further information on how different transformation processes affect NCA dispersion. The roadside measurements showed that NCA concentrations fell more rapidly than the total particle concentrations, especially during the morning. However, a significant amount of NCA particles remained as the aerosol population evolved. Modelling studies showed that, while dilution is the main process acting on the total particle concentration, deposition also had a significant impact. Condensation and possibly enhanced deposition of NCA were the main plausible processes explaining why dispersion is faster for NCA than for total particle concentration, while the effect of coagulation on all size ranges was small. Based on our results, we conclude that NCA may play a significant role in urban environments, since, rather than being scavenged by larger particles, NCA particles remain in the particle population and grow by condensation.Peer reviewe

Chemical and physical characterization of traffic particles in four different highway environments in the Helsinki metropolitan area

Traffic-related pollution is a major concern in urban areas due to its deleterious effects on human health. The characteristics of the traffic emissions on four highway environments in the Helsinki metropolitan area were measured with a mobile laboratory, equipped with state-of-the-art instrumentation. Concentration gradients were observed for all traffic-related pollutants, particle number (CN), particulate mass (PM1), black carbon (BC), organics, and nitrogen oxides (NO and NO2). Flow dynamics in different environments appeared to be an important factor for the dilution of the pollutants. For example, the half-decay distances for the traffic-related CN concentrations varied from 8 to 83aEuro-m at different sites. The PM1 emissions from traffic mostly consisted of organics and BC. At the most open site, the ratio of organics to BC increased with distance to the highway, indicating condensation of volatile and semi-volatile organics on BC particles. These condensed organics were shown to be hydrocarbons as the fraction of hydrocarbon fragments in organics increased. Regarding the CN size distributions, particle growth during the dilution was not observed; however the mass size distributions measured with a soot particle aerosol mass spectrometer (SP-AMS), showed a visible shift of the mode, detected at aEuro-100aEuro-nm at the roadside, to a larger size when the distance to the roadside increased. The fleet average emission factors appeared to be lower for the CN and higher for the NO2 than ten years ago. The reason is likely to be the increased fraction of light-duty (LD) diesel vehicles in the past ten years. The fraction of heavy-duty (HD) traffic, although constituting less than 10aEuro-% of the total traffic flow, was found to have a large impact on the emissions.Peer reviewe

Measurement report : The influence of traffic and new particle formation on the size distribution of 1–800 nm particles in Helsinki – a street canyon and an urban background station comparison

Peer reviewe

The value of plantation forests for plant, invertebrate and bird diversity and the potential for cross-taxon surrogacy

As the area of plantation forest expands worldwide and natural, unmanaged forests decline there is much interest in the potential for planted forests to provide habitat for biodiversity. In regions where little semi-natural woodland remains, the biodiversity supported by forest plantations, typically non-native conifers, may be particularly important. Few studies provide detailed comparisons between the species diversity of native woodlands which are being depleted and non-native plantation forests, which are now expanding, based on data collected from multiple taxa in the same study sites. Here we compare the species diversity and community composition of plants, invertebrates and birds in Sitka spruce- (Picea sitchensis-) dominated and Norway spruce- (Picea abies-) dominated plantations, which have expanded significantly in recent decades in the study area in Ireland, with that of oak- and ash-dominated semi-natural woodlands in the same area. The results show that species richness in spruce plantations can be as high as semi-natural woodlands, but that the two forest types support different assemblages of species. In areas where non-native conifer plantations are the principle forest type, their role in the provision of habitat for biodiversity conservation should not be overlooked. Appropriate management should target the introduction of semi-natural woodland characteristics, and on the extension of existing semi-natural woodlands to maintain and enhance forest species diversity. Our data show that although some relatively easily surveyed groups, such as vascular plants and birds, were congruent with many of the other taxa when looking across all study sites, the similarities in response were not strong enough to warrant use of these taxa as surrogates of the others. In order to capture a wide range of biotic variation, assessments of forest biodiversity should either encompass several taxonomic groups, or rely on the use of indicators of diversity that are not species based