Developing experimental methods to understand atmospheric nucleation

The effects of human civilization on Earth’s climate are undeniable. Some are due to emissions of greenhouse gases, some are due to atmospheric aerosol particles. In this thesis, the focus is on the aerosols. The complexity of the effects of the aerosol particles on the climate strive from high dynamics of the aerosol populations. A fraction of the aerosol particles are formed in the atmosphere by secondary particle formation, but the articles are also emitted to the atmosphere as primary emission for example from engine exhaust or from sea spray. The particles can grow by condensation and coagulation and get lost by deposition due to gravity and wet deposition. All of the processes mentioned lead to a particle population with a highly varying chemical composition. The climatic effects of the aerosol particles can be either direct, by scattering of light by the particles or indirect through cloud formation. In addition to the climatic effects of the aerosol particles, they can have adverse effects on human health. The smallest particles are capable of, not only penetrating deep into the respiratory track and lungs, but also translocate from the nasal surfaces straight into human brain and thus penetrating the blood-brain barrier. This work concentrates on measurement methods of the very smallest particles and how they form in the atmosphere, but also touches upon the size range that is highly relevant with respect to the cloud formation. Gas to particulate phase transitions and interactions play crucial role in climate and are still quite poorly understood. The particles in the atmosphere provide a large surface area that can act as catalyst for gas phase chemistry and contribute to the cloud formation as this typically requires presence of aerosol particles. Majority of the aerosol particles are formed in the atmosphere due to a combination of natural, and anthropogenic precursor gases. In order to understand the climatic effects of these particles, their formation and growth toward larger sizes needs to be investigated. During this thesis work, several new instrumentation methods were developed. First one is a surface tension measurement apparatus that was used to measure the surface tension of atmospherically relevant aqueous mixture of organic acid and inorganic salt. Using a modelling approach, it was discovered that the surface tension had a slight effect on aerosol activation and cloud droplet formation in warm clouds. The second instrument is a new particle measurement system that is capable of counting the particles as small as 1 nm in diameter, making it possible to measure atmospheric new particle formation and their growth as it happens, particle per particle. The work illustrated that the particle counter needs to be able to detect the particles when they are formed in order to get correct information on the formation and growth rates relevant for the atmospheric aerosol population.Ihmiset vaikuttavat Maan ilmastoon monella tavalla. Ilmakehää lämmittävät kasvihuonepäästöt ja toisaalta siihen vaikuttavat myös ilmakehän aerosolihiukkaset. Tässä väitöskirjassa keskitytään ilmakehän aerosolihiukkasiin. Niiden vaikutus ilmakehään on monimutkainen johtuen niiden nopeista muutosprosesseista ilmakehässä. Osa hiukkasista muodostuu vasta ilmakehässä erilaisten kaasumaisten päästöjen hapettumisen johdosta ja osa pääsee ilmakehään suoraan ns. primääreinä hiukkasina, esimerkkeinä diesel moottoreiden nokihiukkaset tai vaikkapa autiomaiden hiekasta muodostuva pöly. Hiukkaset voivat kasvaa ilmakehässä kun niiden pinnalle tiivistyy höyryjä tai jos ne törmäilevät ja kiinnittyvät toisiinsa. Ne voivat poistua ilmakehästä muun muassa sateen vaikutuksesta tai painovoiman johdosta. Aerosolihiukkasiin vaikuttavat prosessit muodostavat aerosolipopulaatioon monimutkaisen kemiallisen koostumuksen riippuen niiden syntytavasta ja paikasta. Hiukkaset vaikuttavat ilmastoon joko suoraan sirottamalla tai absorboimalla auringon valoa, tai epäsuorasti vaikuttamalla pilvien muodostumiseen. Ilmastovaikutusten lisäksi aerosolihiukkaset voivat vaikuttaa ihmisten terveyteen. Pienimpien hiukkasten on todettu kulkeutuvan jopa ihmisten aivokudokseen saakka hermostoa pitkin. Tässä työssä keskitytään hiukkasten mittausmenetelmiin ja kuinka ne syntyvät ilmakehässä, ja toisaalta kuinka ne muodostavat pilvipisaroita. Kaasu-hiukkasmuuntuma vaikuttaa ilmastoon monella tapaa, mutta se tunnetaan kuitenkin vielä suhteellisen huonosti. Aerosolihiukkaset muodostavat ilmakehässä suuren aktiivisen pinta-alan, joka vaikuttaa monella tapaa ilmakehän kemiallisiin prosesseihin ja tätä kautta vaikuttavat esimerkiksi pilvien tai vaikkapa savusumun syntyyn. Suurin osa hiukkasista syntyy ilmakehässä kaasumaisten aineista niiden hapettumisen ansiosta. Jotta tämä prosessi ymmärrettäisiin, on aerosolihiukkaset mitattava juuri kun ne ovat syntyneet, eli noin 1-2 nm kokoisina. Tässä väitöskirjassa kehitettiin uusia mittaustapoja ja laitteita. Ensimmäinen mittalaite kehitettiin mittaamaan ilmakehälle tyypillisien aineiden ja veden yhdisteiden pintajännityksiä. Mittaustuloksia käytettiin arvioimaan pintajännityksen vaikutusta pilvipisaroiden muodostumiseen pilvimallin avulla. Toinen laite kehitettiin mittaamaan ilmakehän pienimpiä, juuri syntyneitä, aerosolihiukkasia aina 1 nm kokoon saakka. Tämän mittalaitteen avulla pystyttiin ensimmäisen kerran mittaamaan ilmakehässä tapahtuvaa hiukkasten muodostumista kaasuista. Tämän työn avulla pystyttiin todistamaan, että hiukkaset on mitattava juuri kun ne syntyvät, jotta muodostumisprosessi voitaisiin täysin ymmärtää. Lisäksi työssä pystyttiin näyttämään, että ilmakehässä tapahtuva hiukkasmuodostus voi vaikuttaa ilmastoon muodostamalla hiukkasia, jotka pystyvät muodostamaan pilvipisaroita ja lopulta pilviä

Indoor Particulate Matter during HOMEChem: Concentrations, Size Distributions, and Exposures.

It is important to improve our understanding of exposure to particulate matter (PM) in residences because of associated health risks. The HOMEChem campaign was conducted to investigate indoor chemistry in a manufactured test house during prescribed everyday activities, such as cooking, cleaning, and opening doors and windows. This paper focuses on measured size distributions of PM (0.001-20 μm), along with estimated exposures and respiratory-tract deposition. Number concentrations were highest for sub-10 nm particles during cooking using a propane-fueled stovetop. During some cooking activities, calculated PM2.5 mass concentrations (assuming a density of 1 g cm-3) exceeded 250 μg m-3, and exposure during the postcooking decay phase exceeded that of the cooking period itself. The modeled PM respiratory deposition for an adult residing in the test house kitchen for 12 h varied from 7 μg on a day with no indoor activities to 68 μg during a simulated day (including breakfast, lunch, and dinner preparation interspersed by cleaning activities) and rose to 149 μg during a simulated Thanksgiving day

Performance of diethylene glycol based particle counters in the sub 3 nm size range [Discussion paper]

When studying new particle formation, the uncertainty in determining the "true" nucleation rate is considerably reduced when using Condensation Particle Counters (CPCs) capable of measuring concentrations of aerosol particles at sizes close to or even at the critical cluster size (1–2 nm). Recently CPCs, able to reliably detect particles below 2 nm in size and even close to 1 nm became available. The corrections needed to calculate nucleation rates are substantially reduced compared to scaling the observed formation rate to the nucleation rate at the critical cluster size. However, this improved instrumentation requires a careful characterization of their cut-off size and the shape of the detection efficiency curve because relatively small shifts in the cut-off size can translate into larger relative errors when measuring particles close to the cut-off size. Here we describe the development of two continuous flow CPCs using diethylene glycol (DEG) as the working fluid. The design is based on two TSI 3776 counters. Several sets of measurements to characterize their performance at different temperature settings were carried out. Furthermore two mixing-type Particle Size Magnifiers (PSM) A09 from Airmodus were characterized in parallel. One PSM was operated at the highest mixing ratio (1 L min−1 saturator flow), and the other was operated in a scanning mode, where the mixing ratios are changed periodically, resulting in a range of cut-off sizes. Different test aerosols were generated using a nano-Differential Mobility Analyzer (nano-DMA) or a high resolution DMA, to obtain detection efficiency curves for all four CPCs. One calibration setup included a high resolution mass spectrometer (APi-TOF) for the determination of the chemical composition of the generated clusters. The lowest cut-off sizes were achieved with negatively charged ammonium sulphate clusters, resulting in cut-offs of 1.4 nm for the laminar flow CPCs and 1.2 and 1.1 nm for the PSMs. A comparison of one of the laminar-flow CPCs and one of the PSMs measuring ambient and laboratory air showed good agreement between the instruments

Assessment of particle size magnifier inversion methods to obtain the particle size distribution from atmospheric measurements

Accurate measurements of the size distribution of atmospheric aerosol nanoparticles are essential to build an understanding of new particle formation and growth. This is particularly crucial at the sub-3 nm range due to the growth of newly formed nanoparticles. The challenge in recovering the size distribution is due its complexity and the fact that not many instruments currently measure at this size range. In this study, we used the particle size magnifier (PSM) to measure atmospheric aerosols. Each day was classified into one of the following three event types: a new particle formation (NPF) event, a non-event or a haze event. We then compared four inversion methods (stepwise, kernel, Hagen-Alofs and expectation-maximization) to determine their feasibility to recover the particle size distribution. In addition, we proposed a method to pretreat the measured data, and we introduced a simple test to estimate the efficacy of the inversion itself. Results showed that all four methods inverted NPF events well; however, the stepwise and kernel methods fared poorly when inverting non-events or haze events. This was due to their algorithm and the fact that, when encountering noisy data (e.g. air mass fluctuations or low sub-3 nm particle concentrations) and under the influence of larger particles, these methods overestimated the size distribution and reported artificial particles during inversion. Therefore, using a statistical hypothesis test to discard noisy scans prior to inversion is an important first step toward achieving a good size distribution. After inversion, it is ideal to compare the integrated concentration to the raw estimate (i.e. the concentration difference at the lowest supersaturation and the highest supersaturation) to ascertain whether the inversion itself is sound. Finally, based on the analysis of the inversion methods, we provide procedures and codes related to the PSM data inversion.Peer reviewe

Laboratory study on new particle formation from the reaction OH + SO2: influence of experimental conditions, H2O vapour, NH3 and the amine tert-butylamine on the overall process

Peer reviewe

Seasonal variation of CCN concentrations and aerosol activation properties in boreal forest

As a part of EUCAARI activities, the annual cycle of cloud condensation nuclei (CCN) concentrations and critical diameter for cloud droplet activation as a function of supersaturation were measured using a CCN counter and a HTDMA (hygroscopicity tandem differential mobility analyzer) at SMEAR II station, Hyytiälä, Finland. The critical diameters for CCN activation were estimated from (i) the measured CCN concentration and particle size distribution data, and (ii) the hygroscopic growth factors by applying κ-Köhler theory, in both cases assuming an internally mixed aerosol. The critical diameters derived by these two methods were in good agreement with each other. The effect of new particle formation on the diurnal variation of CCN concentration and critical diameters was studied. New particle formation was observed to increase the CCN concentrations by 70–110%, depending on the supersaturation level. The average value for the κ-parameter determined from hygroscopicity measurements was κ = 0.18 and it predicted well the CCN activation in boreal forest conditions in Hyytiälä. The derived critical diameters and κ-parameter confirm earlier findings with other methods, that aerosol particles at CCN sizes in Hyytiälä are mostly organic, but contain also more hygrosopic, probably inorganic salts like ammonium sulphate, making the particles more CCN active than pure secondary organic aerosol.Peer reviewe

The standard operating procedure for Airmodus Particle Size Magnifier and nano-Condensation Nucleus Counter

Measurements of aerosol particles and clusters smaller than 3 nm in diameter are performed by many groups in order to detect recently formed or emitted nanoparticles and for studying the formation and early growth processes of aerosol particles. The Airmodus nano-Condensation Nucleus Counter (nCNC), consisting of a Particle Size Magnifier (PSM) and a Condensation Particle Counter (CPC) is a versatile tool to detect aerosol particles and clusters as small as ca. 1 nm in mobility diameter. It offers several different operation modes: fixed mode to measure the total particle number concentration with a fixed, but adjustable lower cut-off size and stepping and scanning modes for retrieving size-resolved information of ca. 1–4 nm particles. The size analysis is based on changing the supersaturation of the working fluid (diethylene glycol) inside the instrument, which changes the lowest detectable size. Here we present a standard operating procedure (SOP) for setting up, calibrating and operating the instrument for atmospheric field measurements. We will also present recommendations for data monitoring and analysis, and discuss some of the uncertainties related to the measurements. This procedure is the first step in harmonizing the use of the PSM/nCNC for atmospheric field measurements of sub-3 nm clusters and particles.Measurements of aerosol particles and clusters smaller than 3 nm in diameter are performed by many groups in order to detect recently formed or emitted nanoparticles and for studying the formation and early growth processes of aerosol particles. The Airmodus nano-Condensation Nucleus Counter (nCNC), consisting of a Particle Size Magnifier (PSM) and a Condensation Particle Counter (CPC) is a versatile tool to detect aerosol particles and clusters as small as ca. 1 nm in mobility diameter. It offers several different operation modes: fixed mode to measure the total particle number concentration with a fixed, but adjustable lower cut-off size and stepping and scanning modes for retrieving size-resolved information of ca. 1–4 nm particles. The size analysis is based on changing the supersaturation of the working fluid (diethylene glycol) inside the instrument, which changes the lowest detectable size. Here we present a standard operating procedure (SOP) for setting up, calibrating and operating the instrument for atmospheric field measurements. We will also present recommendations for data monitoring and analysis, and discuss some of the uncertainties related to the measurements. This procedure is the first step in harmonizing the use of the PSM/nCNC for atmospheric field measurements of sub-3 nm clusters and particles.Peer reviewe

Methods for determining particle size distribution and growth rates between 1 and 3 nm using the Particle Size Magnifier

The most important parameters describing the atmospheric new particle formation process are the particle formation and growth rates. These together determine the amount of cloud condensation nuclei attributed to secondary particle formation. Due to difficulties in detecting small neutral particles, it has previously not been possible to derive these directly from measurements in the size range below about 3 nm. The Airmodus Particle Size Magnifier has been used at the SMEAR II station in Hyytiälä, southern Finland, and during nucleation experiments in the CLOUD chamber at CERN for measuring particles as small as about 1 nm in mobility diameter. We developed several methods to determine the particle size distribution and growth rates in the size range of 1–3 nm from these data sets. Here we introduce the appearance-time method for calculating initial growth rates. The validity of the method was tested by simulations with the Ion-UHMA aerosol dynamic model

Operation of the Airmodus A11 nano Condensation Nucleus Counter at various inlet pressures and various operation temperatures, and design of a new inlet system

Measuring sub-3 nm particles outside of controlled laboratory conditions is a challenging task, as many of the instruments are operated at their limits and are subject to changing ambient conditions. In this study, we advance the current understanding of the operation of the Airmodus A11 nano Condensation Nucleus Counter (nCNC), which consists of an A10 Particle Size Magnifier (PSM) and an A20 Condensation Particle Counter (CPC). The effect of the inlet line pressure on the measured particle concentration was measured, and two separate regions inside the A10, where supersaturation of working fluid can take place, were identified. The possibility of varying the lower cut-off diameter of the nCNC was investigated; by scanning the growth tube temperature, the range of the lower cut-off was extended from 1-2.5 to 1-6 nm. Here we present a new inlet system, which allows automated measurement of the background concentration of homogeneously nucleated droplets, minimizes the diffusion losses in the sampling line and is equipped with an electrostatic filter to remove ions smaller than approximately 4.5 nm. Finally, our view of the guidelines for the optimal use of the Airmodus nCNC is provided.Peer reviewe