Temperature dependence of heterogeneous nucleation

In dieser Arbeit wurde die Temperaturabhängigkeit der Nukleation von n-Nonan Dampf an Natriumchlorid Teilchen untersucht. Nukleation ist ein statistischer Prozess, bei dem Teilchen aus der Dampfphase gebildet werden. Im Fall der heterogenen Nukleation bilden sich die Teilchen nicht spontan aus der Dampfphase, sondern mit Hilfe von sogenannten Kondensationskernen. Vorangegangene Experimente, durchgeführt von Schobesberger, welche sich auf die Temperaturabhängigkeit des Prozesses konzentriert haben, zeigten einen Temperaturtrend, der sowohl der Kelvin- als auch der Fletchertheorie widersprach. Die Aktivierung der Kondensationskerne findet laut Theorie für höhere Temperaturen bei niedrigeren Sättigungsverhältnissen statt. Um herauszufinden, ob dieser umgekehrte Temperaturtrend ein kochsalzspezifischer Effekt ist, oder ob er auf elektrostatischer Wechselwirkung zwischen den Kondensationskernen und dem Dampf beruht (Schobesberger verwendete n-Propanol und Kochsalz, die beide polare Substanzen sind), wurde für diese Arbeit eine nicht polare Flüssigkeit verwendet. Bei der Durchführung der Experimente wurde das n-Nonan verdampft, mit nahezu monodispersen NaCl Teilchen gemischt und in eine zylindrische Messkammer geleitet, welche durch ein Magnetventil mit einem Unterdruckbehälter verbunden war. Durch öffnen des Magnetventils wurde in der Messkammer ein adiabatischer Druckabfall erzeugt, der zu einem Anstieg des Sättigungsverhältnisses führte. Für ausreichend große Sättigungsverhältnisse bildeten sich n-Nonan Tröpfchen an den Kondensationskernen. Dieser Vorgang wurde mit Hilfe von monochromatischer Lichtstreuung untersucht. Der Dampf wurde erzeugt, indem n-Nonan durch eine Mikrodüse geleitet wurde. Der auf diese Weise generierte Flüssigkeitsstrahl verdampfte in einem erhitzten Glaszylinder. Um die Nukleationskeime in der Größenordnung von einigen Nanometern zu erzeugen wurde Kochsalz in einem Rohrofen auf ca. 630°C erhitzt, was zur Folge hatte, dass eine nicht unwesentliche Anzahl von NaCl Molekülen den Kristall verließen und eine breite Teilchenverteilung bildeten. Ein Elektrostatischer Klassifikator diente dazu, eine beinahe monodisperse Fraktion aus der Teilchenverteilung zu extrahieren. Die auf diese Weise klassifizierten Teilchen wurden mit dem Dampf vermischt und in den Size Analyzing Nucleus Counter (SANC) geleitet. Die Aerosolgenerierung ist empfindlich gegenüber Veränderungen der Masseflüsse. Kleine Schwankungen in Flüssigkeits- und Gasmasseflüssen können einen großen Einfluss auf den Nukleationsprozess haben. Der SANC ist ein prozessgesteuertes Messsystem, in dessen Messkammer eine adiabatische Expansion stattfindet, wobei gleichzeitig Druck, Temperatur, gestreutes und transmittiertes Licht gemessen werden. Sind Temperatur, Druckabfall und die genannten Masseflüsse bekannt, kann damit das Sättigungsverhältnis nach der Expansion berechnet werden. Die Messung von gestreutem und transmittiertem Licht kann, unter Verwendung der Constant Angle Mie Scattering (CAMS) Methode, welche auf der mathematischen Beschreibung der Streuung von ebenen Wellen an Sphären basiert, herangezogen werden, um Teilchenwachstum und Konzentration zu ermitteln. Der Vergleich von berechneten Wachstumskurven mit den experimentell ermittelten lässt auf die Aussagekraft des Experiments schließen. In dieser Arbeit wurden Anzahlkonzentrationen aktivierter Teilchen gemessen. Durch Nukleation bildeten sich kritische Cluster an der Oberfläche der NaCl Teilchen, die in der Folge durch Kondensation zu sichtbarer Größe anwuchsen. Je größer das Sättigungsverhältnis ist, umso höher die Anzahl der aktivierten Teilchen. Da die Teilchenzahl in der Messkammer endlich ist, tritt ab einem gewissen Sättigungsverhältnis totale Aktivierung auf. Das Verhältnis von aktivierten zur gesamten Teilchenzahl wird als Aktivierungswahrscheinlichkeit bezeichnet. Die Abhängigkeit der Aktivierung des Sättigungsverhältnisses bei konstanter Temperatur darzustellen, war das Ziel der Messungen. Sie bildet den Ausgangspunkt für weitere Auswertungen der Daten. Aktivierungswahrscheinlichkeitskurven wurden für drei verschiedene NaCl Teilchengrößen (7nm, 10nm und 15nm) und Temperaturen (-10oC, 0oC und +10oC) ermittelt. Da diese Kurven sehr steil sind, war es vergleichsweise einfach jenes Sättigungsverhältnis zu ermitteln, bei dem 50% aller Teilchen aktiviert wurden, das sogenannte Onset Sättigungsverhältnis. Dieses zeigte, entgegen der theoretischen Vorhersage, basierend auf der klassischen Nukleationstheorie, keinen eindeutigen Temperaturtrend. Die Theorie sagt für größere Temperaturen kleinere Onset Sättigungsverhältnisse vorher, zudem waren die experimentell ermittelten Onset Sättigungsverhältnisse deutlich kleiner als die berechneten. Eine mögliche Erklärung dafür könnte die sogenannte Line Tension oder das Surface Diffusion Modell bieten, welche in der klassischen Nukleationstheorie vernachlässigt werden. Die Kelvin Gleichung gibt jene Oberflächenkrümmung eines Tröpfchens an, bei der dieses sich im thermischen Gleichgewicht mit seiner Umgebung befindet. Sie gibt einen Temperaturtrend vor, welcher auch experimentell gefunden werden konnte. Diese Untersuchungen legen nahe, dass die klassische Nukleationstheorie nicht alle Prozesse berücksichtigt, die zur Nukleation beitragen.In this work the temperature dependence of heterogeneous nucleation of n-nonane vapor on sodium chloride seed particles was investigated. Nucleation is a statistical process of particle production from the vapor phase. In the case of heterogeneous nucleation the particles do not form spontaneously from the vapor phase but with help of a so-called seed particle. Previous experiments by Schobesberger focusing on the temperature dependence of the process have shown a temperature trend opposite to the one predicted by the Kelvin equation and the Fletcher theory. For higher temperatures the Kelvin equation predicts activation of the seed particles at lower vapor saturation ratios. To find out whether the above mentioned opposite temperature trend is due to some strange behavior of the seed particles or maybe based on electrostatic interaction (Schobesberger used n-propanol and sodium chloride which both are polar substances) a nonpolar liquid was chosen. The experiments were carried out by evaporating the working fluid, mixing it with nearly monodisperse NaCl particles, passing the particle-vapor mixture on into a cylindrical measurement chamber which was connected to a vessel kept at lower pressure. Connecting the chamber to the vessel results in an adiabatic pressure drop within the chamber leading to a higher saturation ratio. If the saturation ratio is high enough liquid droplets will form on the seed particle surface. This process was observed by laser light scattering. The vapor was produced by passing n-nonane liquid through a micro orifice. The liquid beam, which was generated this way, was evaporated in a heated glass cylinder. For seed particle production in the nanometer size range the sodium chloride was placed in a tube furnace. Heating it up to about 630°C causes a considerable amount of salt molecules to leave the bulk crystal and form a broad particle size distribution. With help of an electrostatic classifier an almost monodisperse fraction was cut out of the particle distribution. The particles classified that way were mixed with the vapor and passed on to the the Size Analyzing Nucleus Counter (SANC). The aerosol production is sensetively dependent on the mass flows. Small uncertainties in the liquid or the gas mass flow may have great influence on the nucleation process. The SANC is a process controlled measurement system, where an adiabatic pressure drop takes place within the measurement chamber. Simultaneously pressure, temperature and scattered as well as transmitted light are recorded. Measurement of pressure and temperature combined with the knowledge of the liquid and gas mass flows allows to determine the saturation rationafter the pressure drop. Scattered and transmitted light measurement is used to determine the particle growth and concentration using the Constant Angle Mie Scattering (CAMS) method. The latter is based on the Mie theory which describes the scattering of light by spherical particles of known refractive index. Comparing droplet growth calculations to the experimental droplet growth provides information on the significance of the experiment. In the present experiments the number of activated seed particles was measured. Due to nucleation critical clusters are formed at the surface of the seed particles. Subsequently, the seed particles grow by condensation to visible sizes. The higher the saturation ratio is the higher the number of activated particles. As the number of seed particles is finite total particle activation will occur above a certain saturation ratio. The ratio of activated over total particle concentration defines the activation probability. The aim of the measurements was to determine the dependence of activation probability on the saturation ratio which is also referred to as activation probability curve. Much effort was put on the attempt to measure the activation probability curves at constant temperature as it forms a basis for further evaluations. Activation probability curves were determined for three different seed particle diameters (7nm, 10nm and 15nm) and temperatures (-10°C, 0°C and +10°C). Since these curves are quite steep it is easy to determine the onset saturation ratio, at which 50% of all particles are activated. The experimentally determined onset saturation ratios for a given particle size did not show a clear temperature trend whereas theoretical calculations based on the classical nucleation theory predict smaller onset saturation ratios for higher temperatures. Furthermore, experimental onset saturation ratios appear to be much smaller than the theoretical ones. Possible reasons could be the so-called line tension or the surface diffusion concept which both are not considered in the classical nucleation theory. The Kelvin equation that does not depend on concepts like line tension predicts the saturation ratio at which a cluster of given size is in thermodynamic equilibrium with the surrounding gas. It predicts a temperature trend which was also found by evaluating the experimental data of this work. These observations seem to suggest that the classical nucleation theory does not consider all processes that play a role in heterogeneous nucleation

Aerosol dynamics simulations of the anatomical variability of e-cigarette particle and vapor deposition in a stochastic lung

Electronic cigarette (EC) aerosols are typically composed of a mixture of nicotine, glycerine (VG), propylene glycol (PG), water, acidic stabilizers and a variety of flavors. Inhalation of e-cigarette aerosols is characterized by a continuous modification of particle diameters, concentrations, composition and phase changes, and smoker-specific inhalation conditions, i.e. puffing, mouthhold and bolus inhalation. The dynamic changes of inhaled e-cigarette droplets in the lungs due to coagulation, conductive heat and diffusive heat/convective vapor transport and particle phase chemistry are described by the Aerosol Dynamics in Containment (ADiC) model. For the simulation of the variability of inhaled particle and vapor deposition, the ADiC model is coupled with the IDEAL Monte Carlo code, which is based on a stochastic, asymmetric airway model of the human lung. We refer to the coupled model as "IDEAL/ADIC_v1.0". In this study, two different ecigarettes were compared, one without any acid ("no acid") and the other one with an acidic regulator (benzoic acid) to establish an initial pH level of about 7 ("lower pH"). Corresponding deposition patterns among human airways comprise total and compound-specific number and mass deposition fractions, distinguishing between inhalation and exhalation phases and condensed and vapor phases. Note that the inhaled EC aerosol is significantly modified in the oral cavity prior to inhalation into the lungs. Computed deposition fractions demonstrate that total particle mass is preferentially deposited in the alveolar region of the lung during inhalation. While nicotine deposits prevalently in the condensed phase for the "lower pH" case, vapor phase deposition is dominating the "no acid" case. The significant statistical fluctuations of the particle and vapor deposition patterns illustrate the inherent anatomical variability of the human lung structure.Peer reviewe

Aerosol mass yields of selected biogenic volatile organic compounds– a theoretical study with nearly explicit gas-phase chemistry

In this study we modeled secondary organic aerosol (SOA) mass loadings from the oxidation (by O-3, OH and NO3) of five representative biogenic volatile organic compounds (BVOCs): isoprene, endocyclic bond-containing monoterpenes (alpha-pinene and limonene), exocyclic double-bond compound (beta-pinene) and a sesquiterpene (beta-caryophyllene). The simulations were designed to replicate an idealized smog chamber and oxidative flow reactors (OFRs). The Master Chemical Mechanism (MCM) together with the peroxy radical autoxidation mechanism (PRAM) were used to simulate the gas-phase chemistry. The aim of this study was to compare the potency of MCM and MCM + PRAM in predicting SOA formation. SOA yields were in good agreement with experimental values for chamber simulations when MCM + PRAM was applied, while a stand-alone MCM underpredicted the SOA yields. Compared to experimental yields, the OFR simulations using MCM + PRAM yields were in good agreement for BVOCs oxidized by both O-3 and OH. On the other hand, a stand-alone MCM underpredicted the SOA mass yields. SOA yields increased with decreasing temperatures and NO concentrations and vice versa. This highlights the limitations posed when using fixed SOA yields in a majority of global and regional models. Few compounds that play a crucial role (> 95% of mass load) in contributing to SOA mass increase (using MCM + PRAM) are identified. The results further emphasized that incorporating PRAM in conjunction with MCM does improve SOA mass yield estimation.Peer reviewe

Positive feedback mechanism between biogenic volatile organic compounds and the methane lifetime in future climates

A multitude of biogeochemical feedback mechanisms govern the climate sensitivity of Earth in response to radiation balance perturbations. One feedback mechanism, which remained missing from most current Earth System Models applied to predict future climate change in IPCC AR6, is the impact of higher temperatures on the emissions of biogenic volatile organic compounds (BVOCs), and their subsequent effects on the hydroxyl radical (OH) concentrations. OH, in turn, is the main sink term for many gaseous compounds including methane, which is the second most important human-influenced greenhouse gas in terms of climate forcing. In this study, we investigate the impact of this feedback mechanism by applying two models, a one-dimensional chemistry-transport model, and a global chemistry-transport model. The results indicate that in a 6 K temperature increase scenario, the BVOC-OH-CH4 feedback increases the lifetime of methane by 11.4% locally over the boreal region when the temperature rise only affects chemical reaction rates, and not both, chemistry and BVOC emissions. This would lead to a local increase in radiative forcing through methane (Delta RFCH4) of approximately 0.013 Wm(-2) per year, which is 2.1% of the current Delta RFCH4. In the whole Northern hemisphere, we predict an increase in the concentration of methane by 0.024% per year comparing simulations with temperature increase only in the chemistry or temperature increase in chemistry and BVOC emissions. This equals approximately 7% of the annual growth rate of methane during the years 2008-2017 (6.6 +/- 0.3 ppb yr-1) and leads to an Delta RFCH4 of 1.9 mWm(-2) per year.Peer reviewe

A modelling study of OH, NO3 and H2SO4 in 2007– 2018 at SMEAR II, Finland : analysis of long-term trends

Major atmospheric oxidants (OH, O3 and NO3) dominate the atmospheric oxidation capacity, while H2SO4 is considered as a main driver for new particle formation. Although numerous studies have investigated the long-term trend of ozone in Europe, the trends of OH, NO3 and H2SO4 at specific sites are to a large extent unknown. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station and has been validated by measurements in several projects. Here, we applied the SOSAA model for the years 2007–2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants OH and NO3, as well as H2SO4 at SMEAR II. The simulations were evaluated with observations from several shorter and longer campaigns at SMEAR II. Our results show that daily OH increased by 2.39% per year and NO3 decreased by 3.41% per year, with different trends of these oxidants during day and night. On the contrary, daytime sulfuric acid concentrations decreased by 2.78% per year, which correlated with the observed decreasing concentration of newly formed particles in the size range of 3– 25 nm with 1.4% per year at SMEAR II during the years 1997–2012. Additionally, we compared our simulated OH, NO3 and H2SO4 concentrations with proxies, which are commonly applied in case a limited number of parameters are measured and no detailed model simulations are available.Peer reviewe

The role of highly oxygenated organic molecules in the Boreal aerosol-cloud-climate system

Over Boreal regions, monoterpenes emitted from the forest are the main precursors for secondary organic aerosol (SOA) formation and the primary driver of the growth of new aerosol particles to climatically important cloud condensation nuclei (CCN). Autoxidation of monoterpenes leads to rapid formation of Highly Oxygenated organic Molecules (HOM). We have developed the first model with near-explicit representation of atmospheric new particle formation (NPF) and HOM formation. The model can reproduce the observed NPF, HOM gas-phase composition and SOA formation over the Boreal forest. During the spring, HOM SOA formation increases the CCN concentration by similar to 10 % and causes a direct aerosol radiative forcing of -0.10 W/m(2). In contrast, NPF reduces the number of CCN at updraft velocities <0.2 m/s, and causes a direct aerosol radiative forcing of +0.15 W/m(2). Hence, while HOM SOA contributes to climate cooling, NPF can result in climate warming over the Boreal forest.Peer reviewe

Enhanced growth rate of atmospheric particles from sulfuric acid

In the present-day atmosphere, sulfuric acid is the most important vapour for aerosol particle formation and initial growth. However, the growth rates of nanoparticles (<10 nm) from sulfuric acid remain poorly measured. Therefore, the effect of stabilizing bases, the contribution of ions and the impact of attractive forces on molecular collisions are under debate. Here, we present precise growth rate measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed under atmospheric conditions in the CERN (European Organization for Nuclear Research) CLOUD chamber. Our results show that the evaporation of sulfuric acid particles above 2 nm is negligible, and growth proceeds kinetically even at low ammonia concentrations. The experimental growth rates exceed the hard-sphere kinetic limit for the condensation of sulfuric acid. We demonstrate that this results from van der Waals forces between the vapour molecules and particles and disentangle it from charge–dipole interactions. The magnitude of the enhancement depends on the assumed particle hydration and collision kinetics but is increasingly important at smaller sizes, resulting in a steep rise in the observed growth rates with decreasing size. Including the experimental results in a global model, we find that the enhanced growth rate of sulfuric acid particles increases the predicted particle number concentrations in the upper free troposphere by more than 50 %

Enhanced growth rate of atmospheric particles from sulfuric acid

In the present-day atmosphere, sulfuric acid is the most important vapour for aerosol particle formation and initial growth. However, the growth rates of nanoparticles (<10 nm) from sulfuric acid remain poorly measured. Therefore, the effect of stabilizing bases, the contribution of ions and the impact of attractive forces on molecular collisions are under debate. Here, we present precise growth rate measurements of uncharged sulfuric acid particles from 1.8 to 10 nm, performed under atmospheric conditions in the CERN (European Organization for Nuclear Research) CLOUD chamber. Our results show that the evaporation of sulfuric acid particles above 2 nm is negligible, and growth proceeds kinetically even at low ammonia concentrations. The experimental growth rates exceed the hard-sphere kinetic limit for the condensation of sulfuric acid. We demonstrate that this results from van derWaals forces between the vapour molecules and particles and disentangle it from charge-dipole interactions. The magnitude of the enhancement depends on the assumed particle hydration and collision kinetics but is increasingly important at smaller sizes, resulting in a steep rise in the observed growth rates with decreasing size. Including the experimental results in a global model, we find that the enhanced growth rate of sulfuric acid particles increases the predicted particle number concentrations in the upper free troposphere by more than 50 %.Peer reviewe

Simulation of cigarette smoke dynamics in denuder tubes considering particle phase chemistry

<p>The aerosol dynamics model ADiC was extended to include chemical reactions. It is used to computationally replicate denuder tube experiments where freshly generated cigarette smoke is drawn through a vertically arranged, acid covered tube to capture alkaline substances. The calculated deposition rates and total deposition are compared to experimental findings from several studies that investigated respective quantities for nicotine (and ammonia). Further, the form of deposition, vaporous and condensed phase, is considered. The model does not apply any parameters changing physico-chemical properties to fit simulation and experimental findings.</p> <p>The only variable parameter used in all simulations is the choice of the amount of acid initially in the system to establish a certain pH value. An initial pH of 5.9 to 6.25 (i.e. the baseline scenarios) allows for replicating the nicotine deposition rate and total deposition in the lower tube sections. For the same simulation, ammonia deposition rate and total deposition are of the order of the experimental data. For the simulation featuring the initially lower pH value, the deposition of ammonia is lower than the experimental data – in the other case it is higher. Increasing the molality of alkaline substances initially in the system by roughly 20% drastically reduces the differences between simulated and measured nicotine deposition rate.</p> <p>The present model describes some aspects of the dynamics of the complex cigarette smoke in a simplified manner; however, since it is independent of experiment specific parameters it may be applied to other environments such as deposition in the respiratory tract.</p