Characteristics-based sectional modeling of aerosol nucleation, condensation and transport

Aerosols can be generated by physical processes such as nucleation, conden- sation and coalescence. To predict spatially varying statistical properties of such aerosols, e.g., the size distribution of the droplets, these processes must be captured accurately. We model nucleation using classical nucleation theory, whereas the con- densational growth is captured with a molecular diffusivity model. The droplet size distribution is discretized using a sectional approach, in which droplets are charac- terized in terms of a number of fixed droplet size bins. Often, in such a formula- tion, the numerical time step restrictions arising from condensation and nucleation are more pronounced than those of the corresponding fluid flow, thereby signifi- cantly limiting the global time step size. We propose a moment-conserving method in which this limitation is avoided, by utilizing the analytical solutions of the spa- tially homogeneous nucleation-condensation subproblem. The method is validated against experimental and numerical data of a laminar flow diffusion chamber, and shows an excellent agreement while being restricted only by a flow-related time step criterion

Dominant aerosol processes during high-pollution episodes over Greater Tokyo

This paper studies two high-pollution episodes over Greater Tokyo: 9 and 10 December 1999, and 31 July and 1 August 2001. Results obtained with the chemistry-transport model (CTM) Polair3D are compared to measurements of inorganic PM2.5. To understand to which extent the aerosol processes modeled in Polair3D impact simulated inorganic PM2.5, Polair3D is run with different options in the aerosol module, e.g. with/without heterogeneous reactions. To quantify the impact of processes outside the aerosol module, simulations are also done with another CTM (CMAQ). In the winter episode, sulfate is mostly impacted by condensation, coagulation, long-range transport, and deposition to a lesser extent. In the summer episode, the effect of long-range transport largely dominates. The impact of condensation/evaporation is dominant for ammonium, nitrate and chloride in both episodes. However, the impact of the thermodynamic equilibrium assumption is limited. The impact of heterogeneous reactions is large for nitrate and ammonium, and taking heterogeneous reactions into account appears to be crucial in predicting the peaks of nitrate and ammonium. The impact of deposition is the same for all inorganic PM2.5. It is small compared to the impact of other processes although it is not negligible. The impact of nucleation is negligible in the summer episode, and small in the winter episode. The impact of coagulation is larger in the winter episode than in the summer episode, because the number of small particles is higher in the winter episode as a consequence of nucleation.Comment: Journal of Geophysical Research D: Atmospheres (15/05/2007) in pres

Explaining global surface aerosol number concentrations in terms of primary emissions and particle formation

We use observations of total particle number concentration at 36 worldwide sites and a global aerosol model to quantify the primary and secondary sources of particle number. We show that emissions of primary particles can reasonably reproduce the spatial pattern of observed condensation nuclei (CN) (R2=0.51) but fail to explain the observed seasonal cycle at many sites (R2=0.1). The modeled CN concentration in the free troposphere is biased low (normalised mean bias, NMB=−88%) unless a secondary source of particles is included, for example from binary homogeneous nucleation of sulfuric acid and water (NMB=−25%). Simulated CN concentrations in the continental boundary layer (BL) are also biased low (NMB=−74%) unless the number emission of anthropogenic primary particles is increased or an empirical BL particle formation mechanism based on sulfuric acid is used. We find that the seasonal CN cycle observed at continental BL sites is better simulated by including a BL particle formation mechanism (R2=0.3) than by increasing the number emission from primary anthropogenic sources (R2=0.18). Using sensitivity tests we derive optimum rate coefficients for this nucleation mechanism, which agree with values derived from detailed case studies at individual sites

Solution of the general dynamic equation along approximate fluid trajectories generated by the method of moments

We consider condensing flow with droplets that nucleate and grow, but do not slip with respect to the surrounding gas phase. To compute the local droplet size distribution, one could solve the general dynamic equation and the fluid dynamics equations simultaneously. To reduce the overall computational effort of this procedure by roughly an order of magnitude, we propose an alternative procedure, in which the general dynamic equation is initially replaced by moment equations complemented with a closure assumption. The key notion is that the flow field obtained from this so-called method of moments, i.e., solving the moment equations and the fluid dynamics equations simultaneously, approximately accommodates the thermodynamic effects of condensation. Instead of estimating the droplet size distribution from the obtained moments by making assumptions about its shape, we subsequently solve the exact general dynamic equation along a number of selected fluid trajectories, keeping the flow field fixed. This alternative procedure leads to fairly accurate size distribution estimates at low cost, and it eliminates the need for assumptions on the distribution shape. Furthermore, it leads to the exact size distribution whenever the closure of the moment equations is exact

Multicomponent aerosol dynamics model UHMA: model development and validation

A size-segregated aerosol dynamics model UHMA (University of Helsinki Multicomponent Aerosol model) was developed for studies of multicomponent tropospheric aerosol particles. The model includes major aerosol microphysical processes in the atmosphere with a focus on new particle formation and growth; thus it incorporates particle coagulation and multicomponent condensation, applying a revised treatment of condensation flux onto free molecular regime particles and the activation of nanosized clusters by organic vapours (Nano-Köhler theory), as well as recent parameterizations for binary H₂SO₄-H₂O and ternary H₂SO₄-NH₃-H₂O homogeneous nucleation and dry deposition. The representation of particle size distribution can be chosen from three sectional methods: the hybrid method, the moving center method, and the retracking method in which moving sections are retracked to a fixed grid after a certain time interval. All these methods can treat particle emissions and atmospheric transport consistently, and are therefore suitable for use in large scale atmospheric models. In a test simulation against an accurate high resolution solution, all the methods showed reasonable treatment of new particle formation with 20 size sections although the hybrid and the retracking methods suffered from artificial widening of the distribution. The moving center approach, on the other hand, showed extra dents in the particle size distribution and failed to predict the onset of detectable particle formation. In a separate test simulation of an observed nucleation event, the model captured the key qualitative behaviour of the system well. Furthermore, its prediction of the organic volume fraction in newly formed particles, suggesting values as high as 0.5 for 3–4 nm particles and approximately 0.8 for 10 nm particles, agrees with recent indirect composition measurements

Simulation and control of nanoparticle size distribution in a high temperature reactor

This work focuses on the modeling, simulation and control of particle size distribution (PSD) during nanoparticle growth with the simultaneous chemical reaction, nucleation, condensation, coagulation and convective transport in a high temperature reactor. Firstly, a model known as population balance model was derived. This model describes the formation of particles via nucleation and growth. Mass and energy balances in the reactor were presented in order to study the effect of particle size distribution for each reaction mechanisms on the reactor dynamics, as well as the evolution of the concentrations of species and temperature of the continuous phase. The models were simulated to see whether the reduced population balance can be used to control the particle size distribution in the high temperature reactor. The simulation results from the above model demonstrated that the reduced population balance can be effectively used to control the PSD. The proposed method “which is the application of reduced population balance model” shows that there is some dependence of the average particle diameter on the wall temperature and the model can thus be used as a basis to synthesize a feedback controller where the manipulated variable is the wall temperature of the reactor and the control variable is the average particle diameter at the outlet of the reactor. The infl uence of disturbances on the average particle diameter was investigated and controlled to its new desired set point which is 1400nm using the proportional-integral-derivative controllers (PID controllers). The proposed model was used to control nanoparticle size distribution at the outlet of the reactor

Numerical Study on Mechanism of Nanoparticle Formation in High Temperature Reactor

Properties of nanoparticles are size-dependent, therefore nucleation and growth of particle is often difficult. The population balance model has been developed to study the mechanism of nanoparticles formation in high temperature reactor. Processes responsible for particle formation and growth are considered by homogeneous nucleation, condensation and Brownian coagulation. Parameters such as temperature, residence time and reactant concentrations influencing particle size are investigated. The population balance model is the dynamic model that describes the evolution of the aerosol size distribution with time. The method of moments was used to solve the dynamic equation under the assumption of log-normal size distribution. The model was validated with existing model

Importance of composition and hygroscopicity of BC particles to the effect of BC mitigation on cloud properties: Application to California conditions

Black carbon (BC) has many effects on climate including the direct effect on atmospheric absorption, indirect and semi-direct effects on clouds, snow effects, and others. While most of these are positive (warming), the first indirect effect is negative and quantifying its magnitude in addition to other BC feedbacks is important for supporting policies that mitigate BC. We use the detailed aerosol chemistry parcel model of Russell and Seinfeld (1998), observationally constrained by initial measured aerosol concentrations from five California sites, to provide simulated cloud drop number (CDN) concentrations against which two GCM calculations – one run at the global scale and one nested from the global-to-regional scale are compared. The GCM results reflect the combined effects of their emission inventories, advection schemes, and cloud parameterizations. BC-type particles contributed between 16 and 20% of cloud droplets at all sites even in the presence of more hygroscopic particles. While this chemically detailed parcel model result is based on simplified cloud dynamics and does not consider semi-direct or cloud absorption effects, the cloud drop number concentrations are similar to the simulations of both Chen et al. (2010b) and Jacobson (2010) for the average cloud conditions in California. Reducing BC particle concentration by 50% decreased the cloud droplet concentration by between 6% and 9% resulting in the formation of fewer, larger cloud droplets that correspond to a lower cloud albedo. This trend is similar to Chen et al. (2010b) and Jacobson (2010) when BC particles were modeled as hygroscopic. This reduction in CDN in California due to the decrease in activated BC particles supports the concern raised by Chen et al. (2010a) that the cloud albedo effect of BC particles has a cooling effect that partially offsets the direct forcing reduction if other warming effects of BC on clouds are unchanged. These results suggests that for regions like the California sites studied here, where BC mitigation targets fossil fuel sources, a critical aspect of the modeled reduction is the chemical composition and associated hygroscopicity of the BC particles removed as well as their relative contribution to the atmospheric particle concentrations

Modélisation CFD de la formation de nanoparticules dans un réacteur de synthèse à plasma

La production de particules dans des réacteurs à plasma thermique revêt un grand intérêt dans la synthèse de poudres. Une vaste gamme de poudres céramiques et métalliques a été pro­ duite en utilisant des réacteurs à plasma. La taille de ces poudres est habituellement de l'ordre du micromètre et parfois au dessous de 0.1 micromètre. Il y a un très grand intérêt pour les méthodes de production de poudres avec une taille moyenne plus petite que 0.1 micromètre. Ces poudres peuvent être utilisées pour créer des matériaux nanophasiques. L'utilisation des réacteurs à plasma thermique permet d'obtenir un taux de génération élevé et des produits de bonne qualité. Les réactifs peuvent être injectés dans le réacteur plasma sous forme de poudres, de liquide pulvérisé ou de gaz. Si les réactifs sont injectés à l'état solide ou liquide, ils complètent leur évaporation dans la zone chaude du plasma. Les espèces gazeuses sont par la suite considérablement dissociées. Le gaz est ensuite refroidi en quittant la zone chaude. Finalement la nucléation et la croissance de particules auront lieu. Cette étude présente la simulation de la formation de poudres métalliques dans un réacteur plasma à induction couplée (ICP). Un gaz porteur transporte les poudres métalliques dans le réacteur où elles sont transformées en gaz. Les produits sont récupérés après un procédé de refroidissement. Le modèle considère la formation de particules par nucléation et la croissance par condensation et coagulation brownienne. Le transport des particules est du à la convection, à la thermophorèse et à la diffusion brownienne. La diffusion axiale et radiale des particules a été considérée. Le code commercial de CFD(Computational Fluids Dyanmics) FLUENT© 6.0 a été utilisé pour les calculs de la mécanique des fluides et pour la croissance des particules. Ce code est couplé avec le modèle mathématique pour la dynamique des aérosols en utilisant les trois premiers moments de la distribution de tailles des particules. La simulation est appliquée à la production de poudres de fer ultrafines. Les résultats montrent le début de la formation de particules dans le réacteur et l'évolution de la taille des particules. Les champs des propriétés macroscopiques de la population d'aérosol et la contribution des différents mécanismes (nucléation, condensation, coagulation) sont analysés pour différentes combinaisons des paramètres d'opération. Une partie des résultats sont partiellement validés avec ceux de Bilodeau.Abstract: The present work reports on the simulation of metal powder formation in a Inductively Coupled Plasma reactor. A carrier gas transports the metal powder into the reactor in where it attains the vapor phase. The products are recovered after a quenching process. The model accounts for particle formation by nucleation and growth by condensation and brownian coagulation. Transport of particles occurs by convection, thermophoresis, and brownian diffusion. Axial and radial diffusion of particles are considered. The commercially computational fluid dynamics code FLUENT Ã 6.0, is used for the detailed calculation of the turbulent fluid flow and particle growth. This code is then completed with a model for aerosol dynamics using the first three moments of the particle size distribution. The simulation is applied for the production of ultrafine iron powders. The fields of the macroscopic properties of the aerosol population and the contribution of the different mechanisms are analyzed under various conditions. The effect of different operating parameters on the properties of the powder generated is studied."--Résumé abrégé par UMI

On the representation of aerosol-cloud interactions in atmospheric models

Anthropogenic atmospheric aerosols (suspended particulate matter) can modify the radiative balance (and climate) of the Earth by altering the properties and global distribution of clouds. Current climate models however cannot adequately account for many important aspects of these aerosol-cloud interactions, ultimately leading to a large uncertainty in the estimation of the magnitude of the effect of aerosols on climate. This thesis focuses on the development of physically-based descriptions of aerosol-cloud processes in climate models that help to address some of such predictive uncertainty. It includes the formulation of a new analytical parameterization for the formation of ice clouds, and the inclusion of the effects of mixing and kinetic limitations in existing liquid cloud parameterizations. The parameterizations are analytical solutions to the cloud ice and water particle nucleation problem, developed within a framework that considers the mass and energy balances associated with the freezing and droplet activation of aerosol particles. The new frameworks explicitly account for the impact of cloud formation dynamics, the aerosol size and composition, and the dominant freezing mechanism (homogeneous vs. heterogeneous) on the ice crystal and droplet concentration and size distribution. Application of the new parameterizations is demonstrated in the NASA Global Modeling Initiative atmospheric and chemical and transport model to study the effect of aerosol emissions on the global distribution of ice crystal concentration, and, the effect of entrainment during cloud droplet activation on the global cloud radiative properties. The ice cloud formation framework is also used within a parcel ensemble model to understand the microphysical structure of cirrus clouds at very low temperature. The frameworks developed in this work provide an efficient, yet rigorous, representation of cloud formation processes from precursor aerosol. They are suitable for the study of the effect of anthropogenic aerosol emissions on cloud formation, and can contribute to the improvement of the predictive ability of atmospheric models and to the understanding of the impact of human activities on climate.Ph.D.Committee Chair: Athanasios Nenes; Committee Member: Amyn S. Teja; Committee Member: Irina Sokolik; Committee Member: Judith A. Curry; Committee Member: Martha A. Grove