In this work, various aerosol particle transport and deposition mechanisms were studied through the computational fluid dynamics (CFD) modeling, including inertial impaction, gravitational effect, lift force, interception, and turbophoresis, within different practical applications including aerosol sampling inlet, filtration system and turbulent pipe flows. The objective of the research is to obtain a better understanding of the mechanisms that affect aerosol particle transport and deposition, and to determine the feasibility and accuracy of using commercial CFD tools in predicting performance of aerosol sampling devices. Flow field simulation was carried out first, and then followed by Lagrangian particle tracking to obtain the aerosol transport and deposition information. The CFD-based results were validated with experimental data and empirical correlations. In the simulation of the aerosol inlet, CFD-based penetration was in excellent agreement with experimental results, and the most significant regional particle deposition occurred due to inertial separation. At higher free wind speeds gravity had less effect on particle deposition. An empirical equation for efficiency prediction was developed considering inertial and gravitational effects, which will be useful for directing design of similar aerosol inlets. In the simulation of aerosol deposition on a screen, a "virtual surface" approach, which eliminates the need for the often-ambiguous user defined functions, was developed to account for particle deposition due to interception. The CFD-based results had a good agreement compared with experimental results, and also with published empirical correlations for interception. In the simulation of turbulent deposition in pipe flows, the relation between particle deposition velocity and wall-normal turbulent velocity fluctuation was quantitative determined for the first time, which could be used to quantify turbulent deposition, without having to carry out Lagrangian particle tracking. It suggested that the Reynolds stress model and large eddy simulation would lead to the most accurate simulated aerosol deposition velocity. The prerequisites were that the wall-adjacent y+ value was sufficiently low, and that sufficient number of prism layers was applied in the near-wall region. The "velocity fluctuation convergence" would be useful criterion for judging the adequacy of a CFD simulation for turbulent deposition.

A two -stage circumferential slot virtual impactor aerosol concentrator system has been developed that is designed for nominal operational conditions of a 2 μm AD cutpoint, an aerosol inflow to the first stage of 100 L/min and a minor flow rate from the second stage of 1 L/min. Each unit was tested separately before being combined in the system. However, because of high inter-stage losses, a sheath air system was inserted between the two stages, wherein a small amount of air was injected into the apex of a cone placed on top of the second stage. The sheath air displaced the stagnation point at the apex of the cone and redirected particles into the sampling zone of the second stage unit. The cutpoint particle size of the system was 2.5 μm AD at the nominal flow rate. The dynamic range (ratio of upper limit to the lower limit of aerodynamic particle diameter associated with transmission efficiencies of 50%) was 5.4, and the largest particle size for which the transmission was at least 50% is 13.6 μm AD. When run at 67 L/min, the cutpoint is 4 μm AD and the dynamic range is 3.75; at 150 L/min the cutpoint is 2.05 μm AD and the dynamic range is not less than 4.74. The pressure drop across the system is 685 Pa (2.75 in. H2O). This yields an ideal power consumption of 0.77 watts.

In this dissertation, a two-moment bulk microphysical scheme with aerosol effects is developed and implemented into the Weather Research and Forecasting (WRF) model to investigate the aerosol-cloud interaction. Sensitivities of cloud properties to the representation of aerosol size distributions are first evaluated using a simple box model and a cloud resolving model with a detailed spectral-bin microphysics, indicating that the three-moment method generally exhibits better performance in modeling cloud properties than the two-moment method against the sectional approach. A convective cloud event occurring on August 24, 2000 in Houston, Texas is investigated using the WRF model, and the simulation results are qualitatively in agreement with the measurements. Simulations with various aerosol profiles demonstrate that the response of precipitation to the increase of aerosol concentrations is non-monotonic. The maximal cloud cover, core updraft, and maximal vertical velocity exhibit similar responses as precipitation. The WRF model with the two-moment microphysical scheme successfully simulates the development of a squall line that occurred in the south plains of the U.S. Model experiments varying aerosol concentrations from the clean background case to the polluted continental case show that the aerosol concentrations insignificantly influence the rainfall pattern/distribution, but can remarkably alter the precipitation intensity. The WRF experiment with polluted aerosols predicts 12.8% more precipitation than that with clean aerosols, as well as more intensive rainfall locally. Using the monthly mean cloudiness from the International Satellite Cloud Climatology Project (ISCCP), a trend of increasing deep convective clouds over the north Pacific in winter from 1984 to 2005 is detected. Additionally, through analyzing the results from the Global Precipitation Climatology Project (GPCP) version 2, we also show a trend of increasing wintertime precipitation over the north Pacific from 1984 to 2005. Simulations with the WRF model reveal that the increased deep convective clouds and precipitation are reproduced when accounting for the aerosol effect from the increasing Asian pollution outflow.

Tests were conducted to characterize mixing in a square and a rectangular duct with respect to suitability for single point sampling of contaminants. Several configurations, such as a straight duct with unidirectional flow at the entrance section and straight ducts preceded by mixing elements (a 90° mitred bend, double 90° bends in S- and U-type configurations) were tested. For a straight duct of square cross section, the COV of tracer gas concentration at 19 duct diameters downstream of the gas release location is 143% (Center release). COVs of velocity and tracer gas concentration downstream of each mixing element in square duct setups were verified throughout this study. In the case of a rectangular duct with a 3:1 (width to height) aspect ratio, COVs of velocity and tracer gas concentration only downstream of a 90° mitred bend were verified. Tests were conducted to develop improved inlets for a Battelle bioaerosol sampling system. New inlets have been developed called the All Weather Inlets (AWI), which are designed to prevent entry of precipitation while maintaining aerosol penetration. The AWI has two inlets - one that samples at a flow rate of 780 L/min and the other one that is operated at a flow rate of 90 L/min. The initial version of the AWI-780 L/min unit featured an internal cone, which was removed because the penetration of the AWI-780 without the bottom chamber was higher than that of the Battelle inlet – 81% with the cone while 86% without the cone for around 9.5 µm AD at 2 km/h. The best bug-screen configuration was verified and a cutpoint management process was performed. The inlets were tested with different wind speeds from 2 to 24 km/h to verify the wind sensitivity of those inlets.

A Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) was used to measure size distributions, hygroscopicity, and volatility during the May 2003 Aerosol Intensive Operational Period at the Central Facility of the Atmospheric Radiation Measurement Southern Great Plains site. Hygroscopic growth factor distributions for particles at eight dry diameters ranging from 0.012 µm to 0.600 µm were measured. These measurements, along with backtrajectory clustering, were used to infer aerosol composition and evolution. The hygroscopic growth of the smallest and largest particles analyzed was typically less than that of particles with dry diameters of about 0.100 µm. Condensation of secondary organic aerosol on nucleation mode particles may be responsible for the minimal growth observed at the smallest sizes. Growth factor distributions of the largest particles typically contained a non-hygroscopic mode believed to be composed of dust. A model was developed to characterize the hygroscopic properties of particles within a size distribution mode through analysis of the fixed-size hygroscopic growth measurements. This model was used to examine three cases in which the sampled aerosol evolved over a period of hours or days. Additionally, size and hygroscopicity information were combined to model the aerosol as a population of multi-component particles. With this model, the aerosol hygroscopic growth factor f(RH), relating the submicron scattering at high RH to that at low RH, is predicted. The f(RH) values predicted when the hygroscopic fraction of the aerosol is assumed to be metastable agree better with measurements than do those predicted under the assumption of crystalline aerosol. Agreement decreases at RH greater than 65%. This multi-component aerosol model is used to derive cloud condensation nuclei (CCN) spectra for comparison with spectra measured directly with two Desert Research Institute (DRI) CCN spectrometers. Among the 1490 pairs of DMA/TDMA-predicted and DRI-measured CCN concentrations at various critical supersaturations from 0.02-1.05%, the sample number-weighted mean R2 value is 0.74. CCN concentrations are slightly overpredicted at both the lowest (0.02-0.04%) and highest (0.80-1.05%) supersaturations measured. Overall, this multi-component aerosol model based on size distributions and size-resolved hygroscopicity yields reasonable predictions of the humidity-dependent optical properties and CCN spectra of the aerosol.

No description available.

The studies in this dissertation aim at advancing our scientific understandings about physical processes involved in the aerosol-cloud-precipitation interaction and quantitatively assessing the impacts of aerosols on the cloud systems with diverse scales over the globe on the basis of the observational data analysis and various modeling studies. Long-term impacts of aerosols on precipitation and lightning over the Pearl River Delta megacity area in China are identified through the analysis of seven-year measurements of precipitation, lightning flashes, and visibility from 2000 to 2006. The cloud resolving - Weather Research and Forecasting (CR-WRF) model with a two- moment bulk microphysical scheme is employed to simulate a mesoscale convective system in the Guangzhou megacity area and to elucidate the effects of aerosols on cloud processes, precipitation, and lightning activity. The responses of hydrometeors and latent heat release to different aerosol loadings reveal the physical mechanism for the precipitation and lightning enhancement in the Guangzhou megacity area, showing more efficient mixed phase processes and intensified convection under the polluted aerosol condition. Sensitivity modeling experiments are performed for maritime warm stratocumulus clouds over the southeast Pacific Ocean to evaluate the microphysical parameterizations for simulations of the aerosol effects in regional and global climate models. The Morrison double-moment bulk microphysical scheme presently implemented in the WRF model is modified by replacing the fixed aerosols in the original bulk scheme with a prognostic double-moment aerosol representation to predict both aerosol number concentration and mass mixing ratio. The impacts of the parameterizations of diffusional growth and autoconversion of cloud droplets and the selection of the embryonic raindrop radius on the performance of the bulk microphysical scheme are also evaluated. The impacts of Asian pollution outflows on the Pacific storm track are assessed utilizing reanalysis data, a hierarchical modeling approach and the multi-scale aerosol- climate modeling frame. Statistical analysis of two sets of reanalysis data suggests a strengthened trend of the storm track intensity over the North Pacific since 1979. The two-month seasonal simulations using a CR-WRF model with a two-moment bulk microphysics are performed to examine the aerosol effects on the Pacific storm track intensity. Subsequently, the anomalies of the diabatic heating rate by the Asian pollution outflow derived from the CR-WRF simulations have been prescribed in the NACR Community Atmosphere Model (CAM5) to provide the aerosol forcing terms. The forced GCM well reproduces an enhancement in the intensity of storm track, compared to the unforced model simulations. Similarly, under the multi-scale aerosol-climate modeling frame, the comparisons of the simulated present day versus pre-industrial climate corresponding to two different aerosol scenarios indicate the increased precipitation and poleward heat transport for the present-day climate reveal invigorated mid-latitude cyclones. The current work illustrates the complexity of the aerosol effects on the cloud systems at the diverse scales with different meteorological conditions. This study also stresses the importance of accurate representation of aerosol forcings in the different types of atmospheric numerical models for future climate projections.

No description available.

No description available.

No description available.