A laboratory model of an instrument concept to measure the absorption of solar radiation by atmospheric aerosols was designed, built and tested. The concept was based on the photophonic phenomenon discovered by Bell and an acoustic resonator developed by Helmholtz. The design consisted of two chambers: an aerosol chamber and a reference chamber combined into a double Helmholtz resonator configuration. The radiation from the visible light source was amplitude modulated by a mechanical chopper. The modulated light beam was passed through the chambers and pressure variations resulted from energy absorbed by the aerosol in the chamber. The pressure signal was sensed by microphones, then the electrical signal amplified and processed by a differential amplifier. The testing showed the instrument had sufficient sensitivity and low enough system noise to measure an absorption coefficient of about 10⁻⁶/meter. Methods of signal improvement and noise reduction were discussed and tested. The results showed the instrument could measure absorption coefficients within the range expected by the earth's atmospheric aerosols. The instrument design was not optimized for maximum signal or minimum noise, but the justifiable conclusion was reached that the concept showed the promise of leading to a useful instrument in the measurement of atmospheric aerosol absorption and an improvement over the present instruments. / Master of Science

A particle counting instrument, the Coherent Optical Particle Spectrometer (COPS) has been developed for measuring particles in aerosol systems. It optically counts and sizes single particles one at a time as they pass through an optically defined inspection region so particle size distributions can be directly measured. COPS uses the coherent nature of light available in a laser beam to measure the phase shift in the scattered light, which is fundamentally different from previous intensity based techniques. The Van-Cittert-Zernike theorem shows that scattered light from small particles will be coherent if viewed upon at the focal point of a gathering lens. Optical homodyne detection can then be used to measure the extent of the phase shift due to the particle. Scattering mechanisms can relate the phase shift to particle diameter so particle size can be determined. An optical inspection region is given by the resolution limited blur spot diameter and depth of focus of the gathering lens. Particles scattering outside this zone will not contribute to measured phase signals. Calculations show that COPS can count in concentrations of 10('9) particles per cubic centimeter with 5% coincidence error. Mie scattering calculations, coupled with homodyne theory, predict a minimum detectable particle diameter ranging from 0.03 to 0.3 micrometers, depending on optical configuration. Theory shows that small, strongly absorbing particles impart a much larger phase shift than refractive particles so a lower detection limit is predicted for particles such as soot and silicon. Particles above one micrometer show classic resonance typical of Mie calculations. An experimental COPS system verified the predicted results from the model. Resolution of particle size ranged from 25 to 60 percent of particle diameter. Preliminary experiments showed that COPS has in situ sampling possibilities and will work for liquid systems as well. Coherent detection of scattered light shows promise for in situ measurement of submicrometer aerosols in high particle laden streams with maximum sensitivity for strongly absorbing particles.

MSc., Faculty of Science, University of the Witwatersrand, 2011 / Atmospheric aerosols have direct and indirect impacts on the earth’s radiation budget and the radiative forcing on the climate system. A large uncertainty exists regarding aerosols and the effect they have on the earth’s radiation budget and global change. The distribution, concentration and types of aerosols are therefore of great importance regarding global warming and climate change. The purpose of this study is to present the atmospheric aerosol characteristics found over the South Atlantic, Southern Ocean and Antarctic continent as well as identify their origin. The aerosol optical properties over the South Atlantic and Southern Ocean region is analysed during the South African National Antarctic Expedition 2007/2008 (SANAE 47) take over cruise on board the M/V S.A. Agulhas. Very low aerosol optical thickness (AOT) values were obtained for the Antarctic Coastal region with a mean AOT500nm of 0.03 and a mean Angstrom exponent of 1.78. The South Atlantic region showed a mean AOT500nm of 0.06 and a mean Angstrom exponent of 0.72. AOT values for the South African coastal region had a mean AOT500nm of 0.07 and a mean Angstrom exponent of 0.76. Data comparisons confirm that the data acquired during the study are consistent with previous research from the study region. Comparisons were made between the dataset and the MODIS satellite aerosol product. A discrepancy was shown to exist between the MODIS aerosol product and the acquired dataset using the Microtops II Sunphotometer. Both MODIS TERRA and AQUA overestimate AOT at 550nm.

Atmospheric aerosols are tiny particles suspended in the atmosphere. They affect global air quality, public health and climate (Boucher et al., 2013; Myhre et al., 2013; Seinfeld and Pandis, 2016), thus playing a key role in the Earth system. However, due to the complexity of aerosol processes and climate change feedbacks, our understanding of aerosols in a changing world is still limited (Boucher et al., 2013). To understand the impact of organic aerosol volatility on particle microphysics and global climate, I developed a new aerosol microphysics scheme, MATRIX-VBS, and its evaluation and application are presented in this dissertation. MATRIX-VBS couples the volatility-basis set (VBS, Donahue et al., 2006) framework with the aerosol microphysical scheme MATRIX (Multiconfiguration Aerosol TRacker of mIXing state, Bauer et al., 2008) that resolves aerosol mass and number concentrations, size, and mixing state. With the inclusion of organic partitioning and photochemical aging of semi-volatile organic aerosols, aerosols are able to grow via organic condensation, a process previously not available in the original model MATRIX, where organic aerosols were treated as nonvolatile. Both MATRIX and MATRIX-VBS can be used as stand-alone box models or within a global model. After the development of MATRIX-VBS in the box model framework, both model’s simulations were performed and assessed on the box and global scales. On the box model scale, idealized experiments were designed to simulate different environments, clean, polluted, urban, and rural. I investigated the evolution of organic aerosol mass concentration and volatility distribution among gas and aerosol phases, and results show that semi-volatile primary organic aerosols evaporate almost completely in the intermediate-volatility range and stay in the particle phase in the low volatility range. I also concluded that the volatility distribution of organics relies on emission, oxidation, and temperature, and the inclusion of organic aerosol volatility changes aerosol mixing state. Comparing against parallel simulations with the original model MATRIX, which treats organic aerosols as nonvolatile, I assessed the effect of gas-particle partitioning and photochemical aging of semi-volatile organics on particle growth, composition, size distribution and mixing state. Results also show that the new model produces different mixing states, increased number concentrations and decreased aerosol sizes for organic-containing aerosol populations. Monte-Carlo type experiments were performed and they offered a more in-depth look at the impact of organic aerosol volatility on activated number concentration, which is the number concentration of aerosols that are activated but has not yet formed into a cloud droplet. By testing multiple parameters such as aerosol composition, mass concentration and number concentration, as well as particle size, I examined the impact of partitioning organic aerosols on activated aerosol number concentration. I found that the new model MATRIX-VBS produces fewer activated particles compared to the original model MATRIX, except in environments with low cloud updrafts, in clean regions at above freezing temperatures, and in polluted areas at high temperature (310K) and extremely low humidity conditions. I concluded that such change is caused by the differences in aerosol number concentration and size between the two models, which would determine how many particles could activate. On the global scale, MATRIX-VBS was implemented in the NASA GISS ModelE Earth systems model. I assessed and evaluated the new model by comparing aerosol mass and number concentrations, activated cloud number concentration, and AOD against output from the original MATRIX model. Further, I evaluate the two models against observations of organic aerosol mass concentration from the aircraft campaign ATom (Atmospheric Tomography Mission), and aerosol optical depth from ground measurement stations from AERONET (Aerosol Robotic Network) as well as satellite retrievals from MODIS (MODerate resolution Imaging Spectroradiometer) and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations). Results show that organics in MATRIX-VBS experience more distant long-range transport, and their mass concentration increase aloft and decrease at the surface as compared to those in MATRIX. There are still underestimations in the vertical profiles of mass concentration in both models, especially in the high latitudes in the Northern Hemisphere and South Pacific Ocean basin, possibly due to the application of universal distribution of mass-based emission factors among different volatilities that perhaps is not realistic in all climate zones, thus affecting organic aerosol lifetime and transport. Just as the box model results, there are more particles and generally more activated ones (except for rare cases such as the highly polluted Eastern China) in MATRIX-VBS than in MATRIX. As for AOD comparisons, MATRIX-VBS have generally lower AOD than MATRIX, which can be due to smaller aerosols and different aerosol composition in the new model, which is also underestimating biomass burning in the Amazon and Congo basins. Compared to satellite retrievals from MODIS and ground measurements from AERONET, both models overestimate aerosol optical depth over anthropogenic polluted regions and biomass regions such as central Africa. Overall, both models also underestimate AOD as compared to AERONET in the winter (DJF), whereas they generally overestimate or estimate it well in other seasons. Even though during its initial evaluation, MATRIX-VBS does not seem to have improved from MATRIX on the global scale in representing the real world, it made the first key step in improving our understanding of organic aerosols on the process level. Changes in mass, number concentration, size distribution, and mixing state (composition) have great implications and impact on climate. Further studies are needed in examining and improving factors linked to the new representation of semi-volatiles in an aerosol microphysics model, including but not limited to the treatment of mass-based emission factor distribution among different organic volatilities and the size distribution of tiny organic particles that have evaporated but not completely. Challenges in evaluations of organic aerosol against measurements remain in that remote regions of significant interest lack available measurements, and additional field campaigns will be important for us to better understand real world conditions and shed light on model performance.

Understanding the composition of atmospheric organic particulate matter (OPM) is essential for predicting its effects on climate, air quality, and health. However, the polar oxygenated fraction (PO-OPM), which includes a significant mass contribution from carboxylic acids, is difficult to speciate and quantitatively determine by current analytical methods such as gas chromatography-mass spectrometry (GC-MS). The method of chemical derivatization and two-dimensional GC with time of flight MS (GC×GC/TOF-MS) was examined in this study for its efficacy in: 1) quantifying a high percentage of the total organic carbon (TOC) mass of a sample containing PO-OPM; 2) quantitatively determining PO-OPM components including carboxylic acids at atmospherically relevant concentrations; and 3) tentatively identifying PO-OPM components. Two derivatization reagent systems were used in this study: BF₃/butanol for the butylation of carboxylic acids, aldehydes, and acidic ketones, and BSTFA for the trimethylsilylation (TMS) of carboxylic acids and alcohols. Three α-pinene ozonolysis OPM filter samples and a set of background filter samples were collected by collaborators in a University of California, Riverside environmental chamber. Derivatization/GC×GC TOF-MS was used to tentatively identify some previously unidentified α-pinene ozonolysis products, and also to show the characteristics of all oxidation products determined. Derivatization efficiencies as measured were 40-70% for most butyl derivatives, and 50-58% for most trimethylsilyl derivatives. A thermal optical method was used to measure the TOC on each filter, and a value of the quantifiable TOC mass using a gas chromatograph was calculated for each sample using GC×GC separation and the mass-sensitive response of a flame ionization detector (FID). The TOC quantified using TMS and GC×GC-FID (TMS/TOCGC×GC FID) accounted for 15-23% of the TOC measured by the thermal-optical method. Using TMS and GC×GC/TOF-MS, 8.85% of the thermal optical TOC was measured and 48.2% of the TMS/TOCGC×GC-FID was semi-quantified using a surrogate standard. The carboxylic acids tentatively identified using TMS and GC×GC/TOF-MS accounted for 8.28% of the TOC measured by thermal optical means. GC×GC TOF-MS chromatograms of derivatized analytes showed reduced peak tailing due in part to the lesser interactions of the derivatized analytes with the stationary phase of the chromatography column as compared to the chromatograms of underivatized samples. The improved peak shape made possible the greater separation, quantification, and identification of high polarity analytes. Limits of detection using derivatization and GC×GC/TOF-MS were μL injected for a series of C2-C6 di-acids, cis-pinonic acid, and dodecanoic acid using both butylation and TMS. Derivatization with GC×GC/TOF-MS was therefore effective for determining polar oxygenated compounds at low concentrations, for determining specific oxidation products not previously identified in OPM, and also for characterizing the probable functional groups and structures of α-pinene ozonolysis products.

Scientists have been observing a change in the climate since the beginning of the 20th century that cannot be attributed to any of the natural influences of the past. Natural and anthropogenic substances and processes perturb the Earth's energy budget, contributing to climate change. In particular, aerosols (particles suspended in air) have long been recognized to be important in processes throughout the atmosphere that affect climate. They directly influence the radiative balance of the Earth's atmosphere, affect cloud formation and properties, and are also key air pollutants that contribute to a variety of respiratory and cardiovascular diseases. Despite their importance, aerosol particles are less well-characterized than greenhouse gases with respect to their sources, temporal and spatial concentration distribution, and physical and chemical properties. This uncertainty is mainly caused by the variable and insufficiently understood sources, formation and transformation processes, and complex composition of atmospheric particles. Instruments that can precisely and accurately measure and characterize the aerosol physical and chemical properties are in great demand. Atmospheric relative humidity (RH) has a crucial impact on the particles' optical properties; the RH dependence of the particle extinction coefficient is an important parameter for radiative forcing and thus climate change modeling. In this work a Humidity-Controlled Cavity Ring-Down (HC-CRD) aerosol optical instrument is described and its ability to measure RH dependent extinction coefficients and related hygroscopicity parameters is characterized. The HC-CRD is capable of simultaneously measuring the aerosol extinction coefficient at three wavelengths (λ = 355, 532, and 1064 nm) and three different RHs (typically 20%, 50%, and 80%). A range of chemicals and their mixtures were used to produce laboratory generated aerosols. Three mixture systems include one inorganic salts mixture system consisting of (NH4)2SO4, NH4HSO4, Na2SO4, NaHSO4 serve as surrogates of the ionic salts found in the atmosphere. Two organic mixture systems were investigated: mixtures of NaCl, D-glucose, sucrose, and glycine are benchmarks for compounds emitted from biomass burning. Finally, mixtures of (NH4)2SO4 (ammonium sulfate, AS) with a series of dicarboxylic acids including malonic acid, adipic acid, and azelaic acid are used as benchmarks to mimic urban pollutants. The extinction coefficients were obtained as a function of RH from the HC-CRD measurements, from which optical growth factors f(RH) and γ(RH) values can be determined to examine their dependence on chemical composition. A volume mixing rule was used to calculate the effective refractive index of the binary substrate mixtures, since both size and composition change during water uptake. The SDA/FMC algorithm developed by O'Neill, et al. 2005 is used to extract the van de Hulst phase shift parameter (Ρeff) from three-wavelength measurements of extinction. The fine mode fraction of extinction (η) and fine mode effective radius (Reff) of laboratory generated aerosol particles can be then determined. An iterative algorithm was developed to retrieve the change in refractive index of particles as function of RH. The calculated Reff of aerosols at different RHs were used to obtain the physical size growth factor (gf), and κ(RH). The size changes as a function of water uptake describe the dependence of aerosol optical properties on chemical composition. This work demonstrates the capability of conducting aerosol optical measurements using HC-CRD to determine the RH dependence of aerosol optical properties. The HC-CRD measurements combined with the SDA/FMC method to retrieve aerosol size for laboratory generated aerosols establish the connection between the optical properties and the aerosol particles' chemical compositions. It also underlines the importance and need for future investigation on the hygroscopic properties of atmospheric aerosols. This work is successfully developed a method that enables using the aerosols optical measurements to predict the compositions; it will greatly contribute to the atmospheric aerosol measurement and global climate modelling.

The problem of filtration of liquid aerosols by both wettable and nonwettable filters has been extensively studied and the results of the theoretical calculations together with the experimental results are presented. More realistic models of filtration by both wettable and nonwettable filters have been developed and verified experimentally. A new instrument has been developed, and used in the experiments, for the measurement of the absolute concentration of aerosols in the gas stream. This instrument is based on the measurement of the initial vapour content of the gas stream simultaneously with the measurement of the vapour content after the total evaporation of aerosol. The concentration of the aerosol is calculated as the difference between these two values. The instrument was developed to provide fast and accurate measurements of aerosol concentration. The main advantages of the instrument are: high accuracy, simplicity of measurement, possibility of use for a wide range of substances, perfect suitability of operation for automatic monitoring technologies, etc. All rights for this instrument have been reserved and the fully automatic version will be available in the near future. It was found that the efficiency of filtration of aerosol on the wettable filter depends on the thickness of the liquid film on the fibre. This parameter was taken into account in the development of a theoretical model of filtration on wettable fibrous filters. The particle breakthrough problem has been solved by the optimisation of the aspect ratio (the ratio of the height by width) of the wettable filter. On this basis, industrial devices have been developed, patented, and implemented in industry. These devices provide a stable operating efficiency of higher than 99%. It was found experimentally that the efficiency of filtration of aerosol on the nonwettable filter depends on the diameter of the drop suspended on the filter, and on the area of the filter blocked by drops: this influences the velocity of filtration. All these parameters were taken into account in the development of a theoretical model of filtration on nonwettable fibrous filters. On the basis of this model, satisfactorily verified by the experiments, an industrial device has been developed. The harnessing of atomisers makes it possible to maintain the efficiency of filtration higher than 99%, even with a relatively high velocity of filtration of 2.7m/s. The new technology is tackling the problem of handling huge amounts of exhaust gases and this is particularly important for cramped installations when the space available for the air pollution control technology is quite limited. A highly efficient gas cleaning technology has been developed. This technology is based on combining two stages (wet scrubber and filter) of currently utilised air pollution control devices by submerging the fibrous filter into the liquid on the plate. The new device provides an effective division of the main gas stream into ultra-small bubbles which increase the contact area between the gas and liquid phases. It was estimated theoretically and verified experimentally that the efficiency of the proposed 'combined' technology, is 45% higher than the efficiency of the two stages technology. The technology has been patented and will be offered for industrial implementation in the near future.

The Indian Ocean Experiment (INDOEX) had as a primary objective determining the radiative forcing due to anthropogenic aerosols over climatologically significant space and time scales: the Indian Ocean during the winter monsoon, January-March. During the winter monsoon, polluted, low-level air from the Asian subcontinent blows over the Arabian Sea and Indian Ocean. As part of INDOEX, aerosol optical depths were derived from Advanced Very High Resolution Radiometer (AVHRR) data for the cloud-free ocean regions. The AVHRR radiances were first calibrated using the interior zone of the Antarctic and Greenland ice sheets, which proved to be radiometrically stable calibration targets. Optical depths were derived by matching the observed radiances to radiances calculated for a wide range of optical depths and viewing geometry. Optical depths derived with the AVHRR were compared with those derived with NASA's Aerosol Robotic Network (AERONET) CIMEL instrument at the Center for Clouds, Chemistry, and Climate's Kaashidhoo Observatory, as well as with other surface and shipboard observations taken in the INDOEX region. The retrieved and surface-based optical depths agreed best for a new 2-channel, 2- aerosol model scheme in which the AVHRR observations at O·64 and O·84 microns were used to determine relative amounts of marine and polluted continental aerosol and then the resulting aerosol mixture was used to derive the optical depths. Broadband radiative transfer calculations for the mixture of marine and polluted continental aerosols were combined with the 0·64 and 0·84-micron AVHRR radiances to determine the radiative forcing due to aerosols in the INDOEX region. Monthly composites of aerosol optical depth and top of the atmosphere, surface, and atmospheric radiative forcing were derived from calibrated AVHRR radiances for January-March 1996-2000. An inter-annual variability in the magnitude and spatial extent of high value regions is noted for derived optical depths and radiative forcing, with highest values reached in 1999, particularly in the Bay of Bengal which during the IFP was covered by plumes from Indochina. Frequency distributions of the optical depth for 1⁰ x 1⁰ latitude-longitude regions are well represented by gamma distribution functions. The day-to-day and year-to-year variability of the optical depth for such regions is correlated with the long term average optical depth. Interannual variability of the monthly mean optical depths for such regions is found to be as large as the day to day. / Graduation date: 2001

Cloud reflectivity is a function of cloud liquid water content and droplet number concentration. Since cloud droplets form around pre-existing aerosol particles, cloud droplet number concentration depends on the availability of particles that can serve as cloud condensation nuclei. Given constant liquid water amount, increased availability of cloud condensation nuclei leads to clouds with a greater droplet number concentration, greater total droplet surface area and consequently, greater reflectivity. The change in cloud reflectivity resulting from the increased availability of condensation nuclei is known as the aerosol indirect effect. The aerosol indirect effect ranks as one of the largest sources of uncertainty in current estimates of global climate change, largely due to difficulties in measurement. Changes in cloud reflectivity resulting from the aerosol indirect effect are typically much smaller than the natural background variability observed in clouds. As a result, the modification signal is very difficult to detect against the background noise. Additionally, since atmospheric aerosols are ubiquitous, it is difficult to find polluted and nonpolluted clouds that are sufficiently alike for reasonable comparison. However, ship tracks seen in satellite images present one opportunity to study the aerosol indirect effect in relative isolation. Ship tracks are regions of enhanced reflectivity in marine stratus, resulting from the addition of aerosols from ship exhaust plumes to preexisting clouds. Ship tracks are a common feature of satellite images of the North Pacific. Since the marine atmosphere has comparatively low background aerosol concentrations, the addition of ship exhaust particles can lead to distinct increases in cloud reflectivity. Ship tracks allow for sampling of polluted and nonpolluted clouds from adjacent regions with similar solar and viewing geometry, cloud temperatures and surface properties, and consequently provide a unique opportunity to study the effects of aerosol modification of cloud reflectivity. Using satellite images of the North Pacific in July 1999, over 1000 ship tracks were identified, logged and analyzed, yielding 504 sets of radiance data matching polluted clouds with nearby nonpolluted clouds. It was expected that increasing the size of the region for selection of nonpolluted clouds would increase the variability in observed reflectivity, and make detection of the modification signal more difficult. In order to study this potential effect of domain size for selection of nonpolluted clouds on measurements of the aerosol indirect effect, three data sets were collected, using domain sizes for selection of nonpolluted clouds of 15, 50 and 100 km. Analysis of retrieved optical depth and droplet effective radius for modified and control pixels shows evidence of a 1-5% increase in visible optical depth of marine stratus following modification by addition of ship exhaust particles, but unexpectedly, shows only slight increases in uncertainty with increasing domain size. A subsequent study revealed that autocorrelation lengths of radiances and retrieved cloud properties were only 8-15 km. This indicates that even the 15 km control domain captured much of the background variability present. Domain sizes smaller than 15 km are difficult to sample automatically while avoiding the inclusion of polluted clouds in the nonpolluted cloud sample. As a result, it remains necessary to analyze large numbers of ship tracks to separate the aerosol modification signal from the background variability. / Graduation date: 2003

Atmospheric particulate matter pollution is a challenging environmental concern in both urban and remote locations worldwide. It is intrinsically difficult to control, given numerous anthropogenic and natural sources (e.g. fossil fuel combustion, biomass burning, dust, and seaspray) and atmospheric transport up to thousands of kilometers after production. In urban regions, fine particulate matter (particles with diameters under 2.5 m) is of special concern for its ability to penetrate the human respiratory system and threaten cardiopulmonary health. A second major impact area is climate, with particulate matter altering Earth s radiative balance through scattering and absorbing solar radiation, modifying cloud properties, and reducing surface reflectivity after deposition in snow-covered regions. While atmospheric particulate matter has been generally well-characterized in populated areas of developed countries, particulate pollution in developing nations and remote regions is relatively unexplored. This thesis characterizes atmospheric particulate matter in locations that represent the extreme ends of the spectrum in terms of air pollution the rapidly-developing and heavily populated Pearl River Delta Region of China, the pristine and climate-sensitive Greenland Ice Sheet, and a remote site in the Colorado Rocky Mountains. In China, fine particles were studied through a year-long field campaign at seven sites surrounding the Pearl River Delta. Fine particulate matter was analyzed for chemical composition, regional variation, and meteorological impacts. On the Greenland Ice Sheet and in the Colorado Rocky Mountains, the carbonaceous fraction (organic and elemental carbon) of particulate matter was studied in the atmosphere and snow pack. Analyses included quantifying particulate chemical and optical properties, assessing atmospheric transport, and evaluating post-depositional processing of carbonaceous species in snow.