Drop size-dependent chemical composition in clouds and fogs

December 2001.Also issued as author's dissertation (Ph.D.) -- Colorado State University, 2002.Includes bibliographical references.Cloud drop composition varies as function of drop size. More sophisticated atmospheric chemistry models predict this and observations at many locations around the world by multiple techniques confirm this. This variation can influence the cloud processing of atmospheric species. Aqueous-phase reaction and atmospheric removal rates for scavenged species, among other processes, can be affected by drop size-dependent composition. Inferences to these processes drawn upon single bulk cloud composition measurements can be misleading according to observations obtained using cloud water collectors that separate drops into two or more size­ resolved fractions. Improved measurements of size-dependent drop composition are needed to further examine these and related issues. Two active multi-stage cloud water collectors were developed for sampling super-cooled drops in mixed-phase clouds and warm cloud drops, respectively. Both use the principles of cascade inertial impaction to separate drops into three fractions (super-cooled drop collector) and five fractions (warm cloud drop collector). While calibration suggests there is more drop overlap between stages than desired, consistently different drop fractions are still collected. FROSTY - the super-cooled drop collector - has been used successfully to obtain size-resolved drop composition information during two field campaigns in Colorado. While the data are limited, FROSTY's field performance appears to be reasonably consistent during individual cloud events, although not predictable based solely upon its collection efficiency curves. Additional factors must be considering in evaluating its performance in future campaigns. Nevertheless, the ability to obtain consistent size-resolved drop composition information from super-cooled clouds was not previously possible. Field data indicate that the warm cloud collector - the CSU 5-Stage - is able to resolve variations in the drop size-dependent composition not discernible with the two-stage size-fractionating Caltech Active Strand Cloud water Collector (sf-CASCC). Field performance evaluations suggest that the 5-Stage and the sf-CASCC compare well to each other for the range of sampling conditions experienced. Both collectors' performances differ from measurements made by the Caltech Active Strand Cloud water Collector #2 (CASCC2) in some specific sampling conditions, but otherwise agreement between the three collectors is good. Where the sf-CASCC indicates little drop variation in an orographic cloud study at Whiteface Mtn., NY, the 5-Stage indicates up to a factor of two difference may exist between the maximum and minimum drop concentrations for the major inorganic ions (ammonium, nitrate and sulfate). The sf-CASCC data suggest that typically a factor of 3 - 5 difference exists between large and small drop species' concentrations in radiation fogs measured in Davis, CA Concurrent 5-Stage samples suggest the actual variation may be up to at least a factor of 4 - 5 greater, and that the smallest drops (approximately < 11 µmin diameter) are principally responsible for the strong observed concentration gradients between sizes. While the data are limited, the 5-Stage's results are consistent for all of the sample sets obtained during both field campaigns. Data from the 5-Stage emphasize that cloud drop chemical composition cannot be considered separately from the sampled cloud's microphysics and dynamics. Interpreting the 5-Stage's results necessarily draws upon both. During the Davis campaign, additional measurements were performed to investigate species removal from the atmosphere via drop deposition and gas/liquid partitioning in-fog. Although subject to confounding effects, these investigations benefited from the additional insight 5-Stage data provided into the processes occurring. In particular, 5-Stage data and between-fog aerosol measurements suggest that deposition of the largest fog drops resulted in the relative removal of coarse mode aerosol particles from the atmosphere. 5-Stage data and gas-phase measurements suggest the ammonia/ammonium system may not be at equilibrium and provide some information about the nitrous acid/nitrite system not otherwise available. The 5-Stage has the potential to be a valuable tool in investigating the effects of fog and fog processing on the fate of ambient species.Sponsored by USEPA under grant NCERQA R82-3979-010; STAR Fellowship U-915364; NSF under grants ATM-9509596, ATM-9712603, and ATM-9980540; and the San Joaquin Valleywide Air Pollution Study Agency

Exploring constrained quantum control landscapes

The broad success of optimally controlling quantum systems with external fields has been attributed to the favorable topology of the underlying control landscape, where the landscape is the physical observable as a function of the controls. The control landscape can be shown to contain no suboptimal trapping extrema upon satisfaction of reasonable physical assumptions, but this topological analysis does not hold when significant constraints are placed on the control resources. This work employs simulations to explore the topology and features of the control landscape for pure-state population transfer with a constrained class of control fields. The fields are parameterized in terms of a set of uniformly spaced spectral frequencies, with the associated phases acting as the controls. Optimization results reveal that the minimum number of phase controls necessary to assure a high yield in the target state has a special dependence on the number of accessible energy levels in the quantum system, revealed from an analysis of the first- and second-order variation of the yield with respect to the controls. When an insufficient number of controls and/or a weak control fluence are employed, trapping extrema and saddle points are observed on the landscape. When the control resources are sufficiently flexible, solutions producing the globally maximal yield are found to form connected `level sets' of continuously variable control fields that preserve the yield. These optimal yield level sets are found to shrink to isolated points on the top of the landscape as the control field fluence is decreased, and further reduction of the fluence turns these points into suboptimal trapping extrema on the landscape. Although constrained control fields can come in many forms beyond the cases explored here, the behavior found in this paper is illustrative of the impacts that constraints can introduce.Comment: 10 figure

Laboratory Transferability of Optimally Shaped Laser Pulses for Quantum Control

Optimal control experiments can readily identify effective shaped laser pulses, or "photonic reagents", that achieve a wide variety of objectives. For many practical applications, an important criterion is that a particular photonic reagent prescription still produce a good, if not optimal, target objective yield when transferred to a different system or laboratory, {even if the same shaped pulse profile cannot be reproduced exactly. As a specific example, we assess the potential for transferring optimal photonic reagents for the objective of optimizing a ratio of photoproduct ions from a family of halomethanes through three related experiments.} First, applying the same set of photonic reagents with systematically varying second- and third-order chirp on both laser systems generated similar shapes of the associated control landscape (i.e., relation between the objective yield and the variables describing the photonic reagents). Second, optimal photonic reagents obtained from the first laser system were found to still produce near optimal yields on the second laser system. Third, transferring a collection of photonic reagents optimized on the first laser system to the second laser system reproduced systematic trends in photoproduct yields upon interaction with the homologous chemical family. Despite inherent differences between the two systems, successful and robust transfer of photonic reagents is demonstrated in the above three circumstances. The ability to transfer photonic reagents from one laser system to another is analogous to well-established utilitarian operating procedures with traditional chemical reagents. The practical implications of the present results for experimental quantum control are discussed

Preparation and characterisation of mixed CeO2-Nb2O5-Bi2O3 nanoparticles

Mixed metal oxides are ionic compounds containing at least two metal ions within an oxide structure. The literature contains a plethora of examples of mixed metal oxides on the bulk scale, which have been well characterised, however, mixed metal oxides on the nanoscale are far less well understood. The work presented here investigates the Bi2O3-CeO2-Nb2O5 mixed oxide system and characterises the resulting nanoparticles and crystal structures. Although the parent oxides are well known and much work has previously been done in analysing their crystal structures, combinations of these oxides have not been well characterised, especially on the nanoscale. Using high resolution electron microscopy (HRTEM), powder X-ray diffraction (PXRD), electron dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) as analytical tools, the structures of the nanoparticles in this system have been explored. As each of the parent oxides possess useful properties, which have been utilised in industrial applications such as electrolyte components in solid oxide fuel cells and as catalysts in a range of chemical reactions, it was hypothesised that if all three metal ions could be contained in one particle they could show novel and interesting characteristics. It was proposed that due to the more relaxed crystal structure in nanoparticles, the solid solubility of the metal ions should be increased, and a solid solution of ions would form. This work presents results showing the synthesis of binary and ternary oxides in the nano-form within the Bi2O3-CeO2-Nb2O5 system, including quantitative analysis of these particles. Secondly, and most importantly, it presents the first successful synthesis of quaternary oxide nanoparticles containing bismuth, cerium and niobium using the low temperature resin-gel method. Finally, the work attempts to explain how and why the ions are ordered in a given arrangement, with bismuth showing a preference for surface site occupation, as shown by XPS data, and describes some preliminary computational results which corroborate the experimental data

Searching for quantum optimal controls in the presence of singular critical points

Quantum optimal control has enjoyed wide success for a variety of theoretical and experimental objectives. These favorable results have been attributed to advantageous properties of the corresponding control landscapes, which are free from local optima if three conditions are met: (1) the quantum system is controllable, (2) the Jacobian of the map from the control field to the evolution operator is full rank, and (3) the control field is not constrained. This paper explores how gradient searches for globally optimal control fields are affected by deviations from assumption (2). In some quantum control problems, so-called singular critical points, at which the Jacobian is rank-deficient, may exist on the landscape. Using optimal control simulations, we show that search failure is only observed when a singular critical point is also a second-order trap, which occurs if the control problem meets additional conditions involving the system Hamiltonian and/or the control objective. All known second-order traps occur at constant control fields, and we also show that they only affect searches that originate very close to them. As a result, even when such traps exist on the control landscape, they are unlikely to affect well-designed gradient optimizations under realistic searching conditions.Comment: 14 pages, 2 figure

Searching for quantum optimal controls under severe constraints

The success of quantum optimal control for both experimental and theoretical objectives is connected to the topology of the corresponding control landscapes, which are free from local traps if three conditions are met: (1) the quantum system is controllable, (2) the Jacobian of the map from the control field to the evolution operator is of full rank, and (3) there are no constraints on the control field. This paper investigates how the violation of assumption (3) affects gradient searches for globally optimal control fields. The satisfaction of assumptions (1) and (2) ensures that the control landscape lacks fundamental traps, but certain control constraints can still introduce artificial traps. Proper management of these constraints is an issue of great practical importance for numerical simulations as well as optimization in the laboratory. Using optimal control simulations, we show that constraints on quantities such as the number of control variables, the control duration, and the field strength are potentially severe enough to prevent successful optimization of the objective. For each such constraint, we show that exceeding quantifiable limits can prevent gradient searches from reaching a globally optimal solution. These results demonstrate that careful choice of relevant control parameters helps to eliminate artificial traps and facilitate successful optimization.Comment: 16 pages, 7 figure

Less is more: latent learning is maximized by shorter training sessions in auditory perceptual learning

Background: The time course and outcome of perceptual learning can be affected by the length and distribution of practice, but the training regimen parameters that govern these effects have received little systematic study in the auditory domain. We asked whether there was a minimum requirement on the number of trials within a training session for learning to occur, whether there was a maximum limit beyond which additional trials became ineffective, and whether multiple training sessions provided benefit over a single session. Methodology/Principal Findings: We investigated the efficacy of different regimens that varied in the distribution of practice across training sessions and in the overall amount of practice received on a frequency discrimination task. While learning was relatively robust to variations in regimen, the group with the shortest training sessions (~8 min) had significantly faster learning in early stages of training than groups with longer sessions. In later stages, the group with the longest training sessions (>1 hr) showed slower learning than the other groups, suggesting overtraining. Between-session improvements were inversely correlated with performance; they were largest at the start of training and reduced as training progressed. In a second experiment we found no additional longer-term improvement in performance, retention, or transfer of learning for a group that trained over 4 sessions (~4 hr in total) relative to a group that trained for a single session (~1 hr). However, the mechanisms of learning differed; the single-session group continued to improve in the days following cessation of training, whereas the multi-session group showed no further improvement once training had ceased. Conclusions/Significance: Shorter training sessions were advantageous because they allowed for more latent, between-session and post-training learning to emerge. These findings suggest that efficient regimens should use short training sessions, and optimized spacing between sessions

ULTRAFAST COHERENT DISSOCIATION DYNAMICS IN NITROTOLUENE RADICAL CATIONS

The ultrafast dynamics of polyatomic radical cations contribute to important processes including initiation of detonation in energetic molecules, radiation-induced DNA damage, and chemical reactions in the upper atmosphere and space. Probing these dynamics in the gas phase is challenging due to the rapid dissociation of many polyatomic radical cations following electron removal. This presentation will discuss how the pump-probe technique of femtosecond time-resolved mass spectrometry (FTRMS) can be a powerful tool for understanding time-resolved vibrational and dissociation dynamics of isolated radical cations and will highlight recent results in our laboratory on 2-, 3-, and 4-nitrotoluene (NT), which serve as model systems for nitroaromatic explosives such as TNT. Our experiments use strong-field, near-infrared (1200--1600 nm) pulses to induce adiabatic tunneling ionization, which prepares a large population of radical cations in the ground state that are amenable to subsequent optical excitation. The resulting electronically cold radical cation is typically prepared in a coherent superposition of highly excited vibrational states, i.e., as a nuclear ``wave packet''. Excitation of the wave packet by the probe pulse at particular time delays accesses electronic excited states that lead to dissociation, thereby resulting in oscillations in the ion yields of the parent and fragment ions as a function of pump-probe delay. These coherent dynamics drive C--\ce{NO2} bond dissociation in all three NT isomers, with each isomer exhibiting a distinct oscillation period depending on the coherently excited vibrational mode. The proximity of the \ce{NO2} and \ce{CH3} moieties in 2-NT also enable a hydrogen atom transfer reaction in the 2-NT cation that proceeds within $\sim20-60$ fs and preserves the initially prepared vibrational coherence, which demonstrates that coherent vibrational dynamics can continue following an intramolecular rearrangement reaction

Au Nanoparticle Synthesis Via Femtosecond Laser-Induced Photochemical Reduction of [AuCl4]−

Laser-assisted metallic nanoparticle synthesis is a versatile “green” method that has become a topic of active research. This chapter discusses the photochemical reaction mechanisms driving AuCl4− reduction using femtosecond-laser irradiation, and reviews recent advances in Au nanoparticle size-control. We begin by describing the physical processes underlying the interactions between laser pulses and the condensed media, including optical breakdown and supercontinuum emission. These processes produce a highly reactive plasma containing free electrons, which reduce AuCl4−, and radical species producing H2O2 that cause autocatalytic growth of Au nanoparticles. Then, we discuss the reduction kinetics of AuCl4−, which follow an autocatalytic rate law in which the first- and second-order rate constants depend on free electrons and H2O2 availability. Finally, we explain strategies to control the size of gold nanoparticles as they are synthesized; including modifications of laser parameters and solution compositions