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D.Phil. (Geography) / The Infrared Atmospheric Sounding Interferometer (lASI), operational in polar-orbit since 2006 on the European MetOp-A satellite, is the most advanced of its kind in space. It has been designed to provide soundings of the troposphere and lower stratosphere at nadir in a spectral interval of 0.25 em" across the range 645-2 760 em". Fine spectral sampling such as this is imperative in the sounding of trace gases. Since its launch, the routine retrievals of greenhouse, species from IASI measurements have made a valuable contribution to atmospheric chemistry studies at a global scale. The main contribution of this thesis is the development of a new trace gas retrieval scheme for IASI measurements. The goal was to improve on the global operational scheme in terms of the algorithm complexity, speed of calculation and spatial resolution achieved in the final solution. This schemedirectly retrieves column integrated trace gas densities at single field-of-view (FOV) from IASI measurements within a 10% accuracy limit. The scheme is built on the Bayesian framework of probability and based on the assumption that the inversion of total column values, as apposed to gas profiles, is a near-linear problem. Performance of the retrieval scheme is demonstrated on simulated noisy measurements for carbon monoxide (CO). Being a linear solution, the scheme is'highly dependent on the accuracy of the a priori. A statistical estimate of the a priori was computed using a principal component regression analysis with 50 eigenvectors. The corresponding root-mean-square (RMS) error of the a priori was calculated to be 9.3%. In general terms, the physical retrieval improved on the a priori, and sensitivity studies were performed to demonstrate the accuracy and stability of the retrieval scheme under a numberof perturbations. A full system characterization and error analysis is additionally preformed to elicidate the nature of this complex problem. The hyperspectral IASI measurements introduce a significant correlation error in the retrieval. The Absorption Line Cluster (ALC) channel selection method was developed in this thesis, to address the correlation error explicitly. When a first neighbour correlation factor of 0.71 is assumed in the measurement error covariance for the clusters of ALC channels, then most of the correlation error is removed in the retrieval. In conclusion, the total column trace gas retrieval scheme developed here is fast, simple, intuitive, transparent and robust. These characteristics together make it highly suitable for implementation in an operational environment intended for air quality monitoring on a regional scale.

The use of biomass as a renewable fuel source can help the United States reduce its dependence on fossil fuels, especially in providing affordable heat for many middle- and low-income households. However, residential wood combustion (RWC) releases pollutants that can negatively impact the environment and human health, especially for those living in the vicinity of wood-burning locations. Current compliance testing methods are insufficient in capturing the actual in-use emissions of residential wood heaters because they do not represent real-life use, leading to higher emissions during actual use. This thesis investigates emissions during realistic operations of wood-fired heaters to identify and quantify the majority of emissions and ways to minimize such emissions. The study focuses on investigating eight different woody biomass fired heaters, including three pellet stoves, a pellet boiler, two wood chip-fired hydronic heaters, and two outdoor cordwood fired hydronic heaters. This research contributes a new knowledge on the impact of combustion strategies, fuel type, and control strategies to minimize emissions. The obtained data can provide information to manufacturers, policy makers, and consumers, guiding low-emission and more efficient use of wood-fired heating devices. In all chapters, variability was evident due to burn phase, fuel type, and operation. The results from the pellet stoves showed that even while using a homogeneous fuel, different burn phases produce different emissions than the overall period. For the pellet boiler studies, the highest efficiencies were achieved during the high load, steady state tests. The burn phase also affects emissions from woodchip boilers, where low output periods are significantly higher in terms of emissions compared to high output periods. Each individual burn phase of the duty cycle produced different emissions in cordwood testing, with steady-state phases having the lowest emissions and highest efficiency. The variability in emissions from different burn phases is a crucial factor in evaluating the performance of wood-burning appliances. Lower moisture content fuels were found to have better performance in terms of PM emissions and efficiency. Fuel type can impact emissions, but it may be overshadowed by burn phase and technology. Relatively high emissions were often related to low or incorrect air-to-fuel ratios. Gasification techniques used in some woodchip boilers during low output periods significantly increased efficiency and reduced CO emissions. Additionally, gasification techniques used during high burn steady states with wet fuel chips resulted in a 77% reduction in PM emissions. Comparing all the primary heaters studied in this thesis, in terms of PM emission output, showed the units that used gasification, integrated catalysts, or thermal storage had the lowest emissions. The results of the study are compiled into data sets that give a more accurate picture of real-world operation of wood-fired heaters that will benefit air quality modelers and policy makers. Such emission data for various biomass heaters in EPA’s AP-42, under realistic operating conditions, currently either does not exist or is limited. Additionally, this research identifies the most important parameters that need to be included in the development of high-resolution models, optimizing the performance of wood-fired devices and supporting the transition from current compliance testing to more realistic testing. In conclusion, this work provides new insights into the performance and emissions of wood-fired heaters during realistic operation. The results of the study can help manufacturers optimize their products for real-life performance and help policy makers and consumers make informed decisions regarding low-emission and more efficient use of wood-fired heating devices. The study highlights the importance of capturing transient phases and the impact of fuel type and control strategies on minimizing emissions.

Tropospheric oxidation modulates pollution chemistry and greenhouse gas lifetimes. The hydroxyl radical (OH) is the primary oxidant and the main sink for methane, the second-most influential anthropogenic contributor to climate change. OH is produced following the photolysis of ozone, an oxidant, respiratory irritant and greenhouse gas. Trends in methane or ozone are frequently attributed to their sources, but sink-driven variability is less often considered. I investigate the influence of fluctuations in turbulent loss to the Earth’s surface, also known as deposition, on tropospheric ozone concentrations and chemistry over the relatively polluted eastern United States. I use idealized sensitivity simulations with the global chemistry-climate model AM3 to demonstrate that coherent shifts in deposition, on the order recently observed at a long-term measurement site, affect surface ozone concentrations as much as decreases in its precursor emissions have over the past decade. I conclude that a sub-regional deposition measurement network is needed to confidently attribute trends in tropospheric ozone. Next, I turn to the remote marine troposphere to evaluate two theoretical proxies for variability in the methane sink, OH, with observations from the NASA Atmospheric Tomography (ATom) aircraft campaign. The low concentration and short lifetime of OH preclude the development of a representative measurement network to track its fluctuations in space and time. This dearth of constraints has led to discrepancies in the methane lifetime across models that project atmospheric composition and climate. Observational and modeling studies suggest that few processes control OH fluctuations in relatively clean air masses, and the short OH lifetime implies that it is at steady-state (total production is equal to loss). I leverage this chemistry by evaluating a convolution of OH drivers, OH production scaled by the lifetime of OH against its sink with carbon monoxide, as a potential “steady-state” proxy. I also assess the predictive skill of formaldehyde (HCHO), an intermediate product of the methane and OH reaction. I find that both proxies broadly reflect OH on sub-hemispheric scales (2 km altitude by 20° zonal bins) relative to existing, well-mixed proxies that capture, at best, hemispheric OH variability. HCHO is produced following methane loss by reaction with OH and reflects the insolation influence on OH, while the steady-state proxy demonstrates a stronger relationship with OH and offers insight into its sensitivity to a wider array of drivers. Few components—water vapor, nitric oxide, and the photolysis rate of ozone to singlet-d atomic oxygen—dominate steady-state proxy variance in most regions of the remote troposphere, with water vapor controlling the largest spatial extent. Current satellite instruments measure water vapor directly, and other retrievals like nitrogen dioxide columns or aerosol optical depth or could be used to infer nitric acid or the rate of ozone photolysis. Thus satellite observations may be used to derive a steady-state proxy product to infer OH variability and sensitivity in the near-term. HCHO is also retrieved from satellite instruments, and an OH product using satellite-observed HCHO columns is already in development. The relatively high fluctuation frequency of HCHO or the steady-state proxy advances our insight into the connection between OH and its drivers. The observed steady-state proxy demonstrates a widespread sensitivity to water vapor along the ATom flight tracks, and I conclude that an improved and consistent representation of the water vapor distribution is a necessary step in constraining the methane lifetime across global chemistry-climate models.