Person:

McKain, Kathryn

Methane emissions from natural gas delivery and end use must be quantified to evaluate the environmental impacts of natural gas and to develop and assess the efficacy of emission reduction strategies. We report natural gas emission rates for 1 y in the urban region of Boston, using a comprehensive atmospheric measurement and modeling framework. Continuous methane observations from four stations are combined with a high-resolution transport model to quantify the regional average emission flux, 18.5 ± 3.7 (95% confidence interval) g CH4⋅m−2⋅y−1. Simultaneous observations of atmospheric ethane, compared with the ethane-to-methane ratio in the pipeline gas delivered to the region, demonstrate that natural gas accounted for ∼60–100% of methane emissions, depending on season. Using government statistics and geospatial data on natural gas use, we find the average fractional loss rate to the atmosphere from all downstream components of the natural gas system, including transmission, distribution, and end use, was 2.7 ± 0.6% in the Boston urban region, with little seasonal variability. This fraction is notably higher than the 1.1% implied by the most closely comparable emission inventory.

Greenhouse gas emission magnitudes, trends, and source contributions are highly uncertain, particularly at sub-national scales. As the world becomes increasingly urbanized, one potential strategy for reducing these uncertainties is to focus atmospheric greenhouse gas measurements in urban areas, where a multitude of emission processes occur, imposing a strong and persistent gradient in the local atmosphere, and contributing a significant fraction of global anthropogenic greenhouse gas emissions. This thesis explores the capabilities and requirements for characterizing and quantifying greenhouse gas fluxes in urban environments using atmospheric measurements and models. The first chapter uses an existing dataset of atmospheric carbon dioxide measurements from Salt Lake City, Utah to assess the capacity of an atmospheric measurement and modeling framework to detect changes in emissions from a city in the context of an emissions verification framework. The results of this work are then used to explore an alternative or complementary measurement strategy of atmospheric column measurements for urban emissions detection, which would be less sensitive than point measurements to the large variability present in urban atmospheres, but would also have more stringent accuracy requirements. The second chapter describes the development and maintenance of a network of greenhouse gas measurement stations in the Boston urban region, which has been continuously running since 2012 and has generated high-quality atmospheric carbon dioxide and methane data that can be used to explore their fluxes across the urban region. The third chapter applies the Boston network data to investigate the magnitude of methane emissions from natural gas infrastructure in the urban region. We find that the natural gas loss rate in 2012-13 was 2.7 ± 0.6 %, two to three times larger than that reported by industry and government. Our findings suggest that natural gas consuming regions may be larger sources of methane than previously thought, and have implications for local and national policies that aim to reduce methane emissions and promote energy-use efficiency. The work presented in this thesis explores general methodological strategies for urban atmospheric measurements and models, and offers example applications of such methods to directed and societally-relevant investigations of urban greenhouse gas emissions.