Improving Observations of Greenhouse Gases, Reactive Nitrogen, and Particulate Matter for Effective Policymaking

Abstract

Greenhouse gases (GHG), reactive nitrogen, and particulate matter (PM) are important atmospheric constituents impacting climate, ecosystems, and air quality. In this thesis, I demonstrate how in-situ measurements from mobile laboratories, flux towers, and surface observational networks can be used to provide representative observations with appropriate spatiotemporal coverage to help create effective policies. Mobile laboratories achieve high spatial resolution but are impacted by non-targeted emissions, especially from nearby vehicles. Eddy covariance (EC) flux measurements allow for high temporal resolution flux measurements in remote areas when combined with lower-power open-path sensors, yet flux corrections due to temperature corrections can be as large as the measured fluxes themselves. Air quality monitoring networks provide long-term trends over wide spatial extents, but there is a lack of supporting measurements for aerosol thermodynamic modeling. To overcome these challenges, I developed new frameworks for sensing technologies and sampling methodologies. The first part of this thesis examined emissions of methane from natural gas vehicles (NGVs) in China, demonstrating a postprocessing framework that reduces impacts from other vehicles. High methane emissions from heavy-duty NGVs in China were observed, suggesting switching to NGVs has increased GHG emissions in China, and more stringent regulations are needed. The second part proposes a new framework for developing open-path sensors that could reduce the temperature-related correction. I developed and deployed the first open-path nitrous oxide sensor for EC measurements over a cornfield. Field results showed that the net temperature-related correction decreased about ten times. Third, ammonia EC fluxes were measured at an alpine meadow downwind of urban and agricultural areas in Colorado. Most dry ammonia deposition was traced to upwind agricultural areas. Finally, observations from multiple air-quality monitoring networks in the U.S. are integrated using aerosol thermodynamic chemistry models to provide the first, long-term observational constraints of inorganic PM2.5 formation in the United States. Due to rapid reductions in SO2 and NOx emissions, PM concentration has become insensitive to ammonia availability in the western U.S. and is approaching the threshold in the eastern U.S. My work shows that representative observations can be a critical part of the weight of evidence for policymaking.