Trends, Sources, and Formation Pathways of Tropospheric Ozone across Southeast Texas
Tropospheric ozone poses significant risks to human health, climate, and agriculture. This dissertation examines long-term variations in ambient levels, sources, and formation mechanisms of surface ozone in the Houston-Galveston-Brazoria (HGB) region. We investigated the reduction in maximum daily 8-hour average ozone (MDA8 O3) levels from 2000 to 2019 and evaluated the impact of emission control policies targeting nitrogen oxides (NOx) and volatile organic compounds (VOCs). By utilizing the positive matrix factorization (PMF) model, regional background ozone was identified as the major contributor to MDA8 O3, followed by natural gas/fuel evaporation, solvent/painting/rubber industries, engine combustion, and petrochemical emissions. Notable reductions in contributions from petrochemical emissions, engine combustion, and natural gas/fuel evaporation were observed, authenticating the effectiveness of emission control policies. In addition to a diverse range of emission sources, meteorology plays a crucial role in driving ozone level exceedances in Southeast Texas. We investigated transport pathways and photochemical formation contributing to ozone exceedances during the TRACER-AQ campaign in September 2021. The trajectory analyses using HYbrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) tool and FLEXible PARTicle (FLEXPART) dispersion model revealed ozone pollution episodes were associated with the transport of air masses from the central/northern US, with local recirculation and pollutant accumulation exacerbating ozone exceedances in Houston. By implementing the Comprehensive Air Quality Model with extensions (CAMx), ozone production hotspots were identified over Houston city and industrial districts of HSC during pollution episodes. Ozone production rates were generally influenced by transported VOC-rich air masses, leading to a transition in ozone formation tendency from VOC-limited to NOx-limited conditions. However, local NOx emissions in Houston maintained a VOC-limited regime. Elevated ozone production along the transport pathways also contributed to ozone exceedances in the region. Through these investigations, this dissertation enhances our understanding about the long-term contribution of different sectors in ozone formation as well as the complex relationship between transport patterns and photochemical formation of ozone in the HGB region during pollution events. The findings provide valuable insights into the synergy between local emissions, transport patterns, and ozone production for developing more precise and tailored policies to further mitigate ozone pollution in Houston.