Plume-scale new particle formation during the Houston TRACER-MAP campaign

July 20, 2026

Hannah Grace Marti

Committee: Jeffrey Pierce (Advisor); Jeffrey Collett; Shantanu Jathar (Mechanical Engineering)

Abstract

New particle formation (NPF), the creation of atmospheric particles from gas-phase precursors, is responsible for about half of the particles at climate-relevant sizes in the lower troposphere, giving this process significant climate impact through the direct and indirect aerosol radiative effects. NPF has long been studied from the stationary perspective, potentially neglecting the impact of inhomogeneity in air masses passing over measurement stations. This stationary approach can especially interfere with the understanding of NPF in urban environments, where air masses are frequently inhomogeneous due to plumes from local sources. To investigate urban NPF, this study employs stationary ground observations from the TRACER-MAP field campaign in Houston as well as the GEOS-Chem-TOMAS chemical transport model to access the Lagrangian perspective of size distributions at the stationary site. In our NPF event classification of the stationary size distributions, we find ~75% of days in both the observations and model showed evidence of NPF but lacked clear particle growth. By evaluating the modeled size distribution along Lagrangian trajectories passing over the stationary site, we find a clear profile of NPF in ~90% of our analysis days. In two Lagrangian case studies, we find that Houston and surrounding point sources initiated NPF when trajectories arrived to the city during daylight hours, and the growth stage of NPF occurred as the air traveled downwind with the urban plume. We evaluate the impact of the nearby WA Parish power plant on Houston NPF and find a relationship between Parish-influenced back trajectories and the occurrence of more clear-like NPF. Removing WA Parish emissions from our model simulation did not significantly impact the NPF classification results of the modeled stationary size distribution but greatly decreased the nucleation strength. Overall, this analysis reveals the high frequency at which Houston served as a source region for NPF during the summer of 2022, and the Lagrangian perspective provides insight into NPF as a plume-scale process relying on urban point source emissions. These results contribute to a growing body of evidence that NPF analysis from stationary sites should be done with caution due to inhomogeneity in air masses, particularly for urban regions.