Probing The Potential Drivers of New Particle Formation and Growth During TRACER in Houston, TX.

The process of new particle formation (NPF) and subsequent particle growth is a significant contributor to cloud condensation nuclei (CCN) concentrations in the atmosphere. The abundance of CCN is important for Earth’s radiative budget through the modulation of cloud properties such as brightness, cloud lifetime, and precipitation efficiency. While significant progress has been made in our understanding of NPF and growth to CCN sizes in a variety of environments, the processes that govern NPF and growth in different urban environments are still not well understood. Urban environments are typically associated with substantial primary particle and NPF precursor emissions, thus making NPF and growth in these environments different than in remote regions, which have been better characterized.

As part of the Tracking Aerosol Convection Interactions Experiment (TRACER) Ultrafine Aerosol Formation and Impacts (UFI; together TRACER-UFI) campaign, the University of California, Riverside’s Captive Aerosol Growth and Evolution (CAGE) chamber operated during July and August of 2022. The CAGE chamber filters out ambient particles, but allows ambient gas-phase aerosol precursors to enter. On most days during TRACER-UFI, oxidation products of these precursor vapors created NPF and growth events in the CAGE chamber, thus providing the unique opportunity to assess what chemical species and mechanisms have the potential to drive NPF and growth in Houston during the summer of 2022. In this work, we use the SOM-TOMAS chemistry and aerosol-microphysics model to represent the CAGE chamber to better understand the driving mechanisms of NPF and growth in the chamber. The SOM-TOMAS model simulates gas-phase oxidation reactions, thermodynamics, gas-particle partitioning, and aerosol microphysics.

SOM-TOMAS generally reproduces the observed NPF events in the chamber. The model results are sensitive to several inputs that each have uncertainties, principally being gas and aerosol wall-loss rates, VOC concentrations, and OH concentrations, which largely drive the oxidation chemistry in the model. Preliminary results suggest that most of the NPF in Houston is driven by a H2SO4-NH3 mechanism; however, particle growth is primarily driven by low-volatility organics derived from anthropogenic VOCs. We will constrain the model further by using size-resolved aerosol composition and gas-phase data measured by the Smith and Collins groups at UC Irvine and UC Riverside, respectively.