Ultrafine Aerosol Particle Formation and Impacts in Houston during TRACER

JEREMY WAKEEN, Xuanlin Du, Sam O'Donnell, Jeffrey R. Pierce, Don Collins, James Smith, University of California, Irvine

     Abstract Number: 474
     Working Group: Aerosol Chemistry

Abstract
New particle formation (NPF) events have been observed in Houston during the summertime, but there are a lack of studies focusing on the gaseous precursors and chemical composition of newly formed particles in this region. Here we describe initial results from the Tracking Aerosol Convection Interactions Experiment-Ultrafine Aerosol Formation and Impacts (TRACER-UFI) campaign, which took place at the La Porte Municipal Airport in La Porte, Texas, from July 1st to August 31st, 2022. The objectives of TRACER-UFI are to understand the chemical species and mechanisms responsible for the formation of ultrafine (sub-100-nm diameter) particles in the Houston atmosphere. Our approach combines direct measurements of low‑volatility precursors and size-resolved ultrafine particles together with measurements of gas-particle partitioning of ambient vapors onto size-selected nanoparticles of known composition, the latter using the UC Riverside Captive Aerosol Growth and Evolution (CAGE) chamber. Our measurements will ultimately be coupled with modeling and co-located measurements of particle hygroscopic properties in order to infer the impacts of ultrafine particles on clouds and climate.
During TRACER-UFI, we measured the composition of ambient 30-60 nm diameter particles using Thermal Desorption Chemical Ionization Mass Spectrometry (TDCIMS) as well as the concentration of ambient Highly Oxidized Molecules (HOMs), including sulfuric acid, with nitrate-CIMS.  Ambient measurements were alternated with measurements of gases and particles participating in NPF and either directly nucleating or partitioning onto 30 nm diameter ammonium sulfate seed particles using CAGE. During the daytime, we observed nucleation and nanoparticle growth each day with or without seed particles, suggesting that conditions exist to produce low-volatility HOMs that, under conditions of low condensation sink, would result in ambient new particle formation and growth events. Initial measurements by the TDCIMS and nitrate-CIMS indicate significant amounts of sulfate and sulfuric acid in ambient nanoparticles and air, respectively.