2024 IGASP Program 

Speaker Abstracts

1.  Xuan Zhang, Life and Environmental Sciences, University of California Merced

Title: Probing the Fate of Highly Oxygenated Molecules in Atmospheric Aerosols

Abstract:

I would like to present our recent work on the fate of highly oxidized molecules (HOMs) in monoterpene SOA under a range of atmospherically relevant conditions. Utilizing a combination of mass spectrometry-based methods, we tracked the trajectory of HOMs at the isomer level across the gas to particle conversion process. A large overlap in the molecular composition was found between the gas and particle phase, yet the time series reveal distinct evolution patterns in each phase. A particular focus of this study was to examine the ultimate fate of HOMs in suspended particles upon the termination of gas phase chemistry. The observed dynamics of HOMs therefore speak directly to their particle-phase reactivity and reflect the strength of condensed-phase chemistry, if any, in modifying their molecular composition. Individual SOA-bound HOMs exhibit remarkably diverse behaviors, and even certain isomers of identical formulas appear to undergo different transformations. A small group of C10 and C20 HOMs decay rapidly in hydrated particles, on a timescale akin to organic hydroperoxides measured in the bulk phase. On the other hand, a subset of C12-18 dimers was found to continuously grow upon forming particles, and the presence of water substantially enhances their growth rates. This growth trend is well captured by box model simulations that incorporate the kinetics of condensed-phase chemistry, in particular the peroxyhemiacetal formation from reactions of hydroperoxides with carbonyls. The majority of HOMs remain structurally stable across all hydrous and photolytic conditions investigated in this study. This persistence underlines the potential of HOMs as a widespread and sustained source of cloud condensation nuclei in the atmosphere.

2.  Sam Silva, University of Southern California

Title: Graph theory for atmospheric chemical mechanisms

Abstract: 

Chemical reactions control the abundance and variability of many key atmospheric pollutants and greenhouse gases. Despite this importance, there are limited methods available for understanding the structure and dynamics of atmospheric chemical reaction mechanisms. Here we present a variety of results from ongoing work to address this research gap using mathematical techniques from the field of graph theory.

We first demonstrate that framing the analysis of atmospheric chemical mechanisms as a graph theoretical problem provides consistent results with existing knowledge in the field. We then go beyond existing work to characterize structural differences across chemical mechanisms by quantifying patterns of gas phase interactions. This interaction quantification can provide useful information for understanding why seemingly similar chemical mechanisms produce very different chemical species abundances. We further explore the dynamics of chemical cycling throughout different regions of the world, tracking all chemical cycles in a representative mechanism. We find nonintuitive results related to the speed of chemical cycling in heavily polluted environments and the fraction of chemical cycles that contain a rate-determining step. Lastly, we describe current research using these graph methods in combination with neural network approaches to learn chemical mechanisms directly from species abundance data.

3. Victoria Barber, Department of Chemistry and Biochemistry, UC Los Angeles

Title: New Atmospheric Chemistry, and Old Atmospheric Chemistry in New Contexts: Stories from Recent Research

Abstract:

Reactive organic carbon (ROC, defined as all organic molecules in the air excluding methane) is present in the Earth’s atmosphere in minute abundance, but exerts an outsized impact on climate and air quality. These impacts are largely controlled by the subsequent physical and chemical processes that govern the evolution of organic molecules in air. This talk will address several studies focused on developing a mechanistic understanding of these processes. In the first portion of the talk, I will discuss the unexpected oxidation mechanisms available to functionalized organic molecules in the Earth’s atmosphere. While the mechanisms associated with hydrocarbon oxidation are at this point reasonably well understood, the fates of functionalized organic species in the atmosphere are considerably more diverse, and more uncertain. We employ automated mechanism generation to develop a novel, systematic approach to exploring atmospheric chemical space, and identify a number of potentially important new chemical pathways for functionalized reactive intermediates. One of these is a previously unreported carbonyl-substituted peroxy radical ring closure reaction. Subsequent laboratory chamber oxidation studies provide the first experimental evidence of this new reaction mechanism, and more generally highlight the continued uncertainty in the oxidation mechanisms of even relatively simple functionalized systems. In the second portion of the talk, I will explore the changes in oxidation chemistry that result from exposure to untraditional wavelengths of light. This work is motivated by recent public health interest in the use of germicidal UV (GUV) light to disinfect indoor air and reduce pathogen transmission. In particular, recent studies suggest that 222 nm is extremely effective at denaturing pathogens, and relatively safe with respect to direct human exposure, but little is known about how 222 nm light affects indoor air quality. In this work, we perform a series of laboratory chamber studies aimed at unraveling how 222 nm light changes oxidation chemistry. We demonstrate that 222 nm light results in the generation of oxidants, leading to the formation of oxidized product species and secondary organic aerosol. A complementary, empirically validated model demonstrates the necessity of employing ventilation and germicidal UV irradiation together to reduce indoor air quality impacts. In the final portion of the talk, I will examine the role of surface partitioning in determining indoor concentrations of biomass-burning VOCs. In particular, I will showcase new data on the time and temperature-dependent off-gassing behavior of biomass burning smoke-derived VOCs from common household materials following smoke exposure- a potential long-term exposure pathway for smoke-impacted communities following wildfire events. The results of this work may help inform mitigation strategies for such exposures.

4. James Davies, UC Riverside

Title: The Applications of Single Particle Levitation in Atmospheric Science

Abstract:

The physicochemical properties of aerosol particles play a pivotal role in dictating their interactions in the atmosphere, influencing important atmospheric processes such as cloud formation and light scattering. These attributes also significantly impact the role of aerosol on air quality at both regional and global scales. This presentation will discuss single particle levitation as an innovative tool for manipulating individual aerosol particles and measuring properties such as hygroscopicity, viscosity, diffusivity, phase state, and reactivity. Particles of a well-defined size and composition are exposed to controlled environmental conditions and their properties are characterized using light scattering. Emphasis will be placed on recent investigations of the properties of particles containing organic molecules and inorganic salts and those containing light-absorbing brown carbon chromophores. The ability to quantify essential morphological and rheological properties of levitated particles is highlighted, with implications extending to both atmospheric effects and their potential role in disease transmission. Furthermore, this presentation will discuss applications of mass spectrometry to single particle analysis and describe recent work comparing reaction rates in solid and liquid particles of the same chemical composition. This holistic characterization is essential for unraveling the nuanced roles of aerosol particles in both atmospheric and indoor environments, ultimately contributing to a more complete understanding of their impact on our surroundings.