Precursor Concentration and Molecular Structure Influence the Viscosity and Volatility of Secondary Organic Aerosol Derived from Sesquiterpene Ozonolysis

SIJIA LIU, Amna Ijaz, ManishKumar Shrivastava, Sergey Nizkorodov, Celia Faiola, Alla Zelenyuk, Pacific Northwest National Laboratory

     Abstract Number: 525
     Working Group: Aerosol Chemistry

Abstract
Sesquiterpenes, with their complex molecular structures, yield SOA with unusually low volatility and high viscosity, but their SOA remain under-characterized compared to SOA from monoterpenes. Moreover, plant stress can increase emissions of acyclic sesquiterpenes, such as β-farnesene, further modulating SOA yield, composition, and phase behavior. To improve our understanding and better characterize SOA from sesquiterpenes, we performed batch-mode chamber ozonolysis of two sesquiterpenes (β-caryophyllene and β-farnesene) at three initial concentrations (300, 100, and 30–50 ppb). We employed the state-of-the-art single-particle laser-ablation mass spectrometer (miniSPLAT) to determine particle composition, morphology, and density, and conducted evaporation-kinetics measurements to quantify volatility and infer viscosity. We also used nanospray desorption electrospray ionization high-resolution mass spectrometry (nano-DESI-HRMS) to elucidate molecular composition of SOA, and thermal desorption- thermal desorption–gas chromatography-quadrupole time-of-flight mass spectrometry (GC-QToF MS) of sorbent tube samples to validate the chamber VOC concentrations. Our results demonstrated that both precursor concentration and molecular structure strongly affect the properties of the resulting SOA. MiniSPLAT evaporation kinetics measurements revealed that lowering the precursor concentration reduced the volume fraction remaining (VFR) from 57 % to 49 % for β-caryophyllene and from 63 % to 59 % for β-farnesene under humidified conditions, but VFR remained similar under dry evaporation conditions, suggesting high relative humidity has a greater influence on the SOA from lower concentration. Nano-DESI‐HRMS analysis indicated that by reducing the initial precursor concentration in both systems decreased the ELVOC fraction and enhanced the relative abundance of SVOCs. β-farnesene SOA had a consistently higher average O/C ratio (0.55) than β-caryophyllene SOA (0.40), and at all concentrations, β-farnesene yielded a larger fraction of LVOC and ELVOC species. These findings will improve our understanding of sesquiterpene-derived SOA and provide critical insights for improving models of organic aerosol formation and phase state in the atmosphere.