Chemical Composition and Volatility Distribution of Secondary Organic Aerosol Formed through Multiphase Chemical Reactions as a Function of Sulfate Seed Acidity
SINING NIU, Miska Olin, Christopher Rapp, Siddharth Gopalakrishnan, Hannah LeClear, Jason Surratt, Daniel Cziczo, Yue Zhang, Texas A&M University
Abstract Number: 349
Working Group: Aerosol Chemistry
Abstract
The climate properties of secondary organic aerosol (SOA) are poorly understood, not only due to variations of their chemical composition but also uncertainties in their volatility distribution and phase state. Previous studies have shown that atmospheric aerosols are often acidic, and these acidic aerosols can also affect the physicochemical properties of SOA, further complicating their climate effects.
In this study, we conducted environmental chamber experiments to investigate the formation of SOA from the oxidation of biogenic volatile organic compounds by hydroxyl radicals in the presence of neutral (ammonium sulfate) and acidic (ammonium bisulfate) seed particles. The molecular composition and thermal desorption behavior of SOA particles were detected and quantified using a Vocus chemical ionization mass spectrometer coupled with a Vaporization Inlet for Aerosols. Volatility distribution of the SOA was derived using the thermogram method established in our previous work, and positive matrix factorization (PMF) was applied to identify components with distinct thermal desorption patterns. In addition, SOA mass and bulk chemical composition were characterized using a Scanning Mobility Particle Sizer and a High-Resolution Aerosol Mass Spectrometer (HR-AMS).
Our results demonstrated enhanced formation of organosulfates (OS) under acidic conditions, with highly functionalized OS exhibiting lower volatility and higher viscosity compared to SOA formed via neutral seeds. In addition, HR-AMS analysis revealed elevated oxygen-to-carbon (O/C) and hydrogen-to-carbon (H/C) ratios under neutral seed conditions, suggesting that the presence of acidic seeds may promote the formation of sulfur-containing oligomers. This highlights the complexity of aerosol acidity effects on SOA composition and the need to decouple OS formation from overall oxidation state metrics. Our findings highlight the importance of accounting for OS formation and the effects of aerosol acidity in atmospheric chemical transport models to better constrain SOA formation, volatility, and phase state, thereby reducing uncertainty in climate and air quality predictions.