Measurements and modeling of urban secondary organic aerosol formation potential as a function of precursor volatility class in the Los Angeles area during summer 2022
Ehrenfels et al.
Aerosol Research Discussions, 2026, [preprint]
DOI: 10.5194/ar-2026-12
A recent study published in Aerosol Research Discussions, led by researchers from the University of Colorado Boulder and including work from Aerodyne Research Inc., used the TOFWERK Vocus 2R to perform air measurements in Summer 2022 in Los Angeles. The results showed that about 31% of secondary organic aerosol (SOA) formation potential comes from lower-volatility compounds (SVOCs and some IVOCs), with overall SOA-FP lower than in 2010 due to greater atmospheric oxidation.
Urban aerosols (mostly organic, with a large fraction being (SOA)) significantly impact air quality and health. In Los Angeles, air quality improvements have slowed in recent years despite past emission reductions, partly due to rising contributions from sources like volatile chemical products and cooking emissions. Oxidation flow reactors (OFRs) have been widely used to measure SOA formation potential (SOA-FP), showing higher formation at night when precursor levels are elevated. However, traditional OFR measurements do not distinguish between precursor types. This study introduces a new method using a volatility-based filter to separate precursor classes and better quantify their contributions to SOA-FP, helping resolve conflicting model predictions about the roles of VOCs versus S/IVOCs.
During the 2022 CalNexT campaign in Pasadena, a Vocus 2R measured gas-phase VOCs at 1-second resolution across multiple sampling pathways (ambient air and OFR outputs), enabling detailed tracking of precursor behavior and oxidation. The Vocus data were carefully calibrated and used to constrain and adjust model inputs (especially VOC concentrations like aromatics and monoterpenes), improving evaluation of SOA formation and chemical mechanisms in both box and CRACMM models.
Vocus measurements were central to characterizing precursor behavior and SOA formation potential during the campaign. They showed near-100% transmission of all detectable VOCs through the inlet, meaning the inlet primarily removed lower-volatility species not measurable by the Vocus. The instrument also tracked chemical changes across pathways, observing depletion of precursors (e.g., aromatics) and formation of oxidation products in the OFRs. Vocus-constrained analyses revealed that ~69% of SOA-FP comes from higher-volatility compounds (VOCs and some IVOCs), while lower-volatility species contribute the remainder. Correlation analysis using Vocus VOC data identified key tracer compounds (e.g., aromatics, carbonyls, sesquiterpenes) linked to SOA-FP, though individually they explain only a fraction of total SOA mass. Importantly, Vocus data showed weak correlations between SOA-FP and biogenic compounds like isoprene and monoterpenes, indicating terpenoids contribute but are not dominant drivers of urban SOA formation.
Overall, the study demonstrates the use of the Vocus 2R for real-time measurement of VOC precursors and their behavior across oxidation pathways, enabling separation of SOA formation potential by volatility.
