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Chemical mechanisms play an important role in simulating the atmosphericchemistry of volatile organic compound oxidation. Comparison of mechanismsimulations with laboratory chamber data tests our level of understanding ofthe prevailing chemistry as well as the dynamic processes occurring in thechamber itself. α-Pinene photooxidation is a well-studied systemexperimentally, for which detailed chemical mechanisms have been formulated.Here, we present the results of simulating low-NO α-pinenephotooxidation experiments conducted in the Caltech chamber with theGenerator for Explicit Chemistry and Kinetics of Organics in the Atmosphere(GECKO-A) under varying concentrations of seed particles and OH levels.Unexpectedly, experiments conducted at low and high OH levels yield the samesecondary organic aerosol (SOA) growth, whereas GECKO-A predicts greater SOA growth under high OH levels.SOA formation in the chamber is a result of a competition among the rates ofgas-phase oxidation to low-volatility products, wall deposition of theseproducts, and condensation into the aerosol phase. Various processes – such asphotolysis of condensed-phase products, particle-phase dimerization, andperoxy radical autoxidation – are explored to rationalize the observations. Inorder to explain the observed similar SOA growth at different OH levels,we conclude that vapor wall loss in the Caltech chamber is likely of order10−5 s−1, consistent with previous experimental measurementsin that chamber. We find that GECKO-A tends to overpredict the contributionto SOA of later-generation oxidation products under high-OH conditions.Moreover, we propose that autoxidation may alternatively resolve some or allof the measurement–model discrepancy, but this hypothesis cannot be confirmeduntil more explicit mechanisms are established for α-pineneautoxidation. The key role of the interplay among oxidation rate, productvolatility, and vapor–wall deposition in chamber experiments is illustrated

SOA formation from the photooxidation of &lt;i&gt;α&lt;/i&gt;-pinene: systematic exploration of the simulation of chamber data

SOA formation from the photooxidation of <i>α</i>-pinene: systematic exploration of the simulation of chamber data