We present the fully automated IONICON Laminar-flow Oxidation reactor (ILOx), a versatile oxidation flow reactor designed for photochemical aging of atmospheric organics. ILOx enables the simulation of multi-day atmospheric oxidation processes within minutes under controlled conditions. The reactor consists of a 110 cm long quartz-glass tube with a usable internal volume of 7.5 l that is irradiated by arrays of UVA and UVC light-emitting diodes (LEDs). Oxidants and precursors are introduced through multiple customizable inlet ports. Computational fluid dynamics (CFD) optimized sample injection suppresses jet formation and quickly establishes near-laminar flow. The reactor outlet permits simultaneous sampling of particles and volatile organic compounds (VOCs). Particle transmission efficiency was verified with dried ammonium sulphate particles, showing only small changes in the size distribution before and after the reactor. VOC core-sampling minimizes wall interactions and potential artefact formation; all wetted surfaces are restricted to inert materials to ensure rapid response even for lower-volatility gas-phase organics.</p> <p>Herein we characterize ILOx with three distinct oxidation modes: ozonolysis, OH induced oxidation via the photolysis of isopropyl nitrite (UVA 365 nm), and OH induced oxidation via the photolysis of O₃ (UVC 275 nm). The ozonolysis of limonene forms reasonable amounts of secondary organic aerosol (SOA) with 25 % of SOA mass yield. NH₄⁺ soft adduct ionization mass spectra of this particulate phase show well distributed products ranging from monomer to trimer fractions. OH induced oxidation of o-xylene via the photolysis of isopropyl nitrite at 365 nm was studied in detail. Box model simulations reveal an OH exposure of 10¹¹ cm^-3 s that is equivalent to approximately 30 h of ambient ageing. We also apply this oxidation scheme to ambient air and study the compounds reacted and formed by proton transfer reaction mass spectrometry in high chemical detail. Finally, with the OH induced oxidation via the photolysis of O₃ we evaluate the limonene SOA mass yield without seed for a series of initial concentrations (5&ndash;48 ppbV) and a large range of OH exposure (0.016 to 10.1 x 10¹¹ cm^-3 s) and ozone levels (60&ndash;2500 ppbV); optimal SOA mass yields were found at an OH_exp of around 0.5 to 2 x 10¹¹ cm^-3 s.</p> <p>Based on the findings we discuss the advantages of LED irradiation, the decoupling of oxidants or precursor production and injection, and the effects of the applied materials on the condensation of reduced volatiles on chamber walls.