A versatile laminar-flow oxidation reactor for studying multiple-day oxidation of atmospheric organics

Herein we introduce the fully automated Ionicon Laminar-flow Oxidation reactor (ILOx) for rapid photochemical oxidation of atmospheric organics, as a useful tool to mimic atmospheric processes of days within minutes. ILOx consists of a 110 cm long quartz glass tube with a total internal volume of 8 l that is irradiated by UVA and UVC LEDs. Oxidants can be introduced through multiple customizable inlet ports. To achieve laminar flow conditions, sample injection is CFD optimized to suppress any formation of injection jets. The outlet of ILOx allows for sampling both particles and VOCs simultaneously. A characterization of the particle transmission through the reactor with dried ammonium sulfate particles showed no significant change in the particle distribution before and after the reactor, proving the highly efficient particle transmission of the system. VOCs are coresampled to reduce wall interactions and potential formations of artifacts. In addition, all wetted surfaces are optimized for purest experiments providing fast response, even for reduced volatility gas-phase organics.

To experimentally confirm the oxidation potential of the reactor, air containing 2 ppbV of toluene is sampled through ILOx while VOCs are monitored by FUSION PTR-TOF 10 (IONICON Analytik, Austria). Just minutes after starting the UVA irradiation, 50% of toluene is oxidized, mimicking atmospheric aging in the range of 2 days. In addition, known toluene oxidation products like methyl glyoxal, cresols or dihydroxymethyl benzene are increasing. MCM 3.3.1 simulations of this experiment result in average OH concentrations of 3x10⁸ cm^-3, which equals to an OH exposure OH_exp of 10¹¹ cm^-3s. 

To characterize the overall efficiency of the system, we study the secondary organic aerosol (SOA) mass-yield of two aerosol precursors, xylene and limonene, respectively. For these experiments, humidified zero air (50% RH, 25°C) containing ~2 ppmV of ozone from an external 185 nm UVC source is used as the carrier gas. ILOx’s integrated 275 nm UVC LED is activated to photolyze O₃ to O₂ and O(¹D) to consequently form OH together with the carrier gas’ humidity. By adding xylene and limonene at atmospherically relevant concentrations of single-digit ppbVs we are able to identify a SOA mass-yield of 25.2 ± 2.7% for xylene and 36 ± 6.4%  for limonene.

Ultimately, we demonstrate the rapid oxidation of ambient air during a summer-time rush-hour event within ILOx. By intercomparison of FUSION PTR-TOF 10 mass spectra pre- and post-ILOx, with and without oxidation, we can quantitatively characterize the chemical compositions of ambient air and identify reacted and formed compounds. We can observe a clear change of chemical composition with a dominant reduction of aromatic and non-aromatic hydrocarbons (e.g. terpenoids) of higher volatility to a production of oxidized species of lower volatility.