Advancing oxidation flow reactor technology: Simulating atmospheric oxidation with UVC LEDs

Oxidation flow reactors (OFRs) are widely used in atmospheric chemistry research to investigate the oxidation of volatile organic compounds (VOCs) and the formation of secondary organic aerosol (SOA). By generating highly oxidizing environments, OFRs enable simulation of hours to days of atmospheric photochemical aging within minutes of real time. Traditionally, oxidants are produced via ozone photolysis using low-pressure mercury discharge lamps. While effective, these lamps present several drawbacks, such as mercury-related environmental, health, and disposal concerns, inefficient, non-directional radiation, significant heat generation, and limited operational lifetime.

Here, we present a novel OFR design employing UVC light-emitting diodes (LEDs) as the photolysis source. Four modules, each equipped with 12 LEDs emitting at 265 nm (Violumas, VC12X1C48LC-265), are mounted around a quartz glass tube with a conical stainless steel inlet and outlet. The minimized radiative heat input from UVC LEDs enables a larger reactor design with an internal volume of 20.5 L (glass tube: length = 40 cm, diameter = 20 cm) by reducing buoyancy-driven convection. Thereby, laminar flow conditions with a typical residence time of ~ 4 minutes (adjustable via input flows) can be achieved, and wall interactions are minimized. Humidified air (RH = 30 %), ozone, and the sample of interest are introduced into the OFR, where ozone photolysis generates OH radicals, confirmed through toluene oxidation experiments. Particle size distributions and ozone concentrations are monitored at the outlet, where particles are also collected on filters. Transmission efficiency was characterized using PSL particles (100, 300, 460 nm) and two CPCs, showing > 80 % transmission, with UVC irradiation and heat generation having no measurable impact.

A movable core-sampling tube is coupled to a multi-scheme chemical ionization (MION2) Orbitrap mass spectrometer, enabling ultra-high-resolution measurements of oxygenated molecules. Experiments on α-pinene and limonene ozonolysis, as well as VOCs from cleaning products and bitumen, demonstrate the versatility of the setup for studying the simulated atmospheric oxidation and new particle formation (NPF) potential of these substances - critical for understanding urban emissions of growing relevance.

With higher efficiency, directional light output, superior thermal management, extended operational lifetime, and enhanced usability compared to conventional mercury lamps, UVC LEDs represent a significant advancement toward safer, more sustainable, and more controllable OFR technology for atmospheric chemistry applications.