A New Oxidation Flow Reactor for Secondary Aerosol Formation Studies

MARKUS NIKKA, Esa Luntta, Oskari Vainio, Erkki Lamminen, Anssi Arffman, Dekati Ltd.

     Abstract Number: 260
     Working Group: Instrumentation and Methods

Abstract
Oxidation flow reactors (OFR) are useful tools for studying the photochemical aging of transient emissions sources due to their high time resolution compared to environmental chambers. The high time resolution stems from the short aerosol residence time compared to environmental chambers. OFR equivalent photochemical aging time can be in the order of several days driven by the high concentration of oxidants compared to atmospheric conditions.

In this study, we present characterization results of a new OFR called Dekati Oxidation Flow Reactor (DOFR). DOFR design is similar to the well-studied Tampere University Secondary Aerosol Reactor (TSAR). The main oxidizer in the DOFR is OH-radical that is formed by UV-C (254 nm) photolysis of injected O₃ and H₂O. The characterizations performed for the DOFR include the determination of the equivalent photochemical age range, residence time distributions (RTD), and the secondary organic aerosol yield. In addition, the DOFR was tested by measuring the secondary aerosol formation from passenger cars running in idle.

Particle size distribution and RTD measurements were conducted using the ELPI+ (Dekati Ltd.) and SMPS (TSI Inc.) instruments. The equivalent photochemical age was determined by measuring CO-trace gas concentration (SICK Ltd., Sidor) with UV lights on/off. For the gaseous species, the RTD was measured producing CO-pulses to the reactor inlet and measuring the output pulse shape. Toluene SOA yield was determined and compared with earlier TSAR reactor studies.

The equivalent photochemical age was determined for several %RHs and UV-light intensities as a function of O₃ concentration. The equivalent photochemical age was found to be in the range of 0.2 – 6 days. Calculations were based on previous studies assuming a rate constant of 2.38*10-3 cm^-3s^-1 for the CO to CO₂ reaction. The RTD of particles and gases was found to match closely the modelled laminar flow reactor RTD.