A new oxidation flow reactor for the measurements of secondary aerosol formation: Characterisation and a case study

Oxidation flow reactors (OFRs) have been extensively used to investigate secondary aerosol formation of the ambient air or emission sources such as biomass burning or vehicular emissions, and they can provide useful information on the mechanisms of secondary aerosol formation. Here we present the design and characteristics of a newly developed OFR, named the Rapid Aerosol Ageing Device (RAAD). The RAAD was specially designed to provide time-resolved and fast-response chemical evolution of aged aerosols and can be applied to investigate how oxidative stress levels change upon ageing. The characterisation experiments included optimisation of the flow rate, residence time distributions for gas and particles, transmission efficiencies of particles, OH exposures with varying O3 concentrations and relative humidity conditions. Results show that the RAAD has near-laminar flow conditions at a total flow rate of 45 slpm with a low surface-area-to-volume ratio of 26.52 m-1. Wall losses for particles with mobility diameters greater than 40 nm are negligible, and narrow residence time distributions of gas and particles were observed. These features allow a better response to the ageing of rapid-changing emission sources, and measurements can be taken directly for instruments demanding high flow rates without diluting the samples. Additionally, the performance of the RAAD was further evaluated by measuring the secondary aerosol formation of biomass burning emissions. The organic aerosol enhancement ratios were comparable with other ageing studies, with an average value of 1.31 ± 0.05. In addition, the absorption Ångström exponent (AAE) value decreased from 1.7 ± 0.05 (primary emissions) to 1.4 ± 0.06 upon ageing. Overall, the RAAD is suitable for studying the formation mechanisms of photochemically aged aerosols from emission sources, and future work will aim to provide more details on the secondary aerosol production from ambient air.