Qualification of an online device for the measurement of the oxidative potential of atmospheric particulate matter

Abstract. Particulate Matter (PM) and gaseous pollutants can carry or induce the production of Reactive Oxygen Species (ROS) in the lung environment, causing oxidative stress, a key factor in the development of cardiovascular and pulmonary outcomes. Over the past decade, numerous techniques have been implemented to assess the Oxidative Potential (OP) of aerosols, i.e., their ability to oxidise the lung environment as an initial proxy of subsequent biological processes. Offline measurements from filters collected from air samplers are widely assessed but are probably underestimating PM redox activity due to the short lifetime of several ROS and/or the loss of the most volatile compounds on filters in a non-proportional and unsystematic way. This study introduces a new device, called ROS-Online, allowing the automatic and near real time measurement of two complementary OP assays, OP Ascorbic Acid (OP^AA) and OP Dithiothreitol (OP^DTT), sensitive to ambient PMs at mass concentrations about [PM₁₀] ~ 20 µg.m^-3. The ROS-Online device is designed to reproduce the exposure and interaction of airborne particles with the respiratory system. ROS-Online consists of three main modules: i) an air sampling module using a BioSampler^® to collect airborne PM, ii) a distribution module that transports samples and reagents to iii) a measurement module that relies on spectrophotometric methods to monitor chemical reactions in real time. Its operation is based on established OP^AA and OP^DTT protocols, ensuring comparability with existing offline OP measurement methods. Compact and transportable (75 × 65 × 170 cm, 85 kg), ROS-Online is designed for deployment in air quality monitoring stations and allows for autonomous operation over two weeks. With a high particle collection efficiency (> 90 % by mass for PM₁ and PM_2.5) and greater sensitivity than offline methods, it provides accurate and reliable results across a wide range of aerosol concentrations, from urban backgrounds to highly polluted environments. The qualification of the device demonstrated an excellent correlation with offline methods for both OP^AA and OP^DTT measurements (r > 0.96), over positive controls, confirming the reliability and specificity of ROS-Online for continuous atmospheric aerosol OP monitoring. ROS-Online was deployed in the field, in an urban background site, where OP^AA of ambient air was measured for 15 continuous days and OP^DTT for 6 continuous days. Results showed a good correlation with ozone (O₃) signal (R² = 0.74), underlying the importance of considering pollutants' interaction in OP measurements, as laboratory experiment showed no OP response when introducing O₃ alone into the instrument. These preliminary results mark an important step towards establishing ROS-Online as a viable and effective tool for OP assessment in future research and monitoring endeavours.