Deploying online instruments to evaluate oxidative properties of combustion and atmospheric aerosols

Outdoor air pollution is one of the greatest environmental risks to public health according to the World Health Organization. As 99% of the world’s population lives in areas which do not meet the recommended limit values, 6.7 million premature deaths per year can be attributed to the adverse health effects caused by the exposure to particulate matter (PM). This is comparable to the effects of direct tobacco use. While the link between adverse health effects and PM has been clearly established after decades of epidemiological studies the exact mechanism, sources and PM properties responsible are still poorly understood. Oxidative stress has been identified as one of the main possible toxicity pathways which is caused by an imbalance of the oxidant-to-antioxidant ratio that favours the former. PM can introduce particle-bound oxidants like reactive oxygen species (ROS) as well as compounds that can produce ROS in situ in the human body after particle inhalation, which is defined as the oxidative potential (OP). OP has been suggested as potential metric that could provide a link between PM with its specific toxicity-relevant composition and negative health effects. ROS and OP are commonly quantified using acellular assays that simulate oxidation reactions in the lungs using surrogate reductants. Conventionally, offline methods with time delays between sample collection and analysis are applied which can lead to significant underestimations. Due to long time delays between sample collection and measurement highly reactive compounds can decompose before analysis. Online adaptations avoid this by extracting PM directly into the reagents. Two recently published instruments applying different online assays have been deployed for several laboratory measurement campaigns investigating interactions of specific PM constituents as well as the effect of atmospheric processes of different anthropogenic combustion emissions (car, ship, aircraft, wood stove). Additionally, ambient measurements near a heavily congested road site and suburban background site as well as in an underground subway station in London, UK, were performed. In between campaigns, technical developments to improve the instrument’s reliability, user friendliness, and limit of detection were implemented. Significant and highly dynamic changes in ROS and OP activity of combustion PM were observed depending on combustion source and conditions (engine type, load, fuel) as well as photochemical ageing. In most cases an increase of ROS and OP with higher ages has been measured compared to primary emissions. This showed that these sources can also play a role
after atmospheric transport beyond the immediate emission. Even in a seemingly stable and isolated environment like an underground public transport system, dynamic changes of OP and ROS were observed in combination with changing physical and chemical properties of PM. These observations were made possible by applying high time resolution instruments that minimize sample degradation. Furthermore, after several deployments their operation as well as data management was streamlined and simplified.