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Reactive Oxygen Species (ROS) in atmospheric aerosols: exploring formation, sources and dynamics of a new air pollution toxicity metric

Periodic Reporting for period 1 - Particle-bound ROS (Reactive Oxygen Species (ROS) in atmospheric aerosols: exploring formation, sources and dynamics of a new air pollution toxicity metric)

Reporting period: 2018-05-21 to 2020-05-20

Ambient particulate air pollution is one of the most severe public health issues worldwide as highlighted in a recent World Health Organization (WHO) report. It is unknown which particle sources and properties are the most health damaging but Reactive Oxygen Species (ROS), present in particles or generated by particle components upon deposition of particles in the human lung, are widely thought to be a significant contributor to particle-related toxicity. However, accurate ROS quantification remains challenging due to the reactive and short-lived nature of many ROS components and the lack of appropriate analytical methods for a reliable quantification, which makes it difficult to gauge their impact on human health. Identifying the ROS components and concentrations in ambient air and ultimately their sources would be crucial for an assessment of their health risks and to formulate improved and efficient air pollution mitigation strategies.

The overall objective of this project is to develop a scientific framework to quantitatively investigate the particle-bound ROS concentrations in polluted urban air, to explore the main sources of ROS and to identify what atmospheric conditions affect ROS formation in a real-world urban environment. We further aim to provide insight into the yield and formation mechanisms of ROS in secondary organic aerosol (SOA), and especially to identify how anthropogenic−biogenic interactions affect the formation and components of ROS in SOA.

The study will provide for the first time knowledge on sources and on characteristics of ROS and will provide essential and critical information for innovative strategies for air pollution mitigation and policies.
1 Explanation of the work carried per work package (WP)
(1) My current host group previously developed the first portable online ROS instrument (OPROSI) based on the HRP/DCFH assay (Atmos. Meas. Tech., 9, 4891–4900, 2016). After started my Marie-curie Fellowship at University of Cambridge, I firstly improved the detection limit of the OPROSI about a factor of 5 compared to the previous version through redesign of some hardware components, adding some new hardware components as well as modification of the software. This makes it sensitive enough to detect ROS levels even at moderate pollution conditions in European cities, which was not possible before.
(2) We then successfully conducted field campaigns during different seasons in two contrasting urban locations, in London, UK and in Padua in Po Valley, Italy. The campaign in London was conducted at a national air quality monitor station in central London during August 2019, which represents urban traffic emission. While in the Po Valley (Italy) in the winter, wood combustion from home heating significantly contributes to atmospheric aerosols. The two field campaigns established for the first time a comprehensive data set of atmospheric particle-bound ROS concentrations and temporal variability, measured with high time resolution, and explored how different sources and atmospheric conditions govern particulate ROS formation.
(3) We further comprehensively studied the evolution and yields of ROS during SOA formation from individual volatile organic compounds (VOCs) under a wide range of atmospherically relevant oxidation conditions by using an atmospheric simulation chamber and oxidation flow reactor. This study allowed us to get a deeper insight into atmospheric transformation processes of anthropogenic and biogenic emissions contributing to aerosol particle toxicity.
(4) In addition to the online ROS measurements, we further established standard protocol to systematically evaluate and compare offline ROS production in aerosol particles from the two European cites (London, UK, and Padua, Italy) and two Asian cities (Guangzhou, China and Kolkata, India), and from laboratory-generated SOA by using four ROS detection techniques. The detailed chemical composition of these samples has also been analyzed to allow us to correlate ROS concentrations with aerosol components.
(5) We have further extended the study to build correlations between ROS concentration in particles and the biological responses (e.g. cytotoxicity, cellular ROS production, gene expression) after exposing human lung cells in particle extracts. The detailed analyses of all these chemical and biological parameters are ongoing.

2 Main results achieved so far
(1) From the campaign in London, we observed clearly that atmospheric photochemical aging of traffic emissions significantly promoted ROS formation in particles. While, the results from the Po Valley’s campaign highlighted that biomass burning particles contain a substantial amount of ROS, which was significantly enhanced during fog episodes, indicating possibly an aqueous phase formation route of ROS during periods with low photochemical activity.
(2) The overall results from atmospheric simulation chamber and oxidation flow reactor experiments indicate that the ROS production in SOA is highly dependent on SOA precursors and formation conditions (e.g. oxidants, NOx and relative humidity).
(3) The results from offline ROS measurement clearly showed that the sources and chemical composition of atmospheric aerosols as well as atmospheric conditions affect aerosol oxidative potential significantly.
3.1 Main achievements
By the end of the project, we (1) gained a quantitative understanding of ambient particle-bound ROS concentrations and temporal variability with high time resolution in two European cities, and identify how atmospheric conditions that govern particle-bound ROS concentrations in the two polluted urban environments, (2) obtained further insight into the formation mechanisms of ROS in SOA, and especially identified how anthropogenic emissions affect their formation, (3) identified the associations between particle-bound ROS production and aerosol particle composition, and (4) evaluated the ability of various chemical assays to quantify ROS or aerosol oxidative potential as particle toxicity metric.
3.2 The potential impact of the project
Some preliminary results and findings have been disseminated to the scientific community, including presentations at international conferences, academic seminars and scientific papers in international peer-reviewed journals. More details can be found in section 5 (Dissemination) below. All the peer-reviewed scientific publications will be published in Open Access journals.
Overall, the scientific knowledge to be gained is crucial and indispensable for public health planners, policymakers and regulators to develop improved health-oriented air quality science and management policy. The research outcomes will promote more scholarly studies and attract regional and international researchers to communicate the health impacts of aerosol pollution.