Periodic Reporting for period 3 - Trop-ClOC (Quantifying the impact of Tropospheric Chlorine Oxidation Chemistry) Période du rapport: 2022-02-01 au 2023-07-31 Résumé du contexte et des objectifs généraux du projet This project addresses a fundamental and yet highly unconstrained question in atmospheric chemistry: What is the impact of atomic chlorine on the composition of the troposphere? Gas phase oxidants control the concentrations of important climate and air pollutants such as methane, ozone and particles. Accurate representation of oxidation chemistry in computational models is paramount to our ability predict and understand past, present and future changes to the Earth system. Over recent decades there have been continual suggestions that the chlorine atom may be a significant tropospheric oxidant, but a lack of observations capable of constraining its chemistry mean that its role remains highly uncertain. Without these underpinning observations, our understanding of atmospheric oxidation and thus our ability to develop effective and timely policies to address air quality and climate change is compromised. This project will provide a step-change in our understanding of atmospheric chlorine chemistry. Capitalising on recent technology advances, two innovative instruments capable of definitively constraining chlorine atom sources and sinks will be developed. These instruments will be deployed at multiple chemically contrasting locations, generating the first comprehensive dataset on tropospheric chlorine atom production and loss. These data will be used to challenge the state-of-the-science representation of chlorine chemistry in atmospheric chemistry models. Ultimately this work will advance our understanding of the fundamental chemistry occurring in the atmosphere and help to direct developments in the next generation of air quality and climate models. Travail effectué depuis le début du projet jusqu’à la fin de la période considérée dans le rapport et principaux résultats atteints jusqu’à présent The first 2-years of the Trop-ClOC work program focused on the development of new instrumentation, including a new optical instrument to measure chlorine source compounds and a chlorine loss rate instrument. These developments have resulted in the successful deployment of a novel optical instrument for the detection of the largest chlorine reservoir compound, hydrochloric acid (HCl), as part of a study to determine the processes controlling air pollution in a northern latitude urban environment (Manchester). These measurements, along with supporting observations, are now being used to quantify the impact of chlorine on local and regional atmospheric chemistry. The Manchester study also enabled us to demonstrate the performance of a new technique for the detection of another key chlorine source compound, nitryl chloride (ClNO2). The newly developed chlorine loss rate instrument has also been developed, and is undergoing laboratory testing ahead of its deployment into the field. Progrès au-delà de l’état des connaissances et impact potentiel prévu (y compris l’impact socio-économique et les conséquences sociétales plus larges du projet jusqu’à présent) The novel instrumentation developed during the initial stages of Trop-ClOC significantly advance our ability to probe tropospheric chlorine chemistry. The field deployments and modelling components of the project will allow Trop-ClOC to deliver the maximum possible scientific impact. By generating the most comprehensive dataset on tropospheric Cl chemistry to date, in a range of contrasting chemical environments, Trop-ClOC will pose the ultimate challenge of our understanding of Cl chemistry. Ultimately this will improve estimations of important Earth system processes, such as the magnitude of global Cl oxidation of CH4 and the effect of Cl atoms on regional air quality, and thus will substantially reduce uncertainty in the next generation of global chemistry climate and air quality forecast models.
