Periodic Reporting for period 5 - Trop-ClOC (Quantifying the impact of Tropospheric Chlorine Oxidation Chemistry)
Période du rapport: 2025-02-01 au 2025-10-31
This project aimed to create a step change in our understanding of the role of atomic chlorine in controlling 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. The approach of Trop-ClOC to tackle this challenge has been through the development of novel measurement technologies and their subsequent deployment in different chemical environments to generate the data required to test and improve our understanding of tropospheric chlorine chemistry. The primary source of chlorine to the atmosphere is from marine aerosols, but mechanisms that liberate this into the gas phase are highly complex and uncertain. Our choice of measurement locations aimed to focus on this aspect of chlorine chemistry, and how this will change geographically. Our field locations thus focussed on a remote marine environment, a polluted marine environment, and a mid-continental location far from any marine sources of particulate chloride. 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.
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. The approach of Trop-ClOC to tackle this challenge has been through the development of novel measurement technologies and their subsequent deployment in different chemical environments to generate the data required to test and improve our understanding of tropospheric chlorine chemistry. The primary source of chlorine to the atmosphere is from marine aerosols, but mechanisms that liberate this into the gas phase are highly complex and uncertain. Our choice of measurement locations aimed to focus on this aspect of chlorine chemistry, and how this will change geographically. Our field locations thus focussed on a remote marine environment, a polluted marine environment, and a mid-continental location far from any marine sources of particulate chloride. 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.
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 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 multiple field deployments in a range of chemical environments along side multiple supporting observations of other chlorine compounds in both the gas and particle phase. The first of these deployments was in the city of Manchester, UK, to study the role of chlorine in a northern latitude urban environment that is heavily influenced by marine air masses. This work has quantified the role of chlorine as a radical source in multiple seasons in this location, which is relevant for many highly populated coastal regions in places such as Europe and SE Asia. A similar study was also undertaken in a remote marine environment on the island of Bermuda, enabling us to study chlorine chemistry in a marine location remote from anthropogenic influences. The final Trop-ClOC deployment was to a mid-continental urban environment, far from marine sources of chlorine, in Toronto. Collectively these data represent the most comprehensive dataset to date on the role of tropospheric chlorine chemistry, and analysis and interpretation is ongoing to extract the most new insight from these data and quantify the impact of chlorine on local and regional atmospheric chemistry. Challenges with some of the supporting observations during the remote marine deployment have also motivated a follow-up study in a similar environment, on the Cabo Verde Islands, which is taking place currently and will provide data to address key uncertainties identified in our previous work.
The novel instrumentation for the detection of both HCl and ClNO2 developed during the initial stages of Trop-ClOC significantly advance our ability to probe tropospheric chlorine chemistry. The field deployments have generated a unique dataset for the interrogation of tropospheric chlorine chemistry and the analysis and interpretation of these data will continue to provide new knowledge and insights for years to come. A major novelty of this work is the study of a range of contrasting chemical environments, providing a unique test of our understanding of key processes and their environmental sensitivities. 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.