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.
