Direct spectroscopic detection of tropospheric chlorine radicals

The extremely reactive chlorine radical contributes to many important chemical processes in the troposphere, including loss of ozone in the Arctic boundary layer, production of ozone in polluted regions, oxidation of mercury, removal of methane and other organic compounds, and formation and aging of secondary organic aerosols. To date, there has been no demonstrated direct method for quantifying the concentrations of these radicals. CIRES research scientist Andrew Rollins and NOAA research physicist Joshua Schwarz propose a unique method for solving this problem. Instead of estimating chlorine (Cl) radical concentration using measurements of other species (which might underestimate or overestimate Cl), the research team plans to build a prototype instrument that uses two photons to sequentially excite Cl from the ground state and then detects blue-shifted fluorescence on a zero background.  Such an instrument would help scientists directly verify the existence of this species, quantify its highly variable concentrations, and constrain the possibility of yet-unidentified chemistry. This would enable significant advances in the study of tropospheric reactive halogen processes, and help reveal anthropogenic changes to this region of the atmosphere.