The role of chlorine in tropospheric chemistry
Seminar | January 30 | 11 a.m.-12 p.m. | Hildebrand Hall, Library Room E
More than 4 million people die prematurely every year by breathing outdoor particulate matter (PM) and ozone, both secondary pollutants formed from tropospheric oxidation chemistry. PM and ozone also play key and uncertain roles in Earth's radiative balance. In order to protect human health and reduce levels of these pollutants, their mechanisms of formation in the atmosphere need to be understood. Hydroxyl (OH) and ozone (O3) are the most abundant tropospheric oxidants, but chlorine atoms are much more reactive and can oxidize functional groups or whole molecules that are resistant to the weaker common oxidants.
Tropospheric chlorine chemistry has not received as much attention because its importance was believed to be limited to coastal areas. However, recent ambient measurements have detected high concentrations of reactive chlorine species in inland and mid-continental regions, suggesting that chlorine chemistry is also important in continental regions. Ambient measurements conducted by our group in New Delhi, India periodically show extremely high concentrations of particulate chlorine, implying an important role of chlorine chemistry in this highly polluted megacity. In addition, due to its use as a disinfectant, chlorine chemistry can be important in the indoor environment.
Laboratory experiments conducted in our group show efficient formation of PM from chlorine-initiated oxidation of different hydrocarbon precursors. Using measurements from a high resolution time of flight chemical ionization mass spectrometer, we are able to track several generations of oxidation chemistry leading to the formation of organic particulate matter, as well as explore the molecular composition of PM. Chlorine-initiated reactions generally form PM at a higher yield than OH-initiated reactions, and the PM formed is often more oxygenated. Organochlorides form from all precursors investigated, even when the initial oxidation occurs via hydrogen-abstraction. Overall, our results suggest important impacts of chlorine chemistry on atmospheric composition which are currently not appropriately represented in air-quality models used to support the development of environmental policies.