Formation of Organic Particulate Matter from Chlorine-Initiated Oxidation of Hydrocarbons (Invited Presentation)

More than seven million people die prematurely every year by particulate matter (PM) in indoor and outdoor environments. Most of the outdoor PM is secondary, formed from the oxidation of hydrocarbons. Hydroxyl (OH) and ozone (O₃) are the most abundant tropospheric oxidants, but chlorine atoms (Cl) are much more reactive and can oxidize functional groups or whole molecules that are resistant to the weaker common oxidants. Cl can also initiate radical propagation pathways which generate OH as secondary radical. Thus, Cl can serve as a quick fuse, rapidly initiating the oxidation cascade that results in the formation of PM, and driving the formation of different products than those derived from OH and O₃. Over the last few years our group has been studying the formation for PM from Cl-initiated photo-oxidation.

Atmospheric chlorine chemistry has not received as much attention in the past 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 high formation of PM from chlorine-initiated oxidation of different biogenic and anthropogenic precursors – experiments so far have included isoprene, alpha-pinene, toluene, xylenes and alkanes. Using measurements from a high resolution time of flight chemical ionization mass spectrometer, we track several generations of oxidation chemistry leading to the formation of organic particulate matter, and analyze 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 – yields of highly oxidized molecules (HOM) are higher from Cl-initiated reactions than from OH-initiated reactions. 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.