10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Organic Aerosol from Chlorine-Initiated Oxidation of Hydrocarbons
LEA HILDEBRANDT RUIZ, Dongyu S. Wang, Surya Venkatesh Dhulipala, Sahil Bhandari, Catherine Masoud, Kanan Patel, University of Texas at Austin
Abstract Number: 1464 Working Group: Aerosol Chemistry
Abstract Chlorine atoms (Cl) are much more reactive than hydroxyl (OH) and ozone (O3) and can oxidize functional groups or whole molecules that are resistant to OH and O3. Cl can also initiate radical propagation pathways which generate OH as secondary radical. Thus, Cl can rapidly initiate the oxidation cascade that results in the formation of organic aerosol (OA). 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. In addition, due to its use as a disinfectant, chlorine chemistry can be important in the indoor environment.
Here we summarize results from laboratory experiments conducted in our group, which show efficient formation of OA from chlorine-initiated oxidation of different hydrocarbon precursors including isoprene, toluene, butyl carbitol and alkanes. Using measurements from a high resolution time of flight chemical ionization mass spectrometer (CIMS), we have tracked several generations of oxidation chemistry leading to the formation of organic particulate matter. We measured known and identified previously unknown reaction products of Cl-initiated oxidation, including highly oxidized molecules (HOMs). Using a filter inlet for gases and aerosols (FIGAERO) we analyze the molecular composition of organic aerosol and have found that Cl-initiated oxidation can form larger and lower-vapor pressure compounds than OH-initiated oxidation, which likely contributes to the larger aerosol mass yields often observed from Cl-initiated chemistry. OA formed from Cl-initiated reactions is often also more oxygenated than OA formed from OH-initiated chemistry.
The fast reaction rate of Cl with hydrocarbons enables complete consumption of all hydrocarbon precursors, including toluene, during an environmental chamber experiment. We evaluated the aging of toluene SOA by adding oxidant precursors after all toluene had been consumed. The addition of oxidants did not result in a significant change in the OA oxidation state, suggesting that the system may have reached an oxidative end-point in the particle phase. Organochlorides formed from all precursors investigated, even when the initial oxidation occurred via hydrogen-abstraction. Overall, our data suggest that chlorine chemistry can enhance atmospheric reactivity and increase the formation and transformation of organic aerosol. Cl-initiated formation of OA is currently not represented in air-quality models used to support the development of environmental policies; we argue that it should be.