Kinetics of Hypochlorous Acid Reactions with Organic and Chloride-Containing Tropospheric Aerosol

SPIRO JORGA, Tengyu Liu, Yutong Wang, Sumaiya Hassan, Han N. Huynh, Jonathan Abbatt, University of Toronto

     Abstract Number: 60
     Working Group: Aerosol Chemistry

Abstract
The Cl atom plays a crucial role in tropospheric oxidation processes, in both the marine and continental environments. Although modeling studies have explored the importance of halogen chemistry, uncertainty remains in associated chemical mechanisms and fundamental kinetics parameters. Prior kinetics measurements of multiphase halogen recycling reactions have been performed with dilute, bulk solutions, leaving unexplored conditions with more realistic chemical systems which have high solute concentrations and are internally mixed with both inorganic and organic components. Here, we address the multiphase kinetics of HOCl using an aerosol flow tube and aerosol mass spectrometer to study its reactions with particulate chloride, using atmospherically relevant particle acidity, solute concentrations, and ionic strength. We also investigate the chemistry that results when biomass burning aerosol components and chloride are internally mixed. Using pH-buffered particles, we show that the rate constant for reaction of HOCl with H⁺ and Cl^- is within a factor of two of the literature value at high relative humidity (RH) (80-85%). However, at lower RH values (60-70%) where the particles become concentrated, the rate constant is an order of magnitude higher. For pure organic compounds commonly found in biomass burning (BB) aerosol, an increase in the aerosol chlorine concentration occurs with HOCl exposure, indicating the formation of organochlorine species. Together, these independent findings explain results for internally mixed aerosol particles with both chloride and BB components present where we observed results consistent with both chloride loss and organochlorine formation occurring simultaneously with HOCl exposure. Overall, we conclude that chlorine recycling via HOCl uptake will occur under a wide range of RH conditions and when reactive BB organic compounds are present in the same particles as chloride.