Reactions initiated by chlorine atoms can enhance the formation of ozone (O-3) and secondary organic aerosol (SOA) in the troposphere. Environmental chamber experiments were conducted to quantify heterogeneous Cl-2 production from NH(4)Cl( )and NaCl particles exposed to O-3 and hydroxyl radicals (center dot OH). Observations are inconsistent with models of Cl-2 production resulting solely from surface-mediated reactions of center dot OH and suggest that O-3 plays a significant role. The production of Cl-2 increased with relative humidity and decreased in the presence of SOA or nitric oxides (NOx). Heterogeneous reactive uptake coefficients for the production of Cl(2 )from O-3 on pure NH 4Cl (gamma(O3)) averaged 1.4 +/- 1.0 X 10(-3). Cl-2 production was six times more efficient on NH(4)Cl( )aerosol than on NaCl aerosol. Model calculations under atmospheric conditions suggest this heterogeneous Cl-2 production could increase peak daily O-3 concentrations by over 10%. (C) 2018 American Institute of Chemical Engineers