Heterogeneous production of Cl 2 from particulate chloride: Effects of composition and relative humidity

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 (•OH). Observations are inconsistent with models of Cl 2 production resulting solely from surface‐mediated reactions of •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 (NO x ). Heterogeneous reactive uptake coefficients for the production of Cl 2 from O 3 on pure NH 4 Cl ( γO 3) averaged 1.4 ± 1.0 × 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%. © 2018 American Institute of Chemical Engineers AIChE J , 64: 3151–3158, 2018