Laboratory Studies of Chlorine Activation by Nitrogen Oxides

Atmospheric chlorine atoms affect the lifetime of volatile organic compounds, including the greenhouse gas methane, and they enhance the rate of ozone production in polluted surface air that results in degraded air quality. We report results from laboratory studies of the heterogeneous chemistry of dinitrogen pentoxide (N₂O₅­), a nighttime reservoir of nitrogen oxide radicals (NO_x) and nitryl chloride (ClNO₂), a morning time Cl atom source produced from reactions of N₂O₅ on chloride-containing aerosol particles.  The main goal of this work is to assess the mechanisms responsible for production of Cl₂ from heterogeneous reactions of ClNO₂.  To mimic nighttime processes occurring in polluted coastal zones, acidic chloride-containing aerosol particles were created through an atomizer and then allowed to interact with ClNO₂ in a steady-state flow reactor.  The resulting gas-phase products were analyzed with a chemical ionization mass spectrometer (CIMS).  We show, for the first time, that Cl₂ is produced from the reaction of ClNO₂ on sub 1-micron aerosol particles containing chloride, and that the ClNO₂ to Cl₂ conversion is likely an equilibrium process under atmospheric conditions.