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 (NOx) 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.