Catalytic efficiencies for atmospheric methane removal in the high-chlorine regime

Luisa Pennacchio, Maarten van Herpen, Daphne Meidan, Alfonso Saiz-Lopez, Matthew S. Johnson

Catalytic production of chlorine atoms from iron salt aerosols (ISA) has been suggested as a means of achieving atmospheric methane reduction (AMR). The feasibility of this approach its efficiency and the optimum conditions for deployment must be determined. Success is not obvious because it depends on nonlinear atmospheric free radical chain reactions; under some conditions added chlorine is known to increase methane lifetime. Here we evaluate the catalytic efficiency of atmospheric methane oxidation, initiated by the photocatalytic conversion of chloride to chlorine by iron chlorides Fe(III)Cl(3−n)n , using a OD box model. While HOx and high NOx behaviours are well known, a new regime is characterized by high ClOx conditions ypified by CH3O2 reacting with ClO rather than NO or HO2. We find that at NOx mixing ratios below 50 ppt or above 390 ppt, methane removal per iron atom is always net positive regardless of the Cl2 addition rate. However, between these NOx mixing ratios and for a chlorine production rate below 1×10^6 Cl2 /(cm3 s) the net effect is negative, increasing CH4 concentrations. The efficiencies seen in the model range from -0.26 to 2.63 CH4/Cl.

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