Catalytic Efficiencies for Methane Removal: Impact of HO<i>x</i>, NO<i>x</i>, and Chemistry in the High-Chlorine Regime

Catalytic production of chlorine atoms from iron salt aerosols has been suggested as a means of achieving atmospheric methane removal. The feasibility of this approach, its efficiency, and the optimum conditions for deployment must be determined, but this is not straightforward as the mechanism involves interlocking 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 under different conditions, initiated by the photocatalytic conversion of chloride to chlorine by iron chlorides Fe(III)Cln(3–n) using a box model. While HOx and high NOx behaviors are well-known, a new regime for tropospheric chemistry is found and described, one characterized by high ClOx conditions. We find that at chlorine production rates below 1 × 106 Cl2 /(cm3 s) and ambient NOx and O3 levels of 4–80 ppt NOx at 14 ppb O3, 8–180 ppt NOx at 30 ppb O3, and 14–200 ppt NOx at 40 ppb O3 the net effect on CH4 is negative, increasing CH4 concentrations. This variation is driven by the formation and hydrolysis of ClONO2 leading to the loss of O3 and NO2. At high rates of Cl2 addition the reaction of CH3OOH with Cl becomes the major source of OH and CH4 is removed. At elevated ClOx, ClO• usurps the role of NO in converting HO2 to OH and CH3O2 to CH3O. The efficiencies seen in the model range from −0.62 to 2.81 CH4/Cl. The modeling shows that due to the dispersion of a ship’s plume into low NOx conditions, iron emitted by ships is likely to increase the lifetime of atmospheric methane.