Photocatalytic Chlorine Production from Iron Chlorides in Atmospheric Aerosols: Strategies for Quantifying Methane and Tropospheric Ozone Control

It was recently discovered that chlorine is produced photocatalytically from mineral dust sea spray aerosols, impacting methane and tropospheric ozone, and an evaluation was made of the climate and environmental impact of a chlorine-based intervention to draw down methane. The generation of chlorine by the iron chlorides Fe(III)Cl(3−n)n will also occur due to iron present in shipping plumes. To study efficiency and environmental implications, there is a need for additional information about the behavior of the process under a range of atmospheric conditions. Here we use box modeling to evaluate whether it is possible to experimentally observe this mechanism in a ship’s plume, or in a plume of pure iron dust, emitted for example from a tower. Detection limits for Cl, Cl2, HOCl, ClO, ClNO3, ClNO2, CO, C2H6, δ13C(CO) and CH2O are determined based on values from the literature. We find that the most promising and low-cost experimental indicators of Cl0 production are the concentration of photoactive iron and the CO:ethane ratio, and Cl2 is a useful indicator if cost is not a limitation. For ships with high NOx emissions, ClNO2 and ClNO3 could also potentially be used, and for towers emitting Fe without NOx the concentration of HOCl and ClO could be used. δ13C(CO) is a very direct method to detect methane removal, but only gives a clear signal for high iron emissions.