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

Abstract

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)${Cl}_n^{\left(3-n\right)}$ 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, ${\text{Cl}}_2$, HOCl, ClO, ${\text{ClNO}}_3$, ${\text{ClNO}}_2$, CO, $C_2{H}_6$, ${\delta }^{13}{C}_{\mathrm{CO}}$, and ${\text{CH}}_{2}O$ are determined based on values from the literature. We find that the most promising and low-cost experimental indicators of chlorine atom production are the concentration of photoactive iron and the CO:ethane ratio, and ${\text{Cl}}_2$ is a useful indicator if cost is not a limitation. For ships with high ${\text{NO}}_x$ emissions, ${\text{ClNO}}_2$, and ${\text{ClNO}}_3$ could also potentially be used, and for towers emitting Fe without ${\text{NO}}_x$ the concentration of HOCl and ClO could be used. ${\delta }^{13}{C}_{\mathrm{CO}}$ is a very direct method to detect methane removal, but only gives a clear signal for high iron emissions.