Quantifying Chlorine Production from Photocatalysis of Iron Chloride Aerosol as a Potential Methane Sink

JIN YAN, Marie K. Mikkelsen, Luisa Pennacchio, Maarten van Herpen, Matthew S. Johnson, Mingyi Wang, University of Chicago

     Abstract Number: 252
     Working Group: Aerosol Chemistry

Abstract
Atmospheric methane is the most important non-CO₂ greenhouse gas. It has a global warming potential 82 times that of CO₂ on a 20-year timescale and is responsible for about 0.5 °C of the present-day 1.5 °C gross warming, with a rapid rising trend. As methane having many diffuse natural sources like wetlands, the changes in atmospheric oxidation ability may greatly affect global methane budget. A recent study has observed that extensive formation of chlorine radicals over the North Atlantic when iron-rich mineral dust mixes with chloride-containing sea spray aerosol significantly contribute to methane removal. As chlorine radicals react with methane about 15 times faster than hydroxyl radicals, photocatalytic reactions of iron chloride may serve as a powerful sink for methane. Understanding the mechanisms behind methane loss is crucial for improving atmospheric models' predictions of the current methane budget and its impact on global warming over the coming decades.

This talk will present recent results from our chamber experiments on the chemical mechanisms driving chlorine radical formation from iron-chloride complexes. First, we quantify the gas-phase chlorine production yields from pure FeCl₃ particles under baseline conditions, using bromide chemical ionization mass spectrometry. We then show how these yields vary over a temperature range from +20 °C to -20 °C, as governed by Henry’s Law constant, to span conditions from the marine boundary layer to the free troposphere. Finally, we discuss how gas- and particle-phase chlorine chemistry is altered by the presence of atmospheric trace gases, including sulfates and inorganic and organic acids.