This work proposes a new photocatalytic mechanism for chlorine atom production that occurs when Sahara dust mixes with sea spray aerosol.

The proposed mechanism is activated by sunlight and we estimate that the resulting atom production is the dominant source of atmospheric chlorine over the North Atlantic, as Saharan dust crosses over the Atlantic, and would increase chlorine production by 41% globally. The study relies on a combination of global modeling and laboratory and field observations, including air samples from Barbados showing seasonal depletion of the stable isotope 13CO, an anomaly which remained unexplained for 20 decades. It was known that the observed changes in 13CO and C18O in Barbados were evidence of chlorine atoms reacting with methane, and that carbon monoxide is the first stable product in atmospheric methane oxidation. But the known sources of atmospheric chlorine could not account for the degree of depletion in 13CO, until now. This new source of chlorine and their unaccounted-for effect on methane loss has implications for the methane budget and its evolution since Saharan dust emissions are changing. The results are published in PNAS: www.pnas.org/doi/10.1073/pnas.2303974120