Global Model of Atmospheric Chlorate on Earth

Naturally occurring chlorate (ClO3 ) has been observed on Earth and potentially plays important roles in hydrology and mineralogy on Mars. However, natural sources of chlorate are uncertain. Here, we quantify the importance of atmospheric sources of chlorate. We use GEOS‐Chem, a global three‐dimensional chemical transport model, to simulate the formation, photochemical loss, transport, and deposition of atmospheric chlorate on present‐day Earth. We also develop a method to estimate the 17O‐excess (∆17O) and the 36 Cl‐to‐total‐Cl ratio (36Cl/Cl) of atmospheric chlorate to interpret the observed isotopic composition of chlorate accumulated in desert soils. The model predicts that gas‐phase chemistry can produce 15 Gg Cl year 1 of chloric acid (HClO3), which predominantly is taken up by aerosols to form particulate chlorate. Comparing the model with observations suggests that particulate chlorate undergoes chemical loss in the atmosphere, which controls the amount reaching Earth's surface. We show that the initial ∆17O that atmospheric chlorate acquires during formation would be erased rapidly in acidic aerosols due to the exchange of oxygen atoms with water. The analysis of 36Cl/Cl does not preclude a partial stratospheric origin for chlorate deposits in the Atacama Desert. In Death Valley, aqueous‐phase oxidation of oxychlorine species and anthropogenic activities potentially have greater influence. Our findings highlight the need for more observations of atmospheric chlorate and laboratory measurements of its reactivity in acidic conditions. Atmospheric chemistry should be considered in the future studies of the origin of chlorate on Mars. Plain Language Summary Chlorate (ClO3 ) is a pollutant in drinking water and food on Earth. On Mars, chlorate could act as an effective antifreeze that allows liquid water to exist under extremely cold and dry conditions and as an oxidant to promote the rusting of iron in surface minerals. Past observations suggested that natural chlorate is present on both planets, but very little is known about how it forms. Inspired by previous studies, we hypothesize that atmospheric chemistry is an important source of natural chlorate. We use 3‐D atmospheric chemical transport simulations to quantify the amount of chlorate that can be produced by the oxidation of chlorine species in air and the amount that can reach the surface on Earth. We also compare model predictions to published measurements of chlorate concentration and isotopic composition. Our analysis shows that atmospheric reactions are indeed a significant source of chlorate. The fraction of atmospheric chlorate that can reach the Earth’s surface is sensitive to the reactivity of chlorate in aerosols, which is currently very uncertain. More field observations of atmospheric chlorate and laboratory measurements of chlorate decomposition in acids will improve our understanding of atmospheric chlorate production and loss.