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Abstract 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 the¹⁷O‐excess (∆¹⁷O) and the³⁶Cl‐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⁻¹of 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 of³⁶Cl/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. 

Abstract Studies of wintertime air quality in the North China Plain (NCP) show that particulate‐nitrate pollution persists despite rapid reduction in NO_xemissions. This intriguing NO_x‐nitrate relationship may originate from non‐linear nitrate‐formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re‐interpret the wintertime observations of¹⁷O excess of nitrate (∆¹⁷O(NO₃⁻)) in Beijing using the GEOS‐Chem (GC) chemical transport model to estimate the importance of various nitrate‐production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NO_ychemistry and [PM_2.5] in observations and model simulations. We find that the model on average has a negative bias of −0.9‰ and −36% for ∆¹⁷O(NO₃⁻) and [O_x,major] (≡ [O₃] + [NO₂] + [p‐NO₃⁻]), respectively, while overestimating the nitrogen oxidation ratio ([NO₃⁻]/([NO₃⁻] + [NO₂])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO₂‐uptake on aerosols and an underestimate in wintertime O₃concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO₂in air‐quality models. The combined assessment of observations and model results suggest that N₂O₅uptake in aerosols and clouds is the dominant nitrate‐production pathway in wintertime Beijing, but its rate is limited by ozone under high‐NO_x‐high‐PM_2.5conditions. Nitrate production rates may continue to increase as long as [O₃] increases despite reduction in [NO_x], creating a negative feedback that reduces the effectiveness of air pollution mitigation.