Stratospheric gas-phase production alone cannot explain observations of atmospheric perchlorate on Earth

Perchlorate has been observed in many environments on Earth and Mars but its sources remain poorly quantified. In this study, we use a global three-dimensional chemical transport model to simulate perchlorate's gas-phase photochemical production, atmospheric transport, and deposition on Earth's surface. Model predictions are compared to newly compiled observations of atmospheric concentrations, deposition flux, and oxygen isotopic composition of perchlorate. We find that the modeled gas-phase production of perchlorate is consistent with reported stratospheric observations. Nevertheless, we show that this mechanism alone cannot explain the high levels of perchlorate observed at many near-surface sites (aerosol concentrations >0.1 ng m⁻³ and deposition fluxes >10 g km⁻² yr⁻¹) or the low ¹⁷O-excess observed in perchlorate sampled from pristine environments (<+18.4‰). We discuss four hypotheses to explain the model-observation discrepancies and recommend laboratory and field observations to address key uncertainties in atmospheric sources of perchlorate.